Mathematics – Logic
Scientific paper
May 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997gecoa..61.2105h&link_type=abstract
Geochimica et Cosmochimica Acta, Volume 61, Issue 10, p. 2105-2118.
Mathematics
Logic
4
Scientific paper
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‰) because these waters have the lower Sr concentration, and, therefore, display the greatest rate of change of 87Sr/86Sr with salinity. Paleosalinities greater than 15‰ 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 (DSr). For the case of seawater-freshwater mixing, DSr is fixed in that seawater must plot on the water mixing line. If the transformation between shell and water mixing lines yields a DSr within the range of modern 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 Sr/Ca 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 environments.
Creaser Robert A.
Holmden Chris
Muehlenbachs Karlis
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