paleos in ancient brackish water systems deter s...

Pergamon Geocbimica et Cosmochimica Acta, Vol. 61, No. 10, pp. 2105-2118, 1997

Copyright 0 1997 Elsevier Science Ltd printed in the USA. All rights reserved

0016-7037/97 $17.00 + .OO

PII SOO16-7037( 97) 00073-2

Paleosalinities in ancient brackish water systems determined by *‘Sr/?3r ratios in carbonate fossils: A case study from the Western Canada Sedimentary Basin

C. HOLMDEN*, R. A. CREASER, and K. MUEHLENBACHS Department of Earth and Atmospheric Sciences, l-26 Earth Sciences Building, University of Alberta, Edmonton, Alberta T6G 2E3, Canada

(Received September 13, 1995; accepted in revised form February 6, 1997)

Abstract-Two strategies for determining paleosalinities in ancient brackish water deposits are presented based on the 87Sr/86Sr ratios in well preserved carbonate fossil shells. The relative contributions of seawater and freshwater can be determined for shells of unknown or presumed ecological affinity by comparing their 87Sr/86Sr ratios to contemporaneous marine and fluviatile sources. The nearly one hundredfold difference in Sr concentration between seawater (7.7 ppm) and freshwater (median = 0.071 ppm) dictates that mixing relations define hyperbolae. Paleosalinities are most precisely determined along the freshwater asymptote (< 15%0) because these waters have the lower Sr concentration, and, therefore, display the greatest rate of change of *‘Sr/*‘?jr with salinity. Paleosalinities greater than 15%0 are resolvable if the 87Sr/86Sr or Sr concentration in the freshwater mixing endmember is greater than the 50th percentile for present-day Sr concentrations in world rivers and lakes.

A second technique utilizes both 87Sr/86Sr and Sr/Ca from a single fossil species and allows the brackish water hypothesis to be tested graphically on a plot of 87Sr/86Sr-Ca/Sr. Linear correlations on this plot are evidence for two-component mixing. Measured Sr/Ca in mollusc shells are related to the original Sr/Ca in the habitat waters by a species specific Sr distribution coefficient (&,). For the case of seawater-freshwater mixing, Ds, is fixed in that seawater must plot on the water mixing line. If the transformation between shell and water mixing lines yields a D, within the range of modem values (0.2-0.3)) a brackish water habitat is implied. Examples from purported brackish water deposits of the early Cretaceous Mannville Group, Canada, illustrate the potential uses of 87Sr/86Sr, Sr, and SrlCa for determining paleosalinities in ancient estuarine and estuarine-like deposits, and the importance of the paleohydrological perspective as a factor influencing the interpretation of ancient depositional environ- me& Cop&ht 0 i997 Elsevier Science Ltd

1. INTRODUCTION

Brackish water systems are difficult to recognize in the sedimentary rock record because sedimentological and paleontological methods are relatively insensitive to salinity. To sub- stantiate brackish water deposition, a wide variety of indicators are used including: ( 1) inferences on the ecological affinities of various macrofauna or microfauna (Pickerill and Brenchley, 1991)) (2) palynology (Banejee and Davies 1988), (3) ichnology (Pemberton and Wightman, 1992), (4) identification of syneresis cracks (Plummer and Gostin, 1981), (5) basin geometry (Zaitlin et al., 1994), and (6) presence of tidal structures (Howard and Frey, 1973; Thomas et al., 1987). Even when ancient sediments can be shown to have brackish water affinities by these methods, what is implied is that neither fully marine nor fully freshwater conditions prevailed during sediment deposition. Quanti- fication of the magnitude of any salinity reduction (relative to contemporaneous seawater) is beyond the capacity of conventional sedimentological and paleontological techniques (Barnes, 1989).

In this paper we show that strontium isotopes can be used to determine accurate and precise paleosalinities in ancient estuarine and estuarine-like systems using well preserved

*Present address: Department of Geological Sciences, University of Saskatchewan, Saskatoon SK S7N 5E2, Canada.

carbonate fossils and limestones. Two strategies are presented using examples from the early Cretaceous Mannville Group of the Western Canada Sedimentary Basin. The first is a comparative technique which utilizes well preserved marine and freshwater carbonate fossils to determine a reference (seawater-freshwater) mixing hyperbola for the depositional system of interest. Paleosalinities for brackish water fossil shells are determined by comparison of their shell 87Sr/86Sr ratios with the reference mixing hyperbola in “Sr/ 86Sr-Sr (salinity) space. The second strategy is a graphical technique which utilizes shell SrlCa ratios in addition to 87Sr/86Sr. The 87Sr/s6Sr-Ca/Sr systematics of shells formed in the seawater-freshwater mixing zone are linearly correlated and constitute the inverse form of a two-component mixing hyperbola. The equation of the hyperbola can be used to evaluate whether seawater constitutes a mixing endmember and, therefore, whether the sediments truly represent brackish water deposition. The principal limitation of using strontium isotopes to determine paleosalinities is that well preserved fossil material is required.

2. PREVIOUS STUDIES OF ESTUARINE PALEOSALINITY

Geochemical methods for distinguishing marine and freshwater deposits in ancient sedimentary rocks have utilized differences in B concentration, and oxygen and carbon

2105

2106 C. Holmden et al.

isotopes preserved in shales and carbonates, respectively. For quantification of estuarine paleosalinity the elemental proxy must also be conservative in the seawater-freshwater mixing zone. Boron is conservative (Liddicoat, 1983 ) and displays large differences in concentration between marine (4.5 ppm) and freshwaters (0.01 ppm) . The decrease in B content of estuarine waters is a linear function of the frac tional dilution of seawater by freshwater. This relationship between B concentration and salinity was thought to be preserved in estuarine shales (Frederickson and Reynolds, 1959; Landergren, 1963 ) : clay minerals being a major geochemical sink for B (Schwartz et al., 1969). Subsequent work re- vealed that additional factors influenced B uptake by clay minerals, compromising the reliability of B in shales as a paleosalinity proxy (Harder, 1970).

The generally contrasting S’*O values between seawater ( O%O, SMOW) and freshwaters ( > -25%0), and 6 “C differences in dissolved inorganic carbon (DIC) between seawater ( -O%O, PDB) and freshwaters (-12 to -32%0), is the basis for distinguishing marine and freshwater sediments through stable isotope analysis of carbonate fossils and limestones (Clayton and Degens, 1959; Keith et al., 1964; Keith and Weber, 1964). Several authors investigated an extension of this approach to cover transitional brackish waters (Sacket and Moore, 1966; Mook and Vogel, 1968; Mook, 1970), demonstrating the approximately conservative behavior of oxygen and carbon isotopes in the seawater-freshwater mixing zone. Several factors limit the widespread application of oxygen and carbon isotopes as proxy measures of paleosalinity: ( 1) S “0 values for marine and freshwaters can be similar in low latitude tropical settings, (2) accuracy is lim- ited by differences in original water temperature for different parts of the estuarine system (causing an uncertainty of I - 2%0 on carbonate-based water S I80 values), (3) evaporation can induce nonconservative effects for S I80 in the seawater- freshwater mixing zone if the climate is dry or water residence times are long, (4) freshwaters can have marine-like 6 13C values if there are older marine carbonates in the watershed, or if equilibration with atmospheric CO2 occurs, and (5) local differences in biological productivity, or rates of organic C respiration and oxidation can induce nonconservative effects in brackish water 6 13C values. Accordingly, there have been few attempts to use oxygen and carbon isotopes to constrain paleosalinity in ancient depositional environments (Lloyd, 1965; Weber et al. 1965; Dodd and Stanton, 1975; Holmden et al., 1997).

The conservative behavior of dissolved Sr and its isotopic variation with salinity has been demonstrated in modem estuarine systems (Ingram and Sloan, 1992; Andersson et al., 1992). The Sr content of brackish water, formed by mixing of high Sr marine water (7.7 ppm) with low Sr freshwater ( -0.071 ppm), varies linearly with salinity. The 87Sr/86Sr ratio of dissolved Sr also varies with salinity because contemporaneous seawater and freshwater generally have contrasting isotopic compositions. Although the ocean has a uniform 87Sr/86Sr at any time because of the long oceanic residence time of Sr (5 X 10’ years), rivers vary widely in

87Sr/86Sr reflecting the isotopic composition of the bedrock and sediment over which they flow (Wadleigh et al.,1985;

Goldstein and Jacobsen, 1987; Palmer and Edmond, 1992). The strontium isotope system has several advantages over previous geochemical methods of paleosalinty determination. Unlike oxygen and carbon isotopes, there is no isotope effect related to the partitioning of Sr into carbonate, so that carbonate fossils record the 87Sr/8hSr of past waters directly. This fact, coupled with the relatively unreactive nature of dissolved aqueous Sr (similar to Ca), enables continental freshwaters to maintain 87Sr/86Sr ratios that reflect contributions from bedrock sources over large distances.

Estuarine paleosalinities may be determined from “Sr/ XhSr ratios in brackish water carbonate fossils, if the 87Sr/ ?Sr and Sr concentration of the seawater and freshwater mixing endmembers are known. Ingram and DePaolo ( 1993) determined paleosalinities for the San Francisco Bay estuary over the past 4300 years by comparing 87Sr/86Sr ratios in estuarine mollusc shells with a reference mixing hyperbola in *‘Sr/‘?lr-Salinity space. The mixing hyperbola was calculated from present-day measurements of “Sr/*‘?jr and Sr in fluvial and coastal marine waters entering the estuary. Similarly, Rosenthal et al. ( 1989) used present-day 87Sr/86Sr and Sr/Ca ratios for fresh and saline water inputs to Israeli lakes of the Jordan River Rift Valley to determine paleosalinities from Quatemary fossil molluscs. The challenge in applying strontium isotopes in older geologic systems is that younger, representative waters are not available for measurement. Therefore, the regional surface paleohydrology must be inferred from isotopic analyses of marine and freshwater carbonate fossils and limestones, before paleosalinities can be determined.

3. THE MANNVILLE GROUP

The early Cretaceous Mannville Group comprises some of the earliest sediments deposited within the Western Canada Sedimentary Basin (WCSB) in Alberta (Hayes et al., 1994). Tectonic loading of the western North American margin initiated development of a foreland basin by late Jurassic (Monger, 1989) and by early Cretaceous the Western Inte- rior Seaway had begun to form with water masses of the Boreal ocean transgressing into Alberta from the north (Koke and Stelck, 1984; Leckie and Smith, 1992). To the south, the foreland basin hosted a major north-flowing river system that drained highlands of the emerging western Cor- dillera (McGookey et al., 1972; Hayes et al., 1994). The early Cretaceous Mannville Group was deposited and re- worked in this complex hydrological interface between northerly flowing rivers of the Foreland Basin Drainage Sys- tem and southerly transgressing marine waters of the epicontinental Moosebar-Clearwater Sea (Fig. 1). Paleo-relief in the region was dominated by several north-south trending valleys (and ridges) incised into underlying Paleozoic carbonates during the previous lowstand of sea level (Hayes et al., 1994; Ranger et al., 1994; Fig. 2).

Although siliciclastics dominate Mannville Group lithologies, a conspicuous -15m interval at the top of the lower Mannville Group, known informally as the Ostracode Zone (or Calcareous Member), records a shift to finer-grained lithologies comprising pyritic dark shales, thin carbonates,

Determination of paleosalinities by 87Sr/86Sr ratios in fossil shell 2107

Fig. 1. Paleogeographic map of early Cretaceous North America showing the location of the Mannville Group, the southerly extent of the early Albian Moosebar-Clearwater Sea (Koke and Stelck, 1984), and the Foreland Basin Drainage System. Figure modified from Kauffman ( 1984) and paleolatitudes from Irving et al. (1993).

and coquinas of gastropods and pelecypods (Banerjee, 1990). The unique carbonate and fossil bearing strata of the Ostracode Zone provide an important stratigraphic reference point for subsurface correlation in the WCSB. Although the Ostracode Zone is most often identified with fossiliferous, carbonate bearing, sediments west of the Wainwright Ridge (Fig. 2)) conspicuous accumulations of mollusc fossils are also found within stratigraphically equivalent sections of the Wabasca sub-basin (developed in a pocket of the Wain- wright Ridge), and the McMurray sub-basin east of the Wainwright Ridge (Fig. 2).

The flooding event responsible for deposition of Ostracode Zone and equivalent strata has been linked to a transgressive pulse of the Moosebar-Clearwater sea, which encroached southwards along the incised valley systems (Banejee, 1990). Although the earliest studies favoured a predomi- nantly lacustrine paleoenvironment based on the occurrence of fossil ostracodes, charophytes, and freshwater gastropods (Loranger, 1951; Glaister, 1959; McLean and Wall, 1981), many recent studies have documented an increasing number of marine indicators in Ostracode strata including, bioturba- tion (Wanklyn, 1985; Banerjee, 1990), echinoid spines (Banerjee, 1990), syneresis cracks and dinoflagellates (Ban- erjee and Davies, 1988), and tidal structures (Wanklyn, 1985; Banerjee and Kidwell, 1991). Other workers have developed or supported brackish water depositional models based primarily on the assumption that the fossil molluscs constitute a mixed assemblage of fresh and brackish water types (Finger, 1983; Mattison, 1987; McPhee, 1994). The depositional environment of the Ostracode Zone has been described as: (1) an expansive brackish bay (Wanklyn, 1985; Mattison, 1987; McPhee, 1994)) (2) a calcareous sea (James, 1985), (3) a lagoon (Wanklyn, 1985), (4) an estuary (Finger, 1983; Wanklyn, 1985), (5) a lagoon/tidal-flat to open marine environment (Banerjee and Davies, 1988), and (6) lacustrine (Farshori, 1983; Hayes, 1986). The wide

range of interpreted paleoenvironments reflect a general lack of consensus on whether seawater or freshwater dominated the depositional paleohydrology.

4. SAMPLING AND ANALYTICAL TECHNIQUES

Since aragonite is easily recrystallized to calcite in the presence of diagenetic fluids, aragonitic fossils were analyzed wherever possible to minimize diagenetic effects. Ostracode Zone fossils were collected from two subsurface cores in central Alberta (Williams, 1960). Fossils of the McMurray sub-basin (Lioplacodes bituminous) were collected from the upper McMurray Formation along the Hang- ingstone River near Ft. McMurray, Alberta (Russell, 1932; Kramers, 1971). Fossils of the Wabasca sub-basin were collected from the upper McMurray Formation in two subsurface cores (Ranger et al., 1994). Two ammonites (Beudenticeras) of the Loon River Forma- tion were used to characterize the marine endmember, and the freshwater endmember was constrained using freshwater limestones from the Kootenai Formation in Montana (Hopkins, 1985). the Peterson Limestone in Wyoming (Glass and Wilkinson, 1980), and aragonitic gastropods from the McLaren and General Petroleums units in the upper Mannville Group of central Alberta (Mattison, 1991).

Most of the strontium and oxygen isotope measurements were performed on different shell composites from the same hand sample. Measurements of the marine ammonites and whole-rocks were con- ducted on the same powder. Aragonite mineralogy was confirmed

Paleotopographic Highs

Upper Mississippian carbonates %@a

Upper Devonian carbonates 0

FBl!# Jurassic sandstones *

4 Mountainous hinterland (schematic)

Erosional limit of early Cretaceous sediments Upper Mannville fossil suites G. mdticarinata & C. ortmanni

Cities

Fig. 2. Paleogeographic map of the Western Canada Sedimentary Basin in the late Aptian to early Albian. Sub-basins containing sediments of the Ostracode Zone (west of the Wainwright Ridge) and stratigraphically equivalent sediments of the Wabasca and McMur- ray sub-basins (east of the Wainwright Ridge) are shown. The Wain- wright Ridge was a local paleotopographic high throughout Mann- ville Group deposition. Figure modified from Leckie and Smith (1992).


Table 1. Strontium, oxygen, and carbon isotope data for carbonate fossils and whole-rocks of the Mannville Group.

Sr 1 OOOSr/Cab 87Sr/86Sr &?)C 6’80 6’80 w,tfzc Sample Phase” (ppm) (atom) (measured) (%oPDB) (%&MOW) (%&MOW) Remarks

Marine environment (ammonites)

BEUD 38081 (1) A BEUD 38081 (2) A BEUD 20-46-34F (1) A repeat - BEUD 20-46-34F (2) A

Transitional environments’

Ostracode sub-basin 7-21-5627W4 3Fl A lo-12-252OW4 C

Wabasca sub-basin 6-17-76-23W4 Fl A

McMurray sub-basin FM 120 I-gast A FM 70 I-gast A FM 70 A repeat A FM 205-l A FM 205-2 A FM 285 A repeat A HS-B A repeat A HS-D I-gast A FMP-0 A

Freshwater environments

Wainwright Ridge Watershed

2284 nd

2494

-0.6 - -0.5 - -

thick fragment, nacreous lustre thick fragment, nacreous lustre thin flakes, nacreous lustre

- nd

2.61 0.707458 (14) - 0.707447 (11)

2.85 0.707508 (18) - 0.707525 (16) - 0.707476 ( 13)

2.1 30.7 nd nd 1.3 30.8

- - nd nd thin flakes. nacreous lustre

nd - 0.708347 ( 17) 2.5 21.1 -9.9 shell cornposited nd - 0.708273 (11) 1.5 23.2 -7.2 recrystallized shell composite”

2290 2.61 0.708095 (14) 4.1 22.5 -8.5 shell cornposited

2022 2.31 0.707707 (11) 4.3 21.1 1768 2.02 0.707825 (11) nd nd 1888 2.16 0.707807 (16) 3.5 20.7 nd - 0.707799 (10) nd nd

1290 1.47 0.708000 (17) 4.4 21.6 1263 1.44 0.708002 (17) nd nd 1778 2.03 0.7078 12 (09) 4.2 20.7 nd - 0.707806 (11) nd nd

1588 1.81 0.707836 (21) 4.5 20.4 nd - 0.707856 (08) nd nd

1601 1.83 0.707880 (09) 4.7 20.7 1549 1.77 0.708155 (11) 5.3 22.3

-9.9 -

-10.3 - -9.4 -

-10.3

single gastropod shell fragmentd single gastropod shell fragmentd shell cornposited

shell cornposited shell cornposited shell cornposited

-10.6 -

-10.3 -8.7

shell cornposited

single gastropod shell fragmentd shell composite”

McLaren Unit (Core 13-l-65-4W4; 383-386 m) C. ortmanni 1 A 1333 1.52 C. ortmunni 2 A 1312 1.50 C. ortmanni 3 A 1331 1.52 L. bituminous A 1591 1.82

Kootenai Formation (Drummond, Montana) Drum 335 C Drum 620 C 414 0.470 Drum 770 C

Average (1 6) Foreland Basin Drainage System

0.708791 (12) 0.54 22.9 -8.1 single gastropod shell fragmentd 0.708733 (08) 0.91 22.6 -8.4 single gastropod shell fragmentd 0.708804 (10) 0.08 23.1 -7.9 single gastropod shell fragmentd 0.708839 (10) 0.21 22.6 -8.4 single gastropod shell fragmentd

0.708740 (14) 0.708594 (14) 0.708633 (13) 0.70873 + 0.001

-.08 17.6 -12.7 whole-rock limestone 0.6 15.4 -14.8 whole-rock limestone 1.5 17.5 -12.8 whole-rock limestone

General Petroleums Unit (Core 16-13-58-5W4; 505 m) G. Multi #4 A 1208 1.38 0.707495 (11) G. Multi #5 A 1459 1.67 0.707550 (09) G. Multi #7 A 1077 1.23 0.707595 (10) G. Multi #9 A 1261 1.44 0.707600 (10)

Peterson Limestone (Wyoming) A-2’ CID 483.1 - 0.707586 (16) A-6’ C/D 547.1 - 0.707619 (14) A-15’ CID 318.1 - 0.707954 (13)

Average (I 6) 0.7076 -c 0.0002

0.9 16.2 -14.6 single gastropod shell fragmentd 0.5 17.3 -13.6 single gastropod shell fragmentd

1 17.2 -13.7 single gastropod shell fragmentd 0.1 16.4 - 14.4 single gastropod shell fragmentd

-2.9 25.5 -3.8 23.2 -5.7 21.2

Wainwright Ridge Dolostones (Devonian)

3-34-86-2OW4 D/C 67.3 - 0.708175 (12) -1.4 UGM-3 D/C nd - 0.708353 (11) -3.3 6-34-81-19W4 D/C 66.4 - 0.709260 (20) 0.7

27.3 24.3 26.2

- - -

whole-rock lime/dolostone whole-rock lime/dolostone whole-rock lime/dolostone

whole-rock dolostone whole-rock dolostone whole-rock dolostone

a Aragonite mineralogy (A) determined by x-ray diffraction with calcite below detection limit. C = calcite. D = dolomite. b Calculated assuming 40 wt % Ca. c 6’ao values calculated using the temperature dependent calcite-water equilibrium fractionation relation of O’Neil et al. (1969), a 0.6%0

corre&io>tifor the aragonite-calcite fractionation (Tanrtani et al., 1969) and a temperature of 14.5”C. d Stable and radiogenic isotope analyses performed on different shells or shell composites from the same hand sample. e Potential transitional (brackish) water environments based on sedimentological and paleontological methods. f Oxygen and carbon isotope analyses by Drummond et al. (1993).

Determination of paleosalinities by 87Sr/86Sr ratios in fossil she11 2109

by x-ray diffraction methods for many of the samples analyzed for stable isotopes, and all samples analyzed for strontium isotopes. For samples identified as aragonite in Table 1, calcite is below detection limits. Shell material was prepared by physically removing adhering detritus and diagenetic overgrowths, followed by brief sonication in ultradistilled water or ethanol. The shells were powdered in an agate mortar. Samples for stable isotope analysis were bleached for >l week to remove organic matter and rinsed and dried prior to sample digestion in 100% phosphoric acid (McCrea, 1950). The CO2 was cryogenically purified and analyzed on a VG 602D mass spectrome- ter. Oxygen and carbon isotope data are reported relative to Standard Mean Ocean Water (SMOW) and Pee Dee Belemnite (PDB), respectively, in delta (6) notation defined as (R.m&R,,ti - 1) 1000, where R = ‘*O/i60 or ‘3C/‘2C. External nrecision (10) for stable isotope analyses is <0.3%0 for 6’*0, and’ <0.2%0 for 6”C, based on replicates of SRM 19 yielding 6°C = 1.98 2 0.14 (PDB) and 6”O = 29.32 ~0.23 (SMOW).

Separate aliquots of powder were weighed for strontium isotope dilution and isotope ratio analysis and dissolved in 0.25 M HCl for 8- 14 h, with the exception of the Peterson Limestone (calcite- dolomite) dissolved in 0.4 M HCl. A weighed amount of mixed 87Rb-84Sr spike was added to the isotope dilution aliquot and left overnight to equilibrate. Any residues were removed by centrifuga- tion, and the supemate evaporated to dryness. Rubidium and strontium were separated and purified by standard cation exchange chro- matography and loaded onto the side filament of a double rhenium filament assembly for mass spectrometric analysis. Isotope dilution analyses were performed using a single-collector Micromass 30 instrument and 87Sr/S6Sr ratio determinations were performed on a multi-collector VG 354 instrument. All analyses are normalized for variable mass-dependent isotope fractionation to a 86Sr/88Sr ratio of 0.1194. Rubidium contents were checked in several samples and were predictably low (2-4 ppm). Therefore, no correction for the decay of “Rb was applied to the measured “Sr/“?Ir data, as this effect is less than the analytical uncertainty. Repeat analyses of National Bureau of Standards SRM 987 over the course of this work yielded 0.71028 c 0.00001 (2~~). with an external precision of -c 0.00002 (2a).

5. PALEOHYDROLOGY OF THE MANNVILLE GROUP

To determine paleosalinities from carbonate shells of potentially brackish dwelling fossil molluscs, it is necessary to measure or estimate the isotopic compositions of marine and freshwater mixing endmembers. Ideally this is accomplished by direct measurement of 87Sr/86Sr ratios in marine and freshwater fossil shells from closely overlying or underlying strata or from stratigraphically equivalent parts of the depositional basin.

5.1. Marine Endmember

The 87Sr/86Sr of the marine endmember can be estimated from the seawater 87Sr/86Sr evolution curve (Burke et al., 1982). Based on palynomorph biostratigraphy, Banejee (1990) assigned a late Aptian to early Albian age for the Ostracode Zone, corresponding to seawater 87Sr/86Sr between 0.7073 and 0.7075 (Ingram et al., 1994). To obtain a better estimate, two samples of the ammonite Beudelzticerus were obtained from stratigraphically equivalent marine sediments of the Moosebar-Clearwater Sea yielding 0.70745 and 0.70750 (Table 1) . Ammonite 6180 values are 30.8 and 30.7%0 (SMOW), corresponding to seawater temperatures of 14-lS’C, and water 6”O values of 0 to - 1%0 (SMOW). The relatively “O-enriched ammonite 6’*0 values indicate that normal marine salinities prevailed in the epicontinental Moosebar-Clearwater Sea at this time.

5.2. Freshwater Endmember

Freshwater aragonitic gastropods overlying the Ostracode Zone (Mattison, 1991) , from the McLaren and General Pe- troleums units (Fig. 2), yielded 87Sr/86Sr ratios of 0.7087- 0.7089 (C. ortmanni) and 0.7075-0.7076 (G. multicari- natu), respectively (Table 1). Compared to contemporaneous seawater, the C. ortmanni fossils are considerably more radiogenic, but the 87Sr/86Sr ratios for the G. multicarinata fossils are not very different from the inferred seawater composition (Table 1). We consider this similarity to be fortu- itous as the freshwater affinity for G. multicarinata is strongly supported by their very low 6r80 values (16.2- 17.3%0), yielding equilibrium water compositions of - 13.6 to - 14.6%0 ( 14.5”C).

Early Cretaceous lacustrine limestones which formed along the axis of the foreland basin (further south of the study area) provide additional constraints on the freshwater paleohydrology. Although limestones are less desirable than aragonitic fossils due to potential diagenetic effects, two widely separated freshwater limestones have 87Sr/86Sr ratios that are similar to aragonitic C. ortmunni and G. multicari- natu fossils in Alberta. The low 87Sr/86Sr ratios (0.7076- 0.7080) for the Peterson Limestone of Wyoming (Glass and Wilkinson, 1980; Drummond et al., 1993, 1996) are similar to the G. multicarinata fossils (0.7075-7076) in that they also closely approach the 87Sr/86Sr of the marine endmember (Table 1) . Unlike G. multicarinata, the Peterson Limestone was situated greater than 1000 km south of the Moosebar- Clearwater sea. Therefore, it is unlikely that the depositional environment of the Peterson Limestone was ever influenced by marine waters. Further north, lacustrine carbonates of the Kootenai Formation, Montana (Hopkins, 1985; Hayes, 1986) yielded 87Sr/86Sr ratios of 0.7086-0.7087, similar to the C. ortmanni fossils (0.7087-0.7088) in Alberta (Table 1).

Based on the above observations and the foreland basin setting, the freshwater paleohydrology of the Mannville Group is interpreted to have comprised fluvial inputs from two watersheds with different mean 87Sr/86Sr ratios. The first (more subordinate) source of freshwaters was locally derived from rivers draining ridges of Devonian carbonate (e.g., the Wainwright Ridge) that dominated local relief during Mannville Group deposition (Fig. 2). Combining mean 87Sr/86Sr ratios for C. ortmanni of the McLaren unit (0.70879) and lacustrine limestones of the Kootenai Forma- tion (0.70866) yields a mean 87Sr/86Sr of 0.7087 + 0.0001 (lo) for the Wainwright Ridge Watershed. Isotopic analyses of Wainwright Ridge dolostone sampled near the Devonian- Cretaceous disconformity surface (0.7082-0.7093; Table 1) confirm that rivers discharging from the Wainwright Ridge delivered Sr to early Cretaceous depositional basins with an isotopic composition of about 0.7087.

The second (more dominant) source of continental freshwaters derived from north flowing rivers of the Foreland Basin Drainage System, sourced primarily from fluvial discharges of the western Cordillera. Combining the mean 87Sr/ 86Sr from G. multicarinuta of the General Petroleums unit (0.70756) and lacustrine carbonates of the Peterson Lime- stone (0.7077) yields a mean 87Sr/86Sr for the Foreland Basin


Drainage System of 0.7076 5 0.0002 (la). This value is similar to the relatively nonradiogenic 87Sr/86Sr for rivers discharging from the Cordillera today (-0.7077), or in the early Cretaceous (-0.7072), after correcting for “Sr production in Cordilleran crust over the past 110 Ma. The nonradiogenic 87Sr/86Sr for present-day (and early Cretaceous) Cordilleran rivers (Table 2) reflects the juvenile character of the Cordilleran crust (Samson and Patchett, 199 1 )

6. PALEOSALINITIES FROM STRONTIUM ISOTOPES

6.1. A Comparative Technique Using 87Sr/86Sr

The paleosalinity of brackish waters formed by seawater- freshwater mixing can be estimated from the measured “‘Sr/ 86Sr of presumed brackish water carbonate fossils if: ( 1) the s7Sr/86Sr ratios of contemporaneous marine and fluviatile waters are known, (2) the Sr concentration of marine water is assumed to be the same as today, (3) a value for the Sr concentration of freshwater is estimated, and (4) seawater and freshwater had salinities of about 35%0 and O%O, respectively.

The most prominent feature of the seawater-freshwater mixing hyperbola is the high degree of curvature exhibited (Fig. 3)) reflecting the nearly one hundred-fold difference in Sr concentration between seawater and the median value for continental freshwaters. Provided that suitable, well preserved fossil material is available, the main source of uncertainty is estimation of the basin-specific freshwater Sr concentration. To estimate this value, 87Sr/86Sr, Sr/Ca, Sr, and Ca in world rivers and lakes are compiled to reveal basic trends (Figs. 4 and 5 ). Percentiles are also computed (Table 3) to enable comparison of estimated values with the range of possible choices. The median values in Table 3 for Sr and Ca contents and 87Sr/86Sr ratios in world rivers and

0 5 10 15 20 25 30 35 40 Salinity (%o)

Fig. 3. Hyperbolic mixing curves between early Albian seawater and freshwaters with differing “SrlR6Sr and Sr concentrations. The symbols on each mixing line represents the potential for each part of the curve to resolve paleosalinities within the stated analytical uncertainty of ~0.00002 (20). The hyperbolic form of the mixing curve causes greater uncertainties for paleosalinities along the seawater asymptote. For most natural waters, paleosalinities <15%0 are resoliabk. Distinguishing paleosalinities->2O%o is possible if the freshwater 87Sr/86Sr and Sr concentration is higher than the median value for world rivers and lakes.

87Sr / 86 Sr

Fig. 4. Histogram of 87Sr/XbSr ratios for dissolved river water Sr. The median value (0.7119) is identical to the weighted average of Palmer and Edmond (1989). Data are from Wadleigh et al. (1985). Goldstein and Jacobsen ( 1987), and Palmer and Edmond ( 1989).

lakes are similar to previously reported weighted averages (Wadleigh et al., 1985; Goldstein and Jacobsen, 1987; Palmer and Edmond, 1989).

The asymptotic character of the mixing curve causes a systematic variation in the uncertainty of paleosalinity determinations, increasing with increasing salinity (Fig. 3). Salin- ities below -15%0 are the most precisely determined. To resolve salinities > 15%0 the contrast in *‘Sr/“Sr and Sr concentration between contemporaneous seawater and freshwater must be larger and smaller, respectively. Since seawater has a uniform 87Sr/86Sr at any time (and we assume a constant seawater Sr concentration), the Sr concentration and 87Sr/86Sr in freshwater controls the shape of the mixing curve and, therefore, the range over which precise paleosalinities can be resolved. The sensitivity of paleosalinity determinations is demonstrated by plotting sequential 87Sr/86Sr ratios on a mixing hyperbola, resolvable within the stated analytical uncertainty of ?0.00002 (20; Fig. 3). Choosing an early Albian seawater 87Sr/86Sr of 0.70745 (Table 1)) the median mixing curve has a first resolvable paleosalinity at 0.70749, or 18.5%0. Decreasing either the riverine Sr concentration, or 87Sr/R6Sr to 20th percentile values in each of their respective distributions lowers the first resolvable paleosalinity to -8%0 (Table 3). In contrast, increasing the riverine X7Sr/XbSr and Sr concentration to 80th percentile values in- creases the first resolvable paleosalinity to 26%a and 29%0, respectively. Since the most accurate paleosalinities are determined from the low end of the salinity spectrum, uncertainty in the riverine Sr concentration has a greater impact on the accuracy of paleosalinity determinations than does equal uncertainty in the marine Sr concentration. Although it is recognized that the Sr content of past seawater may have differed from today (Graham et al., 1982), it is a small consideration compared to the problem of estimating the riverine Sr concentration and does not influence the precision to which differences in paleosalinity may be determined from the same depositional basin.

Determination of paleosalinities by “Sr/% ratios in fossil shell

0 I Rivers

o 1’ Lakes

2111

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0 0 14 28 42 56 70 84 98 8 8 g

WPPm) Frequency

Fig. 5. The relationship between Sr, Ca, and 1OOOSrlCa (atom) is shown using 253 analyses from world rivers (solid circles) and lakes <open circles). Histograms for Sr and Ca are also shown. Rivers and lakes show similar distributions of Sr and Ca. The Sr distribution is skewed with a median value of 0.071 ppm (Table 1 ), similar to the weighted average of 0.078 ppm in Palmer and Edmond ( 1989). The Ca distribution is broadly skewed with a maxima between 14 and 21 ppm, and a median value of 19.7 ppm. River and lake water Sr/Ca range between 0.5 and 5.0 (in agreement with Gdum, 1957a). Few continental waters attain SrlCa greater than seawater (8.5). The median river and lake water SrlCa ratio is 2.3 (Table 1). Data are from Alexander et al. (1954), Odum (1957a), Feulner and Hubble (1960)) Skougstad and Horr (1963), Faure et al. (1967), Wadleigh et al. ( 1985), Goldstein and Jacobsen (1987)) Palmer and Edmond ( 1989). and Rosenthal and Katz (1989). Rivers and lakes with greater than 1 ppm Sr and 98 ppm Ca are omitted from the compilation.

A composite sample of aragonitic shell fragments from the Ostracode Zone yielded s7Sr/*‘Sr of 0.70835, and from another location 300 km distant, a composite of calcite nee- dles (formerly aragonite) yielded 0.70827 (Table 1). A composite sample of aragonitic shells from the Wabasca sub- basin yielded 0.70809, and aragonitic shell material from the Ft. McMurray sub-basin yielded 0.70771-0.70816. To satisfy the brackish water hypothesis for the depositional environment of the Ostracode, McMurray and Wabasca sub- basins, freshwater from the Wainwright Ridge (0.7087) must have dominated fluviatile inputs as it is impossible to mix early Albian seawater with freshwater of the Foreland Basin Drainage System (0.7076) and obtain the 87Sr/86Sr

ratios observed. Using the median value of 0.071 ppm for the Sr concentration of rivers discharging from the Wainwright Ridge, paleosalinities of s1%0 for all three sub-basins (Curve-A, Fig. 6) are obtained. To maximize potential salinities, mixing curve-B (Fig. 6) is constructed using a Sr concentration of 0.226 ppm (80th percentile). This treatment yields paleosalinities s4%0. Considering both mixing curves ignore freshwater contributions from the much larger Fore- land Basin Drainage System, paleosalinities were likely much lower.

If the fossils analyzed are representative of the depositional conditions for which the bulk of the Ostracode, Wa- basca, and McMurray sediments were deposited, the above


Table 2. “‘Sr/“%, Sr, Ca and lOOOSr/Ca (atom) ratios for present-day rivers draining the western North American Cordillera.

River X’Sr/x”Sr (T = 0)

X7Sr/“hSr’ Sr Ca lOOOSr/Ca (T = IlOMa) (ppm) (ppm) (atom)

Skeena” 0.7046 0.704 1 0.071 10.0 3.25 Nass” 0.7054 0.7049 0.097 12.0 3.70 Stikine” 0.7054 0.7049 0.058 17.0 1.56 Sacramento (avg.)b 0.7056 0.705 1 0.088 - - Feathe? 0.7057 0.7052 0.056 - San Joaquin (avg.)” 0.7073 0.7068 0.094 - Merced’ 0.7077 0.7072 0.017 I .35 5.76 Tuolumneh 0.7079 0.7074 0.019 - Bruneau’ 0.7093 0.7088 0.04 1 14.2 1.32 Snake’ 0.7099 0.7094 0.101 15.4 3.00 Fraser” 0.7120 0.7115 0.080 15.0 2.44 Columbia‘ 0.7121 0.7116 0.074 19.0 1.78

Average 0.7077 0.7072 0.066 13.0 2.85 lff &0.003 +0.003 20.03 +5 21.4

a Wadleigh et al. (1985), h Ingram and Sloan (1992), ’ Goldstein and Jacobsen (1987). ” d Early Cretaceous Sr /8%r (T = 110 Ma) calculated using *‘Rb/‘%r = 0.3 (andesite

model composition with Rb = 42 ppm and Sr = 400 ppm (Taylor, 1977).

assessment of paleosalinities (based on the X7Sr/86Sr paleohydrology) strongly favours a lacustrine paleoenvironment. Mixing of freshwaters from north flowing rivers of the Fore- land Basin Drainage System (0.7076) and local fluvial discharges from Wainwright Ridge (0.7087) can account for all the variation in 87Sr/86Sr observed: seawater input is not required.

6.2. A Graphical Technique Using 87Sr/%r and Sr/Ca Ratios

The maximum potential for determining past salinities using strontium isotopes is realized by reintegrating the Sr mass balance, which may be partly accomplished using shell Sr/Ca ratios. The incorporation of Sr into mollusc shells is kinetically controlled, but quantifiable in terms of the distribution coefficient Dsr = Sr/Ca,,,,/Sr/Ca,,,. Laboratory ex- periments show that aragonite precipitated under equilibrium conditions yields DsF = 1.15 at 20°C (Kinsman and Holland, 1969), whereas aragonite Ds, values in mollusc shells are typically lower (0.2-0.3), revealing a strong kinetic fractionation effect. Smaller differences in Dsr are observed between different mollusc species, known as the species effect (Thompson and Chow, 1955; Turekian and Armstrong, 1960). However, the main factor governing Sr/Ca,,,,,( in a single species is the Sr/Caratio of the water from which the shell precipitated (Kulp et al., 1952). Other factors that can influence SrICash,n are carbonate growth temperature (Pilkey and Goodell, 1963) and growth rate (Carpenter and Loh- mann, 1992). Well determined aragonite DsT values for marine and freshwater molluscs are 0.23 (Mytilus edulis, Lorens and Bender, 1980), 0.24 (Limnaea stagnalis; Buchardt and Fritz, 1978), 0.26 (Lumpsilis; Faure et al., 1967), 0.29 (Physa ; Odum, 195 1) , and 0.3 1 (M. tuberculatu ; Rosenthal and Katz, 1989). Although specific Ds7 values cannot be assigned to extinct fossil molluscs, basinal variations in original Sr/Ca,,, can be inferred from members of a single

fossil species, provided differences in Sr/Ca,,,e, are larger than potential variations in Sr/Ca,Yh.l, caused by temperature or growth rate influences.

A property of the hyperbolic mixing curve is that two- component mixtures are linearly correlated on an inverse plot of 87Sr/86Sr vs. Ca/Sr. This allows the two-component mixing hypothesis to be tested graphically (Langmuir et al., 1978). If the endmembers to mixing are seawater and freshwater (which have very different Sr/Ca), the specific DOT, necessary to transform Sr/Ca+,, to SrlCa,,,, is fixed by the condition that seawater must plot on the mixing line. Using the slope and y-intercept of the fossil shell mixing line, and the 87Sr/86Sr and Ca/Sr ratio of paleo-seawater, Dsr is determined. If the value for Da is biologically reasonable (i.e., 0.2-0.3, as above), this is strong evidence that the original shell secreting molluscs inhabited brackish waters. Highly correlated trends on such plots are unlikely to result from kinetic artifacts because factors such as temperature and growth rate do not influence measured 87Sr/86Sr ratios.

Table 3. Percentiles for Sr, Ca, lOOOSr/Ca (atom) and “Sr/%r ratios in world rivers and lakes”.

Percentile X7Sr/X6Sr (T = 0)

Sr (ppm)

Ca (ppm)

lOOOSr/Ca (atom)

0 th 0.70452 0.001 0.2 0.457 10 th 0.707 11 0.010 1.72 0.989 20 th 0.70920 0.017 3.44 1.34 30 th 0.70979 0.03 1 7.96 1.69 40 th 0.71095 0.05 1 14.57 1.93 50 th 0.71190 0.07 I 19.65 2.31 60 th 0.71396 0.096 25.02 2.66 70 th 0.71565 0.135 32.08 3.13 80 th 0.71808 0.226 42.48 3.68 90 th 0.72707 0.429 57.18 4.77

100 th 0.73844 0.949 91.03 8.12

a References below Figure 5.

Determination of paleosalinities by s’SrlssSr ratios in fossil shell 2113

0.7080

0.7078

0.7076

0.7074 0 5 10 15 20 25 30 35 40

Salinity (%0)

Fig. 6. Seawater-freshwater mixing curves for the early Cretaceous Mannville Group based on the 87Sr/86Sr paleohydrology. The solid curve-A is constructed using the median value for the freshwater Sr concentration in world rivers and lakes; the dashed curves are constructed using 80th (curve-B ) and 20th (curve-C) percentile values, respectively. Inferred paleosalinities for carbonate fossils of the Ostracode, Wabasca, and McMurray sub-basins are very low, between 0.5%0 and 4%0 for freshwater Sr contents in the 50th and 80th percentiles, respectively.

In our treatment of the mixing equations, the quantity Ca/ Sr is used rather than 1 /Sr because it is desirable to convert Sr/CastiN to Sr/ChO,=, so that a large database on Sr/Ca variations in natural waters can be used (Fig. 5; e.g., Odum, 1957a,b). Seawater has a lOOOSr/Ca atom ratio of 8.5 (De Villiers et al., 1993), whereas freshwaters typically have values between 0.5 and 5.0 (Odum, 1957a), with a median value of 2.3 (Table 3). Odum (1957a) studied Sr/Ca in natural waters and concluded: ( 1) Sr/Ca in rivers and lakes broadly reflect the Sr/Ca of local bedrock sources, (2) the lower limit of Sr/Ca is set by the congruent weathering of Phanerozoic limestones, (3) rivers draining limestone ter- rains have low Sr/Ca but high Sr and Ca concentrations, (4) rivers from humid regions have low Sr/Ca, (5) rivers with high Sr/Ca are found in regions of arid climate, with volcanic bedrock, or with carbonate hosted evaporites, and (6) closed basins can have higher Sr/Ca than their riverine inputs because of CaC03 precipitation..

6.3. The L bituminous Suite of the McMurray Sub-basin

Isotopic and elemental analyses of eight L. bituminous samples from the Ft. McMurray sub-basin are linearly correlated on a 87Sr/86Sr-Ca/Sr plot (r’ = 0.96), satisfying the criterion for two-component mixing (Fig. 7a). Strontium concentrations in fossil shells (determined by isotope dilution mass spectrometry) are converted to lOOOSr/Ca assuming ideal Ca-stoichiometry for aragonite (40.0 wt%). This assumption was found to be reliable through electron micro- probe analyses of several L. bituminous shell fragments which gave elemental Ca contents between 38.3 and 39.3 wt%. The lowest measured Ca content yields Sr/Ca only 3% higher than stoichiometry, a negligible difference compared to the total variation in the data (Table 1) .

Transforming the L. bituminous shell mixing line to an equivalent water mixing line using 87Sr/86Sr = 0.70745 and Ca/Sr = 0.1176 for early Albian seawater yields an unrealistic Dsr of 0.86. Therefore, the mixing identified in Fig. 7a is not between seawater and freshwater, it is between two sources of freshwater with different 87Sr/86Sr and Ca/Sr ratios. On the inverse 87Sr/86Sr-Sr/Ca plot (Fig. 7b), it is possible to asymptotically derive the 87Sr/86Sr ratio of the

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1 OOOC&r (atom)

A (07087 , 2.38 , 42.5 0.22) B (0.7077 , 8.42 , 10.7 , 0.38)

c (0.70745 8.50 , 415 7.7)

A-C trsshwte, - seawta, rdxhq

0.7072 1 I r I . I . I I . i 2 3 4 5 6 7 8 9

1000Sr/Ca (atom)

Fig. 7. (a) ‘?Sr/?3r - Ca/Sr mixing line for L. bituminous shells of the McMurray sub-basin (r * = 0 96). Shell mixing lines are transformed to water mixing lines using Ds, values of 0.25 and 0.3 1. Early Albian seawater (0.70745) does not plot on either mixing line, indicating that the mixing observed is not between seawater and freshwater, but between two freshwater sources. An unrealistic Dsr value of 0.86 is required to put early Albian seawater on the mixing line. (b) The freshwater mixing line in Fig. 7a is transformed to its hyperbolic form (A-B). The endmember 87Sr/86Sr with the high SrlCa ratio is graphically constrained between 0.7076 and 0.7078, consistent with the mean value for measurements of G. multican’nuta and the Peterson Limestone (0.7076 2 0.0002) interpreted as the mean isotopic composition for rivers of the Foreland Basin Drainage System. To determine the relative contributions of each freshwater source to the McMurray waters, the graphically deduced value of 0.7077 is used for the Foreland Basin Drainage System, and 0.7087 is used for the Wainwright Ridge Watershed. These *‘Sr/?Sr ratios are used to determine corresponding SrlCa ratios from the equation of the water mixing line in 7a (Dsr = 0.31). The Ca concentration for each freshwater endmember is chosen to conform with bedrock type in each watershed (Odum, 1957a), and the Sr concentration is determined from the SrlCa ratio. The fraction of each freshwater source contributing to the McMurray depositional waters is shown on curve A-B: approximately 50-90% of McMurray sub-basin depositional waters were contributed by the Foreland Basin Drainage System. The early Albian seawater-freshwater mixing curve (AD- C) is shown for comparison.


mixing endmember with the highest Sr/Ca ratio. Although the curvature of the mixing line is shallow, a *‘Sr/“Sr ratio between 0.7076 and 0.7078 (-0.7077) is indicated, consistent with the 87Sr/86Sr from fossil shells (0.7076 + 0.0002) interpreted as waters of the Foreland Basin Drainage System (Table 1). We note that if seawater were a mixing endmember, the seawater *7Sr/86Sr ratio could be asymptotically derived assuming the present-day seawater Ca/Sr of 0.1176.

The Ca/Sr ratios for the mixing endmembers are determined from the equation of the mixing line (Fig. 7a) for measured and graphically deduced values of 87Sr/86Sr. To quantify the relative proportions of each freshwater source in the mixture, the Sr and Ca concentrations in the endmember waters must be estimated. As before, Sr concentrations will govern the mass balance. Rather than assuming quantities for these parameters, Ca concentrations are chosen and the Sr concentrations determined from the Sr/Ca ratio. A sensitivity analysis based on increasing Ca contents can be under- taken to test the impact of the assumed Ca concentration on the resulting estimate of mixing proportions, with the additional constraint that the Sr/Ca ratio places some limits on the possible range of Sr concentrations. The Ca concentration of the freshwater endmember can be estimated from the knowledge of bedrock type in the watershed, with the freshwater Sr/Ca and 87Sr/86Sr ratios providing additional constraints. For example, weathering of Precambrian crust yields very radiogenic 87Sr/86Sr, approximately median Sri Ca ratios, and lower than median Ca concentrations (Wad- leigh et al., 1985). Weathering of carbonates yields lower than median 87Sr/86Sr (0.7065-0.7092) and Sr/Ca of -0.5, and higher than median Ca concentrations (Odum, 1957a).

A summary of the measured 87Sr/86Sr, graphically determined Ca/Sr, and estimated Ca concentrations for endmember waters of the Foreland Basin Drainage System and Wain- wright Ridge Watershed are given in Fig. 7b. A Ca concentration in the 80th percentile (Table 3) is chosen for Wainwright Ridge freshwaters based on the carbonate source of Sr. A median Ca concentration is chosen for rivers of the Foreland Basin Drainage System because the high Sr/Ca ratio determined from the mixing line is interpreted to reflect the weathering of juvenile volcanic rocks in the early Creta- ceous Cordillera. The resulting freshwater-freshwater mixing hyperbola (A-B) shows that the McMurray sub-basin was dominated by fluviatile waters of the Foreland Basin Drain- age System which composed 50-90% of the McMurray water mass at any time (Fig. 7b). We note that differentia- tion between curves A-B and A-C in Fig. 7b is possible only because of the high correlation coefficient of the L. bituminous mixing line. For the L. bituminous line to represent seawater-freshwater mixing, the 87Sr/86Sr or Sr/Ca of late Aptian to early Albian seawater must be higher. The former is unlikely in that 87Sr/86Sr of seawater does not approach 0.7077 until the late Cretaceous (McArthur et al., 1994). If the Sr/Ca of early Cretaceous seawater were greater than three times (lOOOSr/Ca = 24) the modem value, the L. bituminous mixing line could represent early Albian seawater-freshwater mixing. However, Sr/Ca ratios for the marine ammonites (Table 1) are consistent with a DsT of 0.2, which is within the range of modem values for

molluscs and, therefore, evidence that the Sr/Ca of seawater in the early Cretaceous was similar to modem seawater.

7. NONCONSERVATIVE BEHAVIOLJR OF OXYGEN AND CARBON ISOTOPES

The correlation observed between 87Sr/86Sr and Sr/Ca for L. bituminous shells of the McMurray sub-basin (Fig. 7a) should be evident in the 6 13C and 6180 systematics, provided that oxygen and carbon isotopes behaved conservatively in the seawater-freshwater mixing zone, and seawater and freshwater were isotopically distinct. However, S”O (20.9 t 0.4%0) and 613C (4.3 ? 0.4%0) do not correlate with 87Sr/ ‘?Sr, being remarkably uniform (Table 1) . This potential for oxygen and carbon isotopes to behave nonconservatively was appreciated by earlier workers (Lloyd, 1965; Sacket and Moore, 1966; Mook and Vogel, 1968, Mook, 1970). Conservative behavior results only if water residence times are short, as in some estuaries. Lakes or lagoons with long water residence times (months-years) are subject to evaporation, driving 6 ‘80wate, values towards ‘80-emiched compositions relative to riverine inputs (Gat, 1981). Long water residence times also promote nonconservative effects in S”Culc through: ( 1) changing primary productivity, (2) equilibration with atmospheric COZ, (3) water stratification, and (4) organic respiration and decay.

Referring to the larger compilation of oxygen and carbon isotope data in Holmden et al. ( 1997), no systematic variation of inferred 613CDIc is observed for the Ostracode (2.5 -+ O.~%C), Wabasca (4.0 2 0.14%0), and McMurray (4.3 ? 0.6%0) waters. Although these fossils inhabited predomi- nantly freshwaters based on their 87Sr/86Sr and 6180 paleohydrologies, their S13C values are higher than marine ammonites of the Moosebar-Clearwater Sea (613C = 1.3-2.1%0). Marine-like S13C in Ostracode, Wabasca, and McMurray fossils likely reflect DIC contributions from the weathering of Devonian marine carbonates comprising the Wainwright Ridge and uplifted Paleozoic carbonates of the early Creta- ceous Cordilleran front range. These relatively ‘3C-enriched riverine inputs were further modified in individual sub-basins by the processes outlined above. Small differences in 613C between sub-basins suggests either diachroneity between deposits or the influence of barriers to inter-basinal water circu- lation such as the Wainwright Ridge (Fig. 2).

Mollusc 6 ’80shelI values are very uniform (20.9 5 0.7%0) for all three sub-basins yielding a mean SL80worer value of - 10.1%0 ( 14.5”C). The uniformity is remarkable considering the stratigraphic uncertainties and complex facies relations between deposits and possible differences in original water temperature between sub-basins. In contrast to 613C, the S “0 paleohydrology displays large differences in oxygen isotope composition between seawater and freshwater endmembers. However, paleosalinity determination using oxygen isotopes is complicated by nonconservative effects, and the difficult task of determining the &I80 of the freshwater endmember. In addition to the known variation of precipitation 6 I80 with latitude, the 6 “0 of meteoric waters is influenced by the continentality of the study area (distance from the coast) and orographic effects (Yurtsever and Gat, 198 1).

Determination of paleosalinities by 8’Sr/86Sr ratios in fossil shell 2115

Holmden et al. ( 1997) compiled data from a variety of sources to show that meteoric waters in the early Cretaceous WCSB ranged widely from -5 to -20%0. Waters from the ‘*O-depleted end of the meteoric water spectrum are attrih- uted to east-flowing paleo-Pacific air masses that were pro- gressively depleted of ‘*O by precipitation over the western Cordillera and subsequently discharged to the foreland basin by rivers. Waters from the ‘80-emiched end of the meteoric water spectrum likely originated as locally derived precipitation with a moisture source in the adjacent Moosebar-Clear- water Sea, with some “O-enrichment due to lake storage effects.

The 87Sr/86Sr results are broadly consistent with this catego- rization of inferred early Cretaceous meteoric water S 180. The Foreland Basin Drainage System is supplied dominantly by rivers discharging from the western Cordillera, characterized by relatively ‘80-depleted oxygen (- 14%0 to - 17%0) and nonradiogenic 87Sr/86Sr (0.7076 ? 0.0002). The S “0 of precipitation over the Wainwright Ridge (6O”N latitude) can be estimated from measurements of C. ortmanni (6180w*ler = -8%0, 145°C) whose 87Sr/86Sr (0.7087) was interpreted to reflect waters discharging from the Wainwright Ridge (0.7082-0.7092). The 6”O of modem coastal precipitation at the paleolatitude of the study area (60”N) is -7 to -8%0 (Fig. 22 of Yurtsever and Gat, 198 1) , consistent with equilibrium water 6’*0 values determined from C. ortmanni even though the early Cretaceous paleoclimate is known to have been warmer. Although the 6 I80 paleohydrology is unable to resolve mixing in the McMurray sub-basin with the detail possible using strontium isotopes, the two paleohydrologies are consistent. Ostracode, Wabasca, and McMurray waters with a uniform 6’*0 of -10%0 could have formed through mixing between freshwaters of the Foreland Basin Drainage System ( - 14%0 to - 17%0) and Wainwright Ridge Watershed (-8%0), with subsequent modification of isotopic signatures by evaporation. The strength of the strontium isotope method lies in its immunity to such nonconservative influences.

8. IMPLICATIONS FOR DEPOSITIONAL ENVIRONMENT OF THE OSTRACODE, WABASCA,

AND McMURRAY SURBASINS

Paleosalinities derived from strontium isotope analyses of well preserved fossil shells of the Ostracode Zone and stratigraphically equivalent sediments of the Wabasca and McMurray sub-basins indicate a dominantly lacustrine paleoenvironment for carbonate bearing units of the Mannville Group. With the exception of Farshori ( 1983) and Hayes (1986), who recognized no marine indicators in their studies, this interpretation conflicts with most other recent inter- pretations of the Ostracode Zone paleoenvironment using paleontological and sedimentological methods (e.g., Finger, 1983; James, 1985; Wanklyn, 1985; Mattison, 1987; Ban- erjee and Davies, 1988; Banerjee, 1990; McPhee, 1994). If marine waters did have access to depositional centers of the Ostracode, Wabasca, and McMurray sub-basins, the contri- bution must have been small, possibly localized, and proba- bly sporadic. Accordingly, the freshwater flux was of suffi- cient magnitude to keep the Ostracode depositional waters fresh most of the time.

A plausible modem analog for the paleohydrology of the Ostracode Zone is Lake Maracaibo, Venezuela, which is a large ( 12,500 km’), shallow (~35 m), intermontane lake with a long narrow connection to the sea. At the entrance to the lake, a 7-12 m sill maintains low stable salinities of about 3.3%0 (Hyne et al., 1979) and a mean 6D = -22 ,3%0 (Freidman et al., 1956; S-values relative to SMOW using *H/‘H = 0.0001558 from Hagemann et al., 1970). In addition, the Catacumbo River supplies high altitude meteoric water (6D = -103%0) to the lake, which differs in isotopic composition from rainfall over the lake (SD = -37). The mean 6D value for Catacumbo river water and Caribbean seawater entering the lake is -63%0 and -7%0, respectively. The residence time for Lake Maracaibo waters is 10 y (Freidman et al., 1956)) and the nonconservative, evaporative increase in 6D of lake waters (relative to riverine waters) is four times greater than the increase in 6D due to mixing with seawater.

The deduced 6 “0 paleohydrology of the Ostracode, Wa- basca, and McMurray sub-basins is similar to that of modem Lake Maracaibo in that the 6”O of freshwater inputs to the lake are wide ranging due to competing orographic and coastal influences on precipitation S180. In addition, the Ma- racaibo basin is separated from the sea by a long narrow channel, just as the Ostracode, Wabasca, and McMurray sub- basins were separated from the Moosebar-Clearwater Sea by incised valleys. These early Cretaceous valleys and ridges were significant topographical features of the study area with several hundred meters of relief (Ranger et al., 1994). Flu- vial waters of the Foreland Basin Drainage System and local river discharges from Wainwright Ridge tlowed through these channels to reach the sea (Fig. 2). As transgressive waters of the Moosebar-Clearwater Sea encroached southwards along the valley systems, the rise in base level reduced fluvial discharge rates causing this part of the WCSB to flood, forming a number of large lakes. The Ostracode, Wa- basca, and McMurray sediments were deposited in these lakes.

9. CONCLUSIONS

Strontium isotope ratios in fossil shells may be used to determine depositional paleosalinities in ancient brackish water systems. Two strategies were investigated for recon- structing the strontium isotope paleohydrology and determining paleosalinities. The first method utilized 87Sr/86Sr ratios in contemporaneous marine and freshwater fossils to con- struct the reference mixing hyperbola. Oxygen isotope analyses were useful in providing increased confidence that the 87Sr/s6Sr results fit within a rational paleohydrological framework. Paleosalinities of potentially brackish water fossils were determined by comparing their 87Sr/86Sr ratios to the mixing hyperbola (having coordinates of 87Sr/86Sr and Salinity). The most important uncertainty is the Sr concentration of the fluviatile endmember which influences the accuracy of paleosalinity determinations, but not their relative magnitude.

A second approach utilized potential variations in 87Srl 86Sr and Sr/Ca ratios from shells of a single fossil species.


Graphical analysis is used to detertnine whether the original shell producing organism inhabited the seawater-freshwater mixing zone. Linearly correlated data constitutes evidence that the original waters were mixtures of two sources. If one source was seawater, Dsr is fixed by the condition that seawater plots on the water mixing line. If the Dsr determined from the mixing line is biologically reasonable (0.2-0.3), compared to modem values, the brackish water affinity of the fossils is confirmed. Compared to sedimentological and paleontological techniques, the main limitation of the strontium isotope paleosalinity method is that well preserved fossil carbonates are required. However, conventional sedimentological and paleontological techniques are unable to quantify paleosalinity, and preservational concerns apply equally to all geochemical techniques that use the chemistry of carbonate fossils to infer the chemistry of past environmental waters.

Both strategies for determining 87Sr/86Sr paleosalinities from mollusc shells of the early Cretaceous Ostracode Zone and stratigraphically equivalent deposits of the Wabasca and McMurray sub-basins of the WCSB yielded consistent results. Although brackish water depositional conditions have been widely proposed, a lacustrine setting is deduced, thus demonstrating the importance of paleohydrological techniques in paleoenvironmental analysis.

Acknowledgments-Mike Ranger is thanked for many discussions concerning aspects of the sedimentology and depositional environment of the Mannville Group and for supplying core descriptions containing some of the fossils analyzed. Appreciation is also ex- tended to Blair Mattison who collected and identified the L. bituminous and G. multicarinata fossil suites and Carl Drummond who supplied samples of the Peterson Limestone. We are grateful to Harold Huebscher for samples of the Wainwright Ridge dolostone. Field assistance from Cathrin Hagey, Blair Mattison, Shawna Vos- sler, Ed Cloutis, and Rose Cloutis is gratefully acknowledged. We thank J. D. Blum, J. L. Banner, and T. K. Kyser for constructive reviews. This study formed part of the doctoral dissertation of C.H. while in receipt of NSERC Doctoral and CSPG Scholarships and received financial support through a NSERC Research Grant to R.A.C and K.M.

Editorial handling: T. K. Kyser

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