PALEOSALINITIES IN ANCIENT BRACKISH-WATER SYSTEMS DETERMINED BY SR-87SR-86 RATIOS IN CARBONATE FOSSILS - A CASE-STUDY FROM THE WESTERN CANADA SEDIMENTARY BASIN/

Two strategies for determining paleosalinities in ancient brackish wat er deposits are presented based on the Sr-87/Sr-86 ratios in well pres erved carbonate fossil shells. The relative contributions of seawater and freshwater can be determined for shells of unknown or presumed eco logical affinity by comparing their Sr-87/Sr-86 ratios to contemporane ous marine and fluviatile sources. The nearly one hundredfold differen ce in Sr concentration between seawater (7.7 ppm) and freshwater(media n = 0.071 ppm) dictates that mixing relations define hyperbolae. Paleo salinities are most precisely determined along the freshwater asymptot e (<15 parts per thousand) because these waters have the lower Sr conc entration, and, therefore, display the greatest rate of change of Sr-8 7/Sr-86 With salinity. Paleosalinities greater than 15 parts per thous and are resolvable if the Sr-87/Sr-86 or Sr concentration in the fresh water mixing endmember is greater than the 50th percentile for present -day Sr concentrations in world rivers and lakes. A second technique u tilizes both Sr-87/Sr-86 and Sr/Ca from a single fossil species and al lows the brackish water hypothesis to be tested graphically on a plot of Sr-87/Sr-86-Ca/Sr. Linear correlations on this plot are evidence fo r two-component mixing. Measured Sr/Ca in mollusc shells are related t o the original Sr/Ca in the habitat waters by a species specie Sr dist ribution coefficient (D-Sr). For the case of seawater-freshwater mixin g, D-Sr is fixed in that seawater must plot on the water mixing line. If the transformation between shell and water mixing lines yields a Ds , within the range of modern values (0.2-0.3), a brackish water habita t is implied. Examples from purported brackish water deposits of the e arly Cretaceous Mannville Group, Canada, illustrate the potential uses of Sr-87/Sr-86, Sr, and Sr/Ca for determining paleosalinities in anci ent estuarine and estuarine-like deposits, and the importance of the p aleohydrological perspective as a factor influencing the interpretatio n of ancient depositional environments. Copyright (C) 1997 Elsevier Sc ience Ltd.