PaleoceanographyoftheLateCretaceous(Maastrichtian ......PaleoceanographyoftheLateCretaceous(Maastrichtian) WesternInteriorSeawayofNorthAmerica: evidencefromSrandOisotopes J. Kirk Cochrana;,Neil

Paleoceanography of the Late Cretaceous (Maastrichtian)Western Interior Seaway of North America:

evidence from Sr and O isotopes

J. Kirk Cochran a;�, Neil H. Landman b, Karl K. Turekian c, Annie Michard d,Daniel P. Schrag e

a Marine Sciences Research Center, State University of New York, Stony Brook, NY 11794-5000, USAb American Museum of Natural History, New York, NY 10024, USA

c Department of Geology and Geophysics, Yale University, New Haven, CT 06520-8109, USAd CEREGE, Europo“le Me¤diterrane¤en de l’Arbois, BP 80, 13545 Aix-en-Provence, France

e Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USA

Received 11 October 2001; received in revised form 15 October 2002; accepted 8 November 2002

Abstract

Well-preserved fossils of the Late Cretaceous Western Interior Seaway (WIS) of North America have beenanalyzed for Sr concentration and Sr and O isotopes in order to decipher paleosalinities and paleotemperatures. Thesamples are from four biofacies within the Seaway (late Maastrichtian): offshore Interior (Pierre Shale), nearshoreInterior (Fox Hills Formation), brackish (reduced salinity; Fox Hills Formation) and freshwater (Hell CreekFormation). Samples were also obtained from the Severn Formation of Maryland (considered to be representative ofthe open ocean). All biofacies (except the freshwater) are demonstrably within the Jeletzkytes nebrascensis ammonitezone (6 1 Ma duration). The 87Sr/86Sr ratios show significant and systematic decreases from marine (mean91 S.D.= 0.7078399 0.000024) to brackish facies (mean9 1 S.D.= 0.7076779 0.000036), consistent with dilution byfreshwater with a lower 87Sr/86Sr ratio than seawater. Such variation disallows using the 87Sr/86Sr ratios of fossil shellmaterial to assign ages to fossils from the Late Cretaceous WIS without knowledge of the salinity in which theorganism grew. The Sr isotope ratios for scaphitid ammonites within a single biofacies are similar to each other anddifferent from those for scaphites in other biofacies, implying that these organisms are restricted in their distributionduring life. The 87Sr/86Sr values of freshwater unionid mussels range widely and are not compatible with thefreshwater endmember 87Sr/86Sr ratio required by the trend in 87Sr/86Sr vs. biofacies established from the othersamples. Paleosalinities for the biofacies are estimated to range from 35x in the open marine to a minimum of 20xin the brackish, based on the presence of cephalopods in all four facies and the known salinity tolerance of moderncephalopods. Producing reasonable 87Sr/86Sr values for the freshwater endmember of a 87Sr/86Sr vs. 1/[Sr] plotrequires a Sr concentration 0.2^0.5 that of seawater for the dominant freshwater input to the WIS. Such high Srconcentrations (relative to seawater) are not observed in modern rivers, and we suggest that the brackish environmentin the WIS arose through the mixing of freshwater and seawater in a nearshore aquifer system. Reactions of thesolution with aquifer solids in this ‘subterranean estuary’ [Moore, Mar. Chem. 65 (1999) 111^125] produced brackishwater with the Sr concentration and isotopic composition recorded in the brackish biofacies. N18O values of the fossils

0031-0182 / 02 / $ ^ see front matter E 2002 Elsevier Science B.V. All rights reserved.PII: S 0 0 3 1 - 0 1 8 2 ( 0 2 ) 0 0 6 4 2 - 9

* Corresponding author. Fax: +1-631-632-3066. E-mail address: [email protected] (J.K. Cochran).

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Palaeogeography, Palaeoclimatology, Palaeoecology 191 (2003) 45^64

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show decreases from the marine to brackish biofacies consistent with increasing temperatures (from V13 to 23‡C) or,if temperatures were relatively constant, to a decrease in the N

18O of the water in which the shell formed. The latterinterpretation is consistent with less-than-fully marine salinities in the nearshore biofacies, but both changes intemperature and the isotopic composition of the water may have occurred in this environment.E 2002 Elsevier Science B.V. All rights reserved.

Keywords: Cretaceous; Western Interior Seaway; North America; scaphites; strontium; oxygen; isotopes; paleoceanography

1. Introduction

The Cretaceous Period was characterized by theexistence of broad continental seaways that con-nected the world oceans. In North America, theWestern Interior Seaway (WIS) connected the Bo-real and Tethys oceans. During the late Maas-trichtian this seaway was closed to the northand disappeared altogether by the end of the Cre-taceous. Because these epicontinental seas have nomodern analogs as far as magnitude and settingare involved, very little is known about theiroceanography. In particular, distributions ofwater masses as de¢ned by temperature and salin-ity variations are poorly characterized althoughreconstructions of WIS paleoceanography and pa-leocirculation have been attempted (e.g. Hay etal., 1993). One approach to reconstructing paleo-temperatures and paleosalinities in such environ-ments is through the record of geochemical trac-ers that are proxies for these parameters and areincorporated in the calcium carbonate shells oforganisms. For example, the determination ofN18O in shell material as an index of paleotemper-atures has been applied to a variety of systemssince Urey (1947) initially proposed that theN18O values of carbonates are a function of boththe temperature and N

18O of the water in whichthe carbonate formed. In the WIS, Tourtelot andRye (1969) and Wright (1987) among others mea-sured N

18O values in well-preserved shell material.Calculated paleotemperatures for benthic organ-isms ranged up to 40‡C and were considered un-realistically high. Moreover, Wright (1987) no-ticed isotopic di¡erences in epifauna comparedwith nekton and infauna and argued that theWIS had an unusual salinity structure, withwarm saline water formed by evaporation in theshallow eastern portion of the sea and subse-

quently transported into the basin interior under-lying colder, less saline intermediate waters.An additional paleoenvironmental indicator is

the ratio of 87Sr to 86Sr in shell material. The 87Sr/86Sr ratio in the open ocean re£ects dominantlythe input of Sr from continents via rivers andmantle-derived sources such as hydrothermalvents (Brass, 1976; Albare'de et al., 1981; Palmerand Edmond, 1989). Variations in the 87Sr/86Srratio over geologic time are due to variations inthe relative importance of these Sr sources, as wellas evolution in the Earth’s 87Sr/86Sr ratio due to87Rb decay. Such variations are recorded in the87Sr/86Sr of marine carbonates, which are charac-teristically low in Rb, and the Phanerozoic evolu-tion of the 87Sr/86Sr ratio in seawater has beenwell documented using these deposits (Petermanet al., 1970; Dasch and Biscaye, 1971; Brass,1976; Veizer and Compston, 1974; Burke et al.,1982; Hess et al., 1986, DePaolo, 1986; Martinand Macdougall, 1991; Veizer et al., 1999). Inmodern ocean^river systems, the 87Sr/86Sr ratioof rivers can be signi¢cantly di¡erent from thatof seawater (0.70916), ranging from values muchlower (e.g. rivers entering San Francisco Bay; In-gram and Sloan, 1992) to much greater than thatof seawater (e.g. rivers entering the Baltic Sea;Andersson et al., 1992, 1994). In the estuarineand coastal environment, the mixture of freshand salt water produces a gradient in 87Sr/86Srratios that can be recorded by organisms livingin a given salinity. Such gradients in 87Sr/86Srvs. salinity are seen in San Francisco Bay andthe Baltic Sea, for example (Ingram and Sloan,1992; Andersson et al., 1992). Thus the 87Sr/86Srratio may be used to reconstruct paleosalinities inareas where salinity gradients, as well as di¡eren-ces in Sr isotope ratios between freshwater andseawater exist.

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We present data for oxygen, carbon and stron-tium isotopes and strontium concentrations inwell-preserved shell material from within theWIS and from coeval material collected fromthe Atlantic Coastal Plain of North America.The samples were collected in the Jeletzkytes ne-brascensis ammonite zone, representing 9 1 Main the late Maastrichtian, around 67 Ma BP(Waage, 1968; Landman and Waage, 1993;Kau¡man et al., 1993; Kennedy et al., 1998),from the Pierre Shale, Fox Hills and Hell CreekFormations in South Dakota. Because of the ex-tensive paleoenvironmental reconstructions forthis region (Gill and Cobban, 1966; Waage,1968), it is possible to separate the samples a pri-ori into environments corresponding to o¡shoreInterior, nearshore Interior and brackish (lessthan fully marine salinity). Samples were collectedfrom the same biozone in the Severn Formation(Maryland; Kennedy et al., 1997) on the AtlanticCoastal Plain and are assumed to represent fullymarine, open-ocean conditions.

2. Methods

2.1. Sample localities

Samples were collected from the Pierre Shaleand Fox Hills Formation in South Dakota andthe Severn Formation in Maryland, all withinthe Jeletzkytes nebrascensis ammonite biozone(Table 1, Fig. 1). The samples were divided intopaleoenvironments according to the reconstruc-tion of Waage (1968). We have labeled theseenvironments as follows: ‘o¡shore Interior’, cor-responding to an open-water biofacies representa-tive of the central seaway with a maximum depthof V200 m; ‘nearshore Interior’, representative ofa more nearshore facies; and ‘brackish’ biofacies,representative of an estuarine, nearshore environ-ment inhabited by organisms with lower salinitytolerances. Samples from the Severn Formationfrom the same biozone (Kennedy et al., 1997)are considered to represent a fully ‘open-ocean’biofacies and include only nektonic organisms(ammonites and nautiloids).Within the WIS, gastropods, pelecypods, am-

monites, nautiloids and belemnites are repre-sented in the fauna. Specimens of two ammonitespecies (J. nebrascensis and Sphenodiscus lobatus)were analyzed from all four paleoenvironments.Additionally, freshwater unionid bivalves fromthe basal Hell Creek Formation were analyzedin an e¡ort to constrain the 87Sr/86Sr ratio infreshwater at that time. However, the Hell Creeksamples lack the biostratigraphic control repre-sented by the other samples. All samples are de-posited in the collections of the Yale PeabodyMuseum, American Museum of Natural History,and the US National Museum. Three samples ofthe rock making up the presumed ambient terrainalso were taken for determination of Sr isotoperatio and concentration. These samples includedthe Elk Butte and Mobridge members of thePierre Shale, which lie stratigraphically belowthe Fox Hills Formation and may have been ex-posed along the paleo-shoreline.

2.2. Sample preservation

Samples were taken from single specimens, ex-cept K15, in which two specimens were com-bined; samples of cephalopods were taken fromthe outer shell, rather than the septa. The shellmaterial of the ammonites and nautiloids showeda distinctive iridescence characteristic of nacreousshell structure. However, prior work has shownthat authigenic deposits on the surfaces of sam-ples can potentially bias isotopic and concentra-tion data (Turekian and Armstrong, 1961; Land-man et al., 1983). Thus, all samples were soni¢edfor several minutes in distilled water to removeloosely adhering surface phases. Samples wereair-dried and the best pieces were selected follow-ing examination with a light microscope. If neces-sary, the soni¢cation step was repeated. Sampleswere then examined by scanning electron micros-copy (SEM) to con¢rm both the absence of sur-face phases and the presence of original shellstructure (Plate I). In the cephalopod samples,the characteristic nacreous microstructure waspreserved. Excellent preservation quality was in-dicated when SEM photos showed nacreous tab-lets of even thickness, without evident alterationor deposition of diagenetic interlayer material. Je-

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letzkytes nebrascensis samples were available fromall four biofacies (Plate IA^D), and of these sam-ples, the best preservation was evident in the near-shore Interior and brackish facies (Plate IC,D).However, the J. nebrascensis samples from the

open-ocean and o¡shore Interior facies (PlateIA,B) showed no evident overgrowths indicativeof diagenetic alteration. Preservation in the nau-tiloid Eutrephoceras dekayi, from the open-oceanbiofacies, was excellent (Plate IE). A ¢nal screen-

Table 1Sample identi¢cation and localitiesa

Sample ID Museum IDb Formation Memberc Species Organism

FreshwaterK6 YPM 38033 Hell Creek Unio sp. bivalveK12 YPM 38034 Hell Creek Unio sp. bivalveK16 YPM 38029 Hell Creek Unio sp. bivalveK17 YPM 38005 Hell Creek Unio sp. bivalveK22 YPM 38044 Hell Creek Unionoida bivalve

Open-oceanK3 AMNH 45406 Severn Eutrephoceras dekayi nautiloidK4 AMNH 45404 Severn Sphenodiscus lobatus ammoniteK35 USNM 520476 Severn Jeletzkytes nebrascensis ammonite

O¡shore InteriorK2 USNM 486542 Pierre J. nebrascensis ammoniteK23 USNM 486528 Pierre Belemnitella cf. bulbosa belemniteK34 AMNH 47462 Pierre S. lobatus ammoniteK37 USNM 520477 Pierre J. nebrascensis ammonite

Nearshore InteriorK5 AMNH 45463 Fox Hills Timber Lake S. lobatus ammoniteK8 YPM 44980 Fox Hills Timber Lake S. lobatus ammoniteK11 YPM 162363 Fox Hills Trail City, C, IC E. dekayi nautiloidK10 YPM 162434 Fox Hills Timber Lake Serrifusus dakotensis gastropodK1 AMNH 45405 Fox Hills Timber Lake J. nebrascensis ammoniteK13 YPM 162433 Fox Hills Timber Lake, CT J. nebrascensis ammoniteK21 YPM 202291 Fox Hills Timber Lake, CT Cymbophora warreni bivalveK33 YPM 32588 Fox Hills Trail City, C Belemnitella cf. bulbosa belemnite

BrackishK7 YPM 46036 Fox Hills Timber Lake, TO Tancredia americana bivalveK9 YPM 162385 Fox Hills Iron Lightning, CL Corbicula sp. bivalveK14 YPM 162374 Fox Hills Iron Lightning, BL J. nebrascensis ammoniteK15 YPM 162397,8 Fox Hills Iron Lightning, CL Lunatia sp. gastropodK18 YPM 162437 Fox Hills Iron Lightning, BL J. nebrascensis ammoniteK19 YPM 200285 Fox Hills Hell Creek Lunatia sp. gastropodK20 YPM 162402 Fox Hills Hell Creek J. nebrascensis ammoniteK36 YPM 44661 Fox Hills Hell Creek S. lobatus ammoniteRockK30 YPMIP loc. D4083c Elk ButteK31 YPMIP loc. C1872 MobridgeK32 YPMIP loc. C479 Mobridge

a Detailed descriptions of localities are available upon request.b YPM=Yale Peabody Museum, AMNH=American Museum of Natural History, USNM=US National Museum, YPMIP

loc. =Yale Peabody Museum Invertebrate Paleontology locality.c C, Cucullaea Assemblage Zone; CT, Cymbophora^Tellina Assemblage Zone; TO, Tancredia^Ophiomorpha biofacies; CL, Col-

gate lithofacies; BL, Bullhead lithofacies; IC, Irish Creek lithofacies.

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ing involved X-ray di¡raction to con¢rm that thesamples were 100% aragonite, with the exceptionof the belemnites, which were 100% calcite.

2.3. Sample analyses

2.3.1. Strontium isotopesCleaned pieces of sample (2^6 mg) were dis-

solved in 1 N HCl and any undissolved residue(largely organic) was separated by centrifugation.Approximately 2 ml of sample was loaded onto acation exchange column (AG50WX8) calibratedfor Sr. After passing 22 ml of 2 N HCl throughthe column, the Sr fraction was collected in 4 mlof 2 N HCl and evaporated to dryness. Totalblank Sr was less than 200 pg compared withV2 Wg Sr in the samples.The matrix samples from the Elk Butte and

Mobridge Members were subjected to a simpleexperiment to determine the Sr isotope ratiothat might be released during chemical weatheringby freshwater. Samples of 0.1^0.2 g were leachedin 10^20 ml distilled water for several hours atroom temperature. The solution was centrifugedand the leachate was separated from the residue,

spiked with a Sr tracer and evaporated. Subse-quent separation of Sr followed the procedureoutlined above.Strontium was loaded with tantalum oxide

(Birk, 1986) onto a single rhenium ¢lament andanalyzed in the dynamic mode on a FinniganMAT 262 mass spectrometer. The static modewas used for the matrix samples. Repeated anal-yses of NBS 987 during the time of sample anal-yses gave a mean 87Sr/86Sr value of 0.71023890.000011 (16 runs) in the dynamic mode and0.7102719 0.000012 (10 runs) in the static mode.The 87Sr/86Sr values of the spiked matrix samples(static mode) have been corrected to the NBSstandard 87Sr/86Sr value of 0.710248. The 87Sr/86Sr values in all samples are normalized to a86Sr/88Sr value of 0.1194.

2.4. Strontium concentrations

Strontium concentrations were measured onseparate aliquots of the samples following theprocedure of Schrag (1999). Samples were dis-solved in 2% HNO3, diluted and Sr was measuredby inductively coupled plasma emission spectros-

Fig. 1. Map showing biofacies sampled relative to the paleo-shoreline (heavy line) of the WIS during the late Maastrichtian zoneof Jeletzkytes nebrascensis (modi¢ed from Kennedy et al., 1998). Key: F= freshwater, B=brackish Interior, S= nearshore Interi-or, D=o¡shore Interior, M=open-ocean.

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Plate I. Shell microstructure in fractured cross section of six samples. (A^D) Jeletzkytes nebrascensis from four di¡erent biofacies.The nacreous microstructure is preserved although some platelets have broken o¡ in fracturing. (A) Fully marine, sample K35(USNM 520476, Severn Formation). (B) O¡shore Interior, sample K37 (USNM 520477, Pierre Shale). (C) Nearshore Interior,sample K1 (AMNH 45405, Fox Hills Formation). (D) Brackish Interior, sample K20 (YPM 162402, Fox Hills Formation).(E) Eutrephoceras dekayi, fully marine, sample K3 (AMNH 45406, Severn Formation), showing the original nacreous microstruc-ture. (F) Serrifusus dakotensis, nearshore Interior, sample K10 (YPM 162434, Fox Hills Formation) with cross-lamellar micro-structure.

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copy on a Jobin-Yvon simultaneous instrument,model 46-P. Analytical precision was determinedto be better than 9 0.2%, based on replicate anal-yses of multiple dilutions of solutions of singlesamples.

2.5. Oxygen and carbon isotopes

Oxygen and carbon isotopic analyses weremade at the University of Michigan Stable Iso-tope Laboratory by L. Wingate. Carbonate sam-ples weighing a minimum of 10 Wg were placed instainless steel boats. Calcite samples were roastedat 380‡C in vacuo for 1 h to remove volatile con-taminants. Aragonite was roasted at a lower tem-perature, 200‡C, to minimize e¡ects of thermaldecarboxylation that can alter the isotopic com-position of the carbonate. Samples were thenplaced in individual borosilicate reaction vesselsand reacted at 76‡ 9 2‡C with three drops of an-hydrous phosphoric acid for 8 min in a FinniganKiel preparation device coupled directly to theinlet of a Finnigan MAT 251 triple-collector iso-tope ratio mass spectrometer. Isotopic enrich-ments were corrected for acid fractionation andoxygen-17 contribution by calibration to a best-¢t regression line de¢ned by two NBS standards,NBS 18 and NBS 19. Precision of data was moni-tored through daily analysis of a variety of pow-dered carbonate standards. At least six standardswere reacted and analyzed daily, bracketing thesample suite at the beginning, middle, and endof the day’s run. Measured precision was main-tained at better than 0.1x for both carbon andoxygen isotope compositions. Data are reportedin x notation relative to VPDB (Vienna PeeDeeBelemnite).

3. Results

3.1. Sr concentrations

Concentrations of Sr in the shell material rangefrom 355 to 4705 ppm (Table 2). The lowest val-ues are recorded in the freshwater bivalves andthe highest values are from ammonites (Spheno-discus lobatus, Jeletzkytes nebrascensis) from the

Seaway. Specimens of the same species from thesame environment have similar Sr concentrations,varying within V10% (e.g. S. lobatus collected inthe nearshore Interior and J. nebrascensis fromboth the nearshore Interior and brackish environ-ments). There is a range of Sr concentrations fordi¡erent organisms from a single environment,presumably re£ecting interspeci¢c di¡erences inthe factors a¡ecting Sr incorporation into shellmaterial (see discussion below). In particular,the benthic gastropods Lunatia and Serrifususand bivalve Cymbophora have lower Sr concentra-tions than those of the ammonites and nautiloidsamples in any given environment. The bivalveCorbicula is conspicuous in having the lowest Srconcentration of the brackish group. This lowvalue, together with its unusually light N18O value(see below), suggests that this specimen was trans-ported into the brackish assemblage from fresh-water. Indeed, present-day Corbicula can live incompletely fresh water (Counts, 1986; Mackie,1986). The Sr concentrations of ammonite andnautiloid fossils measured in this study are com-parable to values measured by Turekian andArmstrong (1961) in material with negligible-to-low percentages of calcite (and thus judged to beminimally altered).Total Sr concentrations in the Elk Butte and

Mobridge Members are greater than those mea-sured in the fossils and range from 5823 to 7434ppm. The Sr released by the leaching experimentswas 646 ppm for K30, 708 ppm for K31 and 20ppm for K32. In the latter case only 0.3% of theSr was leached, whereas the fraction was 9% forK30 and 12% for K31.

3.2. Sr isotope ratios

With the exception of the freshwater bivalves,the 87Sr/86Sr ratios of the samples cluster into dis-crete groups with minimal overlap: 0.707815^0.707862 for the open-ocean group, 0.707796^0.707827 for the o¡shore Interior group,0.707704^0.707795 for the nearshore Interiorgroup and 0.707605^0.707712 for the brackishgroup (Table 2, Fig. 2). In contrast, the 87Sr/86Sr values in the freshwater bivalves range from0.707615 to 0.707986, although the large range

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Table 2Isotopic and concentration data

Species 87Sr/86Sr Sr N18O N

13C Remarks(ppm) (x) (x)

FreshwaterK6 Unio sp. 0.7079409 0.000009 405 320.29 34.69K12 Unio sp. 0.7079039 0.000011 504 319.71 34.99K16 Unio sp. 0.7079429 0.000016 561 319.11 31.73K16 Unio sp. 0.7079869 0.000012 na na na Powdered sampleK17 Unio sp. 0.7076289 0.000015 355 316.87 32.56K17 Unio sp. 0.7076159 0.000014 na na na Powdered sampleK22 Unionoida 0.7077549 0.000016 na na naK22 Unionoida 0.7078699 0.000013 na na na Powdered sample

Open-oceanK3 Eutrephoceras dekayi 0.7078159 0.000007 3261 30.66 31.51K4 Sphenodiscus lobatus 0.7078399 0.000011 3018 0.28 32.95K35 Jeletzkytes nebrascensis 0.7078629 0.000006 1890 na na

O¡shore InteriorK2 J. nebrascensis 0.7078109 0.000008 na 1.43 1.94K23 Belemnitella cf. bulbosa 0.7078019 0.000009 1319 0.31 0.36K34 S. lobatus 0.7078279 0.000008 3859 na naK37 J. nebrascensis 0.7077969 0.000007 2520 na na

Nearshore InteriorK5b S. lobatus 0.7077779 0.000010 4705 30.92 33.64K5g S. lobatus 0.7077689 0.000012 na na na Duplicate sampleK8 S. lobatus 0.7077819 0.000009 4329 31.61 33.34K11 E. dekayi 0.7077859 0.000015 3867 0.18 0.21K10 Serrifusus dakotensis 0.7077739 0.000014 2297 30.49 30.13K1 J. nebrascensis 0.7077409 0.000010 3963 30.04 0.99K13 J. nebrascensis 0.7077509 0.000015 4247 30.06 31.83K21 Cymbophora warreni 0.7077049 0.000012 1898 30.23 1.36 Same locality and concretion

as K13K33 Belemnitella cf. bulbosa 0.7077959 0.000008 1772 31.28 1.91 Same locality as K11

BrackishK7 Tancredia americana 0.7076959 0.000011 2412 31.34 1.16K9 Corbicula sp. 0.7076889 0.000015 667 317.91 34.79K14 J. nebrascensis 0.7076549 0.000014 2545 30.34 1.29K15 Lunatia sp. 0.7076059 0.000010 1589 30.47 0.49 Same locality as K9K18 J. nebrascensis 0.7076889 0.000008 2680 32.44 3.10K19 Lunatia sp. 0.7076999 0.000014 2043 32.40 0.33K20 J. nebrascensis 0.7077039 0.000012 na 30.31 1.63 Same locality and concretion

as K19K36 S. lobatus 0.7077129 0.000007 na na na

RockK30 Elk Butte member 0.7071659 0.000018a 7434 na naK31 Mobridge member 0.7075079 0.000016a 5823 na naK32 Mobridge member 0.7109989 0.000016a 5859 na na

na=not analyzed.a Sr isotope ratio released to solution during leaching experiments. See text.

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tends to be driven by a single sample. There isgenerally close correspondence in the 87Sr/86Srvalues of multiple specimens of the same speciescollected in a given environment; the 87Sr/86Srratios of specimens of the same species within abiofacies are often within the analytical uncer-tainty of one another. This is evident in the multi-ple specimens of J. nebrascensis in the o¡shoreInterior, nearshore Interior and brackish groupsas well as S. lobatus in the brackish group (Table2). In contrast, the values are signi¢cantly di¡er-ent for the same species in di¡erent biofacies.The 87Sr/86Sr ratios leached from the Elk Butte

and Mobridge Member rocks bracket the entirerange of values seen in the fossil shell materialand vary from 0.707165 to 0.710998 (Table 2).The highest ratio corresponds to sample K32, inwhich only 0.3% of the Sr was released to solu-tion.

3.3. Oxygen isotopes

N18O values range from 320.29x to 1.43x

(Fig. 3) and N13C values range from 34.99x to

3.10x. In contrast to the Sr isotope results, thereis less consistency among the paleoenvironmentalgroups, although the data for the WIS samplesshow a tendency toward lighter N

18O values pro-ceeding from o¡shore Interior to brackish envi-ronments : 0.3 to 1.4x for the o¡shore Interior,

0.0 to 31.6x for the nearshore Interior and30.3 to 32.4x, excluding the value of specimenK9, for the brackish group. The N

18O for K9, asample of Corbicula sp., is quite light, 317.9xand is similar to values obtained for the fresh-water bivalves (316.9 to 320.3x). We take theoxygen isotope composition of Corbicula, as wellas its low Sr concentration, to indicate that theshell grew in freshwater. Given that both the N18Oand Sr concentration in a co-occurring specimenof Lunatia are more consistent with the otherbrackish fauna, we believe the sample of Corbicu-la was transported into the brackish environmentafter death.

Fig. 3. N18O (x VPDB) of shell carbonate vs. biofacies (rel-ative scale: decreasing salinity to the right; values in eachgroup are slightly o¡set to improve readability).

Fig. 2. 87Sr/86Sr vs. biofacies (relative scale: decreasing salini-ty to the right; values in each group are slightly o¡set to im-prove readability).

Fig. 4. Shell Sr concentration vs. biofacies for cephalopodspecies. M 9 Open-ocean; OI 9 O¡shore Interior; NI 9

Nearshore Interior; B 9 Brackish.

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4. Discussion

4.1. Strontium concentrations in shells

There is considerable interspeci¢c variation inthe Sr concentrations for organisms collected in asingle paleoenvironment. Despite these di¡erencesin absolute concentration, the trends from open-ocean to brackish biofacies are consistent for thecephalopod species (Fig. 4). The Sr concentrationprogressively increases from open-ocean to o¡-shore Interior to nearshore Interior. Only a singlespecies (J. nebrascensis) from the brackish envi-ronment was analyzed, and the data show a de-crease in Sr concentration relative to the near-shore Interior.The Sr concentration in biogenic carbonate re-

£ects the Sr/Ca ratio in the water in which thecarbonate formed as well as the physiological fac-tors involved in biomineralization. The latter aredependent upon water temperature, salinity, andorganism growth rate (Smith et al., 1979; Stolland Schrag, 2000; Stoll et al., 2002a,b; Weinbau-er et al., 2000). Factors such as water temper-ature and growth rate also may be interdependent(Yatsu et al., 1998; Stoll et al., 2002a). With re-spect to the variation in the Sr/Ca ratio of thewater, the present-day ocean shows a di¡erencebetween the value in the open ocean and inworld-average river water, with the former beingapproximately three times greater than the latter(Palmer and Edmond, 1989; Turekian, 1969). Asimple trend of decreasing shell Sr concentrationsfrom the marine to brackish environments is notseen in our data, suggesting that variation of theSr/Ca ratio in the water is not the ¢rst-order con-trol on Sr/Ca in the shell. Although the exact frac-tionation of Sr/Ca in the shell vs. water will varyamong species, the similarity in trends among allthe cephalopod specimens suggests that funda-mental di¡erences in biomineralization related toenvironmental factors such as temperature, salin-ity or pCO2 are causing the trends.The e¡ects of temperature and other factors on

the Sr/Ca ratio of biogenic carbonates have beendocumented for coral skeletons, ¢sh otoliths,squid statoliths and coccolithophorids (Smith etal., 1979; Yatsu et al., 1998; Stoll et al., 2002b).

The dependence of Sr/Ca on temperature is notconsistent among di¡erent organisms: in herma-typic corals (Porites), the carbonate Sr/Ca de-creases by V10% as the temperature increasesfrom 10 to 25‡C (Alibert and McCulloch, 1997)while in coccoliths the ratio increases by 25% overapproximately the same temperature range (Stollet al., 2002b). It is possible that the water temper-ature of the nearshore WIS environments sampledby us was greater than that of the o¡shore Inte-rior (see discussion below), and the increase inshell Sr from the o¡shore to nearshore biofaciesis consistent with the e¡ect of temperature seen inthe coccolith data (Stoll et al., 2002b). However,the magnitude of the increase predicted from thecoccoliths is less than we observe.

4.2. The 87Sr/86Sr ratio as a paleosalinity indicator

The Sr isotopic evolution of seawater over geo-logic time has been well-documented (Peterman etal., 1970; Veizer and Compston, 1974; Brass,1976; Spooner, 1976; Hess et al., 1986; DePaolo,1986; Martin and Macdougall, 1991; McArthuret al., 1994; Veizer et al., 1999). In the Late Creta-ceous, values of open ocean water increased from0.7073 to 0.7078 (Hess et al., 1986) with a max-imum of V0.7079 at the Cretaceous^Tertiaryboundary (Martin and Macdougall, 1991). Sea-water 87Sr/86Sr increased from V0.7077 toV0.7079 in the last V3 Ma of the Cretaceous,and was V0.70784 at the time represented by oursampling (Martin and Macdougall, 1991). Indeed,within the analytical error, 87Sr/86Sr was con-stant at V0.70784 in the V2 Ma preceding theK/T boundary (Martin and Macdougall, 1991).This value agrees well with the average 87Sr/86Srof the ‘open-ocean’ group: mean9 1 S.D. =0.7078399 0.000024, and suggests that this groupis accurately re£ecting open marine conditions.Values in the o¡shore Interior group are onlyslightly less than the open-ocean group: mean9 1S.D. = 0.7078089 0.000014. The ratio trends to-ward lower values proceeding from o¡shore tonearshore Interior (mean9 1 S.D. = 0.70776390.000030) to the brackish biofacies (mean9 1S.D. = 0.7076779 0.000036). Although valueswithin adjacent groups overlap within the error,

PALAEO 2988 3-1-03


the overall trend is toward progressively lowervalues such that the brackish group is signi¢cantlydi¡erent from the open-ocean group.We note that the trend in 87Sr/86Sr exists even

though each group includes nektonic organismsthat might have ranged from one environmentto another. The ammonite Jeletzkytes nebrascensisis a good example in that this ammonite occurs inall four environments yet specimens from a givenpaleoenvironment possess a signi¢cantly di¡erentisotopic signature from the other environments.This is strong evidence that these organismswere living in the environment in which theywere collected. We conclude that although theseanimals were nektonic, their swimming ability waslimited or the distances were too great to permitmuch migration between biofacies. In contrast, itis interesting to note that the two belemnite speci-mens analyzed (K23 and K33) have nearly iden-tical 87Sr/86Sr ratios and Sr concentrationsalthough they were collected in two di¡erent bio-facies (o¡shore and nearshore Interior; Table2). This may indicate that belemnites were moreactive swimmers, like the present-day squid,and were mobile between various WIS habitats(W. Christensen, Univ. of Copenhagen, personalcommunication, 2000).The trend in 87Sr/86Sr ratios observed among

the paleoenvironments is consistent with a salinityvariation caused by mixing with a freshwater 87Sr/86Sr endmember that is lower than the seawatervalue during late Maastrichtian time. In this re-spect, the trend is similar to that observed in mod-ern San Francisco Bay (Ingram and Sloan, 1992)and in many geologically older systems (Ingramand DePaolo, 1993; Holmden et al., 1997;Schmitz et al., 1991; Vonhof et al., 1998; Vaiani,2000).An alternative explanation is that the trend rep-

resents a temporal di¡erence in sample age. Oursamples were collected within a single ammonitezone, estimated to span V1 Ma of time (Kau¡-man et al., 1993). We reject the possibility that thetrend in 87Sr/86Sr ratios is due to a monotonicchange in the age of the samples re£ecting achange in seawater values of the ratio becausethe brackish fauna is either coeval or younger(but still within the same ammonite biozone)

than the o¡shore Interior samples. The brackishsamples were collected stratigraphically up-sectionor, in a paleogeographic sense, landward of thenearshore Interior samples. Variation in the openocean 87Sr/86Sr ratio for the late Maastrichtiandocumented by Martin and Macdougall (1991)shows that the ratio remains the same(V0.70784) or increases slightly toward the endof the Cretaceous, with a maximum at the K/Tboundary. Thus, if the brackish samples are notcoeval, but are younger, then their 87Sr/86Sr ratioswould be greater than that in the other samples,not lower. On the basis of both stratigraphic re-lationships and the temporal pattern of Sr iso-topes in the Late Cretaceous, we conclude thatthe 87Sr/86Sr ratios in the samples re£ect the pa-leoenvironments.Our e¡orts to use unionids to establish the

freshwater endmember of the 87Sr/86Sr vs. habitattrend were not successful. The values span therange observed in the samples and are generallygreater than values recorded in organisms fromthe WIS (Fig. 2, Table 2). This situation is similarto that observed by Ingram and Sloan (1992) inSan Francisco Bay. Their analyses of the riversdraining into the Bay, coupled with the trend in87Sr/86Sr vs. salinity, show that considerable var-iation in 87Sr/86Sr is observed among the di¡erentfreshwater inputs, with only the dominant inputforming the appropriate endmember for the 87Sr/86Sr vs. salinity trend. Albare'de and Michard(1987) noted a strong dependence of watershedlithology on the 87Sr/86Sr of French rivers drain-ing into the Mediterranean, with signi¢cant var-iation in the ratio (0.708719^0.712765) in a rela-tively small geographic area. We conclude that thesamples of unionids analyzed for this study arenot representative of the dominant freshwater in-puts to the WIS during the Late Cretaceous.A relatively high 87Sr/86Sr ratio (0.709926) in a

freshwater unionid bivalve from the late Maas-trichtian was measured by McArthur et al.(1994) in their detailed investigation of Sr isotopesystematics in the last 30 Ma of the CretaceousWestern Interior of North America. McArthur etal. (1994) did not have the distribution of environ-ments within a single ammonite zone that charac-terizes the present study, and concluded that

PALAEO 2988 3-1-03


freshwater entering the seaway had a higher 87Sr/86Sr ratio than seawater. They further concludedthat the analytical uncertainty in the Sr isotoperatios precluded seeing a perceptible e¡ect ofless-than-fully-marine salinity on the 87Sr/86Sr ra-tios. We conclude in contrast that the di¡erencebetween the 87Sr/86Sr ratios of the open-ocean andbrackish assemblages (vSr of V1609 40U1036)is signi¢cant.

4.3. Estimating paleosalinities in the LateCretaceous Western Interior Seaway

Using 87Sr/86Sr ratios to estimate paleosalinitiesrequires (1) a signi¢cant di¡erence between ma-rine and freshwater Sr concentrations and isotoperatios, (2) knowledge of the endmember valuesand (3) conservative behavior of Sr with respectto freshwater^saltwater mixing. In such condi-tions, a plot of 87Sr/86Sr vs. 1/[Sr] will yield astraight line between the marine and freshwaterendmember values.In this study, we are unable to de¢ne the fresh-

water endmember using the unionids and thefreshwater and marine Sr concentrations are un-known. In the present ocean, the Sr concentration(V90 Wmol/kg) is 10^100 times greater than river-water Sr concentrations (Brass and Turekian,1974; Palmer and Edmond, 1989). Owing to thelarge di¡erence in Sr concentration of the end-members, the 87Sr/86Sr ratio in water of lessthan fully marine salinity should approach the

seawater ratio even in water of fairly low salinity(Schmitz et al., 1991). If we use as the marine87Sr/86Sr the value obtained from the average ofthe open-ocean biofacies samples (0.707839), thelowest 87Sr/86Sr measured in the rock leachates asthe freshwater 87Sr/86Sr (0.707165) and take thedi¡erence between the marine and freshwater Srconcentration to be 10:1, within the range of val-ues in the Recent, we obtain a salinity of V9xfor the brackish facies.The brackish biofacies contains abundant ceph-

alopods that presumably were living in this envi-ronment, as suggested by the similarity of their Srisotope ratios with other organisms from thissame environment. Modern cephalopods poorlytolerate salinities below about 20x (Forsytheand Van Heukelem, 1987; S. von Boletzky, per-sonal communication 1999). If we take this value(20x) as a lower limit on the salinity of thebrackish biofacies, the dominant freshwater in-put to the WIS must have had a 87Sr/86Sr ofV0.70552 if the ratio of marine/freshwater Srconcentration was 10:1 (Table 3). This 87Sr/86Srratio for the freshwater input is considerablylower than the lowest value observed in theunionid samples (0.707615) and is also lowerthan the lowest value measured in the rock leach-ates (0.707165).Indeed, if we take the value of 0.707165 as rep-

resentative of the freshwater input to the WIS, theratio of marine/freshwater Sr concentration re-quired to produce 20x salinity in the brackish

Table 3Paleosalinities in the WIS

Open-ocean O¡shore Interior Nearshore Interior Brackish Freshwater

[Sr]sw :[Sr]fw = 10:187Sr/86Sr 0.707839a 0.707808a 0.707763a 0.707677a 0.705517b

Salinity (x) 35c 30.8e 26.2e 20d 0[Sr]sw :[Sr]fw = 5:187Sr/86Sr 0.707839a 0.707808a 0.707763a 0.707677a 0.706597b

Salinity (x) 35c 31e 26.4e 20d 0[Sr]sw :[Sr]fw = 2:187Sr/86Sr 0.707839a 0.707808a 0.707763a 0.707677a 0.707245b

Salinity (x) 35c 31.5e 27.4e 20d 0a Mean 87Sr/86Sr value in indicated biofacies (Table 2).b Calculated from linear trend of 87Sr/86Sr vs. 1/[Sr] using marine and brackish values, assuming sur¢cial estuarine mixing.c;d Assumed marine and brackish salinities.e Equivalent salinities determined from values of 1/[Sr] calculated from 87Sr/86Sr vs. 1/[Sr] relationship.

PALAEO 2988 3-1-03


facies is V2:1. By ¢xing the brackish salinity at20x, the estimated salinities for the nearshoreand o¡shore Interior facies are 26.2^27.4x and30.8^31.5x, respectively (Table 3). Fig. 5a^cshows the extrapolated values of the freshwater87Sr/86Sr ratios for mixing lines that assume di¡er-ent ratios of marine/freshwater Sr and use ourobserved average 87Sr/86Sr of the open-ocean bio-facies (mean= 0.707839, Table 2) and the con-straint that the brackish biofacies (mean 87Sr/86Sr = 0.707677, Table 2) must be at least 20xsalinity. Values for the freshwater 87Sr/86Sr rangefrom 0.707245 to 0.705517 for marine/freshwaterSr ratios of 2:1 to 10:1 (Table 3). We note thateven lower freshwater 87Sr/86Sr values would berequired if the brackish biofacies correspondedto salinities greater than 20x.The explanation most consistent with the data

is that the mixing curve suggested by the trend in87Sr/86Sr in the di¡erent biofacies has a freshwaterendmember that is Sr-rich compared with modernriver water. One possibility is that the rocks beingweathered at that time were carbonate-rich, withabundant marine fossils and Sr concentrationsand isotope ratios much like those of the rocksamples analyzed (5800^7000 ppm Sr; Table 2).However, as noted above, if the Sr carried byrivers entering the Late Cretaceous WIS had a87Sr/86Sr typical of the lowest value added inour leaching experiments of the matrix samples(0.707165, Table 2), the riverine Sr concentrationmust have been about half that of seawater toproduce brackish salinities of at least 20x. Thereis no evidence in the Recent of such Sr-rich fresh-waters.An alternative explanation is that the brackish

WIS environment is produced by the subterra-nean mixing of groundwater and seawater, withthe Sr concentration and isotope ratio of thebrackish water resulting from both mixing andreaction (i.e. leaching or dissolution of the aquifersolids). Basu et al. (2001) have documented sub-stantial elevations (factors of 10) of Sr even inlow-salinity groundwater from the Bengal Basin,relative to concentrations in the Ganges andBrahmaputra rivers. Similar elevations were seenin Ra (Sarin et al., 1990). Groundwater inputs arebeing increasingly recognized as important in af-

Fig. 5. 87Sr/86Sr ratio vs. 1/[Sr] for mixtures of seawater andfreshwater with di¡erent Sr concentrations. All plots use thepresent-day Sr concentration of V90 Wmol Sr/kg in 35xseawater (Brass and Turekian, 1974) and a 87Sr/86Sr ratio of0.707839, the average measured value of the open-ocean sam-ples (Table 2). The plots are generated for varying Sr con-centrations of freshwater relative to seawater, and lines are¢t through the marine (M) and brackish (B) data, assumingthat the latter represents 20x salinity. (A) [Sr]sw :[Sr]fw =10:1. (B) [Sr]sw :[Sr]fw = 5:1. (C) [Sr]sw :[Sr]fw = 2:1. The o¡-shore Interior (OI) and nearshore Interior (NI) data are plot-ted on the lines as well. The plotted points represent themeans of the 87Sr/86Sr in a given biofacies and the verticalbars denote the range of values for each group.

PALAEO 2988 3-1-03


fecting coastal waters of the modern ocean.Moore (1996), for example, documented high ac-tivities of Ra in shallow shelf waters of the SouthAtlantic Bight (eastern USA), relative to open-ocean values. Although the various Ra isotopescan be introduced to groundwater via recoil asso-ciated with their production (Krishnaswami et al.,1982), Shaw et al. (1998) documented that Ba,which has no comparable input mechanism, isalso enriched in the South Atlantic Bight. Indeed,Basu et al. (2001) argued that groundwater inputsof Sr are important in the oceanic mass balance ofSr.Moore (1999) suggested the concept of a ‘sub-

terranean estuary’ in which groundwater and salt-water mix and interact with aquifer solids to ele-vate concentrations of elements such as Ra andBa. Bryant et al. (1995) documented 87Sr/86Sr val-ues in mollusks collected in the Mississippi Soundthat fell o¡ the mixing line determined by sea-water and the Mississippi River. They attributedthese results to local input of groundwater whoseSr isotope composition was imprinted by reac-tions with older, carbonate-rich rocks. We specu-late that such a process may have been operatingin the Late Cretaceous WIS as well. By this mech-anism, the brackish facies represents an environ-ment in which Sr-rich groundwater and seawatermixed in the subterranean estuary and the waterwas discharged with a greater-than-expected Srconcentration, relative to classical estuarine mix-ing of freshwater and seawater in a sur¢cial estu-ary.We can use our data to test the possibility that

reactions in the subterranean estuary can producethe Sr concentration and isotopic composition ofthe brackish biofacies (Fig. 6). We assume thatthe rocks supplying Sr to solution during mixingof seawater and groundwater have a Sr concen-tration and isotopic composition typical of theElk Butte and Mobridge members. The Sr con-centrations are V7400 ppm (85 Wmol Sr/g) in theElk Butte and V5800 ppm (66 Wmol/g) in theMobridge (Table 2). The leaching experimentsthat released signi¢cant quantities of Sr fromthis material yielded 87Sr/86Sr in solution of0.707165 for the Elk Butte and 0.707500 for theMobridge material (Table 2). Although we do not

know the Sr concentration or isotopic composi-tion of fresh groundwater that might have mixedwith seawater in the subterranean estuary of theWIS, we take as a starting point for calculation[Sr] of 1 WM (average of modern rivers) and 87Sr/86Sr of 0.707848 (average of unionid values, Table2). If we mix this water with seawater of 90 WM Srand 87Sr/86Sr of 0.707839 (average of open-oceanbiofacies, Table 2) to produce 20x brackishwater, we can calculate the amount of Sr thatmust have been added to solution to producethe isotopic signature recorded in the brackishbiofacies (87Sr/86Sr = 0.707681, Table 2). The87Sr/86Sr of the brackish mixture is given by:

Rmix ¼½Sr�fwWf fwWRfw þ ½Sr�swWð13f fwÞWRsw þ ½Sr�rockWRrock

f fwW½Sr�fw þ ð13f fwÞW½Sr�sw þ ½Sr�rockð1Þ

where Rmix;fw;sw;rock are the 87Sr/86Sr in the brack-ish water, freshwater, seawater and released fromaquifer solids, respectively, ffw is the fraction offreshwater in the brackish mixture (0.43 for a sa-linity of 20x), and [Sr]fw;sw;rock are the Wmol Srin freshwater, seawater and released from aquifersolids per kg of water. Solving for [Sr]rock usingthe leached 87Sr/86Sr from Elk Butte or MobridgeMember material produces the 87Sr/86Sr observedin the brackish facies by adding V16 Wmol ElkButte Sr or V45 Wmol Mobridge Sr per kg ofgroundwater. If the freshwater endmember has alower 87Sr/86Sr ratio than the average of theunionids used in the calculation, the amount ofSr released from the rocks to yield the 87Sr/86Sr ofthe brackish water would be less. However, thedi¡erence is not large (only V4% if the fresh-water 87Sr/86Sr is taken to be 0.707165) becausethe ‘seawater’ and ‘rock’ terms dominate in Eq. 1.The Elk Butte and Mobridge Member rocks are

Sr-rich, and as a consequence the calculated addi-tions of Sr from them to the groundwater requireleaching from only about 0.2^0.6 g of rock per kgof groundwater, using the rock Sr concentrationsof Table 2. An aquifer with about 20% watercontent (porosity V0.4) would have a rock/waterratio of about 4000 g rock/kg water, and the cal-culated release of Sr using our data thus repre-sents dissolution of only 0.005^0.015% of theaquifer solids. The resulting Sr concentration inthe brackish mixture is V68 WM if Elk Butte

PALAEO 2988 3-1-03


rocks are reacting or V96 WM if Mobridge ma-terial is supplying the Sr. For comparison, simplemixing of freshwater (with 1 WM Sr) with sea-water (with 35x salinity and 90 WM Sr) to attain20x would yield a Sr concentration of 52 WM.We note that the processes supplying Sr to the

waters of the subterranean estuary need not havebeen (and likely were not) in steady state. Indeed,the subterranean estuary has been viewed as adynamic zone that responds to forcing factorssuch as sea-level change (Moore, 1999). The rockscomprising the coastal aquifer system in the LateCretaceous WIS were deposited earlier and ac-quired a lower 87Sr/86Sr ratio typical of the oceanwhen the carbonates they contain were formed.Subsequent diagenenesis, for example duringtimes of rising sea level when the aquifer was pro-gressively invaded by seawater, could have addedSr to the aquifer solids. During periods of low-ering sea level, such as the Late Cretaceous, theaquifer salinities lower as the saltwater lens re-treats and Sr can be released through dissolution,leaching or desorption.The application of the subterranean estuary

concept to producing brackish salinities in theLate Cretaceous WIS has several implications.First, because the freshwater endmember has littlein£uence on the Sr concentration or isotopic com-position of the mixture, it can readily have theisotopic composition suggested by our analysesof the unionids (Table 2, Fig. 6) and indeed, wehave used the average unionid 87Sr/86Sr value inthe calculations above. This implies that the 87Sr/86Sr of the freshwater entering the WIS by riverscould well be greater than that of the fully marinewaters of the Interior, as suggested by some ofour unionid analyses and as postulated by McAr-thur et al. (1994). We suggest, however, that theLate Cretaceous WIS brackish environment wasnot created by normal, sur¢cial estuarine mixingof freshwater and seawater, but rather that suchmixing took place in the subterranean estuary andthe resultant brackish water was discharged to thenearshore environment.Secondly, mixing in the subterranean estuary

produces brackish water that has an isotopicallylighter N

18O signature than fully marine waters(see discussion below). Epifauna and infauna

that are most exposed to the seepage of brackishgroundwater may thus be isotopically lighter thanthe nekton (depending on mixing in the watercolumn), as observed by Wright (1987).Finally, the concept of the discharge of brack-

ish water produced in the subterranean estuarymay explain both the periodic kill-o¡s of faunaand the formation of concretions that preservethem in the WIS (Waage, 1968). Submarinegroundwater discharge has the potential to locallyalter salinity or dissolved oxygen concentrationsand lead to mortality of organisms. Moreover,as noted by Berner (1971) and others, the alkalineputrefaction of decomposing organic matter maycause the precipitation of calcium soaps or saltsthat subsequently convert to calcitic concretions.Reactions with aquifer solids can release the dis-solved Ca necessary for such precipitation, andgroundwater £ow increases the rate of concretionformation.We now compare our results with the determi-

nations of strontium isotopes in fossils from theWIS made by Whittaker and Kyser (1993) andMcArthur et al. (1994). Whittaker and Kyser(1993) observed that the Sr isotope ratio of the

Fig. 6. Schematic diagram showing the production of 20xbrackish water by mixing of seawater and groundwater inthe WIS. The Sr concentrations of the freshwater and salt-water endmembers are assumed to be the present world-aver-age values (Palmer and Edmond, 1989) and the (87Sr/86Sr)are the mean values measured in the unionids and the ma-rine biofacies, respectively. The (87Sr/86Sr) released from theaquifer solids is from our leaching experiments that releasedsigni¢cant Sr to solution (Table 2) and the [Sr] released fromthe aquifer solids is that necessary to produce the observed(87Sr/86Sr) in the brackish biofacies. See text for discussion.

PALAEO 2988 3-1-03


WIS was similar to the contemporary ocean valueexcept for certain specimens younger than 80 Ma.Such specimens had 87Sr/86Sr ratios (as well asN18O values; Whittaker et al., 1987) that werelower than open-ocean seawater, and Whittakerand Kyser (1993) attributed these values to dilu-tion with freshwater. In contrast, as noted above,McArthur et al. (1994) analyzed freshwater fauna,including Unio sp., from the late Maastrichtianand found higher 87Sr/86Sr values than those offossils from the WIS. Their interpretation of thevariation of 87Sr/86Sr vs. salinity was that the iso-tope ratio was increasing toward lower-salinitywater. McArthur et al. (1994) concluded that,for ratios of marine/freshwater Sr concentrationsof V100:1 and V30:1, variation in the 87Sr/86Srratio from 35 to 20x salinity could not be de-tected within the analytical uncertainty of the iso-tope ratio. This analysis depended on the value ofthe freshwater 87Sr/86Sr ratio. However, McAr-thur et al. (1994) did not have the paleoenviron-mental control of the present study such that var-iation in 87Sr/86Sr within a single time period inthe WIS could be seen. Indeed, the variation in87Sr/86Sr observed among the cephalopod fossilscollected in the di¡erent biofacies sampled in thepresent study is equivalent to approximately 6 Main the Late Cretaceous trend of marine 87Sr/86Srvs. time as de¢ned by McArthur et al. (1994).Thus our results suggest that the use of the 87Sr/86Sr ratio to assign ages with an accuracy of lessthan several Ma to fossils from the Late Creta-ceous WIS must take into account the variation inpaleoenvironment. Similar cautionary notes onthe use of the 87Sr/86Sr ratio in chronostratigra-phy in settings such as the WIS have been cited byBryant et al. (1995) and Vaiani (2000).

4.4. Oxygen and carbon isotopes in the WIS

The N18O values of the samples show signi¢cantvariation among the di¡erent biofacies, with light-er values in the brackish environment (Fig. 3).There is no correlation between N

13C and N18O

(R2 = 0.002). Assuming equilibrium fractionation,the N

18O in the specimens is dependent on boththe temperature of formation and the N

18O of thewater in which the carbonate forms. Thus the

observed trend is consistent with increases in tem-perature or decreases in N

18O of the water (orboth) from marine to brackish environments.There is also a weak trend (R2 = 0.14) of lower87Sr/86Sr ratios with lighter N

18O (Fig. 7) that isconsistent with mixing of seawater and fresh-water. The N

18O values of the unionids are indeedlight, V317 to 320x, although, based on theSr isotope data, we have argued that these shellswere not recording the dominant freshwater inputto the WIS.Earlier studies of N18O in Cretaceous fossil mol-

lusks (Tourtelot and Rye, 1969; Rye and Som-mer, 1980; Wright, 1987) found anomalouslyhigh temperatures when paleotemperatures werecalculated from oxygen isotope data. If we as-sume that the marine-to-brackish trend in N

18Osolely re£ects water temperature changes, we cancalculate the water temperature provided the N18Oof the water is known. We have calculated tem-peratures using a N

18Owater =31x, a value that isoften taken as typical of the Late Cretaceous openocean (e.g. Fatherree et al., 1998). The aragonite^water equation of Grossman and Ku (1986) wasused for all samples except the belemnites, whichare calcite and require the calcite^water equation(Craig, 1965; O’Neil et al., 1969). The resultsshow values of 10.4^11.6‡C in the o¡shore Interi-or waters, 16.3^24.7‡C in the shallow, nearshoreInterior and 18.7^28.6‡C in the brackish environ-ment (Fig. 8). None of the calculated tempera-tures is as anomalously high as those obtainedby Tourtelot and Rye (1969) and Wright (1987),

Fig. 7. 87Sr/86Sr vs. N18O of shell carbonate from the open-ocean and WIS biofacies (R2 = 0.14).

PALAEO 2988 3-1-03


although the temperature anomalies in their stud-ies generally were found in inoceramid bivalvesrather than cephalopods or other mollusks.Although the calculated oxygen isotope temper-

atures are reasonable within and between the dif-ferent habitats, salinity variations in the WISwould likely result in geographic variation ofN18Owater. For example, the temperature calcu-lated for the brackish group averages about23‡C, using the estimated open-ocean CretaceousN18Owater value of 31x (Fig. 8). If the Late Cre-taceous WIS had temperatures throughout thatwere comparable to those calculated for the o¡-shore biofacies (V12‡C) we estimate that thebrackish water had a N

18O of V33.5x. In con-trast, if the N18O of the freshwater that mixed withseawater to produce the water characteristic ofthe brackish biofacies was as light as the valuesobserved in the unionids (e.g. 317x), thenN18Owater values of V38x can be estimatedfor salinities of V20x. This value, when usedas N

18Owater in the temperature equation, yieldsunrealistic (negative) temperatures for the brack-ish group.We have suggested that the brackish water of

the WIS was produced by groundwater^seawatermixing. To the extent that the carbonate fractionof the aquifer solids was comprised of the fossilsof carbonate-secreting organisms, the N

18O ofsuch material likely re£ected equilibrium with sea-water at temperatures characteristic of the earlier

WIS, with values ranging, for example, from V0to 32x. Because the rock/water ratios are typi-cally high in aquifers (see above), isotopic ex-change between the solids and solution in the sub-terranean estuary would have imprinted the waterwith the N

18O characteristic of the dominant oxy-gen reservoir (i.e. the solids) and the N

18O valuesof the brackish mix would be greater than pre-dicted from simple mixing of freshwater withN18O=317x and seawater with N

18O=31x.Thus, mixing in the subterranean estuary, coupledwith reaction between solution and aquifer solidsas implied by the Sr isotope data, producedN18Owater values that were heavier than those re-sulting from normal estuarine mixing. We con-clude that, as with the Sr isotope ratios, theN18O values of the unionid mussels sampled arenot representative of the N

18O of the dominantfreshwater source to the WIS.In her study of N18O in di¡erent habitat groups

in the early Maastrichtian, Wright (1987) noticedthat the epifauna was characterized by lighterN18O relative to nektonic cephalopods and to in-fauna. She explained this as the result of a dense,warm water mass formed in the eastern portion ofthe basin that sank into the basin interior. We donot have su⁄cient detail in the samples analyzedto represent epifauna and infauna signi¢cantly.However, we note that, much like the variationin 87Sr/86Sr ratios, the benthos and nekton do notdisplay signi¢cant o¡sets in their N

18O values inany given habitat group where both types arerepresented (i.e. nearshore Interior and brackish).It is possible that the water column in these loca-tions was too shallow to show much strati¢cationwith respect to temperature or salinity.

5. Conclusions

This study demonstrates that well-preserved,stratigraphically controlled fossils from the LateCretaceous Western Interior Seaway of NorthAmerica preserve a record of environmental var-iation recorded in Sr and O isotopes. The 87Sr/86Sr ratios in such samples display signi¢cant var-iation, with values decreasing from 0.70783990.000024 in fully marine to 0.7076779 0.000036

Fig. 8. Water temperatures determined from N18O analyses of

shell material. Calculations assume that the N18O of the

water is 31x (Fatherree et al., 1998). Values are slightlyo¡set in each group to improve readability.

PALAEO 2988 3-1-03


in brackish environments. When compared withthe well-documented temporal variation in sea-water 87Sr/86Sr in the Late Cretaceous, the ob-served range suggests that using the open-ocean87Sr/86Sr curve to date fossils from the Late Creta-ceous WIS should be applied with caution andmust take into account the paleoenvironmentssampled.It is not possible to evaluate the freshwater

87Sr/86Sr endmember by direct measurement offreshwater mussels, either because the specimensanalyzed do not represent the dominant fresh-water input to the seaway, or do not correspondto the same time because they lack the biostrati-graphic control of the specimens from the Sea-way. If a lower limit of V20x salinity is setfor the brackish biofacies based on salinity toler-ances of modern cephalopods (Forsythe and VanHeukelem, 1987), the 87Sr/86Sr in the dominantfreshwater input to the WIS is estimated to be0.707245^0.705517 for ratios of the marine/fresh-water Sr concentration of 2:1 to 10:1, respec-tively. The former is more consistent with thelikely 87Sr/86Sr ratios of rocks being weatheredat the time, but requires freshwater with a veryhigh Sr concentration compared with modern riv-ers. We conclude that the trend in 87Sr/86Sr in oursamples can be best explained by the mixing offreshwater and seawater in a coastal aquifer andseepage of the brackish water into the nearshoreWIS. Through reactions with aquifer solids in this‘subterranean estuary’, the resultant brackishwater has a Sr concentration enhanced relativeto the conservative behavior of Sr seen in thesur¢cial estuarine mixing of freshwater and sea-water. Moreover, the 87Sr/86Sr ratio of the brack-ish mix re£ects contributions from both seawaterand aquifer solids and thus is lower than might beexpected from sur¢cial estuarine mixing.Values of N

18O decrease from the o¡shore In-terior to brackish biofacies, consistent with a de-crease in N

18O with decreasing salinity and/or in-creasing water temperature. If the e¡ect is entirelydue to temperature, values increase from V10^12‡C in the o¡shore Interior waters of the seawayto 19^28‡C in the brackish environment. If theN18O of the brackish water is isotopically lighterthan the values in Late Cretaceous seawater, as

might be expected from mixing of seawater withisotopically light freshwater, then both the calcu-lated temperatures in the brackish group and thetemperature gradient from the o¡shore Interior tothe brackish environment will be decreased. How-ever, as with the Sr isotopes, simple mixing offreshwater with the N

18O seen in the freshwatermussels with open-ocean seawater to produce20x brackish water yields unrealistic tempera-tures for the brackish environment. Either thedominant freshwater entering the seaway is heav-ier than that suggested by the freshwater musselsor the N

18O of the brackish water has been modi-¢ed by isotopic exchange with aquifer solids in thesubterranean estuary.

Acknowledgements

The strontium isotope measurements weremade while J.K.C. was on sabbatical at theCentre Europe¤en de Recherche et d’Enseigene-ment des Ge¤osciences de l’Environnement (CER-EGE) in Aix-en-Provence, France. Support fromthe State University of New York, the Centre Na-tional de Recherche Scienti¢que (CNRS) and theAmerican Museum of Natural History is grate-fully acknowledged. We thank Y. Noack and D.Borshneck for the X-ray di¡raction analyses andK. Sarg for assistance with SEM. Samples werekindly made available to us by W.A. Cobban(USGS, Denver), and T. White and C. MacClin-tock (Yale Peabody Museum). This manuscripthas bene¢ted from discussions with our colleagueW.A. Cobban and thoughtful reviews by P. Har-ries and F. Kenig.

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