Download Paleosalinities in ancient brackish water systems determined by *`Sr

Geocbimicaet Cosmochimica
Acta,Vol. 61, No. 10,pp. 2105-2118,1997
Copyright0 1997ElsevierScienceLtd
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0016-7037/97
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Pergamon
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 environme&
Cop&ht
0 i997 Elsevier Science Ltd
1. INTRODUCTION
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.
Brackish water systems are difficult to recognize in the sedimentary rock record because sedimentological and paleontological methods are relatively insensitive to salinity. To substantiate 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. Quantification 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
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
Department of Geological Sciences, University
of Saskatchewan, Saskatoon SK S7N 5E2, Canada.
*Present address:
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 revealed 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 limited 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 seawaterfreshwater mixing zone if the climate is dry or water residence times are long, (4) freshwaters can have marine-like
6 13Cvalues 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 13Cvalues. 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/8hSrof past waters directly.
This fact, coupled with the relatively unreactive nature of
dissolved aqueous Sr (similar to Ca), enables continental
freshwaters to maintain 87Sr/86Srratios that reflect contributions from bedrock sources over large distances.
Estuarine paleosalinities may be determined from “Sr/
XhSrratios 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 Interior 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 Cordillera (McGookey et al., 1972; Hayes et al., 1994). The
early Cretaceous Mannville Group was deposited and reworked in this complex hydrological interface between
northerly flowing rivers of the Foreland Basin Drainage System 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,
2107
Determination of paleosalinities by 87Sr/86Srratios in fossil shell
range of interpreted paleoenvironments reflect a general lack
of consensus on whether seawater or freshwater dominated
the depositional paleohydrology.
4. SAMPLING
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 Wainwright 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 predominantly 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, bioturbation (Wanklyn, 1985; Banerjee, 1990), echinoid spines
(Banerjee, 1990), syneresis cracks and dinoflagellates (Banerjee 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
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 Hangingstone 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 Formation 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 conducted on the same powder. Aragonite mineralogy was confirmed
Paleotopographic
Highs
Upper Mississippian carbonates
FBl!#
4
%@a Erosional limit of early
Cretaceous sediments
Upper Devonian carbonates
0
Upper Mannville fossil suites
& C. ortmanni
G. mdticarinata
Jurassic sandstones
*
Cities
Mountainous hinterland
(schematic)
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 McMurray sub-basins (east of the Wainwright Ridge) are shown. The Wainwright Ridge was a local paleotopographic high throughout Mannville Group deposition. Figure modified from Leckie and Smith
(1992).
C. Holmden et al.
2108
Table 1. Strontium, oxygen, and carbon isotope data for carbonate fossils and whole-rocks of the Mannville Group.
Sample
Phase”
Sr
(ppm)
1OOOSr/Cab
(atom)
87Sr/86Sr
(measured)
&?)C
6’80
6’80 w,tfzc
(%oPDB) (%&MOW) (%&MOW)
Remarks
Marine environment (ammonites)
BEUD
BEUD
BEUD
repeat
BEUD
38081 (1)
38081 (2)
20-46-34F (1)
20-46-34F (2)
A
A
A
A
2284
nd
2494
nd
2.61
2.85
-
(14)
(11)
(18)
(16)
( 13)
2.1
nd
1.3
nd
30.7
nd
30.8
nd
-0.6
-0.5
-
thick fragment, nacreous lustre
thick fragment, nacreous lustre
thin flakes, nacreous lustre
0.708347 ( 17)
0.708273 (11)
2.5
1.5
21.1
23.2
-9.9
-7.2
shell cornposited
recrystallized shell composite”
shell cornposited
0.707458
0.707447
0.707508
0.707525
0.707476
thin flakes. nacreous lustre
Transitional environments’
Ostracode sub-basin
7-21-5627W4 3Fl
lo-12-252OW4
Wabasca sub-basin
6-17-76-23W4 Fl
McMurray sub-basin
FM 120 I-gast
FM 70 I-gast
FM 70
repeat
FM 205-l
FM 205-2
FM 285
repeat
HS-B
repeat
HS-D I-gast
FMP-0
-
A
C
nd
nd
A
2290
2.61
0.708095 (14)
4.1
22.5
-8.5
A
A
A
A
A
A
A
A
A
A
A
A
2022
1768
1888
nd
1290
1263
1778
nd
1588
nd
1601
1549
2.31
2.02
2.16
1.47
1.44
2.03
1.81
1.83
1.77
0.707707 (11)
0.707825 (11)
0.707807 (16)
0.707799 (10)
0.708000 (17)
0.708002 (17)
0.7078 12 (09)
0.707806 (11)
0.707836 (21)
0.707856 (08)
0.707880 (09)
0.708155 (11)
4.3
nd
3.5
nd
4.4
nd
4.2
nd
4.5
nd
4.7
5.3
21.1
nd
20.7
nd
21.6
nd
20.7
nd
20.4
nd
20.7
22.3
-9.9
-10.3
-9.4
-10.3
single gastropod shell fragmentd
single gastropod shell fragmentd
shell cornposited
-10.6
-10.3
-8.7
shell cornposited
0.708791
0.708733
0.708804
0.708839
0.54
0.91
0.08
0.21
22.9
22.6
23.1
22.6
-8.1
-8.4
-7.9
-8.4
-.08
0.6
1.5
17.6
15.4
17.5
-12.7
-14.8
-12.8
whole-rock limestone
whole-rock limestone
whole-rock limestone
0.9
0.5
1
0.1
16.2
17.3
17.2
16.4
-14.6
-13.6
-13.7
- 14.4
single
single
single
single
shell cornposited
shell cornposited
shell cornposited
single gastropod shell fragmentd
shell composite”
Freshwater environments
Wainwright Ridge Watershed
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)
(12)
(08)
(10)
(10)
0.708740 (14)
0.708594 (14)
0.708633 (13)
0.70873 + 0.001
single
single
single
single
gastropod
gastropod
gastropod
gastropod
shell
shell
shell
shell
fragmentd
fragmentd
fragmentd
fragmentd
Foreland Basin Drainage System
General Petroleums Unit (Core
G. Multi #4
A
G. Multi #5
A
G. Multi #7
A
G. Multi #9
A
Peterson Limestone (Wyoming)
A-2’
CID
A-6’
C/D
A-15’
CID
Average
16-13-58-5W4; 505
1208
1.38
1459
1.67
1077
1.23
1261
1.44
m)
0.707495
0.707550
0.707595
0.707600
483.1
547.1
318.1
(I 6)
-
0.707586 (16)
0.707619 (14)
0.707954 (13)
0.7076 -c 0.0002
-2.9
-3.8
-5.7
25.5
23.2
21.2
-
0.708175 (12)
0.708353 (11)
0.709260 (20)
-1.4
-3.3
0.7
27.3
24.3
26.2
(11)
(09)
(10)
(10)
gastropod
gastropod
gastropod
gastropod
shell
shell
shell
shell
fragmentd
fragmentd
fragmentd
fragmentd
whole-rock lime/dolostone
whole-rock lime/dolostone
whole-rock lime/dolostone
Wainwright Ridge Dolostones (Devonian)
3-34-86-2OW4
UGM-3
6-34-81-19W4
D/C
D/C
D/C
67.3
nd
66.4
-
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/86Srratios in fossil she11
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 spectrometer. 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 (calcitedolomite) dissolved in 0.4 M HCl. A weighed amount of mixed
87Rb-84Srspike was added to the isotope dilution aliquot and left
overnight to equilibrate. Any residues were removed by centrifugation, and the supemate evaporated to dryness. Rubidium and strontium were separated and purified by standard cation exchange chromatography 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/S6Srratio determinations were performed on a
multi-collector VG 354 instrument. All analyses are normalized for
variable mass-dependent isotope fractionation to a 86Sr/88Srratio 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.
2109
5.2. Freshwater Endmember
Freshwater aragonitic gastropods overlying the Ostracode
Zone (Mattison, 1991) , from the McLaren and General Petroleums units (Fig. 2), yielded 87Sr/86Sr ratios of 0.70870.7089 (C. ortmanni) and 0.7075-0.7076 (G. multicarinatu), 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 fortuitous as the freshwater affinity for G. multicarinata is
strongly supported by their very low 6r80 values (16.217.3%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/86Srratios
that are similar to aragonitic C. ortmunni and G. multicarinatu fossils in Alberta. The low 87Sr/86Sr ratios (0.70760.7080) 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/86Srof the marine endmember
(Table 1) . Unlike G. multicarinata, the Peterson Limestone
was situated greater than 1000 km south of the MoosebarClearwater 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 Formation (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 DevonianCretaceous 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 Limestone (0.7077) yields a mean 87Sr/86Srfor the Foreland Basin
2110
C. Holmden et al.
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 seawaterfreshwater 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 / 86Sr
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). Salinities 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 increases 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
2111
0 I Rivers
o 1’
Lakes
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0
14 28
42
56
70
84
g
98 8
8
Frequency
WPPm)
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 needles (formerly
aragonite)
yielded 0.70827 (Table 1). A
composite sample of aragonitic shells from the Wabasca subbasin 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 subbasins, 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 Foreland 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, Wabasca, and McMurray sediments were deposited, the above
C. Holmden et al.
2112
Table 2. “‘Sr/“ %, Sr, Ca and lOOOSr/Ca (atom) ratios for present-day rivers draining
the western North American Cordillera.
X’Sr/x”Sr
(T = 0)
River
Skeena”
Nass”
Stikine”
Sacramento
Feathe?
San Joaquin
Merced’
Tuolumneh
Bruneau’
Snake’
Fraser”
Columbia‘
Average
lff
(avg.)b
(avg.)”
X7Sr/“hSr’
(T = IlOMa)
0.7046
0.7054
0.7054
0.7056
0.7057
0.7073
0.7077
0.7079
0.7093
0.7099
0.7120
0.7121
0.7077
&0.003
0.704 1
0.7049
0.7049
0.705 1
0.7052
0.7068
0.7072
0.7074
0.7088
0.7094
0.7115
0.7116
0.7072
+0.003
Sr
Ca
(ppm)
(ppm)
0.071
0.097
0.058
0.088
10.0
12.0
17.0
-
0.056
0.094
0.017
0.019
0.04 1
0.101
0.080
0.074
0.066
20.03
I .35
14.2
15.4
15.0
19.0
13.0
+5
lOOOSr/Ca
(atom)
3.25
3.70
1.56
-
5.76
1.32
3.00
2.44
1.78
2.85
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 Foreland 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
Ratios
Technique
Using 87Sr/%r
and Sr/Ca
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 experiments 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 Lohmann, 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 origcan be inferred from members of a single
inal Sr/Ca,,,
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 twocomponent 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)
ratios in world rivers and lakes”.
and “Sr/%r
X7Sr/X6Sr
(T = 0)
Sr
Ca
Percentile
(ppm)
(ppm)
lOOOSr/Ca
(atom)
0
10
20
30
40
50
60
70
80
90
100
0.70452
0.707 11
0.70920
0.70979
0.71095
0.71190
0.71396
0.71565
0.71808
0.72707
0.73844
0.001
0.010
0.017
0.03 1
0.05 1
0.07 I
0.096
0.135
0.226
0.429
0.949
0.2
1.72
3.44
7.96
14.57
19.65
25.02
32.08
42.48
57.18
91.03
0.457
0.989
1.34
1.69
1.93
2.31
2.66
3.13
3.68
4.77
8.12
th
th
th
th
th
th
th
th
th
th
th
a References
below Figure 5.
Determination of paleosalinities by s’SrlssSr ratios in fossil shell
2113
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
0.7080
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/86Srpaleohydrology. 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 terrains 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 microprobe 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) .
0.0
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
1OOOC&r (atom)
0.1
, 42.50.22)
A (07087, 2.38
B (0 . 7077
c
A-C
0.7072 1
2
(0 . 70745
trsshwte,
- seawta, rdxhq
3
4
I
r
, 8 . 42 , 10 . 7 , 0 . 38)
8 . 50 , 415
I . I .
I
5
6
7
1000Sr/Ca (atom)
7 . 7)
I . i
8
9
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/86Srwith 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 (ADC) is shown for comparison.
2114
C. Holmden et al.
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/86Srratio 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 undertaken 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 (Wadleigh 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 Wainwright 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 Cretaceous Cordillera. The resulting freshwater-freshwater mixing
hyperbola (A-B) shows that the McMurray sub-basin was
dominated by fluviatile waters of the Foreland Basin Drainage System which composed 50-90% of the McMurray
water mass at any time (Fig. 7b). We note that differentiation 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/86Srand Sr/Ca for
L. bituminous shells of the McMurray sub-basin (Fig. 7a)
should be evident in the 6 13Cand 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 613CDIcis 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 predominantly freshwaters based on their 87Sr/86Srand 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 Cretaceous 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 circulation such as the Wainwright Ridge (Fig. 2).
Mollusc 6 ’80shelIvalues are very uniform (20.9 5 0.7%0)
for all three sub-basins yielding a mean SL80worervalue 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
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 attrihuted to east-flowing paleo-Pacific air masses that were progressively 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-Clearwater Sea, with some “O-enrichment due to lake storage
effects.
The 87Sr/86Srresults are broadly consistent with this categorization 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%0to - 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.
2115
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, Wabasca, 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 Maracaibo basin is separated from the sea by a long narrow
channel, just as the Ostracode, Wabasca, and McMurray subbasins 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). Fluvial 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, Wabasca, and McMurray sediments were deposited in these
lakes.
8. IMPLICATIONS FOR DEPOSITIONAL
ENVIRONMENT OF THE OSTRACODE, WABASCA,
AND McMURRAY SURBASINS
9. CONCLUSIONS
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 interpretations of the Ostracode Zone paleoenvironment using
paleontological and sedimentological methods (e.g., Finger,
1983; James, 1985; Wanklyn, 1985; Mattison, 1987; Banerjee 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 contribution must have been small, possibly localized, and probably sporadic. Accordingly, the freshwater flux was of sufficient magnitude to keep the Ostracode depositional waters
fresh most of the time.
Strontium isotope ratios in fossil shells may be used to
determine depositional paleosalinities in ancient brackish
water systems. Two strategies were investigated for reconstructing the strontium isotope paleohydrology and determining paleosalinities. The first method utilized 87Sr/86Sr ratios
in contemporaneous marine and freshwater fossils to construct 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.
C. Holmden et al.
2116
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, Dsris 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.
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 extended 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 Vossler, 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.
Acknowledgments-Mike
Editorial handling: T. K. Kyser
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