Abstract This study applied a multianalytical approach to identify active early diagenetic processes during deposition of the Lower Jurassic Bachental bituminous marls (Western Tethys realm). Results point to a major redox control on organic matter (OM) preservation and early diagenetic modifications. Whereas Mn reduction was the main OM oxidation process causing a maximum OM loss of ~1.5% and a lowering of hydrogen index values by up to 400 mgHC/gTOC under suboxic conditions, sulfate reduction was the dominant degradation process under anoxic-euxinic conditions. OM oxidation processes caused an increase of pH values (up to >9) and alkalinity in pore waters. These changes initiated carbonate precipitation in all samples, with high Mn-contents and predominant kutnohorite mineralization in suboxic layers, and presence of Mn-bearing calcite in anoxic units. The actual occurrence of the distinct carbonate phases (dolomite, kutnohorite, Mn-bearing calcite and siderite) reflects the redox pattern established by previous studies. Dissolution of biogenic silica and incipient alteration of pyrite in response to changing pore water pH and alkalinity conditions was found limited to the anoxic-euxinic intervals. In summary, this study provides detailed insights into the early diagenesis of organic-rich marls in a semi-restricted basin setting under varying oxygen depletion.

中文翻译：

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低氧到富氧条件下富含有机质泥灰岩的早期成岩作用：Bächental 盆地下部 Toarcian

摘要 本研究采用多分析方法来确定下侏罗统 Bachental 沥青泥灰岩（西特提斯界）沉积期间活跃的早期成岩过程。结果表明对有机质 (OM) 保存和早期成岩修饰的主要氧化还原控制。Mn 还原是主要的 OM 氧化过程，导致最大 OM 损失约 1.5%，并且在低氧条件下氢指数值降低高达 400 mgHC/gTOC，而硫酸盐还原是缺氧-富氧条件下的主要降解过程。OM 氧化过程导致孔隙水中的 pH 值（高达 >9）和碱度增加。这些变化引发了所有样品中的碳酸盐沉淀，在低氧层中具有高锰含量和主要的 kutnohorite 矿化，和含锰方解石在缺氧单元中的存在。不同碳酸盐相（白云石、钾盐辉石、含锰方解石和菱铁矿）的实际出现反映了先前研究建立的氧化还原模式。发现生物二氧化硅的溶解和黄铁矿随着孔隙水 pH 值和碱度条件的变化而发生变化，仅限于缺氧-富氧期。总而言之，这项研究提供了对不同氧耗竭下半限制性盆地环境中富含有机质泥灰岩的早期成岩作用的详细见解。发现生物二氧化硅的溶解和黄铁矿随着孔隙水 pH 值和碱度条件的变化而发生变化，仅限于缺氧-富氧期。总而言之，这项研究提供了对不同氧耗竭下半限制性盆地环境中富含有机质泥灰岩的早期成岩作用的详细见解。发现生物二氧化硅的溶解和黄铁矿随着孔隙水 pH 值和碱度条件的变化而发生变化，仅限于缺氧-富氧期。总而言之，这项研究提供了对不同氧耗竭下半限制性盆地环境中富含有机质泥灰岩的早期成岩作用的详细见解。