406Lucas, S.G. and Spielmann, J.A., eds., 2007, The Global Triassic. New Mexico Museum of Natural History and Science Bulletin 41.

UPPER TRIASSIC CARBON ISOTOPE STRATIGRAPHY OF THELAGRONEGRO SUCCESSION, SOUTHERN APENNINES, ITALY

LAWRENCE H. TANNER1, GLORIA CIARAPICA2, LETIZIA REGGIANI 2, 3 AND VIOREL ATUDOREI4

1 Department of Biological Sciences, Le Moyne College, Syracuse, NY 13214 USA, email: tannerlh@lemoyne.edu;2 Dipartimento di Scienze della Terra, Univerita di Perugia, Piazza Universita, Perugia 06100 Italy, email: ciarapic@unipg.it, letizia.reggiani@unipg.it;

3 UMR 5125 PEPS, CNRS, France; Université Lyon 1, Campus de la DOUA, Bâtiment Géode, 69622 Villeurbanne Cedex, France;4 Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131 USA, email: atudorei@unm.edu

Abstract—We present a new carbon isotope stratigraphy for the Upper Triassic (Norian) to lowermost Jurassic(Hettangian) strata of the Lagronegro succession in the southern Apennines. Previously published data for thesestrata, the upper Calcari con Selce and lower Scisti Silicei formations, displayed evidence (covariance of 13C and 18O) of diagenetic alteration of the isotopic signature. The new data, from a section on Monte Sirino, appear torecord the primary isotopic signature of the carbonate. These data are most notable for the lack of evidence of anisotopic excursion at the Norian-Rhaetian stage boundary. However, the sampling density does not allow closeinterpretation of the isotopic record at the system boundary.

INTRODUCTION

The decline in diversity during the Late Triassic is now recognizedas the result of progressive and prolonged extinction, rather than as asingle mass-extinction “event” (Hallam, 2002; Tanner et al., 2004; Lucasand Tanner, 2004). Examination of the stratigraphic record reveals thatextinctions took place in a step-wise fashion through the Late Triassic.For example, the more than 150 ammonite genera and subgenera thatexisted during the Carnian were reduced to 90 in the Norian, and reducedagain to 6 or 7 during the Rhaetian (Teichert, 1988). This indicates thatthe most significant ammonite extinctions were during or at the end of theNorian. Inspection of the Late Triassic bivalve record also suggests thatextinctions for this group were distributed episodically throughout thisinterval. Most significant was the extinction of the cosmopolitan andabundant pectinacean Monotis at the end-Norian (Dagys and Dagys,1994; Hallam and Wignall, 1997) Furthermore, conodonts suffered highrates of extinction throughout the Triassic (e.g., Clark, 1983; Sweet,1988; Aldridge and Smith, 1993), and the largest drop in conodont diver-sity took place at the end of the Norian.

Various explanations have been offered for these extinctions, in-cluding both gradualistic and catastrophic mechanisms. Regression dur-ing the Rhaetian, with consequent habitat loss, is compatible with thedisappearance of some marine faunal groups, but may be regional, notglobal in scale. Gradual, widespread aridification of the Pangaean super-continent could explain a decline in diversity of terrestrial fauna duringthe Late Triassic, but suggests little about the marine realm. The pres-ence of multiple impact structures with Late Triassic ages suggests thepossibility of bolide impact-induced environmental degradation as a cata-strophic forcing mechanism. Widespread eruptions of flood basalts ofthe Central Atlantic Magmatic Province (CAMP) apparently were syn-chronous with or slightly preceded the system boundary; emissions ofCO2 and SO2 during these eruptions were substantial, but the specificenvironmental effects of outgassing of these lavas remains to be deter-mined.

In several marine Upper Triassic-Lower Jurassic sections, signifi-cant negative excursions in the isotopic composition of organic matterhave been noted near the system boundary (Hesselbo et al., 2002, 2004).At the New York Canyon section of Nevada, USA, for example, thenegative 13C excursion (about 2.0 o/oo) begins just below the highestoccurrence (HO) of conodonts, Triassic ammonites (Choristocerascrickmayi and Arcestes spp.) and Triassic bivalves (Guex et al., 2004).Similarly, at the Kennecott Point section in the Queen Charlotte Islands,Canada, a pronounced negative 13C excursion (of approximately 1.5 –2.0 o/oo) begins slightly below the HO of Triassic ammonites and radi-olarians, and extends to a level above the lowest occurrence (LO) of

Jurassic radiolarians (Ward et al., 2001, 2004; Williford et al., 2007).Moreover, several European sections display significant negative 13Cexcursions, as at St. Audrie’s Bay, southwest England (2.0 o/oo; Hesselboet al., 2002, 2004), Csövár, Hungary (2.0 o/oo Pálfy et al., 2001), andTiefengraben, Austria (3.0 o/oo; Kuerschner et al., 2007). Notably, in all ofthese sections, the excursion begins below the HO of conodonts and issucceeded by an interval of strata in which the isotopic composition ofthe organic matter returns to previous values, or is enriched. This posi-tive excursion is succeeded, in turn, by an interval again displaying de-pleted isotopic values. Several examples have been offered of a similartrend in the carbon-isotope composition of carbonate (e.g., Pálfy et al.,2001; Galli et al., 2005, 2007).

In contrast to the negative excursion now associated with thesystem boundary, the extinctions at the Norian-Rhaetian boundary havebeen linked to a positive 13C excursion (Sephton et al., 2002; Ward et al.,2004). Increases in 13C are commonly regarded as a product of an in-creased rate of burial of organic carbon, which should produce parallelincreases in the composition of both carbonate and organic carbon (Kumpand Arthur, 1999). Sephton et al. (2002) suggested that ocean anoxia atthe Norian-Rhaetian boundary resulted from conditions of a stratifiedocean, sluggish circulation, and a low latitudinal gradient. These authorsalso presented nitrogen isotope evidence in support of this interpreta-tion. More recently, Ward et al. (2004) have critiqued this work, notingthat the section analyzed by Sephton et al. (2002) is severely condensed,as the entire Rhaetian is represented by only 10 m of section. Ward et al.(2004) also described a positive excursion in the isotopic composition oforganic matter near the Norian-Rhaetian boundary at the Kennecott Pointsection in the Queen Charlotte Islands, also in British Columbia. Further,they noted that the excursion coincides closely with a severe reduction inMonotis in the section and a lithologic change from bioturbated to thinlylaminated facies. Again, increased ocean anoxia is cited as the cause of theisotope enrichment. Of particular note, however, is that while the data ofSephton et al. (2002) suggest a pronounced excursion that is maintainedover a (condensed) stratigraphic interval that corresponds to most of theRhaetian stage, the data of Ward et al. (2004), from a greatly expandedsection, display great complexity, with substantial variation in isotopicvalues over short intervals.

In a continuing attempt to increase understanding of the relation-ship between ocean chemistry and biotic events, we examined the carbonisotope stratigraphy of the Upper Triassic-Lower Jurassic section in theLagronegro basin, southern Apennines, at Monte Sirino (Fig. 1). Tanneret al. (2006) previously examined the carbon isotope (from carbonate)stratigraphy of uppermost Triassic strata at a section near Pignola. Intheir study of 50+ m of strata encompassing the uppermost Norian and

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most of the Rhaetian, they found several negative excursions of 13C ofsignificant magnitude. The largest, occurs slightly above the LO ofMisikella posthernsteini, a conodont that marks the base of the Rhaetianstage (sensu Kozur and Mock, 1991). We note that Dagys and Dagys(1994) instead define the base Rhaetian by the LO of M. hernsteini, butthese condonts co-occur at this stratigraphic interval (H. Kozur, pers.comm.). Another excursion, below the last in situ Triassic conodont, M.ultima, is consistent with the multiple short-duration carbon isotopeexcursions below the system boundary in other sections. Notably, how-ever, the carbon and oxygen isotopes from the section Tanner et al.studied display some covariance, suggesting at least partial recrystalliza-tion of the carbonate from isotopically light formation waters. The sec-tion at Pignola analyzed by Tanner et al. (2006) occupied a medial posi-tion within the Lagronegro basin, and was located considerably moreproximal than the Monte Sirino section studied here.

SETTING AND STRATIGRAPHY

Mesozoic through Cenozoic units of the Lagronegro basin com-prise a substantial portion of the orogenic wedge of the southernApennines (Fig. 1; Passeri et al., 2005; Cirapica, 2007). During the Me-sozoic, the basin, a part of the Ionian Tethys, was located adjacent to oneor more shallow carbonate platforms (e.g., the Apenninic and possiblythe Apulian platforms; Ciarapica and Passeri, 2002; Finetti, 2004, 2005;Ciarapica and Passeri, 2005). The early Mesozoic succession in thisbasin records deepening as the shallow-water siliciclastics and limestonesof the lowermost Monte Facito Formation (Lower to Middle Triassic)are succeeded by deeper water radiolarite facies deposited in the Ladinian(uppermost Monte Facito Formation) and are followed by the UpperTriassic and Jurassic Calcari con Selce and Scisti Silicei formations,possibly a consequence of transtensional tectonics due to the rifting ofthe Ionian Ocean (Ciarapica and Passeri, 2000; 2005).

The Upper Triassic-Jurassic Lagronegro succession is well ex-posed on the western flank of Monte Sirino, near the town of Lagonegro(Fig. 1). Nearly continuous outcrop occurs along a trail that connects theMadonna del Brusco Sanctuary to the Madonna del Sirino Sanctuary(Reggiani et al., 2005). The succession is composed of primarily chertylimestones and siliceous shales and radiolarites. The former characterizethe Calcari con Selce Formation, and the latter the Scisti Silicei Forma-tion. The transitional interval between these two formations is gradualand features interbedded lithologies characteristic of both formations,such as micritic limestones, radiolarian cherts and siliceous shales

(Miconnet, 1983; Amodeo and Baumgartner, 1994; Amodeo, 1996; 1999).The prevalence of deeper water facies in this sequence indicates deposi-tion of the succession in a distal part of the basin (Scandone, 1967;Reggiani et al., 2005).

The Monte Sirino section exposes 45 m of continuous section ofthe upper Calcari con Selce Formation, most of which consists of well-bedded micritic limestones, commonly with chert nodules. The upper-most 18 m are bracketed between two siliceous red shales horizons andare informally designated the “transitional interval” (Fig. 2). This inter-val differs from the major part of the Calcari con Selce by the greatercomponent of siliceous shale and the presence of scattered radiolarites,and is correlatable to many other sections within the Lagronegro basin(Passeri et al., 2005). The red shale beds allow for precise correlation ofthe section between outcrops across switchback turns in the trail. Theupper part of the Calcari con Selce has been well-dated with conodonts,including such upper Norian forms as Epigondolella bidentata, E. posteraand E. matthewi (Reggiani et al., 2005). The Norian-Rhaetian stage bound-ary is placed within the upper red shale bed near the top of the transitionzone based on the LO of M. hernsteini at this level in correlative sections(Dagys and Dagys, 1994; Bertinelli et al., 2005).

The overlying Scisti Silicei Formation is characterized by the ab-sence of micritic limestone beds, consisting primarily of radiolarian-bearing siliceous shales and cherts with minor calcarenites. The lower-most member of the Scisti Silicei, the Buccaglione Member, comprises21 m of thin-bedded red siliceous shales, black shales with green andblack radiolarian chert layers, silicified calcarenites with cherty nodules,and a single 1-m thick calcirudite bed (Reggiani et al., 2005). Siliceousshales in the lower Buccaglione Member have yielded a well-preserved

FIGURE 1. Geologic map of the southern Apennines with locations describedin the text (adapted from Bazzucchi et al., 2005). Units on map as follow:1) mostly Recent sediments; 2) Quaternary volcanics; 3) Cilento units; 4)Mesozoic platform carbonates; 5) units of the Lagronegro succession ofPermian to Early Cretaceous age (5a) and Early Cretaceous to Miocene(5b).

FIGURE 2. Lithostratigraphy and carbon isotope stratigraphy for the MonteSirino section.

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and robust radiolarian assemblage referable to the early Rhaetian. Assem-blages from the upper part of this member are referred to the uppermostpart of the Rhaetian due to the presence of Globolaxtorum tozeri (Reggiani,et al., 2005).

The overlying Nevèra Member consists of ~6 m of black andgreen shales (Fig. 3A) with minor very fine-grained, partially to whollysilicified calcarenites (Fig. 3B). The shales of the Nevèra Member arebarren, but the calcarenites yield pyritized radiolarians that probably areTriassic specimens with twisted spines, and potentially some primitiveJurassic spumellarians (Reggiani et al., 2005). The succeeding SerraMember, which consists mainly of red siliceous shales and thin redradiolarites, was not sampled for this study. Radiolarians in the lowerpart of this member include spumellarians referable to the Lower Juras-sic (Reggiani et al., 2005). Based on this biostratigraphy, the systemboundary occurs within the Nevèra Member between two calcarenitebeds just below the base of the Scisti Silicei (Figs. 2, 3B; Reggiani et al.,2005).

METHODS

We analyzed the carbon and oxygen isotope composition of car-bonate in 40 samples, representing 42 m of section (Fig. 2) from theupper Calcari con Selce (transitional interval) and lower Scisti Silicei(Buccaglione and Nevèra members). Sampling of the section included arepeat of about 2 m where the outcrop was correlated across a turn in thetrail (i.e. the section is discontinuous). The analyses were conducted inthe Stable Isotope Laboratory of the Department of Earth and PlanetarySciences at the University of New Mexico. Carbonate carbon and oxygenisotope ratios were measured by conventional phosphoric acid digestionmethods using elemental analyzer continuous flow isotope ratio massspectrometry with a Carlo Erba Elemental Analyzer coupled to a FinniganMat Delta Plus mass spectrometer. Results are reported in parts perthousand (o/oo) relative to the Vienna Pee Dee Belemnite standard (VPDB).

RESULTS

The isotopic composition of the carbonate demonstrates substan-tial consistency throughout the section (Figs. 2,4), falling within therange of +2.47 o/oo maximum and -0.02 o/oo minimum, maintaining anaverage 13C of +1.56 o/oo. Values of 18O range from -3.58 o/oo to-6.2 o/oo, with an average of -4.35 o/oo (Fig. 4). Several trends are discernablewithin these results. First, the lack of correlation (covariance) between

13C and 18O (Fig. 4), as well as the limited range of 18O values,suggests limited or no alteration of the carbonate by isotopically de-pleted pore waters. Thus, we are relatively confident that the carbon

isotope values reported here represent the primary signature of the car-bonate as originally precipitated from marine waters. By comparison,the data reported for the approximately correlative section at Pignoladisplay a greater range of values of 18O and covariance of 13C and 18Oat a significant level (see Tanner et al., 2006, fig. 6). Furthermore, therange of 13C values at Monte Sirino is substantially smaller than atPignola, where the minimum value of 13C = -3.5 o/oo. We conclude,therefore, that re-equilibration of the isotopic signature of the carbonatein this section was minimal and that the carbon isotope record at MonteSirino represents a record of the primary (original) isotopic compositionof the carbonate sediments. Conversely, the data reported from Pignolalikely represent carbonate in which the isotopic composition possiblyhas been altered by diagenesis.

The significance of these new data is most apparent when com-bined with biostratigraphic constraints on the age of the strata. Sampledensity is greatest in the uppermost Norian and basal Rhaetian portionof the section (transitional interval and basal Buccaglione Member), dueto the greater abundance of limestone beds (Fig. 2). Through this section,from 33 to 46.5 m before the break in the section at the turn in the trail,and 0 to 3 m after the section break, 13C values display very consistentvalues between a minimum of +1.6 o/oo to a maximum of 2.3 o/oo. How-ever, the sample density is considerably lower through most of theRhaetian section (the remainder of the Buccaglione and lower Nevèramembers) due to the substantially lower frequency of carbonate beds.Nevertheless, it is notable that 13C values decrease higher in the sectionand maintain values near 0 o/oo until the system boundary (within theNevèra Member), where 13C recovers to values near and exceeding2.0 o/oo (Fig. 2).

DISCUSSION

Sephton et al. (2002) first drew attention to the perceived posi-tive (organic) carbon isotope excursion at the Norian-Rhaetian boundary,although as pointed out by Ward et al. (2004), the section studied bySephton was condensed, and therefore the stratigraphic placement of thereported excursion is suspect. Ward et al. (2004), as described earlier,also reported a positive carbon isotope excursion at the Norian-Rhaetianboundary although their data across this boundary display a series ofshort-lived fluctuations, rather than a single, pronounced excursion. Inparticular, Ward et al. (2004) related these isotopic fluctuations to thedisappearance of Monotis. Significantly, Williford et al. (2007) re-ana-lyzed the section studied by Ward. They also documented a complex

FIGURE 3. Outcrop views of the Nevèra Member, Scisti Silicei Formation, at Monte Sirino. A) Black shales in the uppermost Rhaetian section about 3 mbelow the system boundary; B) The lower of two calcarenite beds in the upper Nevèra Member. The system boundary is placed at the top of this bed (tothe upper right).

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sequence of variation in 13Corg, but, conspicuously, these data do notindicate any significant excursion in the vicinity of the stage boundary,nor did these authors describe one. Although our data are derived fromanalysis of carbonate, rather than organic carbon, our 13Ccarb valuesappear to represent the primary carbon isotope signature. The 13Cvalues we obtained are consistent across the Norian-Rhaetian boundary,indicating neither positive nor negative excursions. Thus we find littleconvincing evidence for a disruption of the global carbon cycle associatedwith the extinction event at this boundary.

In regard to the system (Triassic-Jurassic) boundary, our dataoffer less information. Due to the lower carbonate content of the Rhaetianstrata, sample frequency was significantly lower (< 1 sample per twometers of section) than in the underlying Norian section (approximately2 samples per meter of section). Notably, the data display consistentlydepleted carbon through the Rhaetian section, as compared to the under-lying Norian section. This differs from the carbonate carbon isotope datapublished for most other sections. We suggest two tentative explana-tions for this difference. One possibility is that the much lighter organiccarbon influences (or contaminates) the analyses in samples with a lowcarbonate yield, resulting in lower isotopic values. An alternative expla-

nation is that some fractionation or bias toward depleted carbon operatesunder conditions of minimal carbonate precipitation or preservation,such as deeper marine environments where the sediment interface is nearor below the CCD. We note, for example, that the 13Ccarb reported fromthe deep-water section at Kennecott Point (Ward et al., 2004) is alsoconsiderably depleted compared to the values reported from marinesections on or marginal to carbonate platforms (e.g., Palfy et al., 2001;Galli et al., 2007). In this view, the lower 13Ccarb values exhibited in theRhaetian section are environmentally biased and do not represent theocean carbon isotope composition of this time. Given this uncertaintyabout the Rhaetian isotope record at Monte Sirino, we cannot interpretthe nature of the abrupt change to heavier 13Ccarb values at the systemboundary. These appear to represent normal values for carbonate de-rived from an adjacent platform, but given the lack of sample resolution,it is impossible to say if these values represent the ocean carbon compo-sition immediately prior to the end-Triassic excursion, or the recovery ofthe carbon cycle following the initial negative excursion observed at mostother sections.

CONCLUSIONS

The carbon isotope stratigraphy for both carbonate and organiccarbon now has been published for numerous Upper Triassic-LowerJurassic sections, including settings ranging from shallow platform tobasinal, in both Tethyan and North American Cordilleran realms. A con-sensus has emerged from these studies that a distinct, short-lived nega-tive isotope excursion occurred shortly before the end of the Triassic.Beyond this, however, there is little than can be agreed upon. Earlierreports of a positive isotope excursion at the Norian-Rhaetian stageboundary are not supported by the results of the present study (or otherrecent studies). Indeed, detailed carbon isotope records for extendedstratigraphic intervals document considerable noise; the isotope stratig-raphy records frequent, and sometimes pronounced shifts of short dura-tion, rather than extended intervals of uniform isotopic composition.Serious questions remain regarding the causes and extent of variation ofthese variations in the isotopic record, and their relation to biotic events.

ACKNOWLEDGMENTS

This research was generously supported by the Le Moyne Col-lege Research & Development Committee, and by MIUR and Universitàdi Perugia (Prin Project 2004). Helpful reviews were provided by Spen-cer Lucas and Leonsevero Passeri.

FIGURE 4. Bivariate plot of 13C and 18O for carbonate from the MonteSirino section.

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