Early diagenesis of shallow-water periplatform carbonate sediments, leeward margin, Great Bahama Bank (Ocean Drilling Program Leg 166)

Mineralogic, petrographic, and geochemical analyses of sediments recovered from two Leg 166 Ocean Drilling Program cores on the western slope of Great Bahama Bank (308 m and 437 m water depth) are used to characterize early m arine diagenesis of these shallow-water, periplatform carbonates. The most pronounced diagenetic products are well-lithified intervals found almost ex clusively in glacial lowstand deposits and interpreted to have formed at or near the seafloor (i.e., hardgrounds), Hardground cements are composed of high-Mg calcite (similar to 14 mol% MgCO3), and exhibit textures typically associated with seafloor cementation. Geochemically, hardgrounds are charac terized by increased delta O-18 and Mg contents and decreased delta C-13, S r, and Na contents relative to their Less lithified counterparts. Despite being deposited in shallow waters that are supersaturated with the common carbonate minerals, it is clear that these sediments are also underg oing shallow subsurface diagenesis, Calculation of saturation states shows that pore waters become undersaturated with aragonite within the upper 10 m at both sites. Dissolution, and likely recrystallization, of metastable ca rbonates is manifested by increases in interstitial water Sr and Sr/Ca prof iles with depth, We infer that the reduction in mineral saturation states a nd subsequent dis-solution are being driven by the oxidation of organic mat ter in this Fe-poor carbonate system. Precipitation of burial diagenetic ph ases is indicated by the down-core appearance of dolomite and corresponding decrease in interstitial water Mg, and the presence of low-Mg calcite ceme nts observed in scanning electron microscope photomicrographs.