Sr concentrations and isotope ratios as tracers of ground-water circulation in carbonate platforms: Examples from San Salvador Island and Long Island, Bahamas

摘要

The depth to which seawater and fresh water circulate through modem carbonate platforms may be estimated with Sr-17/Sr-16 isotope ratios of dissolved Sr2+ that is enriched through carbonate mineral dissolution and recrystallization. In 23 water samples from onshore San Salvador Island and Long Island, Bahamas, carbonate mineral dissolution and aragonite-to-calcite transformations elevate Sr2+ concentrations to twice seawater values in water with near seawater salinity, and to about 130 times the expected value for seawater that has been mixed with fresh water. Carbonate mineral dissolution enriches Ca2+ concentrations to around 30 times seawater concentrations only in the mixed waters; water with seawater salinity has approximately seawater Ca2+ concentrations. Assuming two end-member mixing between seawater Sr2+ and mineral-derived Sr2+, model estimates indicate that mineral-derived Sr2+ of 19 samples have Sr-87/Sr-86 ratios equivalent to modem seawater within error of the measurement, indicating alteration of shallow buried Late Pleistocene to Holocene carbonate minerals. Four samples have mineral-derived Sr2+ with Sr-87/Sr-86 ratios lower than modem seawater value. These low ratios reflect alteration of carbonate minerals that were deposited around 1 mybp, although the measured Sr-87/Sr-86 values could reflect ages as great as 4.6 Ma considering the analytical uncertainty of the measurements. These estimated ages are likely to be minimum values because alteration of modem carbonate minerals at the surface would provide an unknown, but probably large amount of Sr2+ with modem seawater isotope signatures, thereby overprinting any low Sr-87/Sr-86 ratios of non-modem mineral-derived Sr2+. Three of the four samples with low Sr-87/Sr-86 ratios have high salinities and were collected from the interior of the islands. They reflect seawater flow paths at least tens of meters deep may link the ocean to water several kilometers inland. The fourth sample is from the fresh-water lens below a Pleistocene beach ridge (similar to 125 ka) only 100 m from the shore line. This sample suggests the lens may be thicker than expected based on estimates of recharge, hydraulic conductivity and size of the ridge. (C) 2007 Elsevier B.V. All rights reserved.