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

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• 作者：Jonathan B. Martin ; Paul J. Moore
• 关键词：Carbonate platforms ; Sr isotope ratios ; Hydrogeology ; Bahamas ; Aragonite ; Calcite
• 刊名：Chemical Geology
• 出版年：2008
• 出版时间：30 March 2008
• 年：2008
• 卷：249
• 期：1-2
• 页码：52-65
• 全文大小：973 K

文摘

The depth to which seawater and fresh water circulate through modern carbonate platforms may be estimated with ⁸⁷Sr/⁸⁶Sr 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 ⁸⁷Sr/⁸⁶Sr ratios equivalent to modern seawater within error of the measurement, indicating alteration of shallow buried Late Pleistocene to Holocene carbonate minerals. Four samples have mineral-derived Sr²⁺ with ⁸⁷Sr/⁸⁶Sr ratios lower than modern seawater value. These low ratios reflect alteration of carbonate minerals that were deposited around 1 mybp, although the measured ⁸⁷Sr/⁸⁶Sr 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 modern carbonate minerals at the surface would provide an unknown, but probably large amount of Sr²⁺ with modern seawater isotope signatures, thereby overprinting any low ⁸⁷Sr/⁸⁶Sr ratios of non-modern mineral-derived Sr²⁺. Three of the four samples with low ⁸⁷Sr/⁸⁶Sr 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 (~ 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.