冷泉碳酸盐晶格硫（CAS）和黄铁矿多硫同位素组成及其对地质历史时期甲烷通量的指示意义

Sulfate-driven anaerobic oxidation of methane (SD-AOM) are the dominant biogeochemical processes occurring at cold seep (methane-rich environment) in marine settings, which control the greenhouse gas budgets in the geologic past. During geologic history, it’s could be common for the periodic increases of methane flux in marine sediment. In turns, the identification of SD-AOM activity is a prerequisite for a reliable reconstruction of past methane-rich setting as well as its environmental effects. Yet, it remains elusive to identify the SD-AOM activity in the rock record. Recent study showed that the triple sulfur isotope composition (Δ33S-δ34S) of cold-seep barite can be well used to identify the past SD-AOM activity. The fact, however, is that barite is not common minerals in cold-seeps or the methane-rich environment. Here, we propose that the triple sulfur isotope composition of carbonate-associated sulfate (CAS) can provide a promising tool for reconstructing past SD-AOM activity. To test this hypothesis, the present project is designed to measure the triple sulfur isotope composition of CAS of modern and ancient cold-seep carbonates. Given the common effect of pyrite-oxidation on the sulfur isotope composition of CAS during buried period and/or extraction process of CAS, this project is also proposed to investigate the triple sulfur isotope composition of pyrite, combined with the oxygen isotope composition of CAS, major/trace element composition and mineral composition of the studied samples. The obtained data can final constrain how the prosed proxy server as a promising indicator of the SD-AOM activity in records, which can further be used to explore the role of methane in the earth's surface environment evolution.

硫酸盐还原驱动的甲烷厌氧氧化作用（SD-AOM）是海底冷泉系统（富甲烷环境）中最重要的生物地球化学过程，该过程消耗了超过90%的渗漏到海底的甲烷，控制海底向海水乃至大气释放的甲烷通量。地质历史时期甲烷释放通量的阶段性增加可能是普遍发生的，SD-AOM作用的识别可以追踪甲烷释放的沉积记录。最新研究表明，冷泉重晶石多硫同位素组成（Δ33S vs.δ34S）可以识别地质历史时期SD-AOM，但冷泉重晶石并不广泛发育，而碳酸盐和黄铁矿在几乎所有富甲烷环境都有发育。因此，本文拟通过冷泉碳酸盐岩中晶格硫（CAS）和黄铁矿的多硫同位素组成，结合CAS氧同位素、碳酸盐岩主微量元素及矿物组成，探究黄铁矿氧化对CAS多硫同位素组成的影响，建立地质历史时期富甲烷环境的多硫同位素识别体系，为探究甲烷在地球表层环境演化中的角色打开新的视角。

微生物硫酸盐还原作用驱动的甲烷厌氧氧化作用（SD-AOM）地质记录的示踪是探究过去甲烷排放及其在全球碳硫循环中作用的重要前提。前人研究发现甲烷“冒泡区”形成的冷泉重晶石多硫同位素组成Δ33S vs. δ34S具有负相关性，显著区分与正常海洋环境（有机质驱动的硫酸盐还原作用）中的正相关关系，由此奠定了示踪甲烷厌氧氧化作用的多硫同位素方法新视角。然而冷泉重晶石在冷泉和富甲烷沉积环境中并不普遍存在。碳酸盐岩和黄铁矿在富甲烷环境普遍存在。因此，我们开展了冷泉系统中的碳酸盐岩晶格硫酸盐（CAS）、黄铁矿、孔隙水硫酸根和硫化物的多硫同位素组成研究，结果表明：（1）CAS具有高δ34S和低Δ33S的特征，显著区分与正常海洋沉积环境中的孔隙水硫酸盐的多硫同位素组成特征，表明CAS多硫同位素组成可以用来示踪SD-AOM作用；（2）甲烷“冒泡区”的硫化物的多硫同位素组成与富有机质环境中的一致，表明黄铁矿多硫同位素组成示踪高甲烷通量下的SD-AOM作用的局限性；（3）高黄铁矿含量（>0.5wt%）的碳酸盐岩样品具有更低的Δ33S值，二端元混合模型分析表明即使发生黄铁矿氧化CAS多硫同位素组成仍然可以示踪SD-AOM的作用；（4）扩散性甲烷渗漏环境孔隙水硫酸根Δ33S vs. δ34S具有负相关性，表明低甲烷通量下SD-AOM仍然具有低硫同位素分馏特征，且该环境下形成的自生黄铁矿具有更大的Δ33S vs. δ34S分布区间（相比正常海洋沉积环境和高甲烷通量沉积环境的黄铁矿多硫同位素组成）。研究成果突出了电子共同类型对微生物硫酸盐还原过程硫同位分馏的关键性控制因素，解析了富甲烷环境多地质载体中的多硫同位素组成特征及控制因素，建立了示踪SD-AOM的多硫同位素方法体系，为评估甲烷在地球表层环境演化中的作用提供了方法支撑。