A significant proportion of marine calcium carbonate sediments are comprised of metastable minerals that are susceptible to diagenetic alterations during burial. These reactions can reset the geochemical signature of sediments and pore fluids and influence elemental cycling in the ocean. However, the timing and mechanisms by which these reactions take place are poorly constrained. This study uses cores drilled on the slope of the Great Bahama Bank to provide quantitative constraints on important diagenetic reactions; namely respiration-driven dissolution, authigenic carbonate mineral formation, and conversion of aragonite to low Mg calcite (LMC). We perform detailed mineralogical characterization using newly acquired, high resolution X-ray diffraction (XRD) data and over 1000 reanalyzed XRD scans, characterize sediments texturally and elementally using electron microprobe data, calculate pore fluid saturation state with respect to aragonite using a Pitzer ion interaction approach, and use pore fluid chemistry and experimental distribution coefficients to predict authigenic carbonate compositions. These data suggest that aerobic organic matter oxidation, enabled by the advection of oxygenated seawater throughout the upper ∼ 30 m interval of sediment, causes undersaturation and thus dissolution of biogenic high Mg calcite (HMC) and aragonite. Deeper, anaerobic organic matter oxidation takes over and causes supersaturation, promoting authigenic precipitation in the form of HMC, which in turn decreases pore fluid Mg/Ca and promotes aragonite conversion to LMC. One novel aspect of this study is the identification of three types of calcites using the newly acquired XRD and electron microprobe data. Based on their unique crystallographic characteristics and chemical compositions, these types of calcites are interpreted to represent pelagic biogenic LMC, bank-derived biogenic HMC, and authigenic HMC precipitated from pore fluids. Notably, the composition of the authigenic calcite matches that predicted to precipitate from pore fluids using an empirical Mg partition coefficient in calcite. Calcites that form authigenically and from aragonite via replacement are suggested to recrystallize with burial as evidenced by the decrease in Mg content and micro-strain. This process-based geochemical framework assigns diagenetic processes to a specific depth window within the sediment column and paves the way for a mechanistic understanding of carbonate diagenesis, one that is rooted in thermodynamic and kinetic bases.

中文翻译：

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海洋成岩过程中碳酸盐沉积物基于过程的地球化学框架


海洋碳酸钙沉积物的很大一部分由亚稳矿物组成，这些矿物在埋藏过程中容易受到成岩作用的改变。这些反应可以重置沉积物和孔隙流体的地球化学特征，并影响海洋中的元素循环。然而，这些反应发生的时间和机制却很少受到限制。这项研究使用在大巴哈马浅滩斜坡上钻取的岩心来为重要的成岩反应提供定量约束；即呼吸驱动的溶解、自生碳酸盐矿物的形成以及文石向低镁方解石 (LMC) 的转化。我们使用新获取的高分辨率 X 射线衍射 (XRD) 数据和超过 1000 次重新分析的 XRD 扫描进行详细的矿物学表征，使用电子显微探针数据对沉积物进行结构和元素表征，使用 Pitzer 离子相互作用计算文石的孔隙流体饱和状态方法，并使用孔隙流体化学和实验分配系数来预测自生碳酸盐成分。这些数据表明，含氧海水在整个沉积物上部～30 m区间的平流作用下发生需氧有机物氧化，导致欠饱和，从而导致生物高镁方解石（HMC）和霰石的溶解。更深层次的厌氧有机物氧化接管并引起过饱和，促进 HMC 形式的自生沉淀，进而降低孔隙流体 Mg/Ca 并促进文石转化为 LMC。这项研究的一个新颖之处是使用新获得的 XRD 和电子显微探针数据识别了三种类型的方解石。 根据其独特的晶体学特征和化学成分，这些类型的方解石被解释为代表远洋生物成因 LMC、河岸来源的生物成因 HMC 和从孔隙流体中沉淀出来的自生 HMC。值得注意的是，自生方解石的成分与使用方解石中的经验镁分配系数预测的从孔隙流体中沉淀出来的成分相匹配。自生形成的方解石和文石通过置换形成的方解石被认为会随着埋藏而重结晶，镁含量和微应变的降低就证明了这一点。这种基于过程的地球化学框架将成岩过程分配给沉积物柱内的特定深度窗口，并为碳酸盐岩成岩作用的机械理解铺平了道路，碳酸盐岩成岩作用植根于热力学和动力学基础。