Marine carbon burial enhanced by microbial carbonate formation at hydrocarbon seeps@en

Some of the carbon removed from Earth's surface is stored within authigenic carbonate in marine sediments. Methane seeps are crucial sites of global marine carbon cycling sustaining microbial activity, enabling carbonate formation and the transfer of methane-derived carbon to the geosphere. Carbon sequestration rates depend on carbonate precipitation rate, which can be accelerated by mat-forming microorganisms that are ubiquitous at methane seeps and other Earth surface environments today. We investigate a 5-m-long drill core from an active methane seep at 1500 m water depth in the South China Sea with an exceptional abundance of pink and clear aragonite cement derived from the sulphate-driven anaerobic oxidation of methane, yet both cements precipitated under different conditions. Phase-specific 230Th/U-based ages, lipid biomarker compositions, and calcium isotope data suggest that pink aragonite is a product of in situ biofilm mineralization. First estimated precipitation rates of these individual cements in the seep carbonates range from 0.04 cm/ka for clear aragonite to 1.0 cm/ka for pink aragonite, suggesting an up to 25-fold increase in precipitation rates associated with biofilm mineralization. These results provide first kinetic constraints for future quantitative carbon cycle models, emphasizing the role of biofilms in accelerating carbon sequestration in marine authigenic carbonates.从地球表层被剥离的部分碳，会储存在海洋沉积物中的自生碳酸盐（authigenic carbonate）内。甲烷冷泉（methane seeps）是全球海洋碳循环（global marine carbon cycling）的关键场所，可维持微生物活动，促进碳酸盐形成，并将甲烷来源的碳转移至岩石圈（geosphere）。碳封存速率（carbon sequestration rates）取决于碳酸盐沉淀速率（carbonate precipitation rate），而如今广泛分布于甲烷冷泉及其他地球表层环境的菌膜形成微生物（mat-forming microorganisms），可加速该沉淀过程。本研究对采自南海1500米水深活跃甲烷冷泉的5米长钻探岩心（drill core）开展分析，该岩心中赋存丰度极高的粉红色与透明状文石胶结物（aragonite cement），此类胶结物由硫酸盐驱动的甲烷厌氧氧化（sulphate-driven anaerobic oxidation of methane）形成，但两类胶结物的沉淀环境存在差异。基于230Th/U的相特异性定年数据、脂类生物标志物组成以及钙同位素数据表明，粉红色文石是原位生物膜矿化（in situ biofilm mineralization）的产物。本次对该冷泉碳酸盐岩中两类胶结物的沉淀速率首次估算结果显示，其范围从透明文石的0.04 cm/ka到粉红色文石的1.0 cm/ka，表明与生物膜矿化相关的沉淀速率最高可提升25倍。本研究结果为未来的定量碳循环模型提供了首个动力学约束条件（kinetic constraints），同时强调了生物膜在加速海洋自生碳酸盐碳封存过程中的关键作用。