Abstract:
Pleistocene-age sediments recovered from two holes drilled on the western slope of Great Bahama Bank during ODP Leg 166 were examined for mineralogic, elemental, and isotopic variations to determine the nature, timing, and extent of diagenetic alteration. The two sites, 1009 and 1008, are located 4.5 km from the platform edge in 308 m of water, and 7.2 km from the platform edge in 437 m of water, respectively. It is generally assumed that waters overlying such settings are substantially supersaturated with respect to calcites and aragonite, and little or no diagenetic alteration occurs unless exposed to meteoric fluids. However, shipboard interstitial water analyses indicate that diagenetic alteration of these sediments is occurring. Sediments at both sites are carbonate-rich (>90 wt%), composed principally of aragonite, high-Mg calcite (HMC), low-Mg calcite (LMC), with lesser amounts of dolomite. Facies successions contain indications of high frequency sea-level variations typically associated with periplatform carbonates. Neritic, platform-derived sediments (highstand deposits) composed primarily of aragonite and HMC alternate with more pelagic-rich intervals (lowstand deposits) predominately composed of LMC. Samples were collected at 1.5 m intervals at each site as well as from selected lithologies (hardgrounds and more lithified layers). Downcore trends previously attributed to diagenetic alteration in deeper-water periplatform carbonate settings are not observed. However, more lithified samples show evidence of geochemical modification having more positive delta (super 18) O, less positive delta (super 13) C, lower Sr and Na, and higher Mg contents than their unlithified counterparts. Mineralogically, most lithified samples have decreased aragonite but elevated HMC contents suggesting dissolution of aragonite and reprecipitation of HMC cements, which was confirmed by microprobe analyses. Precipitation of HMC cements at these water depths and temperatures is predicted by both empirical and experimental studies. Refinement of the timing of diagenesis awaits development of a more detailed age-model.