Paleoceanography of the Late Cretaceous (Maastrichtian) Western Interior Seaway of North America: Evidence from Sr and O Isotopes

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Palaeogeography, Palaeoclimatology, Palaeoecology 191 (2003) 45^64 www.elsevier.com/locate/palaeo

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, USA b American Museum of Natural History, New York, NY 10024, USA c Department of Geology and Geophysics, Yale University, New Haven, CT 06520-8109, USA d 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, MA02138, USA

Received11 October 2001; receivedin revisedform 15 October 2002; accepted8 November 2002

Abstract

Well-preservedfossils of the Late Cretaceous Western Interior Seaway (WIS) of North America have been analyzed for Sr concentration and Sr and O isotopes in order to decipher paleosalinities and paleotemperatures. The samples are from four biofacies within the Seaway (late Maastrichtian): offshore Interior (Pierre Shale), nearshore Interior (Fox Hills Formation), brackish (reduced salinity; Fox Hills Formation) and freshwater (Hell Creek Formation). Samples were also obtainedfrom the Severn Formation of Maryland(consideredtobe representative of the open ocean). All biofacies (except the freshwater) are demonstrably within the Jeletzkytes nebrascensis ammonite zone ( 6 1 Ma duration). The 87Sr/86Sr ratios show significant andsystematic decreasesfrom marine (mean þ 1 S.D. = 0.707839 þ 0.000024) to brackish facies (mean þ 1 S.D. = 0.707677 þ 0.000036), consistent with dilution by freshwater with a lower 87Sr/86Sr ratio than seawater. Such variation disallows using the 87Sr/86Sr ratios of fossil shell material to assign ages to fossils from the Late Cretaceous WIS without knowledge of the salinity in which the organism grew. The Sr isotope ratios for scaphitidammonites within a single biofacies are similar to each other and different from those for scaphites in other biofacies, implying that these organisms are restricted in their distribution during life. The 87Sr/86Sr values of freshwater unionidmussels range widelyandare not compatible with the freshwater endmember 87Sr/86Sr ratio requiredby the trendin 87Sr/86Sr vs. biofacies establishedfrom the other samples. Paleosalinities for the biofacies are estimatedto range from 35 x in the open marine to a minimum of 20x in the brackish, based on the presence of cephalopods in all four facies and the known salinity tolerance of modern cephalopods. Producing reasonable 87Sr/86Sr values for the freshwater endmember of a 87Sr/86Sr vs. 1/[Sr] plot requires a Sr concentration 0.2^0.5 that of seawater for the dominant freshwater input to the WIS. Such high Sr concentrations (relative to seawater) are not observedin modernrivers, andwe suggest that the brackish environment in the WIS arose through the mixing of freshwater andseawater in a nearshore aquifer system. Reactions of the solution with aquifer solids in this ‘subterranean estuary’ [Moore, Mar. Chem. 65 (1999) 111^125] produced brackish water with the Sr concentration andisotopic composition recordedinthe brackish biofacies. N18O values of the fossils

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

PALAEO 2988 3-1-03 46 J.K. Cochran et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 191 (2003) 45^64 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 N18O of the water in which the shell formed. The latter interpretation is consistent with less-than-fully marine salinities in the nearshore biofacies, but both changes in temperature andthe isotopic composition of the water may have occurredin this environment. ß 2002 Elsevier Science B.V. All rights reserved.

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

1. Introduction quently transportedinto the basin interior under- lying colder, less saline intermediate waters. The Cretaceous Periodwas characterizedby the An additional paleoenvironmental indicator is existence of broadcontinental seaways that con- the ratio of 87Sr to 86Sr in shell material. The 87Sr/ nectedthe worldoceans. In North America, the 86Sr ratio in the open ocean re£ects dominantly Western Interior Seaway (WIS) connectedthe Bo- the input of Sr from continents via rivers and real andTethys oceans. During the late Maas- mantle-derived sources such as hydrothermal trichtian this seaway was closedto the north vents (Brass, 1976; Albare'de et al., 1981; Palmer anddisappearedaltogetherby the endof the Cre- and Edmond, 1989). Variations in the 87Sr/86Sr taceous. Because these epicontinental seas have no ratio over geologic time are due to variations in modern analogs as far as magnitude and setting the relative importance of these Sr sources, as well are involved, very little is known about their as evolution in the Earth’s 87Sr/86Sr ratio due to oceanography. In particular, distributions of 87Rb decay. Such variations are recorded in the water masses as de¢ned by temperature and salin- 87Sr/86Sr of marine carbonates, which are charac- ity variations are poorly characterizedalthough teristically low in Rb, andthe Phanerozoic evolu- reconstructions of WIS paleoceanography andpa- tion of the 87Sr/86Sr ratio in seawater has been leocirculation have been attempted(e.g. Hay et well documented using these deposits (Peterman al., 1993). One approach to reconstructing paleo- et al., 1970; Dasch andBiscaye, 1971; Brass, temperatures andpaleosalinities in such environ- 1976; Veizer andCompston, 1974; Burke et al., ments is through the recordof geochemical trac- 1982; Hess et al., 1986, DePaolo, 1986; Martin ers that are proxies for these parameters andare andMacdougall, 1991; Veizer et al., 1999 ). In incorporatedin the calcium carbonate shells of modern ocean^river systems, the 87Sr/86Sr ratio organisms. For example, the determination of of rivers can be signi¢cantly di¡erent from that N18O in shell material as an index of paleotemper- of seawater (0.70916), ranging from values much atures has been appliedto a variety of systems lower (e.g. rivers entering San Francisco Bay; In- since Urey (1947) initially proposedthat the gram andSloan, 1992 ) to much greater than that N18O values of carbonates are a function of both of seawater (e.g. rivers entering the Baltic Sea; the temperature and N18O of the water in which Andersson et al., 1992, 1994). In the estuarine the carbonate formed. In the WIS, Tourtelot and andcoastal environment, the mixture of fresh Rye (1969) and Wright (1987) among others mea- and salt water produces a gradient in 87Sr/86Sr sured N18O values in well-preservedshell material. ratios that can be recorded by organisms living Calculatedpaleotemperatures for benthic organ- in a given salinity. Such gradients in 87Sr/86Sr isms rangedup to 40‡C andwere consideredun- vs. salinity are seen in San Francisco Bay and realistically high. Moreover, Wright (1987) no- the Baltic Sea, for example (Ingram andSloan, ticedisotopic di¡erences in epifauna compared 1992; Andersson et al., 1992). Thus the 87Sr/86Sr with nekton andinfauna andarguedthat the ratio may be usedto reconstruct paleosalinities in WIS hadan unusual salinity structure, with areas where salinity gradients, as well as di¡eren- warm saline water formedby evaporation in the ces in Sr isotope ratios between freshwater and shallow eastern portion of the sea andsubse- seawater exist.

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We present data for oxygen, carbon and stron- monites, nautiloids and belemnites are repre- tium isotopes andstrontium concentrations in sentedin the fauna. Specimens of two ammonite well-preservedshell material from within the species (J. nebrascensis and Sphenodiscus lobatus) WIS andfrom coeval material collectedfrom were analyzedfrom all four paleoenvironments. the Atlantic Coastal Plain of North America. Additionally, freshwater unionid bivalves from The samples were collectedin the Jeletzkytes ne- the basal Hell Creek Formation were analyzed brascensis ammonite zone, representing 9 1Ma in an e¡ort to constrain the 87Sr/86Sr ratio in in the late Maastrichtian, around67 Ma BP freshwater at that time. However, the Hell Creek (Waage, 1968; Landman and Waage, 1993; samples lack the biostratigraphic control repre- Kau¡man et al., 1993; Kennedy et al., 1998), sentedby the other samples. All samples are de- from the Pierre Shale, Fox Hills andHell Creek positedin the collections of the Yale Peabody Formations in South Dakota. Because of the ex- Museum, American Museum of Natural History, tensive paleoenvironmental reconstructions for andthe US National Museum. Three samples of this region (Gill andCobban, 1966; Waage, the rock making up the presumedambient terrain 1968), it is possible to separate the samples a pri- also were taken for determination of Sr isotope ori into environments corresponding to o¡shore ratio andconcentration. These samples included Interior, nearshore Interior andbrackish (less the Elk Butte andMobridge members of the than fully marine salinity). Samples were collected Pierre Shale, which lie stratigraphically below from the same biozone in the Severn Formation the Fox Hills Formation andmay have been ex- (Maryland; Kennedy et al., 1997) on the Atlantic posedalong the paleo-shoreline. Coastal Plain andare assumedto represent fully marine, open-ocean conditions. 2.2. Sample preservation

Samples were taken from single specimens, ex- 2. Methods cept K15, in which two specimens were com- bined; samples of cephalopods were taken from 2.1. Sample localities the outer shell, rather than the septa. The shell material of the ammonites andnautiloidsshowed Samples were collectedfrom the Pierre Shale a distinctive iridescence characteristic of nacreous andFox Hills Formation in South Dakota and shell structure. However, prior work has shown the Severn Formation in Maryland, all within that authigenic deposits on the surfaces of sam- the Jeletzkytes nebrascensis ammonite biozone ples can potentially bias isotopic andconcentra- (Table 1, Fig. 1). The samples were divided into tion data (Turekian andArmstrong, 1961; Land- paleoenvironments according to the reconstruc- man et al., 1983). Thus, all samples were soni¢ed tion of Waage (1968). We have labeledthese for several minutes in distilled water to remove environments as follows: ‘o¡shore Interior’, cor- loosely adhering surface phases. Samples were responding to an open-water biofacies representa- air-dried and the best pieces were selected follow- tive of the central seaway with a maximum depth ing examination with a light microscope. If neces- of V200 m; ‘nearshore Interior’, representative of sary, the soni¢cation step was repeated. Samples a more nearshore facies; and‘brackish’ biofacies, were then examinedby scanning electron micros- representative of an estuarine, nearshore environ- copy (SEM) to con¢rm both the absence of sur- ment inhabitedby organisms with lower salinity face phases andthe presence of original shell tolerances. Samples from the Severn Formation structure (Plate I). In the cephalopodsamples, from the same biozone (Kennedy et al., 1997) the characteristic nacreous microstructure was are considered to represent a fully ‘open-ocean’ preserved. Excellent preservation quality was in- biofacies andincludeonly nektonic organisms dicated when SEM photos showed nacreous tab- (ammonites andnautiloids). lets of even thickness, without evident alteration Within the WIS, gastropods, pelecypods, am- or deposition of diagenetic interlayer material. Je-

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Table 1 Sample identi¢cation and localitiesa Sample ID Museum IDb Formation Memberc Species Organism Freshwater K6 YPM 38033 Hell Creek Unio sp. bivalve K12 YPM 38034 Hell Creek Unio sp. bivalve K16 YPM 38029 Hell Creek Unio sp. bivalve K17 YPM 38005 Hell Creek Unio sp. bivalve K22 YPM 38044 Hell Creek Unionoida bivalve

Open-ocean K3 AMNH 45406 Severn Eutrephoceras dekayi nautiloid K4 AMNH 45404 Severn Sphenodiscus lobatus ammonite K35 USNM 520476 Severn Jeletzkytes nebrascensis ammonite

O¡shore Interior K2 USNM 486542 Pierre J. nebrascensis ammonite K23 USNM 486528 Pierre Belemnitella cf. bulbosa belemnite K34 AMNH 47462 Pierre S. lobatus ammonite K37 USNM 520477 Pierre J. nebrascensis ammonite

Nearshore Interior K5 AMNH 45463 Fox Hills Timber Lake S. lobatus ammonite K8 YPM 44980 Fox Hills Timber Lake S. lobatus ammonite K11 YPM 162363 Fox Hills Trail City, C, IC E. dekayi nautiloid K10 YPM 162434 Fox Hills Timber Lake Serrifusus dakotensis gastropod K1 AMNH 45405 Fox Hills Timber Lake J. nebrascensis ammonite K13 YPM 162433 Fox Hills Timber Lake, CT J. nebrascensis ammonite K21 YPM 202291 Fox Hills Timber Lake, CT Cymbophora warreni bivalve K33 YPM 32588 Fox Hills Trail City, C Belemnitella cf. bulbosa belemnite

Brackish K7 YPM 46036 Fox Hills Timber Lake, TO Tancredia americana bivalve K9 YPM 162385 Fox Hills Iron Lightning, CL Corbicula sp. bivalve K14 YPM 162374 Fox Hills Iron Lightning, BL J. nebrascensis ammonite K15 YPM 162397,8 Fox Hills Iron Lightning, CL Lunatia sp. gastropod K18 YPM 162437 Fox Hills Iron Lightning, BL J. nebrascensis ammonite K19 YPM 200285 Fox Hills Hell Creek Lunatia sp. gastropod K20 YPM 162402 Fox Hills Hell Creek J. nebrascensis ammonite K36 YPM 44661 Fox Hills Hell Creek S. lobatus ammonite Rock K30 YPMIP loc. D4083c Elk Butte K31 YPMIP loc. C1872 Mobridge K32 YPMIP loc. C479 Mobridge a Detaileddescriptions 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, Bullheadlithofacies; IC, Irish Creek lithofacies. letzkytes nebrascensis samples were available from open-ocean ando¡shore Interior facies ( Plate all four biofacies (Plate IA^D), andof these sam- IA,B) showed no evident overgrowths indicative ples, the best preservation was evident in the near- of diagenetic alteration. Preservation in the nau- shore Interior andbrackish facies ( Plate IC,D). tiloid Eutrephoceras dekayi, from the open-ocean However, the J. nebrascensis samples from the biofacies, was excellent (Plate IE). A ¢nal screen-

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Fig. 1. Map showing biofacies sampledrelative to the paleo-shoreline (heavy line) of the WIS duringthe late Maastrichtian zone of 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. ing involvedX-ray di¡ractionto con¢rm that the spikedwith a Sr tracer andevaporated.Subse- samples were 100% aragonite, with the exception quent separation of Sr followedthe procedure of the belemnites, which were 100% calcite. outlinedabove. Strontium was loaded with tantalum oxide 2.3. Sample analyses (Birk, 1986) onto a single rhenium ¢lament and analyzedin the dynamic modeon a Finnigan 2.3.1. Strontium isotopes MAT 262 mass spectrometer. The static mode Cleanedpieces of sample (2^6 mg) were dis- was usedfor the matrix samples. Repeatedanal- solved in 1 N HCl and any undissolved residue yses of NBS 987 during the time of sample anal- (largely organic) was separatedby centrifugation. yses gave a mean 87Sr/86Sr value of 0.710238 þ Approximately 2 ml of sample was loaded onto a 0.000011 (16 runs) in the dynamic mode and cation exchange column (AG50WX8) calibrated 0.710271 þ 0.000012 (10 runs) in the static mode. for Sr. After passing 22 ml of 2 N HCl through The 87Sr/86Sr values of the spikedmatrix samples the column, the Sr fraction was collectedin 4 ml (static mode) have been corrected to the NBS of 2 N HCl andevaporatedto dryness.Total standard 87Sr/86Sr value of 0.710248. The 87Sr/ blank Sr was less than 200 pg comparedwith 86Sr values in all samples are normalizedto a V2 Wg Sr in the samples. 86Sr/88Sr value of 0.1194. The matrix samples from the Elk Butte and Mobridge Members were subjected to a simple 2.4. Strontium concentrations experiment to determine the Sr isotope ratio that might be releasedduringchemical weathering Strontium concentrations were measuredon by freshwater. Samples of 0.1^0.2 g were leached separate aliquots of the samples following the in 10^20 ml distilled water for several hours at procedure of Schrag (1999). Samples were dis- room temperature. The solution was centrifuged solvedin 2% HNO 3, diluted and Sr was measured andthe leachate was separatedfrom the residue, by inductively coupled plasma emission spectros-

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Plate I. Shell microstructure in fracturedcross section of six samples. (A^D) Jeletzkytes nebrascensis from four di¡erent biofacies. The nacreous microstructure is preservedalthough 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 microstructure. (F) Serrifusus dakotensis, nearshore Interior, sample K10 (YPM 162434, Fox Hills Formation) with cross-lamellar microstructure.

PALAEO 2988 3-1-03 J.K. Cochran et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 191 (2003) 45^64 51 copy on a Jobin-Yvon simultaneous instrument, Seaway. Specimens of the same species from the model 46-P. Analytical precision was determined same environment have similar Sr concentrations, to be better than þ 0.2%, basedon replicate anal- varying within V10% (e.g. S. lobatus collectedin yses of multiple dilutions of solutions of single the nearshore Interior and J. nebrascensis from samples. both the nearshore Interior andbrackish environments). There is a range of Sr concentrations for 2.5. Oxygen and carbon isotopes di¡erent organisms from a single environment, presumably re£ecting interspeci¢c di¡erences in Oxygen andcarbon isotopic analyses were the factors a¡ecting Sr incorporation into shell made at the University of Michigan Stable Iso- material (see discussion below). In particular, tope Laboratory by L. Wingate. Carbonate sam- the benthic gastropods Lunatia and Serrifusus ples weighing a minimum of 10 Wg were placedin andbivalve Cymbophora have lower Sr concentra- stainless steel boats. Calcite samples were roasted tions than those of the ammonites andnautiloid at 380‡C in vacuo for 1 h to remove volatile con- samples in any given environment. The bivalve taminants. Aragonite was roastedat a lower tem- Corbicula is conspicuous in having the lowest Sr perature, 200‡C, to minimize e¡ects of thermal concentration of the brackish group. This low decarboxylation that can alter the isotopic com- value, together with its unusually light N18O value position of the carbonate. Samples were then (see below), suggests that this specimen was trans- placed in individual borosilicate reaction vessels portedinto the brackish assemblage from fresh- andreactedat 76‡ þ 2‡C with three dropsof an- water. Indeed, present-day Corbicula can live in hydrous phosphoric acid for 8 min in a Finnigan completely fresh water (Counts, 1986; Mackie, Kiel preparation device coupled directly to the 1986). The Sr concentrations of ammonite and inlet of a Finnigan MAT 251 triple-collector iso- nautiloidfossils measuredin this studyare com- tope ratio mass spectrometer. Isotopic enrich- parable to values measuredby Turekian and ments were correctedfor acidfractionation and Armstrong (1961) in material with negligible-to- oxygen-17 contribution by calibration to a best- low percentages of calcite (andthus judgedtobe ¢t regression line de¢ned by two NBS standards, minimally altered). NBS 18 andNBS 19. Precision of datawas moni- Total Sr concentrations in the Elk Butte and toredthrough dailyanalysis of a variety of pow- Mobridge Members are greater than those mea- dered carbonate standards. At least six standards suredin the fossils andrange from 5823 to 7434 were reactedandanalyzeddaily,bracketing the ppm. The Sr releasedby the leaching experiments sample suite at the beginning, middle, and end was 646 ppm for K30, 708 ppm for K31 and20 of the day’s run. Measured precision was main- ppm for K32. In the latter case only 0.3% of the tainedat better than 0.1 x for both carbon and Sr was leached, whereas the fraction was 9% for oxygen isotope compositions. Data are reported K30 and12% for K31. in x notation relative to VPDB (Vienna PeeDee Belemnite). 3.2. Sr isotope ratios

With the exception of the freshwater bivalves, 3. Results the 87Sr/86Sr ratios of the samples cluster into dis- crete groups with minimal overlap: 0.707815^ 3.1. Sr concentrations 0.707862 for the open-ocean group, 0.707796^ 0.707827 for the o¡shore Interior group, Concentrations of Sr in the shell material range 0.707704^0.707795 for the nearshore Interior from 355 to 4705 ppm (Table 2). The lowest val- group and0.707605^0.707712 for the brackish ues are recorded in the freshwater bivalves and group (Table 2, Fig. 2). In contrast, the 87Sr/ the highest values are from ammonites (Spheno- 86Sr values in the freshwater bivalves range from discus lobatus, Jeletzkytes nebrascensis) from the 0.707615 to 0.707986, although the large range

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Table 2 Isotopic andconcentration data Species 87Sr/86Sr Sr N18O N13C Remarks (ppm) (x)(x) Freshwater K6 Unio sp. 0.707940 þ 0.000009 405 320.29 34.69 K12 Unio sp. 0.707903 þ 0.000011 504 319.71 34.99 K16 Unio sp. 0.707942 þ 0.000016 561 319.11 31.73 K16 Unio sp. 0.707986 þ 0.000012 na na na Powdered sample K17 Unio sp. 0.707628 þ 0.000015 355 316.87 32.56 K17 Unio sp. 0.707615 þ 0.000014 na na na Powdered sample K22 Unionoida 0.707754 þ 0.000016 na na na K22 Unionoida 0.707869 þ 0.000013 na na na Powdered sample

Open-ocean K3 Eutrephoceras dekayi 0.707815 þ 0.000007 3261 30.66 31.51 K4 Sphenodiscus lobatus 0.707839 þ 0.000011 3018 0.28 32.95 K35 Jeletzkytes nebrascensis 0.707862 þ 0.000006 1890 na na

O¡shore Interior K2 J. nebrascensis 0.707810 þ 0.000008 na 1.43 1.94 K23 Belemnitella cf. bulbosa 0.707801 þ 0.000009 1319 0.31 0.36 K34 S. lobatus 0.707827 þ 0.000008 3859 na na K37 J. nebrascensis 0.707796 þ 0.000007 2520 na na

Nearshore Interior K5b S. lobatus 0.707777 þ 0.000010 4705 30.92 33.64 K5g S. lobatus 0.707768 þ 0.000012 na na na Duplicate sample K8 S. lobatus 0.707781 þ 0.000009 4329 31.61 33.34 K11 E. dekayi 0.707785 þ 0.000015 3867 0.18 0.21 K10 Serrifusus dakotensis 0.707773 þ 0.000014 2297 30.49 30.13 K1 J. nebrascensis 0.707740 þ 0.000010 3963 30.04 0.99 K13 J. nebrascensis 0.707750 þ 0.000015 4247 30.06 31.83 K21 Cymbophora warreni 0.707704 þ 0.000012 1898 30.23 1.36 Same locality andconcretion as K13 K33 Belemnitella cf. bulbosa 0.707795 þ 0.000008 1772 31.28 1.91 Same locality as K11

Brackish K7 Tancredia americana 0.707695 þ 0.000011 2412 31.34 1.16 K9 Corbicula sp. 0.707688 þ 0.000015 667 317.91 34.79 K14 J. nebrascensis 0.707654 þ 0.000014 2545 30.34 1.29 K15 Lunatia sp. 0.707605 þ 0.000010 1589 30.47 0.49 Same locality as K9 K18 J. nebrascensis 0.707688 þ 0.000008 2680 32.44 3.10 K19 Lunatia sp. 0.707699 þ 0.000014 2043 32.40 0.33 K20 J. nebrascensis 0.707703 þ 0.000012 na 30.31 1.63 Same locality andconcretion as K19 K36 S. lobatus 0.707712 þ 0.000007 na na na

Rock K30 Elk Butte member 0.707165 þ 0.000018a 7434 na na K31 Mobridge member 0.707507 þ 0.000016a 5823 na na K32 Mobridge member 0.710998 þ 0.000016a 5859 na na na = not analyzed. a Sr isotope ratio releasedto solution duringleaching experiments. See text.

PALAEO 2988 3-1-03 J.K. Cochran et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 191 (2003) 45^64 53 tends to be driven by a single sample. There is generally close correspondence in the 87Sr/86Sr values of multiple specimens of the same species collectedin a given environment; the 87Sr/86Sr ratios of specimens of the same species within a biofacies are often within the analytical uncertainty of one another. This is evident in the multiple specimens of J. nebrascensis in the o¡shore Interior, nearshore Interior andbrackish groups as well as S. lobatus in the brackish group (Table 2). In contrast, the values are signi¢cantly di¡erent for the same species in di¡erent biofacies. The 87Sr/86Sr ratios leachedfrom the Elk Butte andMobridgeMember rocks bracket the entire range of values seen in the fossil shell material Fig. 3. N18O(x VPDB) of shell carbonate vs. biofacies (rel- andvary from 0.707165 to 0.710998 ( Table 2). ative scale: decreasing salinity to the right; values in each The highest ratio corresponds to sample K32, in group are slightly o¡set to improve readability). which only 0.3% of the Sr was releasedto solution. 0.0 to 31.6x for the nearshore Interior and 30.3 to 32.4x, excluding the value of specimen 3.3. Oxygen isotopes K9, for the brackish group. The N18O for K9, a sample of Corbicula sp., is quite light, 317.9x N18O values range from 320.29x to 1.43x andis similar to values obtainedfor the fresh- (Fig. 3) and N13C values range from 34.99x to water bivalves (316.9 to 320.3x). We take the 3.10x. In contrast to the Sr isotope results, there oxygen isotope composition of Corbicula, as well is less consistency among the paleoenvironmental as its low Sr concentration, to indicate that the groups, although the data for the WIS samples shell grew in freshwater. Given that both the N18O show a tendency toward lighter N18O values pro- andSr concentration in a co-occurring specimen ceeding from o¡shore Interior to brackish envi- of Lunatia are more consistent with the other ronments: 0.3 to 1.4x for the o¡shore Interior, brackish fauna, we believe the sample of Corbicu- la was transportedinto the brackish environment after death.

Fig. 2. 87Sr/86Sr vs. biofacies (relative scale: decreasing salini- Fig. 4. Shell Sr concentration vs. biofacies for cephalopod ty to the right; values in each group are slightly o¡set to im- species. M 9 Open-ocean; OI 9 O¡shore Interior; NI 9 prove readability). Nearshore Interior; B 9 Brackish.

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4. Discussion The dependence of Sr/Ca on temperature is not consistent among di¡erent organisms: in herma- 4.1. Strontium concentrations in shells typic corals (Porites), the carbonate Sr/Ca decreases by V10% as the temperature increases There is considerable interspeci¢c variation in from 10 to 25‡C (Alibert andMcCulloch, 1997 ) the Sr concentrations for organisms collectedin a while in coccoliths the ratio increases by 25% over single paleoenvironment. Despite these di¡erences approximately the same temperature range (Stoll in absolute concentration, the trends from open- et al., 2002b). It is possible that the water temper- ocean to brackish biofacies are consistent for the ature of the nearshore WIS environments sampled cephalopodspecies ( Fig. 4). The Sr concentration by us was greater than that of the o¡shore Inte- progressively increases from open-ocean to o¡- rior (see discussion below), and the increase in shore Interior to nearshore Interior. Only a single shell Sr from the o¡shore to nearshore biofacies species (J. nebrascensis) from the brackish envi- is consistent with the e¡ect of temperature seen in ronment was analyzed, and the data show a de- the coccolith data (Stoll et al., 2002b). However, crease in Sr concentration relative to the near- the magnitude of the increase predicted from the shore Interior. coccoliths is less than we observe. The Sr concentration in biogenic carbonate re- £ects the Sr/Ca ratio in the water in which the 4.2. The 87Sr/86Sr ratio as a paleosalinity indicator carbonate formedas well as the physiological factors involvedin biomineralization. The latter are The Sr isotopic evolution of seawater over geo- dependent upon water temperature, salinity, and logic time has been well-documented (Peterman et organism growth rate (Smith et al., 1979; Stoll al., 1970; Veizer andCompston, 1974; Brass, andSchrag, 2000; Stoll et al., 2002a,b; Weinbau- 1976; Spooner, 1976; Hess et al., 1986; DePaolo, er et al., 2000). Factors such as water temper- 1986; Martin andMacdougall,1991; McArthur ature andgrowth rate also may be interdependent et al., 1994; Veizer et al., 1999). In the Late Creta- (Yatsu et al., 1998; Stoll et al., 2002a). With re- ceous, values of open ocean water increasedfrom spect to the variation in the Sr/Ca ratio of the 0.7073 to 0.7078 (Hess et al., 1986) with a max- water, the present-day ocean shows a di¡erence imum of V0.7079 at the Cretaceous^Tertiary between the value in the open ocean andin boundary (Martin andMacdougall, 1991 ). Sea- world-average river water, with the former being water 87Sr/86Sr increasedfrom V0.7077 to approximately three times greater than the latter V0.7079 in the last V3 Ma of the Cretaceous, (Palmer and Edmond, 1989; Turekian, 1969). A andwas V0.70784 at the time representedby our simple trendof decreasingshell Sr concentrations sampling (Martin andMacdougall, 1991 ). Indeed, from the marine to brackish environments is not within the analytical error, 87Sr/86Sr was con- seen in our data, suggesting that variation of the stant at V0.70784 in the V2 Ma preceding the Sr/Ca ratio in the water is not the ¢rst-order con- K/T boundary (Martin andMacdougall, 1991 ). trol on Sr/Ca in the shell. Although the exact frac- This value agrees well with the average 87Sr/86Sr tionation of Sr/Ca in the shell vs. water will vary of the ‘open-ocean’ group: mean þ 1 S.D. = among species, the similarity in trends among all 0.707839 þ 0.000024, andsuggests that this group the cephalopodspecimens suggests that funda- is accurately re£ecting open marine conditions. mental di¡erences in biomineralization related to Values in the o¡shore Interior group are only environmental factors such as temperature, salin- slightly less than the open-ocean group: mean þ 1 ity or pCO2 are causing the trends. S.D. = 0.707808 þ 0.000014. The ratio trends to- The e¡ects of temperature andother factors on wardlower values proceedingfrom o¡shore to the Sr/Ca ratio of biogenic carbonates have been nearshore Interior (mean þ 1 S.D. = 0.707763 þ documented for coral skeletons, ¢sh otoliths, 0.000030) to the brackish biofacies (mean þ 1 squidstatoliths andcoccolithophorids( Smith et S.D. = 0.707677 þ 0.000036). Although values al., 1979; Yatsu et al., 1998; Stoll et al., 2002b). within adjacent groups overlap within the error,

PALAEO 2988 3-1-03 J.K. Cochran et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 191 (2003) 45^64 55 the overall trendis towardprogressively lower than the o¡shore Interior samples. The brackish values such that the brackish group is signi¢cantly samples were collectedstratigraphically up-section di¡erent from the open-ocean group. or, in a paleogeographic sense, landward of the We note that the trendin 87Sr/86Sr exists even nearshore Interior samples. Variation in the open though each group includes nektonic organisms ocean 87Sr/86Sr ratio for the late Maastrichtian that might have rangedfrom one environment documented by Martin and Macdougall (1991) to another. The ammonite Jeletzkytes nebrascensis shows that the ratio remains the same is a goodexample in that this ammonite occurs in (V0.70784) or increases slightly towardthe end all four environments yet specimens from a given of the Cretaceous, with a maximum at the K/T paleoenvironment possess a signi¢cantly di¡erent boundary. Thus, if the brackish samples are not isotopic signature from the other environments. coeval, but are younger, then their 87Sr/86Sr ratios This is strong evidence that these organisms wouldbe greater than that in the other samples, were living in the environment in which they not lower. On the basis of both stratigraphic re- were collected. We conclude that although these lationships andthe temporal pattern of Sr iso- animals were nektonic, their swimming ability was topes in the Late Cretaceous, we conclude that limitedor the distances were too great to permit the 87Sr/86Sr ratios in the samples re£ect the pa- much migration between biofacies. In contrast, it leoenvironments. is interesting to note that the two belemnite speci- Our e¡orts to use unionids to establish the mens analyzed(K23 andK33) have nearly iden- freshwater endmember of the 87Sr/86Sr vs. habitat tical 87Sr/86Sr ratios andSr concentrations trendwere not successful. The values span the although they were collectedin two di¡erentbio- range observedin the samples andare generally facies (o¡shore andnearshore Interior; Table greater than values recorded in organisms from 2). This may indicate that belemnites were more the WIS (Fig. 2, Table 2). This situation is similar active swimmers, like the present-day squid, to that observedby Ingram andSloan (1992) in andwere mobile between various WIS habitats San Francisco Bay. Their analyses of the rivers (W. Christensen, Univ. of Copenhagen, personal draining into the Bay, coupled with the trend in communication, 2000). 87Sr/86Sr vs. salinity, show that considerable var- The trendin 87Sr/86Sr ratios observedamong iation in 87Sr/86Sr is observedamong the di¡erent the paleoenvironments is consistent with a salinity freshwater inputs, with only the dominant input variation causedby mixing with a freshwater 87Sr/ forming the appropriate endmember for the 87Sr/ 86Sr endmember that is lower than the seawater 86Sr vs. salinity trend. Albare'de and Michard value during late Maastrichtian time. In this re- (1987) noteda strong dependenceof watershed spect, the trendis similar to that observedin mod- lithology on the 87Sr/86Sr of French rivers drain- ern San Francisco Bay (Ingram andSloan, 1992 ) ing into the Mediterranean, with signi¢cant var- andin many geologically oldersystems ( Ingram iation in the ratio (0.708719^0.712765) in a rela- andDePaolo, 1993; Holmdenet al., 1997; tively small geographic area. We conclude that the Schmitz et al., 1991; Vonhof et al., 1998; Vaiani, samples of unionids analyzed for this study are 2000). not representative of the dominant freshwater in- An alternative explanation is that the trendrep- puts to the WIS during the Late Cretaceous. resents a temporal di¡erence in sample age. Our A relatively high 87Sr/86Sr ratio (0.709926) in a samples were collectedwithin a single ammonite freshwater unionidbivalve from the late Maas- zone, estimatedto span V1 Ma of time (Kau¡- trichtian was measuredby McArthur et al. man et al., 1993). We reject the possibility that the (1994) in their detailed investigation of Sr isotope trendin 87Sr/86Sr ratios is due to a monotonic systematics in the last 30 Ma of the Cretaceous change in the age of the samples re£ecting a Western Interior of North America. McArthur et change in seawater values of the ratio because al. (1994) did not have the distribution of environ- the brackish fauna is either coeval or younger ments within a single ammonite zone that charac- (but still within the same ammonite biozone) terizes the present study, and concluded that

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Table 3 Paleosalinities in the WIS Open-ocean O¡shore Interior Nearshore Interior Brackish Freshwater

[Sr]sw :[Sr]fw =10:1 87Sr/86Sr 0.707839a 0.707808a 0.707763a 0.707677a 0.705517b Salinity (x)35c 30.8e 26.2e 20d 0 [Sr]sw :[Sr]fw =5:1 87Sr/86Sr 0.707839a 0.707808a 0.707763a 0.707677a 0.706597b Salinity (x)35c 31e 26.4e 20d 0 [Sr]sw :[Sr]fw =2:1 87Sr/86Sr 0.707839a 0.707808a 0.707763a 0.707677a 0.707245b Salinity (x)35c 31.5e 27.4e 20d 0 a Mean 87Sr/86Sr value in indicated biofacies (Table 2). b Calculatedfrom linear trendof 87Sr/86Sr vs. 1/[Sr] using marine andbrackish values, assuming sur¢cial estuarine mixing. c;d Assumedmarine andbrackish salinities. e Equivalent salinities determined from values of 1/[Sr] calculated from 87Sr/86Sr vs. 1/[Sr] relationship. freshwater entering the seaway hada higher 87Sr/ seawater ratio even in water of fairly low salinity 86Sr ratio than seawater. They further concluded (Schmitz et al., 1991). If we use as the marine that the analytical uncertainty in the Sr isotope 87Sr/86Sr the value obtainedfrom the average of ratios precluded seeing a perceptible e¡ect of the open-ocean biofacies samples (0.707839), the less-than-fully-marine salinity on the 87Sr/86Sr ra- lowest 87Sr/86Sr measuredin the rock leachates as tios. We conclude in contrast that the di¡erence the freshwater 87Sr/86Sr (0.707165) andtake the between the 87Sr/86Sr ratios of the open-ocean and di¡erence between the marine and freshwater Sr brackish assemblages (vSr of V160 þ 40U1036) concentration to be 10:1, within the range of val- is signi¢cant. ues in the Recent, we obtain a salinity of V9x for the brackish facies. 4.3. Estimating paleosalinities in the Late The brackish biofacies contains abundant ceph- Cretaceous Western Interior Seaway alopods that presumably were living in this environment, as suggestedby the similarity of their Sr Using 87Sr/86Sr ratios to estimate paleosalinities isotope ratios with other organisms from this requires (1) a signi¢cant di¡erence between ma- same environment. Modern cephalopods poorly rine andfreshwater Sr concentrations andisotope tolerate salinities below about 20x (Forsythe ratios, (2) knowledge of the endmember values andVan Heukelem, 1987 ; S. von Boletzky, per- and(3) conservative behavior of Sr with respect sonal communication 1999). If we take this value to freshwater^saltwater mixing. In such condi- (20x) as a lower limit on the salinity of the tions, a plot of 87Sr/86Sr vs. 1/[Sr] will yielda brackish biofacies, the dominant freshwater in- straight line between the marine andfreshwater put to the WIS must have hada 87Sr/86Sr of endmember values. V0.70552 if the ratio of marine/freshwater Sr In this study, we are unable to de¢ne the fresh- concentration was 10:1 (Table 3). This 87Sr/86Sr water endmember using the unionids and the ratio for the freshwater input is considerably freshwater andmarine Sr concentrations are un- lower than the lowest value observedin the known. In the present ocean, the Sr concentration unionidsamples (0.707615) andis also lower (V90 Wmol/kg) is 10^100 times greater than river- than the lowest value measuredin the rock leach- water Sr concentrations (Brass andTurekian, ates (0.707165). 1974; Palmer and Edmond, 1989). Owing to the Indeed, if we take the value of 0.707165 as rep- large di¡erence in Sr concentration of the end- resentative of the freshwater input to the WIS, the members, the 87Sr/86Sr ratio in water of less ratio of marine/freshwater Sr concentration re- than fully marine salinity shouldapproach the quiredto produce20 x salinity in the brackish

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facies is V2:1. By ¢xing the brackish salinity at 20x, the estimatedsalinities for the nearshore ando¡shore Interior facies are 26.2^27.4 x and 30.8^31.5x, respectively (Table 3). Fig. 5a^c shows the extrapolatedvalues of the freshwater 87Sr/86Sr ratios for mixing lines that assume di¡erent ratios of marine/freshwater Sr anduse our observedaverage 87Sr/86Sr of the open-ocean biofacies (mean = 0.707839, Table 2) andthe con- straint that the brackish biofacies (mean 87Sr/ 86Sr = 0.707677, Table 2) must be at least 20x salinity. Values for the freshwater 87Sr/86Sr range from 0.707245 to 0.705517 for marine/freshwater Sr ratios of 2:1 to 10:1 (Table 3). We note that even lower freshwater 87Sr/86Sr values wouldbe requiredif the brackish biofacies corresponded to salinities greater than 20x. The explanation most consistent with the data is that the mixing curve suggestedby the trendin 87Sr/86Sr in the di¡erent biofacies has a freshwater endmember that is Sr-rich compared with modern river water. One possibility is that the rocks being weatheredat that time were carbonate-rich, with abundant marine fossils and Sr concentrations andisotope ratios much like those of the rock samples analyzed(5800^7000 ppm Sr; Table 2). However, as notedabove, if the Sr carriedby rivers entering the Late Cretaceous WIS hada 87Sr/86Sr typical of the lowest value added in our leaching experiments of the matrix samples (0.707165, Table 2), the riverine Sr concentration must have been about half that of seawater to produce brackish salinities of at least 20x. There is no evidence in the Recent of such Sr-rich fresh- waters. An alternative explanation is that the brackish Fig. 5. 87Sr/86Sr ratio vs. 1/[Sr] for mixtures of seawater and WIS environment is produced by the subterra- freshwater with di¡erent Sr concentrations. All plots use the nean mixing of groundwater and seawater, with V W x present-day Sr concentration of 90 mol Sr/kg in 35 the Sr concentration andisotope ratio of the seawater (Brass andTurekian, 1974 ) anda 87Sr/86Sr ratio of 0.707839, the average measuredvalue of the open-ocean sam- brackish water resulting from both mixing and ples (Table 2). The plots are generatedfor varying Sr con- reaction (i.e. leaching or dissolution of the aquifer centrations of freshwater relative to seawater, andlines are solids). Basu et al. (2001) have documented sub- ¢t through the marine (M) andbrackish (B) data,assuming stantial elevations (factors of 10) of Sr even in x that the latter represents 20 salinity. (A) [Sr]sw :[Sr]fw = low-salinity groundwater from the Bengal Basin, 10:1. (B) [Sr]sw :[Sr]fw = 5:1. (C) [Sr]sw :[Sr]fw = 2:1. The o¡- shore Interior (OI) andnearshore Interior (NI) dataare plot- relative to concentrations in the Ganges and tedon the lines as well. The plottedpoints represent the Brahmaputra rivers. Similar elevations were seen means of the 87Sr/86Sr in a given biofacies andthe vertical in Ra (Sarin et al., 1990). Groundwater inputs are bars denote the range of values for each group. being increasingly recognizedas important in af-

PALAEO 2988 3-1-03 58 J.K. Cochran et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 191 (2003) 45^64 fecting coastal waters of the modern ocean. know the Sr concentration or isotopic composi- Moore (1996), for example, documented high ac- tion of fresh groundwater that might have mixed tivities of Ra in shallow shelf waters of the South with seawater in the subterranean estuary of the Atlantic Bight (eastern USA), relative to open- WIS, we take as a starting point for calculation ocean values. Although the various Ra isotopes [Sr] of 1 WM (average of modern rivers) and 87Sr/ can be introduced to groundwater via recoil asso- 86Sr of 0.707848 (average of unionidvalues, Table ciatedwith their production( Krishnaswami et al., 2). If we mix this water with seawater of 90 WMSr 1982), Shaw et al. (1998) documented that Ba, and 87Sr/86Sr of 0.707839 (average of open-ocean which has no comparable input mechanism, is biofacies, Table 2) to produce 20x brackish also enriched in the South Atlantic Bight. Indeed, water, we can calculate the amount of Sr that Basu et al. (2001) arguedthat groundwater inputs must have been added to solution to produce of Sr are important in the oceanic mass balance of the isotopic signature recorded in the brackish Sr. biofacies (87Sr/86Sr = 0.707681, Table 2). The Moore (1999) suggestedthe concept of a ‘sub- 87Sr/86Sr of the brackish mixture is given by: terranean estuary’ in which groundwater and salt- ½SrfwWf fwWRfw þ½SrswWð13f fwÞWRsw þ½SrrockWRrock water mix andinteract with aquifer solidsto ele- Rmix ¼ ð1Þ f W½Sr þð13f ÞW½Srsw þ½Sr vate concentrations of elements such as Ra and fw fw fw rock 87 86 87 86 Ba. Bryant et al. (1995) documented Sr/ Sr val- where Rmix;fw;sw;rock are the Sr/ Sr in the brack- ues in mollusks collectedin the Mississippi Sound ish water, freshwater, seawater andreleasedfrom that fell o¡ the mixing line determined by sea- aquifer solids, respectively, ffw is the fraction of water andthe Mississippi River. They attributed freshwater in the brackish mixture (0.43 for a sa- these results to local input of groundwater whose linity of 20x), and[Sr] fw;sw;rock are the Wmol Sr Sr isotope composition was imprintedby reac- in freshwater, seawater andreleasedfrom aquifer tions with older, carbonate-rich rocks. We specu- solids per kg of water. Solving for [Sr]rock using late that such a process may have been operating the leached 87Sr/86Sr from Elk Butte or Mobridge in the Late Cretaceous WIS as well. By this mech- Member material produces the 87Sr/86Sr observed anism, the brackish facies represents an environ- in the brackish facies by adding V16 Wmol Elk ment in which Sr-rich groundwater and seawater Butte Sr or V45 Wmol Mobridge Sr per kg of mixedin the subterranean estuary andthe water groundwater. If the freshwater endmember has a was discharged with a greater-than-expected Sr lower 87Sr/86Sr ratio than the average of the concentration, relative to classical estuarine mix- unionids used in the calculation, the amount of ing of freshwater andseawater in a sur¢cial estu- Sr releasedfrom the rocks to yieldthe 87Sr/86Sr of ary. the brackish water wouldbe less. However, the We can use our data to test the possibility that di¡erence is not large (only V4% if the fresh- reactions in the subterranean estuary can produce water 87Sr/86Sr is taken to be 0.707165) because the Sr concentration andisotopic composition of the ‘seawater’ and‘rock’ terms dominatein Eq. 1. the brackish biofacies (Fig. 6). We assume that The Elk Butte andMobridge Member rocks are the rocks supplying Sr to solution during mixing Sr-rich, and as a consequence the calculated addi- of seawater andgroundwaterhave a Sr concen- tions of Sr from them to the groundwater require tration andisotopic composition typical of the leaching from only about 0.2^0.6 g of rock per kg Elk Butte andMobridgemembers. The Sr con- of groundwater, using the rock Sr concentrations centrations are V7400 ppm (85 Wmol Sr/g) in the of Table 2. An aquifer with about 20% water Elk Butte and V5800 ppm (66 Wmol/g) in the content (porosity V0.4) wouldhave a rock/water Mobridge (Table 2). The leaching experiments ratio of about 4000 g rock/kg water, andthe cal- that releasedsigni¢cant quantities of Sr from culatedrelease of Sr using our datathus repre- this material yielded 87Sr/86Sr in solution of sents dissolution of only 0.005^0.015% of the 0.707165 for the Elk Butte and0.707500 for the aquifer solids. The resulting Sr concentration in Mobridge material (Table 2). Although we do not the brackish mixture is V68 WM if Elk Butte

PALAEO 2988 3-1-03 J.K. Cochran et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 191 (2003) 45^64 59 rocks are reacting or V96 WM if Mobridge material is supplying the Sr. For comparison, simple mixing of freshwater (with 1 WM Sr) with seawater (with 35x salinity and90 WM Sr) to attain 20x wouldyielda Sr concentration of 52 WM. We note that the processes supplying Sr to the waters of the subterranean estuary neednot have been (and likely were not) in steady state. Indeed, the subterranean estuary has been viewedas a dynamic zone that responds to forcing factors such as sea-level change (Moore, 1999). The rocks comprising the coastal aquifer system in the Late Cretaceous WIS were deposited earlier and ac- x quireda lower 87Sr/86Sr ratio typical of the ocean Fig. 6. Schematic diagram showing the production of 20 brackish water by mixing of seawater andgroundwater in when the carbonates they contain were formed. the WIS. The Sr concentrations of the freshwater andsalt- Subsequent diagenenesis, for example during water endmembers are assumed to be the present world-aver- times of rising sea level when the aquifer was pro- age values (Palmer and Edmond, 1989) andthe ( 87Sr/86Sr) gressively invaded by seawater, could have added are the mean values measuredin the unionidsandthe ma- 87 86 Sr to the aquifer solids. During periods of low- rine biofacies, respectively. The ( Sr/ Sr) releasedfrom the aquifer solids is from our leaching experiments that released ering sea level, such as the Late Cretaceous, the signi¢cant Sr to solution (Table 2) andthe [Sr] releasedfrom aquifer salinities lower as the saltwater lens re- the aquifer solids is that necessary to produce the observed treats andSr can be releasedthrough dissolution, (87Sr/86Sr) in the brackish biofacies. See text for discussion. leaching or desorption. The application of the subterranean estuary that are most exposedto the seepage of brackish concept to producing brackish salinities in the groundwater may thus be isotopically lighter than Late Cretaceous WIS has several implications. the nekton (depending on mixing in the water First, because the freshwater endmember has little column), as observedby Wright (1987). in£uence on the Sr concentration or isotopic com- Finally, the concept of the discharge of brack- position of the mixture, it can readily have the ish water produced in the subterranean estuary isotopic composition suggestedby our analyses may explain both the periodic kill-o¡s of fauna of the unionids (Table 2, Fig. 6) and indeed, we andthe formation of concretions that preserve have usedthe average unionid 87Sr/86Sr value in them in the WIS (Waage, 1968). Submarine the calculations above. This implies that the 87Sr/ groundwater discharge has the potential to locally 86Sr of the freshwater entering the WIS by rivers alter salinity or dissolved oxygen concentrations couldwell be greater than that of the fully marine andleadto mortality of organisms. Moreover, waters of the Interior, as suggestedby some of as notedby Berner (1971) andothers, the alkaline our unionidanalyses andas postulatedby McAr- putrefaction of decomposing organic matter may thur et al. (1994). We suggest, however, that the cause the precipitation of calcium soaps or salts Late Cretaceous WIS brackish environment was that subsequently convert to calcitic concretions. not createdby normal, sur¢cial estuarine mixing Reactions with aquifer solids can release the dis- of freshwater andseawater, but rather that such solvedCa necessary for such precipitation, and mixing took place in the subterranean estuary and groundwater £ow increases the rate of concretion the resultant brackish water was discharged to the formation. nearshore environment. We now compare our results with the determi- Secondly, mixing in the subterranean estuary nations of strontium isotopes in fossils from the produces brackish water that has an isotopically WIS made by Whittaker andKyser (1993) and lighter N18O signature than fully marine waters McArthur et al. (1994). Whittaker andKyser (see discussion below). Epifauna and infauna (1993) observedthat the Sr isotope ratio of the

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WIS was similar to the contemporary ocean value except for certain specimens younger than 80 Ma. Such specimens had 87Sr/86Sr ratios (as well as N18O values; Whittaker et al., 1987) that were lower than open-ocean seawater, and Whittaker andKyser (1993) attributedthese values to dilution with freshwater. In contrast, as notedabove, McArthur et al. (1994) analyzedfreshwater fauna, including Unio sp., from the late Maastrichtian andfoundhigher 87Sr/86Sr values than those of fossils from the WIS. Their interpretation of the variation of 87Sr/86Sr vs. salinity was that the isotope ratio was increasing towardlower-salinity Fig. 7. 87Sr/86Sr vs. N18O of shell carbonate from the open- water. McArthur et al. (1994) concluded that, ocean andWIS biofacies ( R2 = 0.14). for ratios of marine/freshwater Sr concentrations of V100:1 and V30:1, variation in the 87Sr/86Sr observedtrendis consistent with increases in tem- ratio from 35 to 20x salinity couldnot be de- perature or decreases in N18O of the water (or tectedwithin the analytical uncertainty of the iso- both) from marine to brackish environments. tope ratio. This analysis depended on the value of There is also a weak trend( R2 = 0.14) of lower the freshwater 87Sr/86Sr ratio. However, McAr- 87Sr/86Sr ratios with lighter N18O(Fig. 7) that is thur et al. (1994) did not have the paleoenviron- consistent with mixing of seawater andfresh- mental control of the present study such that var- water. The N18O values of the unionids are indeed iation in 87Sr/86Sr within a single time periodin light, V317 to 320x, although, basedon the the WIS could be seen. Indeed, the variation in Sr isotope data, we have argued that these shells 87Sr/86Sr observedamong the cephalopodfossils were not recording the dominant freshwater input collectedin the di¡erentbiofacies sampledin the to the WIS. present study is equivalent to approximately 6 Ma Earlier studies of N18O in Cretaceous fossil mol- in the Late Cretaceous trendof marine 87Sr/86Sr lusks (Tourtelot andRye, 1969; Rye andSom- vs. time as de¢ned by McArthur et al. (1994). mer, 1980; Wright, 1987) foundanomalously Thus our results suggest that the use of the 87Sr/ high temperatures when paleotemperatures were 86Sr ratio to assign ages with an accuracy of less calculatedfrom oxygen isotope data.If we as- than several Ma to fossils from the Late Creta- sume that the marine-to-brackish trendin N18O ceous WIS must take into account the variation in solely re£ects water temperature changes, we can paleoenvironment. Similar cautionary notes on calculate the water temperature provided the N18O the use of the 87Sr/86Sr ratio in chronostratigra- of the water is known. We have calculatedtem- 18 phy in settings such as the WIS have been citedby peratures using a N Owater = 31x, a value that is Bryant et al. (1995) and Vaiani (2000). often taken as typical of the Late Cretaceous open ocean (e.g. Fatherree et al., 1998). The aragonite^ 4.4. Oxygen and carbon isotopes in the WIS water equation of Grossman andKu (1986) was usedfor all samples except the belemnites, which The N18O values of the samples show signi¢cant are calcite andrequire the calcite^water equation variation among the di¡erent biofacies, with light- (Craig, 1965; O’Neil et al., 1969). The results er values in the brackish environment (Fig. 3). show values of 10.4^11.6‡C in the o¡shore Interi- There is no correlation between N13C and N18O or waters, 16.3^24.7‡C in the shallow, nearshore (R2 = 0.002). Assuming equilibrium fractionation, Interior and18.7^28.6‡C in the brackish environ- the N18O in the specimens is dependent on both ment (Fig. 8). None of the calculatedtempera- the temperature of formation andthe N18O of the tures is as anomalously high as those obtained water in which the carbonate forms. Thus the by Tourtelot andRye (1969) and Wright (1987),

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WIS, with values ranging, for example, from V0 to 32x. Because the rock/water ratios are typi- cally high in aquifers (see above), isotopic exchange between the solids and solution in the subterranean estuary wouldhave imprintedthe water with the N18O characteristic of the dominant oxygen reservoir (i.e. the solids) and the N18O values of the brackish mix wouldbe greater than predicted from simple mixing of freshwater with N18O=317x andseawater with N18O=31x. Thus, mixing in the subterranean estuary, coupled with reaction between solution andaquifer solids Fig. 8. Water temperatures determined from N18O analyses of as implied by the Sr isotope data, produced N18 shell material. Calculations assume that the O of the N18O values that were heavier than those re- water is 31x (Fatherree et al., 1998). Values are slightly water o¡set in each group to improve readability. sulting from normal estuarine mixing. We conclude that, as with the Sr isotope ratios, the N18O values of the unionidmussels sampledare although the temperature anomalies in their stud- not representative of the N18O of the dominant ies generally were foundin inoceramidbivalves freshwater source to the WIS. rather than cephalopods or other mollusks. In her study of N18O in di¡erent habitat groups Although the calculatedoxygen isotope temper- in the early Maastrichtian, Wright (1987) noticed atures are reasonable within andbetween the dif- that the epifauna was characterizedby lighter ferent habitats, salinity variations in the WIS N18O relative to nektonic cephalopods and to in- wouldlikely result in geographic variation of fauna. She explainedthis as the result of a dense, 18 N Owater. For example, the temperature calcu- warm water mass formedin the eastern portion of latedfor the brackish group averages about the basin that sank into the basin interior. We do 23‡C, using the estimatedopen-ocean Cretaceous not have su⁄cient detail in the samples analyzed 18 N Owater value of 31x (Fig. 8). If the Late Cre- to represent epifauna andinfauna signi¢cantly. taceous WIS hadtemperatures throughout that However, we note that, much like the variation were comparable to those calculatedfor the o¡- in 87Sr/86Sr ratios, the benthos andnekton donot shore biofacies (V12‡C) we estimate that the display signi¢cant o¡sets in their N18O values in brackish water hada N18OofV33.5x. In con- any given habitat group where both types are trast, if the N18O of the freshwater that mixedwith represented(i.e. nearshore Interior andbrackish). seawater to produce the water characteristic of It is possible that the water column in these loca- the brackish biofacies was as light as the values tions was too shallow to show much strati¢cation observedin the unionids(e.g. 317x), then with respect to temperature or salinity. 18 N Owater values of V38x can be estimated for salinities of V20x. This value, when used 18 as N Owater in the temperature equation, yields 5. Conclusions unrealistic (negative) temperatures for the brackish group. This study demonstrates that well-preserved, We have suggestedthat the brackish water of stratigraphically controlledfossils from the Late the WIS was produced by groundwater^seawater Cretaceous Western Interior Seaway of North mixing. To the extent that the carbonate fraction America preserve a recordof environmental var- of the aquifer solids was comprised of the fossils iation recorded in Sr and O isotopes. The 87Sr/ of carbonate-secreting organisms, the N18Oof 86Sr ratios in such samples display signi¢cant var- such material likely re£ectedequilibrium with sea- iation, with values decreasing from 0.707839 þ water at temperatures characteristic of the earlier 0.000024 in fully marine to 0.707677 þ 0.000036

PALAEO 2988 3-1-03 62 J.K. Cochran et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 191 (2003) 45^64 in brackish environments. When comparedwith might be expectedfrom mixing of seawater with the well-documented temporal variation in sea- isotopically light freshwater, then both the calcu- water 87Sr/86Sr in the Late Cretaceous, the ob- latedtemperatures in the brackish group andthe servedrange suggests that using the open-ocean temperature gradient from the o¡shore Interior to 87Sr/86Sr curve to date fossils from the Late Creta- the brackish environment will be decreased. How- ceous WIS shouldbe appliedwith caution and ever, as with the Sr isotopes, simple mixing of must take into account the paleoenvironments freshwater with the N18O seen in the freshwater sampled. mussels with open-ocean seawater to produce It is not possible to evaluate the freshwater 20x brackish water yields unrealistic tempera- 87Sr/86Sr endmember by direct measurement of tures for the brackish environment. Either the freshwater mussels, either because the specimens dominant freshwater entering the seaway is heav- analyzed do not represent the dominant fresh- ier than that suggestedby the freshwater mussels water input to the seaway, or do not correspond or the N18O of the brackish water has been modi- to the same time because they lack the biostrati- ¢edby isotopic exchange with aquifer solidsin the graphic control of the specimens from the Sea- subterranean estuary. way. If a lower limit of V20x salinity is set for the brackish biofacies basedon salinity tolerances of modern cephalopods (Forsythe andVan Acknowledgements Heukelem, 1987), the 87Sr/86Sr in the dominant freshwater input to the WIS is estimatedto be The strontium isotope measurements were 0.707245^0.705517 for ratios of the marine/fresh- made while J.K.C. was on sabbatical at the water Sr concentration of 2:1 to 10:1, respec- Centre Europe¤en de Recherche et d’Enseigene- tively. The former is more consistent with the ment des Ge¤osciences de l’Environnement (CER- likely 87Sr/86Sr ratios of rocks being weathered EGE) in Aix-en-Provence, France. Support from at the time, but requires freshwater with a very the State University of New York, the Centre Na- high Sr concentration comparedwith modernriv- tional de Recherche Scienti¢que (CNRS) and the ers. We conclude that the trend in 87Sr/86Sr in our American Museum of Natural History is grate- samples can be best explainedby the mixing of fully acknowledged. We thank Y. Noack and D. freshwater andseawater in a coastal aquifer and Borshneck for the X-ray di¡raction analyses and seepage of the brackish water into the nearshore K. Sarg for assistance with SEM. Samples were WIS. Through reactions with aquifer solids in this kindly made available to us by W.A. Cobban ‘subterranean estuary’, the resultant brackish (USGS, Denver), andT. White andC. MacClin- water has a Sr concentration enhancedrelative tock (Yale Peabody Museum). This manuscript to the conservative behavior of Sr seen in the has bene¢tedfrom discussions with our colleague sur¢cial estuarine mixing of freshwater andsea- W.A. Cobban andthoughtful reviews by P. Har- water. Moreover, the 87Sr/86Sr ratio of the brack- ries andF. Kenig. ish mix re£ects contributions from both seawater andaquifer solidsandthus is lower than might be expectedfrom sur¢cial estuarine mixing. References Values of N18O decrease from the o¡shore In- terior to brackish biofacies, consistent with a de- Albare'de, F., Michard, A., 1987. Evidence for slowly changing 18 crease in N O with decreasing salinity and/or in- 87Sr/86Sr in runo¡ from freshwater limestones of southern creasing water temperature. If the e¡ect is entirely France. Chem. Geol. 64, 55^65. due to temperature, values increase from V10^ Albare'de, F., Michard, A., Minster, J.F., Michard, G., 1981. 87 86 12‡C in the o¡shore Interior waters of the seaway Sr/ Sr ratios in hydrothermal waters and deposits from the East Paci¢c Rise at 21‡N. Earth Planet. Sci. Lett. 55, to 19^28‡C in the brackish environment. If the 229^236. 18 N O of the brackish water is isotopically lighter Alibert, C., McCulloch, M.T., 1997. Strontium/calcium ratios than the values in Late Cretaceous seawater, as in modern Porites corals from the Great Barrier reef as a

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