Composition of hardgrounds and of pore-water parameters in ODP Holes 166-1008 and 166-1009

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 marine diagenesis of these shallow-water, periplatform carbonates. The most pronounced diagenetic products are well-lithified intervals found almost exclusively 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 (~14 mol% MgCO3), and exhibit textures typically associated with seafloor cementation. Geochemically, hardgrounds are characterized by increased d18O and Mg contents and decreased d13C, Sr, 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 undergoing 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 carbonates is manifested by increases in interstitial water Sr and Sr/Ca profiles with depth. We infer that the reduction in mineral saturation states and subsequent dissolution are being driven by the oxidation of organic matter in this Fe-poor carbonate system. Precipitation of burial diagenetic phases is indicated by the down-core appearance of dolomite and corresponding decrease in interstitial water Mg, and the presence of low-Mg calcite cements observed in scanning electron microscope photomicrographs.对采自大巴哈马滩西部斜坡两处大洋钻探计划（Ocean Drilling Program，ODP）第166航次岩芯（水深分别为308米与437米）的沉积物开展矿物学、岩石学及地球化学分析，以此表征这套浅水环境台地周缘碳酸盐岩的早期海洋成岩作用。其中最显著的成岩产物为几乎仅产出在冰期低水位沉积中的强固结层段，被认为形成于海底或近海底环境（即硬底（hardgrounds））。硬底胶结物由高镁方解石（high-Mg calcite，约含14 mol% MgCO₃）组成，其结构特征通常与海底胶结作用相关。地球化学层面，相较于固结程度更低的沉积物，硬底的δ¹⁸O与Mg含量显著升高，而δ¹³C、Sr及Na含量则有所降低。 尽管这些沉积物沉积于常见碳酸盐矿物饱和的浅水环境，但显然它们同样经历了浅埋藏成岩作用。饱和度状态计算结果显示，两处站位的孔隙水在上部10米范围内便出现文石不饱和现象。亚稳定碳酸盐的溶蚀与大概率伴随的重结晶作用，可通过孔隙水Sr含量及Sr/Ca比值随深度增加的特征得到体现。我们推断，在这一贫铁碳酸盐体系中，有机质氧化作用驱动了矿物饱和度状态降低及后续的溶蚀过程。埋藏成岩相的沉淀现象则表现为：随岩芯深度增加依次出现白云石（dolomite）、孔隙水Mg含量相应下降，且扫描电子显微镜（scanning electron microscope，SEM）显微照片中可见低镁方解石（low-Mg calcite）胶结物。