Carbonate concretions provide unique records of ancient biogeochemical processes in marine sediments, and have the potential to reflect seawater chemistry indirectly. In fine-siliciclastic settings, they preferentially form in organic-rich mudstones, owing to a significant fraction of the bicarbonate required for carbonate precipitation resulted from the decomposition of organic matter in sediments. In the Member IV of the Xiamaling Formation (ca. 1.40–1.35 Ga), North China, however, carbonate concretions occur in organic-poor green silty shales (avg. TOC = ~ 0.1 wt%). In order to elucidate the mechanism of the concretion formation and their environmental implications, a thorough study on the petrographic and geochemical compositions of the concretions and their host rocks was conducted. Macro- to microscopic fabrics, including deformed shale laminae surrounding the concretions, “cardhouse” structures of clay minerals and calcite geodes in the concretions, indicate that these concretions are of early diagenetic origin prior to the significant compaction of clay minerals. The carbon isotope compositions of the concretions (− 1.7‰ to + 1.5‰) are stable and close to or slightly lower than that of the contemporaneous seawater, indicating that the bicarbonates required for the concretion formation were mainly sourced from seawater by diffusion rather than produced by methanogenesis or anoxic oxidation of methane (AOM); the rare occurrence of authigenic pyrite grains in the concretions likely indicates that bacterial sulfate reduction (BSR) did not play a significant role in their formation either. Almost all the calcite in the concretions has low Mn–Fe in nuclei but high Mn–Fe in rims with average Mn/Fe ratio close to 3.3. The calcite shows positive Ce anomalies (avg. 1.43) and low Y/Ho ratios (avg. 31). This evidence suggests that Mn reduction is the dominant process responsible for the formation of calcite rims while nitrate reduction probably triggered the precipitation of calcite nuclei. Prominence of Mn reduction in the porewater likely indicates that there was sufficient oxygen to support active Mn-redox cycling in the overlying seawater.

中文翻译：

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华北~1.4嘎下马岭组碳酸盐结核生长机制及环境影响

碳酸盐结核提供了海洋沉积物中古代生物地球化学过程的独特记录，并有可能间接反映海水化学。在细硅质碎屑岩环境中，它们优先形成于富含有机物的泥岩中，因为沉积物中有机物分解产生的碳酸盐沉淀所需的碳酸氢盐占很大比例。然而，在华北的下马岭组 IV 段（约 1.40-1.35 Ga），碳酸盐结核发生在有机质贫乏的绿色粉质页岩中（平均 TOC = ~ 0.1 wt%）。为了阐明结核形成的机制及其对环境的影响，对结核及其主岩的岩石学和地球化学成分进行了深入研究。宏观到微观织物，包括结核周围变形的页岩层、粘土矿物的“卡屋”结构和结核中的方解石晶界，表明这些结核是粘土矿物显着压实之前的早期成岩成因。结核（- 1.7‰至+ 1.5‰）的碳同位素组成稳定，接近或略低于同期海水，表明结核形成所需的碳酸氢盐主要来自海水扩散而不是生产通过甲烷生成或缺氧氧化（AOM）；结核中罕见的自生黄铁矿颗粒可能表明细菌硫酸盐还原 (BSR) 在它们的形成中也没有发挥重要作用。几乎所有结核中的方解石在核中具有低 Mn-Fe，但在边缘具有高 Mn-Fe，平均 Mn/Fe 比接近 3.3。方解石显示出 Ce 正异常（平均 1.43）和低 Y/Ho 比（平均 31）。该证据表明，Mn 还原是导致方解石边缘形成的主要过程，而硝酸盐还原可能引发方解石核的沉淀。孔隙水中 Mn 还原的显着性可能表明有足够的氧气支持上覆海水中活跃的 Mn-氧化还原循环。该证据表明，Mn 还原是导致方解石边缘形成的主要过程，而硝酸盐还原可能引发方解石核的沉淀。孔隙水中 Mn 的显着减少可能表明有足够的氧气支持上覆海水中活跃的 Mn-氧化还原循环。该证据表明，Mn 还原是导致方解石边缘形成的主要过程，而硝酸盐还原可能引发方解石核的沉淀。孔隙水中 Mn 的显着减少可能表明有足够的氧气支持上覆海水中活跃的 Mn-氧化还原循环。