GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 159-3
Presentation Time: 9:00 AM-6:30 PM


BADDOUH, M'bark1, CARROLL, Alan R.2, BEARD, Brian L.3 and JOHNSON, Clark, M.3, (1)Department of Geoscience, University of Wisconsin-Madison, 1215 W. Dayton St, Madison, WI 53706; Department of Atmospheric, Oceanic and Earth Sciences, George Mason University, 44000 University Dr, Farifax, VA 22030, (2)Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706, (3)Department of Geoscience, University of Wisconsin-Madison, NASA Astrobiology Institute, 1215 W. Dayton St, Madison, WI 53706,

Positive covariance of δ18O and δ13C in ancient and modern lacustrine carbonate facies has commonly been interpreted to reflect variable residence time of water in endorheic (underfilled) lake basins. We test this hypothesis by integrating δ18O and δ13C values with 87Sr/86Sr ratios from Eocene lake-marginal carbonate facies preserved at Little Mesa. These deposits mark the maximum expansion of a long-lived hypersaline lake. Carbonate facies comprise a complex mosaic of grainstone to boundstone (microbialite mound) fabrics, with widely varying proportions of calcite versus dolomite. δ13C ranges from -1.77 to 3.37 ‰ VPDB, and δ18O ranges from -7.20 to -3.10 ‰ VPDB (n=27). δ18O and δ13C are strongly positively correlated (r2=0.85). Higher δ18O and δ13C values generally occur in more calcitic samples, which is surprising in light of the expected greater fractionation of 18O into dolomite. This result also appears to be inconsistent with previous dolomitization models invoking evaporative concentration of mudflat pore waters.

 87Sr/86Sr ratios are the lowest yet measured for the deposits of Eocene Lake Gosiute, ranging from 0.71022 to 0.71140, consistent with previous studies showing that lakewater became less radiogenic as lake levels rose. 87Sr/86Sr ratios are strongly positively correlated with both δ13C (r2=0.83), and δ18O (r2=0.74). Because meteoric processes do not significantly fractionate 87Sr/86Sr, this suggests that the observed covariance between δ18O and δ13C may reflect varying relative contributions of water from two geochemically distinct sources, rather than evaporative distillation. Baddouh et al. (2016) proposed that lake highstands reflect increased drainage from thick, Sr-rich marine carbonate strata located west of the basin. In contrast, more radiogenic waters drained from Precambrian-cored uplifts located to the north, south, and east. 87Sr/86Sr ratios in the Little Mesa carbonates are also weakly correlated with carbonate mineralogy, with lower ratios associated with more dolomitic samples. This could indicate that dolomitization was preferentially associated with elevated Mg/Ca ratios in waters draining from the west. Dolomitization may also have been influenced by the ecology of the mounds themselves, via organic matter mediation, or by burial diagenesis.