Abstract

The geochemistry of meteoric calcite cements and their parent waters on three small Holocene ooid-sand islands in the Schooner Cays, Bahamas, is explicated in terms of observed hydrologic and hydrochemical processes. Dissolved Mg is derived soley from admixed seawater or aerosols. The Mg contents of the cements (0.4 to 3.0 mole %) are thus a salinity indicator and suggest cementation in waters composed of < 1% to 12% seawater. Dissolved Sr is derived from aragonite dissolution; thus Sr contents of the cements (600 to 3,700 ppm) reflect the amount of aragonite-to-calcite alteration in the pore waters prior to precipitation of the cements (< 3 to > 16 mmoles/liter per yr). Variations in Mg and Sr contents across cement mosaics reflect long-term temporal fluctuations in the amount of aragonite-to-calcite alteration and incursions of brackish waters. The pore-water oxygen reservoir is water-buffered and derived from meteoric rainfall and seawater mixing, but is seasonally enriched in 18 O by evaporation. The delta 18 O values of the cements (-3.4 to -5.0 per thousand PDB) reflect the relative proportions of cements formed during and immediately after the rainy season versus those formed during the dry season. The pore-water carbon is derived from organic respiration, atmospheric carbon, and dissolved aragonite. Organic respiration dominates the delta 18 C of the vadose cements (-7.8 to + 1.1 per thousand PDB) and the delta 13 C of phreatic groundwaters (-2.2 to -14.0% per thousand PDB). In contrast, 13 C-enriched phreatic cements (-1.2 to +3.7% per thousand PDB) reflect carbon from dissolved aragonite, and are not in carbon isotopic equilibrium with the present groundwaters. This case example reveals the complex geochemical signal that can result from even a very simple diagenetic history, such as a single phase of meteoric diagenesis acting on a purely aragonitic sediment.

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