--> Abstract: Carbon and Strontium Isotopic Variations in Pleistocene Carbonates and Modern Groundwaters of Barbados, West Indies: A Temporal Record of Carbonate Diagenetic Fluids, by J. L. Banner, M. Musgrove, and J. D. Humphrey; #90987 (1993).

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BANNER, JAY L., and MARYLYNN MUSGROVE, University of Texas, Austin, TX; and JOHN D. HUMPHREY, Colorado School of Mines, Golden, CO

ABSTRACT: Carbon and Strontium Isotopic Variations in Pleistocene Carbonates and Modern Groundwaters of Barbados, West Indies: A Temporal Record of Carbonate Diagenetic Fluids

Uplifted Pleistocene coral-reef terraces on Barbados comprise an aquifer that is built on a confining unit of Tertiary deep sea sediments. Meteoric recharge is transmitted by topographically-driven flow from the central, elevated (and older) portion of the island radially outward to the younger, coastal portions. This flow system has produced extensive alteration of the reef carbonates, and provides a setting to examine the evolution of diagenetic fluids from the Pleistocene to present day using carbon and strontium isotopic variations.

Groundwaters have isotope 87Sr = -20 to -1 and isotope 13C = -12 to -6, while Pleistocene carbonates have isotope 87Sr = -10 to 2 and isotope 13C = -9 to 0 (modern seawater has isotope 87Sr = 0 and isotope 13C = 1.4). Tertiary sediments have very low isotope 87Sr of -200 to - 10, and reflect the Sr isotopic evolution of seawater. Soils have similar Sr and C isotopic compositions to their host Pleistocene carbonates, indicating a dominant lithogenic component. Groundwaters and carbonates from the older, updip portions of the aquifer have lower isotope (87Sr and 13C) values and Sr/Ca ratios than those from the younger, down gradient part of the aquifer. At a given elevation, groundwaters have lower values than the carbonates.

These results indicate a groundwater evolution involving the aquisition of: (1) low isotope 13C from soil-gas CO2 during rainfall recharge, and (2) low isotope 87Sr via interaction with Tertiary sediments during flow along the base of the Pleistocene aquifer. The pronounced isotopic differences at high elevations are consistent with limited host rock influence on groundwater chemistry in this more mineralogically-mature part of the island. However, most of the waters in the younger, more aragonitic terraces still have notably lower isotope values than their host rocks. The persistence of allochthonous updip isotopic signatures in groundwaters, even during migration through metastable carbonate terrains, may reflect thermodynamic (i.e., equilibration with updip carbonates) and/or kinet c (i.e., high flow rates) barriers to groundwater-limestone interaction. This has significant implications for modeling ancient diagenetic systems presumed to be `rock-buffered' during early diagenesis. Diagenetic phases have generally lower isotope values than their Pieistocene host rocks, indicating that the Pieistocene pore fluid system of Barbados transported dissolved ions along similar flow paths to the modern groundwater system.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.