the geochemistry of oceanic anoxic events

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Volume 11, Number 3

9 March 2010

Q03004, doi:10.1029/2009GC002788

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Geochemistry of oceanic anoxic events

Hugh C. JenkynsDepartment of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK ([email protected])

[1] Oceanic anoxic events (OAEs) record profound changes in the climatic and paleoceanographic state ofthe planet and represent major disturbances in the global carbon cycle. OAEs that manifestly caused majorchemical change in the Mesozoic Ocean include those of the early Toarcian (Posidonienschiefer event,T‐OAE, ∼183 Ma), early Aptian (Selli event, OAE 1a, ∼120 Ma), early Albian (Paquier event, OAE 1b,∼111 Ma), and Cenomanian–Turonian (Bonarelli event, C/T OAE, OAE 2, ∼93 Ma). Currently availabledata suggest that the major forcing function behind OAEs was an abrupt rise in temperature, induced byrapid influx of CO2 into the atmosphere from volcanogenic and/or methanogenic sources. Global warm-ing was accompanied by an accelerated hydrological cycle, increased continental weathering, enhancednutrient discharge to oceans and lakes, intensified upwelling, and an increase in organic productivity. Anincrease in continental weathering is typically recorded by transient increases in the seawater values of87Sr/86Sr and 187Os/188Os ratios acting against, in the case of the Cenomanian‐Turonian and early AptianOAEs, a longer‐term trend to less radiogenic values. This latter trend indicates that hydrothermally andvolcanically sourced nutrients may also have stimulated local increases in organic productivity. Increasedflux of organic matter favored intense oxygen demand in the water column, as well as increased rates ofmarine and lacustrine carbon burial. Particularly in those restricted oceans and seaways where densitystratification was favored by paleogeography and significant fluvial input, conditions could readilyevolve from poorly oxygenated to anoxic and ultimately euxinic (i.e., sulfidic), this latter state being geo-chemically the most significant. The progressive evolution in redox conditions through phases of deni-trification/anammox, through to sulfate reduction accompanied by water column precipitation of pyriteframboids, resulted in fractionation of many isotope systems (e.g., N, S, Fe, Mo, and U) and mobilizationand incorporation of certain trace elements into carbonates (Mn), sulfides, and organic matter. Sequestrationof CO2 in organic‐rich black shales and by reaction with silicate rocks exposed on continents would ulti-mately restore climatic equilibrium but at the expense of massive chemical change in the oceans and overtime scales of tens to hundreds of thousands of years.

Components: 20,549 words, 8 figures.

Keywords: geochemistry.

Index Terms: 1051 Geochemistry: Sedimentary geochemistry (1605); 4901 Paleoceanography: Abrupt/rapid climate change(1605); 4924 Paleoceanography: Geochemical tracers; 4948 Paleoceanography: Paleocene/Eocene thermal maximum.

Received 19 August 2009; Revised 2 December 2009; Accepted 17 December 2009; Published 9 March 2010.

Jenkyns, H. C. (2010), Geochemistry of oceanic anoxic events, Geochem. Geophys. Geosyst., 11, Q03004,doi:10.1029/2009GC002788.

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http://dx.doi.org/10.1029/2009GC002788

1. Introduction

[2] Although the concept of the oceanic anoxicevent (OAE) was introduced more than 30 yearsago [Schlanger and Jenkyns, 1976], the forcingfunctions behind such phenomena remain prob-lematic. The general presumption, given in earlyworks on this topic, was that during discrete inter-vals of geological time an expanded and intensi-fied oxygen minimum zone characterized much ofthe World Ocean, a chemical state accompaniedby enhanced accumulation of marine organic‐richsediment. Although OAEs were initially recognizedby the presence of apparently coeval Cretaceousmarine carbon‐rich sediments (primarily black shales)exposed in Europe and below the seafloor of partic-ularly the Pacific Ocean (half a world away), recenthigh‐resolution studies have highlighted problemswith a definition of the phenomenon relying purelyon the stratigraphic distribution of organic matter.Local variations in depositional and diagenetic con-ditions have manifestly affected the preservation anddilution of suchmaterial to some degree [Tsikos et al.,2004a].

[3] As with many carbon‐rich deposits, there havealso been controversies as to the relative roles ofincreased organic productivity and increased pres-ervation [Demaison and Moore, 1980; Pedersenand Calvert, 1990], with the balance now tippingin favor of the former as generally the moreimportant factor, even though the evidence fromcalcareous nannofossils, when taken as produc-tivity indicators, has proven problematic [Kuyperset al., 2002a; Jenkyns, 2003; Erba, 2004; Browningand Watkins, 2008; Mattioli et al., 2008]. Althoughthe number of OAEs recorded in Cretaceous stratahas multiplied over time from the those initiallyrecognized [Schlanger and Jenkyns, 1976; Coccioniet al., 1987; Arthur et al., 1990; Leckie et al., 2002;Erba et al., 2004; Baudin, 2005] only one otherdefinitive example is currently identified fromthe rest of the Mesozoic Era, namely that of theearly Toarcian in the Jurassic [Jenkyns, 1980,1985, 1988]. A number of equivalent events arerecognized from the Paleozoic Era, of which theKellwasser event/s of the Devonian is/are perhapsthe best documented [Büggisch, 1991]. All suchevents, which are accompanied by major chemicalchanges in the ocean, seem to be characteristic ofa warm, equable “greenhouse Earth” [Jenkyns,2003].

[4] Discussion is here primarily limited to eventssuggested to be of global significance (Figure 1),

namely those of the early Toarcian (Posido-nienschiefer event, T‐OAE, ∼183 Ma), earlyAptian (Selli event, OAE 1a, ∼120 Ma), earlyAlbian (Paquier event, OAE 1b, ∼111 Ma) andCenomanian–Turonian (Bonarelli event, C/T OAE,OAE 2, ∼93 Ma). Other Cretaceous OAEs havebeen recognized, particularly from the Tethyandomain (OAE 1c, OAE 1d [Arthur et al., 1990]),OAE 1c being somewhat problematic in that therecord is patchy and horizons assigned to this eventmay not be coeval and likely record episodes oflocal basinal restriction and density stratification[Tiraboschi et al., 2009]. However, OAE1d (lateAlbian Breistroffer event), initially recognized,like the lower Albian Niveau Paquier, from a hori-zon in the Vocontian Trough of southeast France[Breistroffer, 1937;Bréhéret, 1997], has equivalentsin the Atlantic Ocean as well as being identified inpelagic limestones of the Franciscan accretionarycomplex of California [Wilson and Norris, 2001;Bornemann et al., 2005; Robinson et al., 2008],suggesting a potentially global imprint. Similarly,the Early Cretaceous Valanginian Weissert event,with a patchy organic‐rich record known from theTethyan region, as exemplified from the SouthernAlps in Italy, the Atlantic and the central Pacific,would seem to qualify as an OAE of some sig-nificance in terms of global environmental change[Lini et al., 1992; Erba et al., 2004; Bornemannand Mutterlose, 2008; Brassell, 2009]. The latestHauterivian Faraoni event, recognized in the Tethyanand Atlantic domains and possibly beyond, has alsobeen characterized as an OAE [Cecca et al., 1993;Baudin, 2005; Bodin et al., 2007]. OAE 3, of lateConiacian to Santonian age, appears to be representedlargely by a widespread and stratigraphically exten-sive black shale record in the Atlantic, althoughoccurrences beyond this region, for instance in theWestern Interior of the United States, are known[Arthur et al., 1990; Wagner et al., 2004].

[5] Computing the exact durations of OAEs de-pends critically on how they are defined but, inthe case of the T‐OAE, early Aptian OAE, earlyAlbian OAE and the C/T OAE, estimates basedon cyclostratigraphy and notional durations offaunal zones, typically fall in the range of a fewhundred thousand years, or more in the case of theearly Aptian OAE [Sageman et al., 2006; Li et al.,2008; Suan et al., 2008; Voigt et al., 2008;Sabatino et al., 2009]. For the early Albian OAE,the duration seems to have been somewhat lessthan a hundred thousand years [Wilson and Norris,2001; Jenkyns, 2003; Erba, 2004]. Even thoughthe sedimentary record of OAEs varies from place

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Figure 1. Time scale [Gradstein et al., 2005] illustrating the stratigraphic position and nomenclature of OAEsdiscussed in the text. The major OAEs, identified by the extent of associated chemical change, are denoted by thelarger pink‐colored ellipses. Discussion in the text concentrates on these three major OAEs, the early Albian PaquierEvent (OAE 1b), and the PETM.


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to place and can be represented locally by depositslargely lacking organic matter, particularly in shal-low water facies, it can be hypothesized that anincrease in organic productivity was globally syn-chronous, in turn requiring generally enhancedlevels of nutrient availability in the photic zoneacross much of the World Ocean for many tens orhundreds of thousands of years. In well‐oxygenatedcarbonate platform settings not conducive to accu-mulation of organic matter, the extinction of cer-tain biota nevertheless suggests the upwelling/overturning of nutrient‐rich waters, changing shal-low water environments from oligotrophic to eutro-phic and eliminating organisms unable to survive insuch detrimental conditions [Parente et al., 2008].Understanding how ocean waters could maintainthemselves in such a fertile state over relativelylong time scales, albeit with periodic fluctuations innutrient availability, offers a major paleoceano-graphic challenge [Meyer and Kump, 2008]. In thefollowing account, major Mesozoic OAEs areexamined from the youngest to the oldest, reflectingthe density of currently available geochemical data.

2. Organic Carbon as the KeySedimentary Signature of the OAE

[6] The defining characteristic of the Cenomanian–Turonian OAE is given by the outcrops aroundGubbio, in the Marche–Umbrian Apennines ofcentral Italy. Here, a 1 m thick level of blacklaminated organic‐rich shale (Total Organic Car-bon (TOC) contents locally >30 wt %) and brownsucrosic radiolarian sand (Livello Bonarelli, namedafter its discoverer, Guido Bonarelli, who describedit as uno strato di scisto nero bituminoso dellospessore di un metro circa (a bed of black bitu-minous shale about a meter thick)) interrupts reg-ularly bedded cherty pink and white pelagiccoccolith‐rich limestones [Bonarelli, 1891; Arthurand Premoli Silva, 1982; Tsikos et al., 2004a;Bernoulli and Jenkyns, 2009]. A number of othernorthern and southern European outcrops also il-lustrate a similar sedimentary signature: in theEnglish Upper Cretaceous, for example, the keyinterval is locally (in northeast England) reduced toa 10 cm thick level of black, partly laminated,partly bioturbated shale encased above and belowby white pelagic chalk [Schlanger et al., 1987]. Insouthern England, the chalk largely lacks organicmatter at the same biostratigraphically defined level,even though the facies become darker colored andrelatively rich in clay [Tsikos et al., 2004a].

[7] In parts of Sicily, some outcrops resemble theLivello Bonarelli; others record this interval, andsubjacent and suprajacent strata, as a continuoussequence of black, organic‐rich shale and lime-stone [Scopelliti et al., 2008], as also developed inTunisia and Morocco [Nederbragt and Fiorentino,1999; Kolonic et al., 2005]. In these North Africanlocalities, where more continuous sedimentation oforganic‐rich shale is recorded, the interval charac-teristic of the OAE may be identified both by high‐resolution biostratigraphy and relatively elevatedcontents of TOC [Tsikos et al., 2004a]. A similarincrease in TOC is recorded in coeval materialcored from the North Atlantic [Kuypers et al.,2002a]. However, such relationships are not uni-versal. In the Cretaceous of the Western Interior,U.S.A., Cenomanian–Turonian boundary strataexhibit only modest enrichment in organic mattercompared with the strata above, even though pri-mary production and bulk sedimentary rates, bothfactors that should have aided preservation, wererelatively high [Meyers et al., 2001, 2005]. Accu-mulation of organic matter was clearly influencedby a variety of environmental factors peculiar tothe basin, among which the lack of sulphidic bot-tom waters may be critical [Sageman et al., 1997;Meyers, 2007]. Generally, however, whereverhigh‐resolution biostratigraphy exists, the mostorganic‐rich units can be shown to be specificallyassigned to the latest Cenomanian–earliest Turonianand can be considered as having been depositedduring the OAE [Tsikos et al., 2004a; Scopelliti etal., 2004, 2008].

[8] Deep marine pelagic Atlantic, Caribbean andAlpine‐Mediterranean areas provide the spatiallymost extensive and most carbon‐rich facies of theC/T interval, but the stratigraphic thickness andrichness of these black shales are reduced as out-crops are followed eastward across North Africainto Asia [Lüning et al., 2004]. In Tibet, TOCvalues of Cenomanian–Turonian boundary strataare generally <2 wt % [Wang et al., 2001]. Con-tinuing further east and entering the southernhemisphere, on the Exmouth Plateau off westernAustralia, black shales of this age reappear withTOC values locally rising to >25 wt % [Rullkötteret al., 1992; Pancost et al., 2004]. In the PacificOcean, coeval organic‐rich sediments are recordedfrom oceanic plateaus (e.g., Shatsky Rise: TOC of∼9 wt %; Hess Rise; the sediment burned whenheated) as well as areas with greater paleodepth,such as the Mariana Basin, where TOC rises to∼10 wt %. In the latter case, the 2 cm thick


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radiolarian‐rich black shale occurs in a sectioncontaining redeposited sediment and may itselfhave been transported basinward from shallowersubmarine sites [Schlanger et al., 1987].

[9] The original oversimplified concept of an OAEextending through the Aptian and Albian stages ofthe Cretaceous [Schlanger and Jenkyns, 1976;Jenkyns, 1980] has since been modified by therecognition of a number of discrete events, ofwhich those of the early Aptian (Selli event, OAE 1a[Coccioni et al., 1987]) and early Albian (Paquierevent, OAE 1b, first recognized in the VocontianTrough of southeast France [Bréhéret, 1985]) havethe most extensive geological record [Arthur et al.,1990]. Both these events are well represented byorganic‐rich shales in the Alpine‐Mediterraneanregion (TOCs typically in the 2–18 wt % for theLivello Selli [Coccioni et al., 1987; Pancost et al.,2004; Baudin et al., 1998], locally extending to∼30 wt % for the equivalent to the Paquier Level inwestern Greece [Tsikos et al., 2004b]). Becausethese Aptian and Albian organic‐rich shales typi-cally occur interbedded with dark‐colored argilla-ceous units they are not as obvious at outcrop as isthe Livello Bonarelli, whose color difference withthe enclosing limestones is more dramatic. Althoughwell represented in the Atlantic, the Livello Sellior its equivalent (OAE 1a) is also recorded as adistinctive black shale from a number of sites inthe central and north Pacific Ocean, representing aconsiderable range of paleodepths (Manihiki Plateau,Shatsky Rise, Resolution Guyot and deeper parts ofthe Mid‐Pacific Mountains), and in the allochtho-nous Calera Limestones of the Franciscan Complexin California [Sliter, 1989; Bralower et al., 1993,1994; Jenkyns and Wilson, 1999; Dumitrescu andBrassell, 2006; Robinson et al., 2008]. In theselocalities, the black shale contrasts sharply withenclosing lithologies of pelagic limestone and chert,hence resembling the Cenomanian–Turonian LivelloBonarelli in its type locality. The sediment of theLivello Selli equivalent cored from Shatsky Riseis extraordinarily rich in organic matter (max TOC∼40 wt % [Dumitrescu and Brassell, 2006]),implying a dramatic flux of organic matter tothe equatorial Pacific seafloor over the time intervalin question.

[10] Although the Paquier event has not beenidentified as such in the Pacific, the stratigraphiclevel that records the OAE 1b corresponds with anincrease in the relative abundance of biogenic chert[Robinson et al., 2004]. Given that black shalesformed during OAEs are generally accompanied by

an increase in radiolarian silica, the factor incommon would seem to be an increase in the fer-tility of ocean waters, promoting a shift away fromcalcareous nannofossils and foraminifera towardsiliceous and organic‐walled phytoplankton and(predatory) zooplankton [Jenkyns, 2003]. Hence,this OAE potentially had global reach. The organicmatter of the black shales formed during the Paquierevent, as represented by outcrops in France andGreece and in Atlantic sites, is dominated by cer-tain isoprenoidal biomarkers indicating that archaeawere a principal component of the plankton [Kuyperset al., 2002b; Tsikos et al., 2004b]. In this way, itdiffers fundamentally from the organic matter (thusfar analyzed) constituting the black shales of theearly Aptian (Selli) and C/T OAEs, which had adominant phytoplanktonic source [Kuypers et al.,2004; Ohkouchi et al., 2006; van Breugel et al.,2007]. Clearly, the organisms that flourished in thewater column during OAEs were not necessarily thesame.

[11] The record of the early Toarcian OAE is nec-essarily limited to outcrops and wells from formershelves and continental margins because of theabsence of Lower Jurassic oceanic crust on theplanet. However, the global spread of organic‐richsediments of this age is impressive [Jenkyns, 1988;Jenkyns et al., 2002] and, in one locality in Japan,black organic‐rich radiolarian cherts assigned tothe OAE occur as elements of accretionary com-plexes derived from the paleo–Pacific Ocean [Hori,1997]. In Europe, the TOC patterns show a strikingpolarity between northern and southern Europe: inareas such as Britain (Figure 2), Germany,Switzerland and France, the organic carbon levelslocally rise to ∼20 wt % [Küspert, 1982; Jenkynsand Clayton, 1997; Röhl et al., 2001] whereas, insouthern or Tethyan Europe, the values rarelyexceed 5 wt % in pelagic sediments [Jenkyns,1985, 1988; Jenkyns et al., 2001; Pancost etal., 2004; Sabatino et al., 2009]. In coastalPortugal, a hemipelagic carbonate‐rich section islargely devoid of organic matter except for an8 cm bed of black shale whose TOC value is ∼2wt%[Hesselbo et al., 2007;Hermoso et al., 2009a]. UsingTOC levels alone, correlation across Europe thatconforms with available biostratigraphy is bestachieved by taking the level of maximum enrich-ment in organic carbon as a tie point [Jenkyns et al.,2002]: such a relationship implies that carbon flux,likely reflecting highest regional organic produc-tivity, was the first‐order variable in this case andsedimentation rates were less significant. As well as


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local factors such as spatial variations in produc-tivity and redox conditions in the water column,the greater water depths in which pelagic Tethyansediments were deposited would have meant greatertransit times to the seafloor for organic matter andconsequent greater likelihood of oxidation beforefinal burial. The early Toarcian oceanic anoxicevent, as defined by both relatively high TOCvalues and ammonite and nannofossil biostratig-raphy, is clearly synchronous across Europe andwill undoubtedly prove to be so in a global context,given its recent recognition in Argentina [BucefaloPalliani et al., 2002; Jenkyns et al., 2002; Sabatinoet al., 2009; Al‐Suwaidi et al., 2009].

[12] One factor that results in relative elevation ofTOC values is the drop in carbonate that typifiesmost black shales that were formed during OAEs:an extreme case in point would be the LivelloBonarelli itself, which is effectively devoid ofcalcite, although its exact correlatives in Sicily doyield nannofossils and foraminifera with averageCaCO3 values of ∼20 wt % and TOC values typi-cally in the 12–25 wt % range [Scopelliti et al.,2004, 2006]. To what extent this relative paucityof carbonate is related to oceanic acidification from

assumed introduction of atmospheric carbon diox-ide and to what extent it is due to a move awayfrom calcareous toward siliceous and organic‐walled plankton under high‐fertility conditions isunclear, but both factors likely play a role [Erba,2004]. To some extent there seems to be a tradeoff in terms of thickness and TOC values in that theequivalent level to the Livello Bonarelli in Greeceis reduced to a mere 35 cm of carbonate‐free blackshale whose TOC values are as high as 44.5 wt %[Karakitsios et al., 2007].

3. Carbon Isotope Signatures of OAEs

[13] Because an OAE describes a phenomenoncharacterized by anomalously high burial rates ofmarine organic carbon (reduced reservoir, rela-tively 12C‐rich) an increase in the d13C values ofmarine and atmospheric carbon (oxidized reser-voir) is predicted. Hence, the recognition, in1980, of a pronounced regionally developedpositive carbon isotope excursion in d13Ccarbonate

across the Cenomanian–Turonian boundary con-formed perfectly to expectations [Scholle andArthur, 1980]. The carbon isotope signatures of

Figure 2. Total organic carbon (TOC), organic carbon isotope, and molybdenum isotope stratigraphy of the LowerToarcian black shales cropping out on the Yorkshire coast, northeast England [Jenkyns and Clayton, 1997; Hesselboet al., 2000; Kemp et al., 2005; Pearce et al., 2008]. The OAE is primarily defined by the relatively high TOC levels;a lower interval of values rising to a maximum (darker band) can be distinguished from a higher interval (lighter band)where values are lower but still distinctly above background. The stepped negative excursion in the organic carbonisotope profile, attributed to influx of isotopically light carbon into the ocean–atmosphere system, is characteristic ofthe early Toarcian OAE and, in fact, punctuates a broad positive excursion that may be attributed to massive burial ofmarine organic matter on a global scale [Jenkyns, 1985, 1988, 2003]. When conditions were sufficiently euxinic,molybdenum isotope ratios should have recorded the signature of ambient seawater because, in the absence of oxicsinks for this element, quantitative removal would have taken place [Neubert et al., 2008]. However, the extent towhich the geochemistry of early Toarcian seawater in the north European seaway reflected the composition of theglobal ocean remains problematic [McArthur et al., 2000, 2008].


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the early Albian, early Aptian and early ToarcianOAEs are, however, more problematic in thatsignals from both carbonate, organic matter andspecific biomarkers display both positive and pro-nounced negative excursions [Jenkyns and Clayton,1986, 1997; Menegatti et al., 1998; Schouten et al.,2000; Jenkyns, 2003;Herrle et al., 2004], indicatingthat, as well as enhanced carbon burial drivingglobal d13C to heavier values, an input of isotopi-cally light carbon must be responsible for move-ment in the opposite direction. Within the broadpositive excursion associated with the C/T OAE 2,all sections illustrate a subdued negative shift: thisis particularly pronounced in the organic carbonisotope record at the base of the Livello Bonarelliwhere a fall of 3‰ is recorded [Kuroda et al., 2007].Introduction of some isotopically light carbon intothe ocean–atmosphere system during the C/T OAEis thus likely.

[14] Possible processes supplying isotopically lightcarbon at the onset of or during OAEs includelarge‐scale venting of volcanogenic carbon dioxideand/or dissociation of gas hydrates and/or thermalmetamorphism of coals, either singly or in combi-nation, with no consensus yet achieved as to therelative importance of these processes [Hesselbo etal., 2000; Jahren et al., 2001; Jenkyns, 2003;McElwain et al., 2005; Kuroda et al., 2007]. Thecharacteristic isotopic signature of the early Aptianis of a negative excursion that coincides with thelowest stratigraphic levels of the organic‐rich blackshale, after which the d13C profile returns to rela-tively elevated values that continue into the higherlevels of the stage [Menegatti et al., 1998; Jenkynsand Wilson, 1999; Jenkyns, 2003] (Figure 3). Theearly Toarcian OAE has a broadly similar patternof a positive excursion with an abrupt negative“bite” in its central portion: in most Europeanlocalities, the negative excursion begins at thelevel where TOC values start to rise [Jenkyns andClayton, 1997; Jenkyns et al., 2001; Jenkyns,2003; Hermoso et al., 2009b; Sabatino et al.,2009] (Figure 2). The atmosphere as well as theocean was affected by all these OAE‐related isoto-pic anomalies because the characteristic signaturesare registered in terrestrial carbon, including mac-roscopic wood [Hasegawa, 1997; Gröcke et al.,1999; Kuypers et al., 1999; Hesselbo et al., 2000,2007; Jahren et al., 2001; Hasegawa et al., 2003;Ando and Kakegawa, 2007; Wagner et al., 2008;Uramoto et al., 2009]. A distinctive feature of boththe early Aptian and early Toarcian negative d13Cexcursions in marine organic carbon and carbonate,found in all successions analyzed at high resolution,

is a pronounced stepped descent, suggesting thatisotopically light carbon was supplied in pulses tothe ocean–atmosphere system [Menegatti et al.,1998; Jenkyns, 2003; Kemp et al., 2005].

[15] Critical new high‐resolution (centimeter scale)data on the negative excursion associated with ablack shale attributed (perhaps erroneously, be-cause two regionally developed black shales, withaccompanying negative carbon isotope excursions,characterize the lower Albian [Herrle et al., 2004])to the early Albian Paquier event (OAE 1b) showthat movement to lower values took place earlier(estimated as 1–3 kyr) in terrestrially derived leafwax n‐alkanes than inmarine carbonate, bulk organicmatter and algal sterenes [Wagner et al., 2008].Such data might suggest that the isotopic signalpropagated downward relatively slowly from theatmosphere into marine waters. The transient dis-equilibrium between the atmosphere and near‐surface ocean implied by such relationships maybe explained by fixing of most pelagic carbonateand planktonic organic matter below the mixed layer.An alternative explanation might lie in upwelling,whereby deeper seated waters were continuallymixed with those of the shallower zones that wereexchanging directly with atmospheric carbon diox-ide [cf. Böhm et al., 1996]. In addition, any resultantincrease in plankton productivity would have tendedto move the d13C values of the local water mass togreater values, due to fixation of relatively isotopi-cally light organic matter, and counteracted thenegative trend. No comparable lead or lag has beenrecognized in the early Aptian OAE [van Breugelet al., 2007; Méhay et al., 2009], for which datafrom Italy and Pacific DSDP sites have been ana-lyzed, but the sampling resolution may not havebeen of sufficiently high resolution to identify anysuch phenomenon. Whether there is any phase lagbetween the deeper marine and atmospheric responseto the negative excursion of the early Toarcian OAEis as yet unknown.

[16] The amplitude of both negative and positivecarbon isotope excursions is generally but not in-variably greater in marine and terrestrial organiccarbon than in carbonate, locally by as much as afactor of two or more [Arthur et al., 1988; Kuyperset al., 1999; Tsikos et al., 2004a; Sageman et al.,2006; Hesselbo et al., 2007; Scopelliti et al.,2008], posing a problem for quantitative model-ing of the carbon cycle in that the amount of rel-atively 12C‐rich material either removed from orinjected into the ocean–atmosphere system cannotbe accurately determined. Explanations for thedisparity in the size of the excursions have been


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sought in differential fractionation effects associ-ated with certain marine planktonic biota and ter-restrial organic components, diagenesis, changes inthe carbon dioxide content of the atmosphere, theconcentration of the carbonate ion in seawater, astate of transient isotopic nonequilibrium betweenthe oceans and atmosphere, and greater wateravailability in the terrestrial environment [Arthur etal., 1988; Freeman and Hayes, 1992; Spero et al.,1997;Kuypers et al., 1999; Jenkyns, 2003;Hesselboet al., 2007]. As far as terrestrial organic matter isconcerned, data from the Paleocene–Eocene bound-

ary of the Arctic show a greater negative shift ind13C with angiosperms than with conifers [Schoutenet al., 2007]: differences in fractionation that couldalso have applied to land‐dwelling trees and plantsduring the Cretaceous. In the case of the d13Cexcursion in carbonate that characterizes the C/TOAE 2, the amplitude of the positive shift in asection from Tibet is lower by ∼1.5‰ from thattypically registered in Europe and North Africa,interpreted as due to dilution of and/or aging ofisotopically positive water masses generated in anddispersed from the Atlantic Ocean, where burial of

Figure 3. Carbon isotope (smoothed with a five‐point moving average) and strontium isotope profile through the1.6 km shallow water platform carbonate section of Resolution Guyot, mid‐Pacific Mountains (ODP Site 866), show-ing the characteristic signatures (negative followed by positive d13C excursion) of the early Aptian OAE [Jenkynsand Wilson, 1999; Jones and Jenkyns, 2001]. Accompanying osmium isotope data derive from an Aptian (OAE1a, Selli Level) pelagic black shale from central Italy [Tejada et al., 2009]. Correlation between the two data sets isestablished on the basis of their characteristic carbon isotope curves. The Pacific section does contain a thin (∼10 cm)black shale, but the OAE is primarily defined (darker band) by reference to outcrops in the Alpine‐Mediterraneanregion where the organic‐rich sediments are well studied and accompanying high‐resolution carbon isotope data exist[Menegatti et al., 1998]; the lighter band extends to the stratigraphic level where there is a maximum in d13C values.The persistence of relatively high d13C values into the mid‐Aptian is not readily explicable in terms of marine carbonburial, as far as the European record is concerned, although black shales of this age are known locally in theallochthonous Calera Limestone of the Franciscan Complex in California [Sliter, 1999].While the beginning of the OAEbroadly correlates with the onset of declining 87Sr/86Sr values, suggesting increasing relative importance of hydro-thermally sourced mantle‐derived strontium into seawater, the more radiogenic value at the level of the negative carbonisotope excursion may be real, given the increase in osmium isotope ratios at this interval recorded from the section inItaly. A pulse of relatively increased continental weathering appears to be a feature of most, if not all, OAEs.


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marine organic matter was apparently concentrated,as shown by the remarkably close match betweenthe positive carbon isotope excursion and the TOCstratigraphic profile in this region [Kuypers et al.,2002a; Li et al., 2006; Erbacher et al., 2005].

[17] In areas outside the Atlantic region, the mostorganic‐rich Cenomanian–Turonian strata are seento be slightly diachronous when cross calibratedagainst the accompanying carbon isotope curves, ifthe latter are viewed as synchronous [Tsikos et al.,2004a]: an observation that compromises the origi-nal definition of the OAE based on assumed coevaldeposition of organic‐rich sediment in a globalcontext. Thus the isotope curve, given that it largelyrepresents a global summation of changes in therate of carbon burial, potentially supplies a moreobjective descriptor of an OAE; theoretically thegeochemical record of an OAE could be taken tobegin where the curve begins its ascent and finishwhere the curve begins its descent, if these points ofinflection can be objectively defined. However, thepresence of interposed negative excursions mani-festly complicates and confounds the issue. Further-more, skeletal carbonate, bulk carbonate carbon,organic carbon and compound‐specific carbon iso-tope curves do not necessarily move exactly inparallel; vital effects, diagenesis, changes in therelative proportions of both carbonate and organiccarbon constituents and disequilibrium betweenoceans and atmosphere can affect the total isotopicsignal. Hence, examination of the stratigraphic pat-tern of TOC values, together with accompanyingcarbon isotope curves, is necessary to elucidate theduration of an OAE in the most objective manner.Caveats, however, still remain: the onset of anoxicconditions, albeit with total water column and sea-floor oxidation of organic matter, will certainly havepreceded initial deposition of black shale and therelated global positive carbon isotope shift. Hence,major chemical changes in the oceans must havetaken place before the onset of the OAE, as con-ventionally defined.

4. Geochemical Proxies forPaleotemperature Change (d18O, Mg/CaRatios, and TEX86) During OAEs

[18] Oxygen isotope paleothermometry, based onbulk and skeletal carbonates, suggests that allOAEs are associated with relative thermal maxima,but the exact timing of temperature change withrespect to the OAE is not always apparent. In thecase of the C/T OAE, oxygen isotope data from

north European pelagic chalks and well‐preservedbrachiopods contained therein suggest that tem-peratures in the midlatitudes rose ∼6–7°C duringthe latest Cenomanian to achieve relative maximaat the stage boundary and in the mid‐Turonian[Jenkyns et al., 1994; Voigt et al., 2006]. In detail,however, the pattern is relatively complicated: fol-lowing the initial rise in carbon isotope values thatmarks the beginning of the OAE, oxygen isotopedata suggest two cooling episodes of 2–3°C, one ofwhich is also marked by the southward invasion ofboreal faunas [Voigt et al., 2004, 2006]. Maximummidlatitude seafloor temperatures during the OAE,as recorded by epifaunal brachiopods, are estimatedas ∼17–21°C.

[19] In the tropical equatorial Atlantic, the organicgeochemical proxy, TEX86, and oxygen isotopedata from well‐preserved foraminifera show tem-perature fluctuations of ∼4°C during the C/T OAE2 [Forster et al., 2007]; sea surface temperatures of∼33°C warmed rapidly to 35–36°C at its onset,then cooled by some 4°C, then warmed again, withthis warmth persisting into the Turonian. The ex-trapolated Arctic Ocean sea surface paleotemperatureduring the C/T OAE is ∼25°C, suggesting a subduedequator‐to‐pole thermal gradient of ∼10°C [Jenkynset al., 2004]. Cooling events are generally ascribedto drawdown of CO2 due to excess carbon burial[Arthur et al., 1988; Jenkyns et al., 1994; Kuyperset al., 1999, 2002a; Jenkyns, 2003] but, as dis-cussed below, increased weathering of continentalcrust is likely to have also been a significant mech-anism. The d13C values of various biomarkers,used to estimate pCO2 levels, indicate a drop from∼1300 ppmv before the OAE to ∼1000 ppmv duringthe interval of maximum carbon burial [SinningheDamsté et al., 2008].

[20] Data from nannofossil populations have beenused to identify an increase in paleotemperatureacross the early Albian Paquier black shale level atits type locality in France, and bulk rock oxygenisotope data suggest as much as 8°C warming, afigure that may be exaggerated by diagenesis ofcarbonate materials [Herrle et al., 2003]. Oxygenisotope data from Atlantic deep‐sea drilling coressimilarly suggest abrupt sea surface warming dur-ing the interval represented by the base of the blackshale, followed by cooling [Erbacher et al., 2001].

[21] High‐resolution oxygen isotope data for aDSDP Site in the mid‐Pacific Mountains, based onlow‐carbonate samples considered to have sufferedminimal diagenetic overprint, suggest a pulse ofrapid warming of ∼8°C in the runup to the early


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Aptian OAE 1a, followed by a cooling trend [Andoet al., 2008]. Data for the early Aptian OAE 1ausing the TEX86 proxy from another equatorialPacific location similarly suggest high (30–36°C)but fluctuating temperatures during deposition ofthe organic matter, with at least two episodes ofcooling [Dumitrescu et al., 2006]. Isotopic and pal-ynological data from the Vocontian Trough, south-east France also indicate relatively modest coolingacross the latter part of the interval represented bythe black shale [Heimhofer et al., 2004]. Exactpolar temperatures are not as yet established for thisinterval, but high‐latitude Early to mid‐Cretaceouswarmth in the southern hemisphere, based on d18Odeterminations on bulk fine‐fraction carbonateand well‐preserved “glassy” foraminifera, indicatea generally subdued equator‐to‐pole thermal gra-dient during this period of geological time [Huberet al., 1995; Clarke and Jenkyns, 1999], similar tothat prevailing during the C/T OAE 2.

[22] Paleotemperature data across a range of lati-tudes are not available for the Toarcian OAE. It isnot certain whether or not the TEX86 parameter canbe applied to sediments of this age and, in theabsence of planktonic foraminifera, paleo-temperature determinations have been undertakenon macrofossil skeletal calcite, principally be-lemnites. Analysis of this material from northernEurope, both within and between individual fossils,invariably produces scattered oxygen isotope data[Jenkyns et al., 2002]. However, compilations doshow, coincident with the early Toarcian OAE, adistinct minimum in d18O values, and a relativemaximum in Mg:Ca ratios, the latter interpreted asrecording a paleotemperature increase of 6–7°C[Bailey et al., 2003]. Because the reconstructedoxygen isotope paleotemperatures from Yorkshire,northeast England are consistently warmer thatthose determined from the more southerly latitudeof northern Spain, it is apparent that fluvial inputwas significant in freshening the surface waters ofthe north European epicontinental seas [Rosales etal., 2004] and that, in this instance, Mg:Ca ratiosmay provide the more useful paleothermometer.However, salinity variations can also compromisethe use of Mg:Ca ratios in skeletal carbonates, buttypically by lowering estimated paleotemperatureswhere salinities are reduced [e.g., Ferguson et al.,2008]. Influx of fresh water was presumably in-strumental in stratifying the water column innorthern Europe, hence enhancing preservation oforganic matter compared with deeper water areasrepresented by pelagic deposits in the Alps andApennines, which contain relatively low TOC

contents, typically in the 1–5 wt % range [Jenkyns,1985, 1988; Sælen et al., 1996; Jenkyns et al.,2001; Mailliot et al., 2009; Sabatino et al., 2009].

5. Organic Geochemical Evidence ofEuxinic Conditions During OAEs

[23] Investigation of the changing redox state ofoceans and seas during OAEs has been stimulatedby the recognition that the black shales formedduring these intervals contain biomarkers derivedfrom different strains of green sulfur bacteria thatdemand both light and free hydrogen sulfide in thewater column. These compounds, comprisingdegradation products of the carotenoids iso-renieratene and chlorobactene, and chlorophyll andbacteriochlorophylls, have been found in Cen-omanian–Turonian, Aptian and Toarcian blackshales deposited during OAEs [Sinninghe Damstéand Köster, 1998; Schouten et al., 2000; Kuyperset al., 2002a; Pancost et al., 2004; Schwark andFrimmel, 2004; Bushnev, 2005: Kolonic et al.,2005; van Breugel et al., 2006; van Bentum etal., 2009]. The relative abundance of these bio-markers is variable: they are particularly abundantin the Cenomanian–Turonian of the equatorialproto‐Atlantic region during peak OAE conditions,particularly during intervals of relatively hightemperature when water mass stratification mayhave been enhanced [van Bentum et al., 2009].Chlorobactene, which is found exclusively in thegreen‐colored strain of green sulfur bacteria thatdemand high light intensity, indicates that sulfidicconditions, periodically at least, extended to within∼15 m of the sea surface in some areas [Kuypers etal., 2002a]. In sediments deposited in the WesternInterior Seaway of North America, the degradationproducts of isorenieratene are more abundant in themore northerly (Canadian) sections than they are inthe United States, implying a better mixed watercolumn in southerly areas [Simons et al., 2003].

[24] Traces of biomarkers characteristic of photiczone euxinic conditions have been found in lowerAptian pelagic black shales in Italy but they are notabundant [Pancost et al., 2004; van Breugel et al.,2007], whereas coeval sediments, deposited inmore restricted paleoenvironments on the RussianPlatform, contain abundant isorenieratene deriva-tives, which indicate the former presence of anilluminated sulfidic water column [Bushnev, 2005].Derivatives of both isorenieratene and chlorobacteneare similarly found in Toarcian black shales depositedin the shelf seas of northern Europe during the


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OAE [Schwark and Frimmel, 2004]; biomarkersderiving from chlorophyll and bacteriochlorophyllsoccur in some but not all pelagic Toarcian blackshales in Italy, suggesting that sulfidic conditionsin the higher part of the water column were lessfrequent and/or less intense in this deeper waterregion [Pancost et al., 2004]. Overall, the evidencesuggests that the paleogeography of narrow oceansand epicontinental seas, in so far as it controlledwater column stratification, was a significant fac-tor in promoting photic zone euxinic conditions,as well as intensified carbon flux from zones ofupwelling and high productivity [Meyer and Kump,2008] (Figure 4). An absence of these signaturecompounds has been reported from those lowerAlbian black shales from the Atlantic (Paquierevent, OAE 1b) that are rich in archaeal biomarkers[Kuypers et al., 2002b], further underscoring thenonuniform nature of the planktonic biota thatthrived during OAEs.

6. Inorganic Geochemical Evidence ofRedox Conditions During OAEs

[25] A number of redox‐sensitive trace metalsare, with respect to average shale, concentrated inorganic‐rich sediments deposited during OAEs. Inthe case of the C/T OAE 2, the concentration of As,Bi, Cd, Co, Cr, Cu, Mo, Ni, Sb, Tl, V, and U in theLivello Bonarelli and coeval sections fromMoroccoand DSDP/ODP sites from the Atlantic, suggestsfixation of certain of these elements in organicmatter or as insoluble sulfide phases or by substi-

Figure 4. Generalized paleogeographic maps of marineareas (Cenomanian–Turonian South Atlantic, withDSDP/ODP Sites; Early Aptian Russian Platform andEarly Toarcian north European seaway) influenced byoceanic anoxic events, indicating areas where euxinicconditions in the water column are likely to have beenfrequent and prolonged. Euxinic conditions were fosteredby restriction of the ocean or seaway and/or intense up-welling and/or enhanced freshwater input favoring strat-ification of the water column. Degradation products ofisorenieratene and chlorobactene, indicating photic zoneeuxinia, are also found in Cenomanian–Turonian blackshales of Site 530 in the South Atlantic, but they arenot as abundant as in the periequatorial sites [Forsteret al., 2008]. Data from Baudin et al. [1990], Smith etal. [1994], Sinninghe Damsté and Köster [1998],Schouten et al. [2000], Gavrilov et al. [2002], Pancost etal. [2004], Schwark and Frimmel [2004], Bushnev[2005], Kolonic et al. [2005], Bowden et al. [2006],Mutterlose et al. [2009], and van Bentum et al. [2009].


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tution within the pyrite structure, implying thepresence of euxinic conditions below the seafloorand/or in the water column [Kuypers et al., 2002a;Kolonic et al., 2005; Brumsack, 2006; Scopelliti etal., 2006, 2008; Turgeon and Brumsack, 2006;Forster et al., 2008; Hetzel et al., 2009]. A numberof these elements are also enriched in a section froma C/T boundary section in Colorado, but there theyhave been interpreted as a signature of hydro-thermal activity from a large igneous province[Snow et al., 2005], calling into question the relativeimportance of the actual source of the metals asopposed to the redox state of ambient seawater. Thestratigraphic distribution of some of these elementsshows an initial peak, characteristic of the onset ofthe OAE, followed by a fall to very low values,indicating regional or possibly global drawdownof the geochemical species in question under anoxicto euxinic conditions and indicating the importanceof the local environment [Hetzel et al., 2009]. Zinc,which is typically concentrated in C/T OAE blackshales, has been labeled as of possible hydrothermalorigin [Brumsack, 2006].

[26] Cd, Mo, Ni, Pb and Se, elements that formstable sulfides, are also relatively enriched in fish‐bearing black shales from northwest Germanyformed during the early Aptian OAE [Hild andBrumsack, 1998]; relative enrichment in Mn in-dicates the presence of a manganese carbonatephase. Coeval lower Aptian black shales from theRussian platform are similar in composition,showing particular enrichment in Ag, Mo and Se[Gavrilov et al., 2002]. Black shales depositedduring the T‐OAE on the north European shelf(Germany) also show relative enrichment in manyof the same trace metals, including manganese[Brumsack, 1991].

[27] Manganese can, however, be depleted in manyorganic‐rich deposits, including the Livello Bo-narelli, formed during the C/T OAE 2, suggestingthat this element could not be readily fixed in thesediment under the prevailing redox conditions,possibly because of a lack of available carbonatealkalinity in calcite‐free sediments or diffusion, assoluble Mn2+, into the overlying water column[Brumsack, 2006; Turgeon and Brumsack, 2006;Hetzel et al., 2009]. However, in some areas, Mn‐enriched carbonates are stratigraphically associatedwith black shales or represent their lateral equiva-lents [Schlanger et al., 1987; Pratt et al., 1991;Dickens and Owen, 1993; Hetzel et al., 2009]. Inthe pelagic shelf‐sea chalks of northern Europe, anincrease in the manganese content of carbonateclosely tracks but is offset below the positive car-

bon isotope excursion of the OAE, presumablyreflecting local redox conditions in the water mass[Pearce et al., 2009]. In the Carpathian region ofPoland, manganoan carbonates occur strati-graphically above the black shales in a pelagic tohemipelagic sequence, and apparently record thegeochemistry of interstitial waters in a state tran-sitional between that of the C/T OAE proper andmore normal oxygenated conditions [Bak, 2007].Manganese‐rich carbonates are also locally re-corded, in southern France, from the calcareousshales and limestones deposited during the earlyAptian OAE 1a [Renard et al., 2005].

[28] With respect to the Toarcian OAE, manganoancarbonates, of economic importance in Hungary,are widely developed in pelagic sediments of theAlpine‐Carpathian region, occurring either strati-graphically below, or cyclically interbedded withinthe principal black shale units [Jenkyns et al., 1991;Polgári et al., 1991; Ebli et al., 1998]. In the for-mer case, manifested also as a manganese “spike”stratigraphically below black shales in northernEurope [Hermoso et al., 2009a], the metal‐richcarbonate must reflect conditions in interstitialwaters immediately preceding the most anoxic/eu-xinic episodes; in the latter case, regular changes inthe chemical state of interstitial waters are implied,possibly forced by orbital climatic cycles that gov-erned organic productivity in near‐surface watersand controlled the nature of the sediment accumu-lating on the seafloor [Claps et al., 1995; Bellancaet al., 1999]. Manganese carbonates occur strati-graphically above deep water radiolarian cherts in aLower Toarcian allochthonous section from Japan,in this case indicating the declining phase of theOAE [Hori, 1997]. A key factor in all of theseoccurrences must be the presence of a laterally con-tinuous zone of deoxygenated but not necessarilysulfidic water (oxygen minimum zone), typicallywhere reduction of nitrate is taking place, allowinglateral transport of soluble divalent manganesefrom continental or hydrothermal sources that canultimately react to form early diagenetic carbonateminerals at or just below the sediment–water inter-face [Jenkyns et al., 1991;Huckriede and Meischner,1996].

7. Changes in Redox‐Sensitive IsotopeSystems (N, S, Fe, Mo, U, and [Nd])During OAEs

[29] Nitrogen isotope data from black shalesformed during OAEs generally plot in the −4 to


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2‰ range: values that are considerably lower thanthose found in Quaternary to recent sediments.Such relatively low values are recorded fromCenomanian–Turonian, lower Aptian and Toarcianblack shales and are interpreted as recording asubstantial contribution of organic matter fromnitrogen‐fixing cyanobacteria, which essentiallyreflect the isotopic composition of atmosphericdinitrogen (d15N = 0) [Jenkyns et al., 2001, 2007;Kuypers et al., 2004; Ohkouchi et al., 2006;Dumitrescu and Brassell, 2006; Junium and Arthur,2007; Kashiyama et al., 2008]. In C/T, Aptian andToarcian examples, the presence of specific mem-brane lipids (2‐methyl hopanoids) has been recorded,representing a geochemical fingerprint for the for-mer presence of these organisms, known to becomeextremely abundant in areas of the present‐dayocean where high surface production and develop-ment of intense oxygen minima has led to totalnitrate depletion [Ohkouchi et al., 1997, 2006;Farrimond et al., 2004; Kuypers et al., 2004;Dumitrescu and Brassell, 2006].

[30] Related phenomena, also characteristic of de-oxygenated upwelling zones, include denitrifica-tion where nitrate has replaced oxygen as the primeoxidant for falling organic matter and anaerobicoxidation of ammonium by nitrite (anammox pro-cess): reactions that liberate elemental dinitrogenand/or nitrous oxide to the atmosphere [Kuypers etal., 2005; Thamdrup et al., 2006; Ward et al.,2009]. These phenomena are assumed to causerelative isotopic enrichment of dissolved residualnitrate which, when upwelled into the photic zone,is relayed to the planktonic population, some ofwhich ultimately becomes part of the sedimentaryrecord. This chain of events, which is necessarily afunction of vigorous recycling of water masses,produces a positive correlation between organiccarbon values (TOC) and nitrogen isotope ratios:relationships that have been documented forToarcian and C/T OAE black shales in Europe andelsewhere [Jenkyns et al., 2001, 2007]. Given theevidence for both the presence and the absence ofdissolved nitrate, it is apparent that the redox stateof the middle to upper water column was highlyvariable during OAEs. Particularly significant isthe formation of nitrous oxide during the denitri-fication steps (NO3

− → NO2− → NO → N2O→ N2),

given that this product is a greenhouse gas whoseeffect is ∼200 times more powerful than that ofcarbon dioxide [Lashof and Ahuja, 1990]. Wide-spread denitrification could hence have fosteredsignificant warming of global climate. The ultimatestep in the denitrification process, whereby nitrous

oxide is converted to nitrogen, requires an enzymecontaining copper: were this metal ion to have beendrawn down during periodic euxinification, theratio of N2O to N2 in gases expelled to the atmo-sphere would have likely increased [Buick, 2007].

[31] Euxinia represents the most extreme condition,where sulfate has replaced nitrate as the oxidant oforganic matter and free H2S exists in the watercolumn. Abundant inorganic and organic geo-chemical evidence exists for the former presence ofsulfidic waters, as detailed above. Sulfur isotoperatios are also informative, the highest‐resolutionrecord being that across the Cenomanian–Turonianboundary, in Marche–Umbria, Italy where a 8‰positive shift in d34S in structurally substitutedsulfate in carbonate (Carbonate Associated Sulfate:CAS) is recorded in limestones below and abovethe Livello Bonarelli [Ohkouchi et al., 1999]. Ashift of somewhat lesser magnitude (∼6‰), albeitdefined by far fewer points, is recorded frombelemnite calcite in Lower Toarcian black shalesfrom Yorkshire, England [Kampschulte and Strauss,2004] and, at higher resolution but with much lowerabsolute values, in coeval diagenetically modifiedmanganoan carbonates and organic‐rich shales fromHungary [Ebli et al., 1998]. With both the C/T andT‐OAE, deposition of increased quantities of pyrite,whose bacterially mediated precipitation favors thelighter isotope 32S over 34S, is credited with causing aglobal positive excursion in seawater sulfate. Thepresence of micron‐scale framboids in the LivelloBonarelli and Toarcian black shales from Yorkshireindicates that some pyrite was precipitated in thewater column in the presence of free hydrogen sul-fide, as presently happens in the euxinic waters ofthe Black Sea [Wilkin et al., 1996; Wignall et al.,2005; Kuroda et al., 2005; Jenkyns et al., 2007].

[32] The low‐resolution sulfur isotope curve for theCretaceous, based on sedimentary barite fromDSDP/ODP cores and CAS from pelagic lime-stones in Italy, indicates a negative step changeover the interval represented by the early AptianSelli event [Paytan et al., 2004; Turchyn et al.,2009]. Such a shift, apparently in the oppositesense to those of the early Toarcian and C/T OAEs,is counterintuitive and needs to be confirmed byhigh‐resolution studies of appropriate material ofearly Aptian age. One explanation for the contrarybehavior of the Aptian sulfur isotope curve relatesto loss of available seawater sulfate in the globaloceanic reservoir, which might otherwise havebeen available to form pyrite, due to deposition ofmassive amounts of evaporite minerals, such as


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gypsum and anhydrite, in the proto–South Atlantic[Wortmann and Chernyavsky, 2007].

[33] Correlation between the long‐term low‐resolution Phanerozoic d13C and d34S trends isbroadly negative, interpreted as due to an approxi-mate balancing of the carbon and sulfur cycles,resulting in relatively constant atmospheric oxy-gen levels [Veizer et al., 1980]. However, for theshorter‐term Toarcian and C/T OAEs, the manifestlypositive correlation between accelerated regionalcarbon burial and movement of seawater d34S toheavier values implies a rise in the abundance ofatmospheric oxygen during these intervals.

[34] Iron isotope data from bulk sediment, onlyavailable for the C/T OAE 2, show a positiveexcursion through the Livello Bonarelli. Althoughthis excursion probably indicates some isotopicevolution of the dissolved iron pool, there is alsoevidence for a change in the mechanism ofpyrite formation from dissimilatory iron reduction(a bacterially mediated suboxic process that takesplace within the sediment) to bacterial sulfatereduction within a euxinic water column to formpyrite framboids [Jenkyns et al., 2007]. Hence onlyunder euxinic conditions are iron isotopes poten-tially likely to record paleoceanographic ratherthan diagenetic signatures. Similar complicationsarise in the case of molybdenum, the isotopicratios of which in black shales can reflect reac-tions that took place in the sediment as well aschanges in the composition of seawater itself[Neubert et al., 2008; Poulson Brucker et al., 2009].Periodic decreases and increases in the d98/95Mosignature of black shales of the T‐OAE (sectionfrom Yorkshire, northeast England) have beeninterpreted as reflecting cyclic expansion and con-traction of the global volume of sulfidic watersand the concomitant contraction and expansionof oxic sinks, into which the lighter isotope ofmolybdenum is preferentially removed [Pearceet al., 2008] (Figure 2): an interpretation thatassumes a local environment euxinic enough tocapture the Mo isotope signature of ambient sea-water. Indeed, d98/95Mo values as low as 0.8‰in these shales have been taken to imply near totalseafloor euxinia in the world ocean [Archer andVance, 2008], but this interpretation hinges criti-cally on the notion that the isotopic values recordthe signature of the world ocean rather than thatof a restricted or semirestricted water mass. At thepresent time, the Mo isotope signature of euxinicsediments formed under a water column with acritical minimum content of dissolved sulfide in

the Black Sea is essentially that of global seawater(d98/95Mo ∼ 2.3‰ [Neubert et al., 2008]), but iffurther restriction of this water body were totake place, this value would undoubtedly evolve.Because Mo is captured in euxinic black shales,extreme restriction of a water mass would causethe concentrations of this element in local seawaterto be lowered, its residence time reduced and itssensitivity to isotopic change enhanced. In thiscontext, the relative depletion, with respect to sub-jacent and suprajacent shale facies, of Mo inorganic‐rich sediments deposited during OAEs isnotable [Algeo and Lyons, 2006; McArthur et al.,2008; Hetzel et al., 2009]. Given the evidence formassive freshwater input to the north Europeanarea during the T‐OAE, coupled with evidence forregionally developed euxinia (Figure 4), somedegree of salinity and density stratification seemslikely; hence isotopic values, in some parts of thewater column at least, may well have departed fromthe global signature. Indeed, crossplots of Moagainst TOC for Lower Toarcian black shales fromYorkshire deposited during the OAE suggest adegree of restriction ∼10 times greater than thatexisting in the present‐day Black Sea [McArthur etal., 2008]. These considerations suggest a generalprinciple: if the generation of euxinic conditionsautomatically implies some degree of basinalrestriction, d98/95Mo signatures from sedimentsformed under these conditions may be the onlyindicators that accurately record seawater values,but these values will only be those of the worldocean if the levels of dissolved sulfide are highenough to ensure quantitative removal of Mo fromthe water mass but restriction is not severe enoughto cause seawater d98/95Mo values to develop alocal signature. Analyses of Mo isotope ratios incoeval black shales from different parts of worldare necessary to address this uncertainty.

[35] Uranium isotopes also show potential as globalindicators of redox conditions, because anoxic/euxinic environments not only draw down the con-centration of this element but also selectively removethe heavier isotope, 238U, from seawater, hencelowering the 238U/235U ratio (d238U) in the marinereservoir: black shales, containing UIV are isotopi-cally heavier than ambient seawater containing UVI.Cenomanian–Turonian boundary black shales coredfrom the western Atlantic illustrate this effect inthat they are isotopically lighter than those stra-tigraphically above and below and hence recordexpansion of oxygen‐depleted waters and “blackshale sinks” in the world ocean during OAE 2[Montoya Pino et al., 2010].


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[36] A positive excursion of 8 "Nd units (to higher143Nd/144Nd ratios) is recorded in phosphatic fishdebris in black shales deposited during the C/TOAE 2 in the northwestern South Atlantic[MacLeod et al., 2008]. This signature, whichimplies relative enrichment in mantle‐derived radio-genic neodymium, closely tracks the carbon isotopecurve. Given that Nd isotope values are variablywater mass–dependent in the past and the present,and that fish skeleta acquire their neodymiumpostmortally while resting on the seabed [Stille etal., 1996; Martin and Haley, 2000], this excursioncould suggest a major change in the position ofbottom currents. Alternatively, or additionally, theexcursion may be directly related to the presenceof euxinic water masses (Figure 4), indicated bythe abundance of biomarkers for green sulfurbacteria in Cenomanian–Turonian black shalesfrom the periequatorial Atlantic region [SinningheDamsté and Köster, 1998; Kuypers et al., 2002a].Although, in an oxic ocean, any influx of mantle‐derived radiogenic neodymium, derived from aspreading ridge or an erupting large igneous prov-ince, would have been rapidly scavenged by ironmanganese oxyhydoxides, such phases would nothave been stable in the presence of dissolved sulfide[Halliday et al., 1992; Jenkyns et al., 2002]. Henceeuxinic bottom waters could have potentiallyallowed dispersal of fluids with a high 143Nd/144Ndratio that could be incorporated into fish remainsaccumulating on the seafloor.

8. Role of Phosphorus During theGenesis of OAEs

[37] The role of nutrients in the genesis of OAEs,which is relevant to the debate over the “enhancedproductivity” as opposed to the “enhanced preser-vation” model for black shales [Demaison andMoore, 1980; Pedersen and Calvert, 1990], hasbeen underscored by detailed studies of the geo-chemistry of phosphorus in a number of sequencesthat cross the Cenomanian–Turonian boundary[Nederbragt et al., 2004; Mort et al., 2007, 2008].In the sections analyzed, the determined massaccumulation rates of phosphorus reach a maxi-mum at intervals where the initial rise in carbonisotope ratios is registered, then return to back-ground values close to the beginning of the firstcarbon isotope peak, typically at levels where thesediments have become increasingly rich in organicmatter: essentially recording the interval where thesedimentary evidence suggests that the C/T OAE 2had “strengthened its global grip.” This phenome-

non is in line with the recognition that the burialefficiency of phosphorus declines when bottomwaters become progressively more anoxic (due toredox‐dependent changes in iron phases carryingphosphorus, as well as microbial activity) andimplies that, as the effects of the OAE becamemore intense and widespread, phosphorus couldbe recycled from bottom waters to fuel furtherplankton productivity [van Cappellen and Ingall,1994]. Essentially, this model requires intenseupwelling of bottomwaters to sustain large planktonpopulations, which could well have been dominatedby nitrogen‐fixing cyanobacteria, were nitrate tohave been totally depleted. Under such circum-stances, fluvial input would not have been the onlysource of nutrients necessary to maintain the con-ditions characteristic of the OAE. Comparablegeochemical data are available for the early AptianOAE, implying similar recycling of phosphorusunder anoxic conditions [Westermann et al., 2007].

9. Indices of Enhanced ContinentalWeathering (Sr and Os Isotopes) DuringOAEs

[38] Given that OAEs correspond with relativepaleotemperature maxima, in all probability accom-panied by an accelerated hydrological cycle andglobal increase in weathering rates [Jenkyns, 2003],the isotopic ratios of those isotopes concentrated incontinental crust as opposed to the mantle shouldshow relative enrichment in seawater. Hence the87Sr/86Sr and 187Os/188Os ratios of strontium andosmium, respectively, should act as fingerprints forthe relative importance of fluvial supply of theseelements from the continents to the marine environ-ment versus seawater–ocean crust interaction[Elderfield, 1986; Peucker‐Ehrenbrink and Ravizza,2000]. In this respect, however, the OAEs are notall alike in their geochemical response.

[39] For the C/T OAE 2, a correlation of the eventwith the onset of a decline in the Sr isotope curve(derived from skeletal and bulk carbonate) isobserved, implying a relative increase in the supplyof mantle‐derived strontium from hydrothermal orother mafic igneous sources [Bralower et al., 1997;Jones and Jenkyns, 2001]. The Os isotope record,derived from the Livello Bonarelli in Umbria–Marche, Italy supports this interpretation by illus-trating a dramatic drop in values just below the baseof the black shale [Turgeon and Creaser, 2008].However, very detailed Sr isotope analysis ofprimary bivalve (rudist) calcite in Italian shallow


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water limestones defines an interval of more radio-genic values where carbon isotope ratios, definingthe onset of the OAE, rise into the positive excursion[Frijia and Parente, 2008] (Figure 5). These criticaldata suggest the existence of an interval when globalweathering rates increased enough to override thenonradiogenic trend in 87Sr/86Sr ratios. The increasein Sr isotope values (i.e., the upward limb of theexcursion) can be approximated to 400,000 years byreference to the cyclostratigraphically determinedduration of the OAE itself [Voigt et al., 2008]. In thecontext of a present‐day residence time of strontium

in the oceans of 2–4 Myr, this excursion is remark-ably rapid, most likely implying a reduced marinereservoir of Sr during this part of the Cretaceous,perhaps related to the widespread secretion of bio-logical aragonite in the many carbonate platformscharacteristic of the Period. The absence of accom-panying Os isotope evidence for this pulse of rela-tively radiogenic strontium into seawater from theLivello Bonarelli is most likely due to the presenceof a hiatus at the base of the black shale: theremarkably abrupt jump in a number of geochemicalparameters (d13Corg, d

15N, d57Fe) at the base of this

Figure 5. Carbon and strontium isotope profile across the interval characterized by the Cenomanian–Turonian oce-anic anoxic event. The generalized Sr isotope profile indicates progressively falling values across the stage boundary[Jones and Jenkyns, 2001]. However, very detailed work on immaculately preserved bivalve (rudist) calcite fromItalian carbonate platforms indicates a pulse of relatively radiogenic strontium (four data points plotted) at the onset ofthe OAE [Frijia and Parente, 2008], a trend to higher values that dominates the entire 87Sr/86Sr profile of the earlyToarcian OAE (Figure 6) and which, in both cases, can be attributed to increased continental weathering during aperiod of accelerated hydrological cycling. The carbon isotope profile shows unsmoothed data from the English Chalk[Jarvis et al., 2006]; the characteristic positive excursion is used to plot the stratigraphic position of the Italian Srisotope data.


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unit suggests a lack of sedimentary record whendeposition was changing from carbonate to non-carbonate [Jenkyns et al., 2007; Kuroda et al.,2007]. The 187Os/188Os ratios of Cenomanian–Turonian boundary black shales from Umbria–Marche, Italy are not identical with approximatelycoeval samples from Colorado, USA and fromGermany, potentially implying that seawater duringthis interval was not isotopically homogenous withrespect with osmium [Selby et al., 2009], althoughsome of these differences may result from rapidisotopic variation during the OAE that will onlybe captured by future systematic high‐resolutionchemostratigraphic studies.

[40] In the case of the early Aptian Selli OAE 1a,the general trend of the Sr isotope curve is amovement to less radiogenic values, as with theC/T OAE 2, implying that hydrothermal or othermantle‐derived sources of strontium were becom-ing increasingly important in governing seawaterchemistry: the onset of the decline in 87Sr/86Srvalues corresponds closely with that of the nega-tive carbon isotope excursion [Jones and Jenkyns,2001] (Figure 3). It is, however, possible thatdetailed analysis of this interval will reveal an earlypulse of relatively radiogenic strontium to theoceans, as is the case with the C/T event. In fact,detailed 187Os/188Os profiles through the LivelloSelli in Italy clearly indicate a pulse of radiogenicosmium to the oceans interrupting a trend to lowervalues (Figure 3), interpreted as due to the erup-tion of submarine volcanic products with attendanthydrothermal activity [Tejada et al., 2009]. Althoughthere are no high‐resolution data specifically onthe Albian Paquier event (OAE 1b), Sr isotopevalues over this interval [Bralower et al., 1997]indicate a relative increase in radiogenic strontium tothe oceans, which could be interpreted as reflectingaccelerated continental weathering in response toelevated global temperature.

[41] The early Toarcian Sr isotope signature isfundamentally different from those of the Creta-ceous OAEs. 87Sr/86Sr ratios, derived from Euro-pean belemnite calcite, indicate a shift to moreradiogenic values during the OAE [Jones et al.,1994; McArthur et al., 2000; Jones and Jenkyns,2001; Jenkyns et al., 2002], suggesting an increasein the relative importance of global continentalweathering on the chemistry of seawater. Confir-mation of this interpretation derives from osmiumisotope ratios determined from Toarcian blackshales in Yorkshire, England, which similarly showan excursion to more radiogenic values over the

interval characterized by the negative carbon isotopeexcursion (Figure 6): a transient increase in weath-ering rates of several hundred percent has beensuggested [Cohen et al., 2004]. There are, however,important differences between the Sr isotope andOs isotope curves; with respect to the former, theshift to more radiogenic values is maintained afterthe initial excursion and 87Sr/86Sr values increasegradually through the rest of the Toarcian, a trendthat is recorded from different localities acrossEurope [McArthur et al., 2000; Jenkyns et al.,2002]. In the case of the Os isotope ratios, how-ever, the excursion is relatively short lived withbackground values attained by the end of the OAE.Although both isotopic proxies primarily respondto changes in fluxes from continental weatheringand hydrothermal activity, the residence time in theoceans of osmium is presently some 2 orders ofmagnitude less than that of strontium, such that187Os/186Os will potentially respond to oceano-graphic perturbations on a scale of tens of thousandsof years and may also register some degree ofwater mass dependence [Peucker‐Ehrenbrink andRavizza, 2000]. Because osmium is concentratedin organic‐rich sediments, deposition of such faciesunder anoxic to euxinic conditions during an OAEwould have drawn down the inventory of marinedissolved Os, hence reducing its residence time andpotentially leading to local differences in isotopicsignatures [McArthur et al., 2008]. Consequently,the Os isotope curve from Toarcian black shalesof Yorkshire may not necessarily reflect a globalsignature but could be recording the geochemistryof an epicontinental seaway at times stronglyaffected by local fluvial input [Jenkyns, 1988, 2003;Sælen et al., 1996; Bailey et al., 2003; Rosales et al.,2004; McArthur et al., 2008]. Hence, the extremelylarge estimated increases in continental weatheringrates may record local rather than global environ-mental change.

[42] In summary, therefore, there is geochemicalevidence for increased continental weathering,accompanied by increase in temperature, in the caseof the early Toarcian, early Aptian, early Albian andCenomanian/Turonian OAEs even though, in theearly Aptian and C/T events, the seawater signaturethat ultimately dominated must have been derivedfrom mantle sources. Whether the increase in non-radiogenic osmium and strontium derived fromelevated hydrothermal fluxes from ocean ridges,perhaps related to accelerated seafloor spreadingand subduction rates, or whether it was related tothe extrusion of large igneous provinces (LIPs),


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or, perhaps most likely, both of these phenomena,remains unresolved [Larson and Erba, 1999; Jonesand Jenkyns, 2001; Leckie et al., 2002; Jenkyns,2003; Erba, 2004; Kuroda et al., 2007; Turgeonand Creaser, 2008]. The early Aptian and Cen-omanian/Turonian OAEs have been correlated withextrusion of the Ontong Java and CaribbeanPlateaus, respectively, both of which were built onocean crust and probably accompanied by acceler-ated seafloor spreading rates, whereas the earlyToarcian OAE correlates with the extrusion of sub-aerial flood basalts in the Karoo–Ferrar province,related to the breakup of Gondwana [Pálfy andSmith, 2000]. Direct venting of volcanogenic car-bon dioxide and other greenhouse gases into theatmosphere from subaerial flood basalts may haveproduced particularly rapid and intense warmingand accelerated continental weathering, hence ac-counting for the dominant radiogenic Sr and Osisotope signatures characteristic of this Jurassicevent (Figure 6).

[43] Increased weathering and fluvial activitywould have elevated the nutrient flux to the oceans,favoring an increase in organic productivity, par-

ticularly in areas close to continents, and therebysetting in train one of the critical phenomena thatresulted in an oceanic anoxic event. Stratificationof oceans and seaways would also have beenpromoted where paleogeography favored relativerestriction of the water mass, as was the case duringthe Early Toarcian in northern Europe, the Russianplatform during the early Aptian and the periequa-torial Atlantic region during Cenomanian–Turonianboundary time (Figure 4). Stimulation of planktonicbiota by hydrothermally sourced elements may havebeen a significant (but presently unquantifiable)variable [Sinton and Duncan, 1997; Larson andErba, 1999; Jones and Jenkyns, 2001; Erba, 2004;Snow et al., 2005].

10. Impact of OAEs in LacustrineSystems

[44] Increased fluxes of terrestrially derived nutrientswould not only have been confined to marine areasbut would also have affected those larger and deeperlakes into which substantial drainage was takingplace. Lacustrine black shales, coeval with their

Figure 6. Total organic carbon (TOC) and osmium isotope stratigraphy of the Lower Toarcian black shales croppingout on the Yorkshire coast, northeast England [Jenkyns and Clayton, 1997; Hesselbo et al., 2000; Cohen et al., 2004;Kemp et al., 2005]: Sr isotope data from belemnites contained therein [McArthur et al., 2000]. The OAE is primarilydefined by the relatively high TOC levels; a lower interval of values rising to a maximum (darker band) can bedistinguished from a higher interval (lighter band) where values are lower but still distinctly above background. Therise in Sr isotope values conforms to patterns found elsewhere in Europe (e.g., Portugal [Jenkyns et al., 2002;Hesselbo et al., 2007]) and must represent a global signal of an accelerated hydrological cycle and increase incontinental weathering, whereas the osmium isotope record may record an amplified, albeit local, response to thesame phenomenon in the more restricted waters of the north European seaway. Drawdown of osmium into euxinicblack shales would have decreased the residence time of this element in seawater, and the impact of increased fluvialdrainage on 187Os/188Os ratios could have been profound.


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marine counterparts, may thus have developedduring time intervals characterized by OAEs, whichmay explain some of the Cretaceous organic‐richdeposits formed in the rift basins of the proto–SouthAtlantic and northeast China [Demaison andMoore, 1980; Smith, 1990; Rodrigues, 2005;Neumann et al., 2003; Wu et al., 2009]. With thedevelopment of anoxic conditions in such lakes, amajor environmental prerequisite for the formationof Fossillagestätten, including those containing bothfreshwater and terrestrial elements, would have beenmet. A case in point would be the famous LowerCretaceous Jehol biota of northeastern China, whichcontains a spectacular range of fossils, includingplants, insects, birds, feathered dinosaurs, fish andcrustaceans that were preserved under seasonallyanoxic conditions [Fürsich et al., 2007]. Absoluteage dating of tuffs and basalts from this successionindicates that it covers the interval of the earlyAptian OAE 1a [He et al., 2004; Chang et al.,2009]. In the lacustrine Songliao Basin of north-east China, the C/T OAE 2 is recognized as aninterval of extremely carbon‐rich laminated pyritic“oil shale” displaying a positive carbon isotopeexcursion [Wu et al., 2009]. For the early Aptianand C/T events, the evidence from radiogenicisotope ratios (87Sr/86Sr and 187Os/188Os) wouldsuggests that the most favorable conditions forgenerating fertile lakes existed during the earlyintervals of OAEs (Figures 3 and 5) when maxi-mum weathering and/or fluvial input was takingplace.

11. PETM: Insights From OAEs

[45] The Paleocene–Eocene Thermal Maximumcan lay claim, if not to being the last OAE of thePhanerozoic, to at least sharing many features incommon (Figure 1). Although initially recognizedprimarily because of its abrupt rise in global tem-peratures [Kennett and Stott, 1991], the PETM ischaracterized by a number of related phenomena:deposition of organic matter in many shelf‐sea re-gions (Egypt and North American margin) and theArctic Ocean, a stepped negative carbon isotopeexcursion that propagated downward from theshallowest levels of the ocean, a positive sulfurisotope shift, an osmium isotope excursion to moreradiogenic values, implying increased continentalweathering, elevation of the calcite compensationdepth, and correlation in time with large‐scalevolcanic activity (North Atlantic Large IgneousProvince) [Speijer and Wagner, 2002; Jenkyns,2003; Kurtz et al., 2003; Erba, 2004; Cohen et

al., 2007; John et al., 2008; Sluijs et al., 2008].The disposition of marine organic matter, concen-trated particularly on the shelf rather than the deepsea recalls particularly the characteristics of theearly Toarcian OAE, less so the early Aptian andCenomanian–Turonian examples [Jenkyns, 1985;John et al., 2008; Sluijs et al., 2008]. Nannofossilevidence equally suggests that relatively elevatedorganic productivity was concentrated in shelfregions during the PETM, as opposed to the earlyAptian OAE that was characterized by the devel-opment of more extensive productive oceanic areas[Tremolada et al., 2007].

[46] The propagation of the oxygen and carbonisotope signature from surface‐ to thermocline‐dwelling foraminifera and ultimately to benthicspecies during the PETM [Thomas et al., 2002]finds a parallel in the nonsynchronous negativecarbon isotope shift, seen first in terrestrial organicmatter and subsequently in marine organic matter,in a lower Albian Atlantic black shale [Wagner etal., 2008]. To date, this is the only OAE‐relatednegative carbon isotope excursion that has beeninvestigated in sufficient stratigraphic detail toresolve this phenomenon. One may thus ask: willthis lag in response time of the deeper ocean withrespect to the shallower ocean and the atmosphereturn out to be a defining parameter of other largenegative carbon isotope excursions in the strati-graphic record? Was isotopically light carbonprimarily injected into the atmosphere, as methaneand/or carbon dioxide, before being progressivelyabsorbed into the deeper levels of the ocean?Because the estimated rise in temperature duringthe PETM is too large to have been forced bycarbon dioxide alone, albeit originally in the formof methane [Dickens et al., 1995; Dickens, 2000;Zeebe et al., 2009], another greenhouse amplifierneeds to be sought. Here, OAEs may offer a clue:was the PETM characterized by enhanced rates ofshelfal denitrification, with concomitant release ofnitrous oxide to the atmosphere?

[47] As to why the PETM did not develop into afull‐blown OAE, two possibilities suggest them-selves. First, and most importantly, climatic forcingwas probably not of sufficient intensity to fertilizeextensive areas of the ocean; and, second, paleo-geography was not, except in the case of the ArcticOcean [Sluijs et al., 2008], conducive to watercolumn stratification and development of extensiveeuxinic conditions. Even though there is a record ofrelatively enhanced organic matter burial duringthe PETM and the suspicion of a positive d13C


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excursion in some sections preceding the mainevent [Bains et al., 1999;Cohen et al., 2007; Sluijs etal., 2008], the carbon isotope signature dominantlyrecords only the injection of isotopically lightcarbon into the ocean–atmosphere system.

12. Postscript: Insidious Advance ofRecent Oceanic Anoxia

[48] Oceanic anoxic events represent some of themost profound disturbances in the global carboncycle that have taken place during Phanerozoictime. At the simplest level, they may be interpreted

as a response to a rise in temperature that set intrain a concatenation of sedimentary, geochemicaland biological events, which allowed removal ofexcess carbon dioxide from the atmosphere by bothcontinental weathering and deposition of marineand lacustrine organic matter, ultimately restoringbalance to global ecosystems, at the expense ofmassive chemical change (Figures 7 and 8). Giventhat the PETM and, by implication, OAEs devel-oped in less than 2000 years [Jenkyns, 2003], someof the more subtle effects of recent global warmingtake on particular relevance. Global warming hasbeen accompanied by an approximately −1.4‰d13C negative shift in atmospheric carbon dioxide

Figure 7. Model to illustrate the variety of geochemical processes characteristic of OAEs. Volcanism, through theventing of greenhouse gases, initiates global warming; increased acidification of the oceans from dissolution of CO2

and SO2 causes increased carbonate dissolution; and methane release from gas hydrates, triggered either by warmingof bottom waters and the subjacent sedimentary pile and/or synsedimentary faulting, produces further increase in tem-perature in seawater and atmosphere. The hydrological cycle accelerates with increased nutrient flux to the oceans;upwelling intensifies, as does organic productivity. As depicted in this early phase of the OAE, oxygen depletionhas advanced to a state where denitrification and the anammox process have reduced nitrate and nitrite, such that ni-trous oxide (a potent greenhouse gas that would promote further global warming) and elemental dinitrogen are beinglost from the ocean. Organic‐rich sediments change from bioturbated to laminated (rich in fish remains) as the benthosis excluded from the sea bottom by the spread of anoxic conditions. Manganese is fixed as early diagenetic carbonatephases at or just below the seafloor; iron is fixed as pyrite below the seafloor. The carbon isotope profile illustratedshows the effect of a global increase in carbon burial, causing a positive d13C excursion interrupted by a negativeexcursion produced by the input of isotopically negative methane and its oxidation product carbon dioxide. Inspiredby Weissert [2000].


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over the last two centuries with a lesser shift in thesurface ocean [Böhm et al., 1996; Francey et al.,1999; Al‐Rousan et al., 2004]. In addition, phenom-ena such as locally intensified wind‐driven upwelling[Bakun, 1990; Goes et al., 2005; McGregor et al.,2007], increase in monsoonal rainfall [Anderson etal., 2002] and the expansion of anoxic waters inmany parts of the world ocean [Whitney et al., 2007;Stramma et al., 2008] have all been identified. Manymarine water masses have recorded a marked ex-pansion of the oxygen minimum zone over the lastdecades, recalling exactly the model originally in-voked for the genesis of Cretaceous OAEs

[Schlanger and Jenkyns, 1976], underscoring notonly the importance of the uniformitarian principlebut also the role of the past as a harbinger of thefuture.

Acknowledgments

[49] I am grateful to Anthony Cohen, Elisabetta Erba, StephenHesselbo, Ros Rickaby, Chris Siebert, and Tom Wagner formany discussions concerning the causes and consequences ofOceanic Anoxic Events; to Evelyn Polgreen for reading themanuscript; and to David Kemp for supplying the original data

Figure 8. Model to illustrate the variety of geochemical processes characteristic of OAEs. As depicted in this moreextreme stage of the OAE, large tracts of the water column have advanced to sulfate reduction (free H2S: euxinicconditions), with the precipitation of pyrite framboids in the water column. In the absence of dissolved nitrate, butin the presence of upwelled phosphate released from organic‐rich sediments, nitrogen‐fixing cyanobacteria can thrivein near‐surface illuminated environments, as can green sulfur bacteria in slightly deeper parts of the photic zone wherefree H2S is present. Molybdenum and osmium are drawn down into pyritic organic‐rich shales under euxinic condi-tions. The isotopic ratios of osmium, neodymium, and strontium reflect the balance between fluvial input, related tothe weathering of continental materials, and fluxes from volcanic and hydrothermal sources. In euxinic bottom watershydrothermally sourced radiogenic neodymium can be dispersed laterally, rather than being precipitated in metalliferous(Fe–Mn) oxyhydroxides (such phases would not be stable), and find its way into fish skeleta accumulating on the sea-floor. An intensified hydrological cycle and increase in continental weathering would stimulate organic productivity notonly in oceans but also in large lakes, thereby favoring development of coeval lacustrine organic‐rich facies. The carbonand sulfur isotope profiles illustrated show the effect of a global increase in marine carbon burial, causing a positive d13Cexcursion, and an increase in global pyrite fixation, causing a positive d34S excursion.


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used in generating Figure 2. Constructive reviews were furn-ished by Michael Arthur and Tim Lyons.

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the geochemistry of oceanic anoxic events

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