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Tracing ground-water evolution in a limestone aquifer using Sr isotopes: Effects of multiple sources of dissolved ions and mineral-solution reactions

¹ Department of Geological Sciences, University of Texas, Austin, Texas 78712
² Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125

Uplifted Pleistocene coral-reef terraces on Barbados, West Indies, constitute an aquifer that is built on low-permeability Tertiary pelagic rocks that overlie the Barbados accretionary prism. The downdip segments of the aquifer are composed of younger reef limestones that contain more aragonite and have higher ⁸⁷Sr/⁸⁶Sr and Sr/Ca ratios than the updip parts of the aquifer. Ground waters and host limestones display similar stratigraphic trends in ⁸⁷Sr/⁸⁶Sr and Sr/Ca. The ground waters have lower ⁸⁷Sr/⁸⁶Sr values, however, indicating that they acquire a significant fraction of their dissolved Sr through interaction with components of Tertiary rocks, which compose the underlying aquitard and parts of overlying soils. Geochemical modeling results indicate that ground-water evolution is controlled by (1) variations in the age and composition of the aquifer and aquitard rocks and (2) the relative roles of calcite dissolution, calcite recrystallization, and the transformation of aragonite to calcite. Sr isotopes can provide unique information for tracing ground-water evolution, which requires consideration of the multiple components and processes that make up even relatively simple limestone aquifer systems.

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