terrestrial gypsum from alaska and greenland in glacially influenced marine sediments

Terrestrial gypsum from Alaska and Greenlandin glacially influenced marine sediments

K.E.K. St. John*, E.A. Cowan1

Department of Geology, Appalachian State University, Boone, NC 28608, USA

Received 7 June 1999; accepted 23 March 2000

Abstract

Gypsum grains were identified in Miocene–Pleistocene sediment cores from two deep-water ODP sites, Site 918 off the SEGreenland margin and Site 887 in the Gulf of Alaska, and in Holocene sediment cores from shallow-water localities inDisenchantment Bay and Muir Inlet in southern Alaska. Although initial morphologic and textural observations suggested acomplex system in which the gypsum may have had more than one origin, quantitative sulfur isotope analyses of the gypsumprovide evidence of its detrital nature.d 34S values in gypsum from southern Alaska range between10.0 and17.1‰. Gypsumhasd 34S values between227.1 and227.5‰ in the Gulf of Alaska and values between228.5 and10.2‰ off the SE Greenlandmargin. All of these isotopic signatures are too highly depleted ind 34S to have precipitated from seawater, present or past. Inaddition there is no significant change ind 34S values for gypsum crystals with differing physical characteristics (abraded vs.unabraded) from the same stratigraphic horizon, suggesting all the gypsum is detrital regardless of the degree of abrasion. Theisotopic and physical evidence, in combination with the onshore geology the environmental setting, and site characteristics ofthe gypsum-bearing marine localities, lead us to propose that the ultimate source of the gypsum is precipitation from freeze-induced terrestrial sediment or soil brines. Furthermore the combined evidence suggests that the subsequent occurrence ofgypsum in glacimarine sediments results from ice-rafting (by icebergs or sea ice) of the frozen regolith and/or, in the proximalglacimarine setting of southern Alaska, very rapid burial via turbidity currents.q 2000 Elsevier Science B.V. All rightsreserved.

Keywords: gypsum; ice-rafted debris; sulfur-isotopes; Alaska; Greenland; glacimarine

1. Introduction

Gypsum is not usually found as a constituent ofdeep marine sediments, reflecting the typical condi-tion that sea water is undersaturated with respect togypsum (Briskin and Schreiber, 1978). In fact the

classic model for the origin of marine gypsumrequires an arid climate and a semi-restricted basin(Rothwell, 1989), such as the Mediterranean Seaduring the Messinian salinity crisis and the nearshoreenvironments of the Persian Gulf today. Nevertheless,compositional analyses of coarse sand-sized grains inMiocene–Pleistocene sediment cores from two deep-water Ocean Drilling Program (ODP) sites, Site 918off the SE Greenland margin and Site 887 in the Gulfof Alaska, have unexpectedly identified gypsum as aminor constituent. Furthermore, gypsum crystals withsizes, shapes, and surface textures strikingly similar to

Sedimentary Geology 136 (2000) 43–58

0037-0738/00/$ - see front matterq 2000 Elsevier Science B.V. All rights reserved.PII: S0037-0738(00)00083-X

www.elsevier.nl/locate/sedgeo

* Corresponding author. Tel.:1828-262-6739; fax:1828-262-6503.

E-mail addresses:[email protected] (K.E.K. St. John),[email protected] (E.A. Cowan).

1 Tel.: 1828-262-6739; fax:1828-262-6503.

those recovered from the Gulf of Alaska and the SEGreenland margin have also been identified in shallowcores of Holocene sediments from two glaciatedmarine embayments in southern Alaska.

The occurrence of gypsum in these marine cores isboth unusual and intriguing, especially consideringtheir cool climate, mid to high latitude settings. Ofprimary importance in understanding these gypsumoccurrences is determining the origin of the gypsum,as it could be either authigenic or detrital in nature.Investigations into whether these gypsum crystals areauthigenic or detrital provides us with an opportunityto consider a new model for evaporite formation andmarine deposition in glacially influenced mid to highlatitude climates.

In this paper, physical grain characteristics andsulfur isotope measurements are used to explain theoccurrence and origin of coarse-grained gypsum.Although initial morphologic and textural observa-tions of samples containing both abraded and unab-raded forms suggested a complex system in whichgypsum appears to have both detrital and authigeniccharacteristics, simple quantitative sulfur isotopeanalyses of the gypsum provide evidence of its detritalnature. The physical and isotopic evidence, in combi-nation with the onshore geology the environmentalsetting, and site characteristics of the gypsum-bearingmarine localities, lead us to propose that the ultimatesource of the gypsum is precipitation from freeze-induced terrestrial sediment or soil brines. Further-more the combined evidence suggests that thesubsequent occurrence of gypsum in glacimarine sedi-ments results from ice-rafting (by icebergs or sea ice)of the frozen regolith and/or, in the proximal glaci-marine setting of southern Alaska, very rapid burialvia turbidity currents.

2. Stratigraphic and geographic distribution ofgypsum

Two deep water gypsum-bearing sites are consid-ered in the study: ODP Site 918 in the western Irmin-ger Basin off SE Greenland (Fig. 1A), and ODP Site887 on the Patton–Murray Seamount in the Gulf ofAlaska (Fig. 1B). Both of these sites contain robustregional ice-rafted debris (IRD) records for the LateCenozoic (Krissek, 1995; St. John and Krissek, 2000,

in review), and gypsum crystals were recognizedinitially at these locations during preliminary compo-sitional analyses of the coarse-sand fractions for ice-rafted debris studies. At Site 918, gypsum crystalswere visually identified in 17 of the 495 coarse-sandsamples investigated for IRD; gypsum was the domi-nant constituent in four of these sample and was aminor constituent in 13 of these samples (Fig. 2A).Gypsum was visually identified in only one coarse-sand sample from Site 887, yet it was the dominantgrain type in this sample (Fig. 2B). The gypsum-bear-ing sediments range in age from Late Miocene to LatePleistocene at Site 918, and the gypsum-bearingsample at Site 887 is of Late Pleistocene age. Thelithology of the upper 600 m at Site 918 is dividedinto five subunits of muds and silts with dropstones(Fig. 2A). Whereas the Pleistocene sediment lithologyat Site 887 is composed of siliceous silty clay anddiatom oozes with dropstones (Fig. 2B). Foraminiferaare common in Site 918 sediments and rare to absentin Site 887 sediments. Each site has an organic carboncontent of,1.0 wt% and pyritized burrows are some-times present. Detailed description of the sampledlithologic units of Sites 918 and 887 can be found inthe appropriate chapters in Rea et al. (1993) andLarsen et al. (1994). No correlations between gypsumabundance and sediment age, subbottom depth, orlithologic character are recognized for either ofthese sites (Fig. 2A and B). In addition, gypsum crys-tals were not consistently associated with ice-rafteddebris of any particular composition.

The remaining sites examined in this study arenearshore, relatively shallow-water locations proxi-mal to glacial termini; these occur in DisenchantmentBay, near Yakutat in southern Alaska (Fig. 1C) and inMuir Inlet of Glacier Bay National Park and Preserve(Fig. 1D). Investigations of these sites focused onmodern glacimarine depositional processes usingshallow gravity cores (Cowan et al., 1997, 1999).Gypsum crystals were initially identified duringroutine examination of the.63mm fraction duringparticle size analysis. The gypsum-bearing sites(YB10, YB11, YB19, and YB21) in DisenchantmentBay are located in less than 250 m water depth and areup to 20 km from the terminus of Hubbard Glacier.The gypsum-bearing sites (GB4, GB5, GB6, andGB7) in Muir Inlet are located in less than 260 mwater depth, 7–10 km from the terminus of Muir

K.E.K. St. John, E.A. Cowan / Sedimentary Geology 136 (2000) 43–5844

K.E.K. St. John, E.A. Cowan / Sedimentary Geology 136 (2000) 43–58 45

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Fig. 1. Site location maps of gypsum-bearing samples at (A) ODP Site 918, off SE Greenland, and (B) Site 887 in the Gulf of Alaska, and in twosouthern Alaska fjords: (C) Disenchantment Bay, near Yakutat, and (D) Muir Inlet in Glacier Bay National Park.

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Fig. 2. (A) Composite diagram of Site 918 Unit I lithology, gypsum abundance, IRD abundance and composition plotted against depth (mbsf, meters belowsea floor) and calculatedage (ka). The lithologic subunits consist of the following: IA, interbedded soupy, fining-upward sand and firmer silt with dropstones present throughout (open circles representdropstones); IB two sets of fining-upward sand-silt couplets with dropstones present throughout; IC, silt enriched in dropstones; ID, silt with dropstones; IE, silt (Larsen et al., 1994).Gypsum-bearing samples that also contained Foraminifera or pyritized burrows are indicated by “f.” and “p.b.”, respectively. Coarse sand ice-rafted debris (CS IRD) consists ofquartz, granitic rock fragments, basaltic rock fragments, coarse grained mafic rock fragments, and sedimentary rock fragments (St. John and Krissek, 2000, in review). (B) Compositediagram of the Site 887 lithology, gypsum abundance, and IRD abundance plotted against depth (mbsf, meters below sea floor) and calculated age (ka) forthe gypsum-bearinginterval. Lithologic subunit IA consists of siliceous silty clay, with discrete intervals diatom ooze; lithologic subunit IB consists of diatom oozes with dropstones (Rea et al., 1993).Coarse sand ice-rafted debris (CS IRD) abundances are expressed in terms of mass accumulation rates (St. John and Krissek, 1999).

Glacier. Because the Disenchantment Bay and MuirInlet sites are proximal to glacial termini biologicalproductivity is low, reflected by an organic carboncontent of,1.0 wt% and rare to few foraminifer incores from each of these sites. Pyrite is absent in thecores. Glacimarine sediments in Muir Inlet and Disen-chantment Bay are dominated by Holocene melt-water-deposited rhythmically laminated terrigenousmud with turbidite sand beds. Annual units (varves)are identified by couplets of laminated sediments anddiamicton deposited by rainout of pebbles, granulesand course-sand during the winter months (Cowan etal., 1997). Sediment accumulation rates are extremelyhigh in both Muir Inlet and Disenchantment Bay anddecay exponentially with distance downfjord from theglaciers. For example, in Disenchantment Bay aver-age accumulation rates range from 48 cm/yr, 3.4 kmfrom Hubbard Glacier at the head of the bay to 14 cm/yr, 15 km away (Cowan et al., 1997).

Gypsum crystals occur throughout the Disenchant-ment Bay and Muir Inlet cores but they occur mostfrequently in laminated mud lithofacies and in thinmassive or graded sand beds (Fig. 3). Laminatedmud is formed by centimeter to millimeter scale inter-

laminations with very find sand or silt (Cowan et al.,1998). In some samples most of the sand fraction oflaminated mud is composed of gypsum crystals. Sandbeds are normally graded with sharp lower contactsand gradational upper contacts.

Although site-specific characteristics, such assediment ages, lithofacies, water depths, and distanceto land, differ among the gypsum-bearing sites insouthern Alaska the Gulf of Alaska, and the IrmingerBasin, sediments at all of these localities show astrong glacial influence at the time of deposition.Diamictons and temperate glacimarine rhythmites atthe shallow water sites, and abundant IRD at thedeeper water offshore locations record deposition ina range of proximal to distal glacimarine environ-ments.

3. Investigative approach

3.1. Sample preparation and gypsum identification

Disenchantment Bay and Muir Inlet samples weretaken immediately after core recovery and required no


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refrigerated storage. Whereas, samples from deepwater sites 918 and 887 were taken at cold-storagecore repositories two and three years post-recovery,respectively. Similar laboratory methods werefollowed for each sample processed for this study.Samples were dried at 608C, weighed, disaggregatedultrasonically, and wet-sieved at 2 mm and 250mmfor samples from the two deep water sites, and at63mm for samples from coastal Alaska. The coarse-sand fraction of each sample was then dried at 608Cand examined under a binocular microscope for initialmineral identification. Gypsum crystals that repre-sented the full range of observed sizes, shapes, andsurface textures were selected for further study. Quali-tative analysis of crystal morphology and surfacetexture was conducted using a scanning electronmicroscope (SEM). Elemental analysis using theenergy dispersive X-ray capability (EDS) of theSEM was used to confirm the chemical compositionof the crystals.

3.2. Sulfur isotope analysis

After the SEM analyses were completed and themorphological and textural information wasreviewed, gypsum crystals from different locationsand core depths, and with a range of surface textureswere selected for sulfur stable isotope ratio analysis(SIRA). Because a minimum sample size of 1.2 mg ofgypsum was needed for such analysis, compositesamples of multiple crystals from a single site and asingle stratigraphic position frequently were used(Table 1). When composite samples were made,care was taken to group crystals of similar morphol-ogy and surface texture. The analytical precision forthis method is 0.3‰ based on multiple analyses ofhomogeneous standards.

3.2.1. Why use sulfur isotopes to determine the originof gypsum?

Sulfur is present in nearly all natural environments:it is a minor constituent in igneous and metamorphicrocks, primarily as sulfides; it occurs in the biosphereand in related organic substances such as crude oil andcoal; it occurs in ocean water as sulfate and in marinesediments as both sulfate and sulfide (Hoefs, 1987). Inaddition to the presence of sulfur in a variety ofnatural environments, sulfur has a wide range of


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34S/32S ratios in nature. This range of34S/32S ratiosprimarily exists because of isotopic fractionationcaused by sulfur-reducing bacteria (Faure, 1986;Hoefs, 1987). Through metabolic isotopic exchangereactions, bacteria reduce sulfate ions to H2S, enrich-ing the sulfide product in32S and, at the same time,enriching the remaining sulfate in34S. This isotopicfractionation and corresponding variability ofd 34S arebelieved to be characteristic of sulfur that has passedthrough the sedimentary cycle, which explains thewide ranged 34S values in sedimentary rocks, fromabout240 to 150‰ (Fig. 4), while thed 34S valuesof igneous rocks are close to 0‰.

While fractionation effects produce a wide range of

d 34S values in nature the range ofd 34S values in marinesulfate is more narrowly restricted and is uniform atany one time throughout the world’s oceans. For exam-ple, as shown in Figs. 4 and 5, marine sulfate has rangedonly between about110 and130‰ during Phanero-zoic time, and modern marine sulfate has ad 34S valueof about120‰, regardless of location (Faure, 1986).The isotopic composition of marine sulfate can bedetermined by measuring the isotopic composition ofmarine sulfate minerals which have essentially thesame isotopic composition as the sea water at theirtime of formation. This relationship exists because nobacterial steps are involved in mineral crystallizationto cause S-isotopes fractionate appreciably (Thode andMonster, 1965). For example, gypsum precipitatingfrom modern sea water would have ad 34S value ofabout120‰ the same as modern parent sea water.Whereas, Permian marine gypsum would have ad 34Svalue of about112‰ the same as Permian parent seawater (see Fig. 5).

Unlike sea water, nonmarine brines have a widerange ofd 34S values, and are not globally uniformin their S-isotope composition at any point in timegiven the heterogeneity of the many source rocks.However because no bacterial steps are involved,gypsum precipitating from a nonmarine brine willhave essentially the same isotopic composition asthe brine from which it precipitated. For example,gypsum that precipitated from a nonmarine brine inwhich the brine S was a product of weathering anderosion of a granitic rock would have ad 34S valueclose to 0‰ the samed 34S value as the graniticrock (see Fig. 4). Conversely, gypsum that precipi-tated from a nonmarine brine in which the brine Swas a product of weathering and erosion of a32S-enriched pyrite-bearing black shale would have ad 34S value that is also enriched in32S the same asthe pyrite, and reflecting the degree of bacterial actionthat occurred in the earlier sulfide-forming solution(i.e. pyrite-forming solution).

In sum, marine gypsum is restricted to a narrowrange of d 34S values (those between110 and130‰). With rare exception (see Discussion), valuesoutside this marine range, and especially valuesenriched in 32S, indicate that gypsum precipitatedfrom nonmarine brines. In this way,d 34S values canprovide a more definitive answer to the question ofgypsum origin than SEM analyses alone.


Table 1Results of the sulfur-isotope analyses of gypsum samples from theSE Greenland margin, southern Alaska, and Gulf of Alaska

Descriptiona d 34S

Samples: Site 918, Greenland Margin17H1, 99–100 cm 4 grains 223.831×5, 47–49 cm 1 grain: unabraded 10.2

5 grains: unabraded 21.231×5, 47–49 cm 2 grains: abraded 21.0

4 grains: abraded 22.122R3, 92–94 cm 3 grains 228.525R5, 22–24 cm 2 grains 210.629R2, 107–100 cm 2 grains 212.633R1, 37–39 cm 5 grains 210.4

Sample: Site 887, Gulf of Alaska5H1, 141–143 cm 5 grains: abraded 227.1

6 grains: abraded 227.5

Samples: Disenchantment Bay, AKAH94 YB11, 55 cm 1 grain: unabraded 16.5AH94 YB11, 55 cm 2 grains: abraded 16.8AH94 YB11, 220 cm 3 grains 17.1AH94 YB11, 270 cm 4 grains 16.6

Samples: Muir Inlet, AKAH93 GB5, 13 cm 1 grain: unabraded 12.2AH93 GB5, 13 cm 1 grain: abraded 10.7

2 grains: abraded 10.4AH93 GB5, 26 cm 5 grains: unabraded 11.0AH93 GB5, 26 cm 2 grains: abraded 10.0AH93 GB5, 50 cm 3 grains: unabraded 13.6AH93 GB5, 50 cm 5 grains: abraded 12.8AH93 GB7, 60 cm 1 grain: abraded 15.2AH93 GB7, 180 cm 1grain 14.3

a Includes the number of gypsum grains used for each isotopicanalysis and the grain form (abraded or unabraded) for somesamples.

4. Results

4.1. SEM

Results of a representative EDS elemental analysisof a gypsum crystal are shown in Fig. 6. The onlyabundant elements indicated are sulfur and calcium,which confirms the visual interpretation that thesecrystals are gypsum. When examined at high magni-fication the gypsum grains show a wide variety ofcrystal morphologies and surface textures, as shownin Fig. 7(A–H). Crystals are euhedral to subhedraltabular forms, ranging from classically monoclinicto diamond shaped. In several cases two (or more)crystals are intergrown (twinned), either in right-angle pairs or, less commonly, as intergrown rosettes.The long axis of the crystals averages 1 mm in length,and no crystals are greater than 1.6 mm long. (Notethat the lower end of the size range cannot be definedbecause the samples from which gypsum was identi-fied had previously been wet sieved at either at 63or 250mm for other studies thus removing the finematerial.) Subhedral forms show some degree ofrounding, and breakage surfaces are sometimespresent. Surface features include linear striations andpitting, although smooth planar surfaces are alsoobserved. Based on these morphological and texturalobservations, rounded, subhedral crystals are consid-ered “abraded”, whereas, euhedral crystals withsmooth planer surfaces are considered “unabraded”.

An additional textural feature is the presence of


marine evaporate sulfate

modern ocean water

sedimentary rocks

metamorphic rocks

granite rocks

basaltic rocks

gypsum crystals, this study

50 40 30 20 10 0 -10 -20 -30 -40

Fig. 4. 34S/32S ratios in some geologically important materials and gypsum crystals analyzed in this study (d 34S relative to Can˜on Diablotroilite; modified from Hoefs, 1987).

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Fig. 5. Variations ofd 34S of marine sulfate minerals from LatePrecambrian to the Present. Thed 34S are relative to Can˜on Diablotroilite (modified from Faure, 1986).

clay to silt-sized inclusions in many of the gypsumcrystals (Fig. 7). On some crystals these inclusionsappear to cluster at the ends of the gypsum crys-tals, whereas in others the inclusions are distribu-ted randomly. In a few cases the inclusions covernearly the entire gypsum crystal. EDS analyses ofthe inclusions show the presence of a wide varietyof elements, but are dominated by Si, Al, and Ca(Fig. 8), suggesting that the inclusions are clayminerals.

At each of the sites there is no stratigraphic trend incrystal morphology or surface texture, and gypsumcrystals from the same sample (i.e. the same strati-graphic horizon) can show the entire range ofmorphologies and surface textures (e.g. Fig. 7A). Inaddition, gypsum crystals from the shallow-watersouthern Alaska sites and the deep-water SE Green-land site are indistinguishable, having similar texturaland morphological characteristics. Crystals from thesingle gypsum-bearing sample at Site 887 in the Gulfof Alaska show many of the same textural andmorphological characteristics as are observed ingypsum crystals from the other marine locations,including subhedral, intergrown (primarily twinned,but also some untwinned) forms (Fig. 9A and B),

but in addition the gypsum crystals at Site 887 alsocontain siliceous microfossil inclusions (diatoms; Fig.9C and D). No inclusions or intergrowths of calcar-eous microfossils are observed in gypsum from any ofthe sites in this study.

4.2. S-isotope

Stable sulfur isotope ratio analyses were performedon a total of 20 gypsum samples: three from Disen-chantment Bay, eight from Muir Inlet, seven from Site918, and one from Site 887. As shown in Table 1 thed 34S values range from228.5 to17.1‰. The south-ern Alaska sites have the most uniformd 34S values;samples from Disenchantment Bay only rangebetween16.5 and17.1‰, and samples from MuirInlet range between10.0 and15.4‰. Samples fromthe Greenland margin and the Gulf of Alaska havealmost exclusively negative values, withd 34S valuesranging between228.5 and10.2‰ for Site 918, andad 34S value slightly greater than227‰ for Site 887.

There is no significant change ind 34S values forcrystals with differing physical characteristics (i.e.“abraded” vs. “unabraded”) from the same strati-graphic horizon. For example, abraded and unabraded


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Fig. 6. Representative EDS analysis of a gypsum crystal in sample 31×5, 47–49 cm from ODP Site 918, SE Greenland margin.


Fig. 7. Examples of morphology and surface texture of gypsum from ODP Site 918, SE Greenland, and from Muir Inlet and DisenchantmentBay, southern Alaska. (A) Low magnification showing the variety of crystal morphologies and textures within one sample, 31×5, 47–49 cm,Site 918; bar scale� 1 mm. (B) Nearly perfect euhedral crystal, AH94 YB11, 270 cm, Disenchantment Bay; long axis� 0.91 mm. (C) Clayinclusions at end of an otherwise euhedral crystal, 31×5, 47–49 cm, Site 918; bar scale� 100mm. (D) Twinned, rounded subhedral crystals,Muir Inlet; long axis�,1.0 mm. (E) Irregular, intergrown crystals (rosette), 31×5, 47–49 cm, Site 918; bar scale� 100mm. (F) Twinnedcrystals nearly completely covered with clay inclusions, 31×5, 47–49 cm, Site 918; bar scale� 100mm. (G) Euhedral crystal with clay–siltinclusions on all faces, AH94 YB10, 120 cm, Disenchantment Bay; long axis� 1.55 mm. (H) Crystal with broken edge, surface pitting, androunding, AH93 GB5, 26 cm, Muir Inlet; long axis� 0.86 mm.

crystals from sample AH94 YB11, 55 cm in Disen-chantment Bay haved 34S values of16.8 and16.5‰,respectively, and abraded and unabraded crystalsfrom sample AH93 GB5, 26 cm in Muir Inlet haved 34S values of10.0 and11.0‰, respectively.

Sample variation was also measured by analyzingcrystals of the same type (e.g. two “abraded” crystals)from the sample, and the resulting values are verysimilar. For example, unabraded crystals from sample31×5, 47–49 cm at Site 918 range between21.2 and10.2‰, abraded crystals from sample AH93 GB5,13 cm in Muir Inlet range between10.4 and10.7‰, and abraded crystals in the only gypsum-bearing sample at Site 887 range between227.5and227.1‰.

5. Discussion

Traditionally, detrital gypsum has been distin-guished from authigenic gypsum on the basis ofgrain morphology; detrital gypsum typically wasseen to occur as subhedral to anhedral crystals withcleaved and abraded surfaces, whereas authigenic

gypsum was considered to occur as euhedral unbrokencrystals, commonly with abundant clay inclusions(Rothwell, 1989). Based on these apparent distin-guishing characteristics the gypsum identified in thisstudy would be interpreted as having a mixed origin-mixed to the extent that detrital gypsum and authi-genic gypsum even occur in the same samples.However thed 34S values suggest that this is not thecase. Although the analyzed crystals have a range ofd 34S values, all are too highly depleted ind 34S (orenriched in 32S) to have been precipitated frommodern sea water, Late Cenozoic sea water, or anyother sea water of the past. The isotopic data indicatesthat the gypsum crystals did not form in situ at thetime of sediment deposition.

Another possibility to consider is that thed 34S-depleted gypsum formed sometime after sedimentdeposition from the combination of oxidized H2S,originally derived from sulfate-reducing bacteria,with Ca21 ions derived from dissolved calcareousmicrofossils (Briskin and Schreiber, 1978; Schnitkeret al., 1980) Such a scenario requires: (1) iron-sulfideaccumulation in carbonate-containing sediment; and(2) oxygen intrusion either at the sediment–water


Cou

nts

(x1

0)

3

Range ( keV )

2

1

01 2 3 4 5 6 7 8 9 10

O

Na

MS SSP C1K

Ca

CaFe

A1

Si

Fig. 8. Representative EDS analysis of a clay inclusion in gypsum. From sample 31×5, 47–49 cm from ODP Site 918, SE Greenland margin.

interface, after burial from bioturbation, or byatmospheric exposure in storage after core recovery.In general, condition 1 is not met by the gypsum-bearing sediment from the localities in this study.Disenchantment Bay and Muir Inlet cores containno pyrite (iron-sulfide) and only rare to few foramini-fer. The lithology of the gypsum-bearing sample fromthe Gulf of Alaska is siliceous, not calcareous, and nopyrite was identified. And, only three of the 17gypsum-bearing samples from the Irminger Basincontained pyritized burrows, while just one gypsum-bearing sample contained abundant foraminifer (Fig.2). In addition, as stated in the core descriptionsection, no gypsum crystals in this study containedcalcareous microfossil inclusions.

Even if the condition of iron-sulfide accumulationin carbonate-containing sediment was met in a few ofthe gypsum-bearing core horizons from these local-ities, condition 2 of the above scenario is difficult tosatisfy. Sufficient oxidation at the sediment–water

interface is unlikely given the relatively rapid burialby IRD rainout and/or turbidity currents in each of thegypsum-bearing settings. In addition the laminatedsediments in Disenchantment Bay and Muir Inlet arenot bioturbated and those in the Gulf of Alaska andIrminger Basin are only slightly bioturbated; thus thepossibility of in situ oxygen-intrusion after burial isunlikely. And, while sediments were certainlyexposed to oxygen after core recovery the generallack of carbonate material and iron-sulfide in the sedi-ment cores makes the possibility of post-recoveryprecipitation of the gypsum crystals unlikely.Therefore, the difficulty of meeting both conditions1 and 2 of this scenario indicates that little, if any,gypsum in our study is authigenic.

Non-authigenic gypsum must then be detrital,regardless of its morphology, its surface texture, orthe presence or absence of clay and diatom inclusions.Furthermore, the isotopic results show that thesephysical characteristics are not definitive indicators


Fig. 9. (A,B) Examples of gypsum crystals from ODP Site 887, Gulf of Alaska. Bar scale is 100mm. (C,D) Close-up of diatom inclusions ingypsum crystals from ODP Site 887. Bar scale is 10mm.

of the authigenic or detrital nature of gypsum, andtherefore identification of grain origin based solelyon these physical characteristics should be madewith caution. Thed 34S values of the gypsum crystalsalso rule out some potential detrital sources ofgypsum. Because thed 34S values indicate that thegypsum did not precipitate directly from any seawater of the past this gypsum could also not bederived simply by eroding ancient marine evaporiteoutcrops, such as exposed Permian gyprock whichwould have an isotopic signature far too enriched ind 34S (,12‰, see Fig. 5). Instead another land-basedsource for this detrital gypsum must exist.

We propose that the gypsum source is precipitationfrom freeze-induced terrestrial sediment brines. Theargument for this source is as follows: gypsum preci-pitation in any setting requires that Ca21 and SO22

4

ions must be present and that their concentrationsmust exceed the solubility product of gypsum.Previous studies have shown that these conditionscan be met in the pore spaces of soil (Timpson etal., 1986) and in particular, during freezing of soilbecause of salt exclusion as the pore fluids freeze(Steinwand and Richardson, 1989). It is certainlyconceivable that pore water solutions in the terrestrialsediments of southern Alaska and southeast Green-land could similarly experience at least seasonalfreezing.

The isotopic variability among gypsum samplesfrom Greenland and Alaska is an indicator of differ-ences in local bedrock mineralogy (the unweatheredsource of sulfur in the terrestrial sediment or soil porewaters that precipitated to form gypsum) and in frac-tionation histories of the sulfur (Fig. 4). Gypsumoccurring in nearshore, restricted marine embayments(i.e. Muir Inlet and Disenchantment Bay) would beexpected to have rather limited sediment provenancedue to the geographic restrictions, and therefore theleast variability ofd 34S values. The sediment prove-nance for the sites more distant from land likely wouldbe more complex and mixed, resulting in morevariabled 34S values. That all of thed 34S values ofgypsum show an enrichment in32S (regardless ofdegree of enrichment) implies that the sulfur presentlyin the gypsum was derived ultimately from weather-ing of 32S-enriched sulfide minerals in southernAlaska and southeast Greenland. For example thesulfur may have originated from weathering of shales

containing pyrite, which was enriched in32S bysulfur-reducing bacteria during pyrite formation.Oxidation of the pyrite during subsequent weatheringwould combine sulfur previously enriched in32S withoxygen to produce similarly32S-enriched sulfate ions.The 32S-enriched sulfate ions then combined withCa21 ions under the terrestrial sediment (or soil)freezing conditions previously described to producesimilarly 32S-enriched gypsum. In addition, sulfurthat is only moderately enriched in32S, such as thegypsum at the southern Alaska sites, may have origi-nated from the oxidation of sulfide minerals inigneous and metamorphic rocks during weathering.More specific identification of the source(s) of thesulfur presently contained in the gypsum is not possi-ble without a detailed study of thed 34S values of rocksin southern Alaska and SE Greenland.

The presence of clay inclusions in gypsum, whichtraditionally has been used to interpret an authigenicorigin of gypsum in marine settings (Rothwell, 1989),alternatively can be interpreted as evidence of gypsumformation in terrestrial sediment or soil. Clay inclu-sions indicate that the gypsum formed in a setting inwhich fine loose sediment was present and gypsumgrowth was restricted to the open spaces in and aroundthe loose sediment. During gypsum growth these pre-existing bounding grains could be entrapped withinthe growing crystal, resulting in clay inclusions.Both soil and unconsolidated glacifluvial sedimentsurrounding a tidal flat or beach would provide suchsettings for the formation of gypsum within an openframework of unconsolidated grains.

The presence of diatom inclusions can be explainedwithin the bounds of our proposed hypothesis ofterrestrial gypsum formation. Preliminary observa-tions (J. Snyder, personal communication) of diatomsincorporated in gypsum crystals from Site 887 suggestthat they resemble marine pelagic forms, but fresh orbrackish water forms could not be excluded, espe-cially considering that nonmarine diatoms arecommon in modern Arctic lakes (Koivo and Ritchie,1978; Young and King, 1989; Abelmann, 1992;Douglas et al., 1996). If the diatoms are fresh orbrackish water species the sediment brines in whichthe gypsum precipitated may have been remnants ofephemeral periglacial lakes or ponds. If the diatomsare pelagic marine species, our hypothesis wouldrequire that they either have an eolian origin or were


eroded from exposed former marine sediments.Previous studies have shown that marine pelagicdiatoms can be transported by wind to various terres-trial environments (e.g. Burckle et al., 1988; Kelloggand Kellogg, 1996). Preliminary observations (J.Snyder, personal communication) also suggest thatboth Late and Early Cenozoic forms are present.The inclusion of pre-Pleistocene diatom species inPleistocene marine sediment may indicate that thediatoms were eroded from raised marine deposits.

The occurrence of detrital gypsum in marine sedi-ments requires a transport mechanism that is capableof distributing coarse sand-sized grains long distancesto sites on isolated bathymetric highs (i.e. Site 887),without complete destruction (via abrasion or dissolu-tion) of such a soft and soluble mineral. Given theserequirements, and considering the glacially dominatedenvironment, in which the gypsum formed, a logicalmode of transport is ice-rafting. IRD abundancestudies of Late Cenozoic sediments at Site 918 (St.John and Krissek, 2000, in review) and Site 887(Krissek, 1995; St. John and Krissek, 1999) indicatethat transport by icebergs has been an important trans-port process for land-derived material at both deep-water localities. In fact, because Site 887 is situated ona seamount, iceberg rafting is the only feasible modeof transport for supplying land-derived coarse sand-sized material to this location during the Pleistocene.Provenance studies of the IRD from Site 887 indicatethe ice-rafted grains were derived from glaciers insoutheastern Alaska (McKelvey et al., 1995; St.John and Krissek, 1999).

The presence of detrital gypsum crystals in south-ern Alaska proximal glacimarine sites can also beexplained by rafting of debris-laden icebergs or seaice. Iceberg rafting is a dominate process in Disen-chantment Bay and sea ice rafting occurs commonlyduring the winter in Muir Inlet (Cowan et al., 1997,1999). The terminus of Muir Glacier is separated fromthe sea by an ice-contact delta, but sea ice transportssediment from the delta and beaches along theshoreline. Large tides (7 m range at spring tide) enablesea ice to incorporate sediment when grounded onshore and then be floated off into the basin. In bothDisenchantment Bay and Muir Inlet the most commonlithofacies containing gypsum is laminated mud withscattered isolated pebbles, granules, and sand. Lami-nated mud is deposited by suspension settling of silt

and clay from turbid meltwater plumes and coarsesediment is added by melting or overturn of debrisladen icebergs or sea ice. Very rapid deposition andburial in the proximal glacimarine setting may be animportant control on gypsum preservation. This mayexplain why gypsum crystals were most commonlyfound in laminated mud lithofacies and not in diamic-ton which has higher concentrations of IRD but aslower sedimentation rate.

Turbidity currents deposit thin graded sand bedscontaining gypsum crystals in both southern Alaskafjords. In Muir Inlet turbidity currents may form bytidal drawdown on the ice-contact delta as describedby Smith et al. (1990) and Phillips et al. (1991). In thismodel turbidity currents are generated cyclicallywhen the delta lip and plain are exposed during extremelow spring tides. These flows could transport gypsumcrystals that formed within pore spaces of delta sandsinto the basin and preserve this detrital gypsum byrapid burial, thus providing an additional mechanismfor transporting and preserving terrestrial gypsum inthe southern Alaska proximal glacimarine setting.

In addition to rafting the gypsum out to sea, thepresence of glacial or sea ice would protect the gypsumcrystals from complete disintegration and dissolutionduring transport and deposition. If the gypsum formsin freezing terrestrial sediment or soil then clumps offrozen regolith could be incorporated into glacial ice (orsea ice) asa solid clast. The clast or regolith clump that isfrozen into the ice, but not transported as basal ice,would be protected from destruction. Rafting ofterrestrial “till balls” by shorefast ice has been docu-mented in the eastern Greenland Sea (Goldschmidt,1994), suggesting that similar ice-rafting of frozen,gypsum-bearing terrestrial regolith is also possiblein southeast Greenland and southern Alaska.

6. Conclusions

Sulfur isotope analyses show that gypsum crystalsfound in mid to high latitude marine cores from south-ern Alaska, the southeast Greenland margin and theGulf of Alaska are detrital and precipitated fromnonmarine brines. We conclude that the nonmarinesource of this gypsum is precipitation from freeze-induced terrestrial sediment or soil brines. Thegypsum subsequently was transported to the present


marine settings via ice-rafting of frozen regolith and/or, in the proximal glacimarine setting of southernAlaska, very rapid burial via turbidity currents.

Some broader conclusions can also be drawn fromthe results of this study

1. When considering the environments of evaporiteformation, it is important to remember that aridconditions are not solely temperature dependent;gypsum can form in low latitude coastal andtrade wind deserts, but also forms in glacial andperiglacial environments.

2. The data presented here is from multiple mid tohigh latitude marine locations of various litholo-gies, ages, water depths and continental associa-tions. This variety suggests that our proposedscenario for gypsum formation and transport maybe a common, albeit previously unrecognized,process in ice-dominated environments. Whenrecognized, gypsum of this type can serve as amineralogical indicator of specific environmentaland geochemical conditions on land.

3. Physical characteristics, such as crystal morphol-ogy, surface texture, and presence or absence ofclay inclusions, may be insufficient for distinguish-ing detrital from authigenic gypsum found inmarine sediments. Analysis of the isotopiccomposition of sulfur in gypsum, however,provides a straightforward, relatively inexpensive,and more definitive means of determining theauthigenic or detrital nature of such gypsum.

Acknowledgements

We thank the Ocean Drilling Program and the ship-board scientists of ODP Legs 145, 152, and 163 formaking this research possible. Analyses of ODP Site918 samples were funded by JOI-USSSP post-cruiseresearch grant to St. John. Analyses of the ODP Site887 samples were partially funded by a grant fromThe Friends of Orton Hall Foundation, Ohio StateUniversity. Cores from southern Alaska werecollected during cruises on the R/V Alpha Helix in1993 and 1994 as part of a project supported byNational Science Foundation grants OPP-9223992 toCowan and OPP-9223990 to Ross D. Powell at North-ern Illinois University. Special thanks to V. Leanne

Spurgeon and Michelle Brewer, geology students whohelped with the laboratory analysis of gypsum atAppalachian State University. Special appreciationgoes to Gunter Faure, Charlotte Schreiber, and LarryKrissek for their very helpful discussions and to JeffSnyder for his assistance with diatom identification.Our thanks to Ruth Dewel at Appalachian StateUniversity and John Mitchell at Ohio State Universityfor their assistance with SEM analyses. This manu-script benefited from constructive comments made byRuediger Henrich and one anonymous reviewer.Isotopic analyses were conducted at GeochronLaboratories, and were supported by a SIRA awardgranted to St. John by Geochron Laboratories of Krue-ger Enterprises.

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terrestrial gypsum from alaska and greenland in glacially influenced marine sediments

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