Geochemistry of sediment cores GeoB13820-1 and GeoB13863-1 from the western South Atlantic

Here, we present results from sediments collected in the Argentine Basin, a non-steady state depositional marine system characterized by abundant oxidized iron within methane-rich layers due to sediment reworking followed by rapid deposition. Our comprehensive inorganic data set shows that iron reduction in these sulfate and sulfide-depleted sediments is best explained by a microbially mediated process-implicating anaerobic oxidation of methane coupled to iron reduction (Fe-AOM) as the most likely major mechanism. Although important in many modern marine environments, iron-driven AOM may not consume similar amounts of methane compared with sulfate-dependent AOM. Nevertheless, it may have broad impact on the deep biosphere and dominate both iron and methane cycling in sulfate-lean marine settings. Fe-AOM might have been particularly relevant in the Archean ocean, >2.5 billion years ago, known for its production and accumulation of iron oxides (in iron formations) in a biosphere likely replete with methane but low in sulfate. Methane at that time was a critical greenhouse gas capable of sustaining a habitable climate under relatively low solar luminosity, and relationships to iron cycling may have impacted if not dominated methane loss from the biosphere.

本研究报道了阿根廷盆地沉积物的分析结果。阿根廷盆地为非稳态沉积海洋系统，因沉积物经再搬运后快速沉积，其富甲烷层内富含氧化态铁。本研究的全套无机数据集显示，在这些硫酸盐与硫化物耗尽的沉积物中，铁还原过程最符合微生物介导的机制——即甲烷厌氧氧化耦合铁还原（anaerobic oxidation of methane coupled to iron reduction，Fe-AOM）为最可能的核心作用机制。尽管铁驱动的Fe-AOM在诸多现代海洋环境中具有重要意义，但与依赖硫酸盐的AOM相比，其消耗的甲烷量或不及后者。尽管如此，该过程仍可能对深部生物圈产生广泛影响，并在硫酸盐匮乏的海洋环境中主导铁与甲烷的生物地球化学循环。Fe-AOM在距今25亿年以上的太古代海洋中可能尤为关键——当时的生物圈以甲烷富集、硫酸盐匮乏为特征，海洋中广泛生成并沉积氧化铁（形成铁建造）。彼时甲烷作为关键温室气体，可在太阳光度相对较低的条件下维持宜居气候；而铁循环与甲烷的关联，即便未完全主导生物圈的甲烷流失过程，也可能对其产生显著影响。