Biohopanoid markers as tracers of methane emission and oxidation events in the Quaternary ocean

甲烷是一种强烈的温室气体，直接从微生物或化石碳长期加热的沉积物中产生。众所周知，向海洋和大气释放甲烷在过去改变了全球气候和营养循环，特别是当甲烷从被称为海洋天然气水合物的冻结地下水库大量释放时，特别是在很短的时间内。随着全球变暖，天然气水合物等不稳定的来源对海洋环境、气候和社会构成了潜在的巨大威胁。因此，认识过去甲烷排放增加的机制至关重要。同样重要的是从地下了解甲烷的来源和路径，查明循环释放甲烷的过程，以及量化随后的海洋-大气和生物地球化学反馈。该研究项目制定了一项雄心勃勃的有机地球化学方案，以探索第四纪晚期最后100万年海洋中甲烷的好氧微生物氧化过程，并评估其对气候的影响。到目前为止，目标过程几乎没有受到关注，但可能在海洋的碳循环和氧气供应方面发挥着比通常认为的更重要的作用。作为过去气候研究中的一种新工具，我们建议使用专门由以甲烷为食的细菌(甲烷氧化细菌)产生的分子化合物(特定的生物类化合物，BHP)。我们最近分析了热带非洲刚果河前一个巨大的深海沉积物扇的沉积记录中的BHP(深至100米，相当于大约100万年)。这些新数据将大量甲烷释放及其有氧微生物氧化过程的直接证据推回到地质过去；之前的研究表明，类似的机制仅限于最近45ka。新的刚果球迷数据令人鼓舞，并为热带大西洋东部以前未被识别的甲烷排放事件和有氧周转提供了强有力的支持。在此基础上，我们将重点研究两个截然不同的沉积物记录，(1)刚果和亚马逊扇区，它们在整个研究期间的水条件是氧气的；(2)地中海。后者是众所周知的，因为在过去的几百万年里，它的氧合作用发生了显著而迅速的变化，在有氧和氧气之间交替变化(缺氧)，甚至经常是有毒的(硫酸盐/正辛酸)条件，导致形成异常富含有机碳的沉积物，通常被称为腐泥岩。我们在这里提出了一种多学科的方法，利用必和必拓的标记物和其他地球化学和同位素证据并进行建模。这一综合方法将使我们能够充分评估第四纪海洋中以前未被识别的甲烷排放事件和随后甲烷氧化的存在。结合大气-陆地-海洋耦合模拟，这些数据将使我们能够处理目标过程在氧气环境和接近无氧条件的其他环境中的相关性。然后，这将提供对排放事件排放的温室气体量的第一次定量估计，并探索气候影响。

Methane is a strong greenhouse gas produced in sediments directly from microorganisms, or from prolonged heating of fossil carbon. Release of methane to the ocean and atmosphere is known to have shifted global climate and nutrient cycles in the past, in particular when released in massive quantities and over short time periods from frozen subsurface reservoirs known as marine gas hydrates. With global warming, unstable sources such as gas hydrates pose a potentially large threat to the marine environments, climate, and thus society. It is therefore pivotal to recognize mechanisms of increased methane emission in the past. Equally important is understanding its sources and pathways from the subsurface, identification of processes which cycle released methane, and quantification of subsequent ocean-atmosphere and biogeochemical feedbacks. This research project sets out an ambitious organic geochemical programme to explore the process of aerobic microbial oxidation of methane in oceans over the last 1 million years of the late Quaternary and assess its effects on climate. The target process has received little attention to date but possibly plays a more important role in carbon cycling and oxygen availability in the ocean than commonly considered. As a new tool in past climate research we propose to use molecular compounds (specific biohopanoids, BHPs) generated exclusively by bacteria which feed on methane (methane-oxidising bacteria). We recently analysed BHPs in the sedimentary record (down to 100 m depth equivalent to about 1 million years) from a giant deep sea sediment fan in front of the Congo river in tropical Africa. These new data push direct evidence for the process of massive methane release and its aerobic microbial oxidation far back into the geological past; previous studies suggesting a similar mechanism were limited to the last 45 ka. The new Congo fan data are encouraging and provide strong support for previously unrecognised methane emission events and aerobic turnover in the eastern tropical Atlantic. Building on that, we will focus our research on two contrasting sediment records, (1) the Congo and Amazon Fans which had oxygenated water conditions throughout the study period and (2) the Mediterranean. The latter is well known for its pronounced and rapid changes in oxygenation throughout the past few million years which alternated between oxygenated and oxygen-fee (anoxic), and frequently even toxic (sulfidic/euxinic) conditions leading to the formation of sediments exceptionally rich in organic carbon, commonly termed sapropels. We here propose a multidisciplinary approach, utilising BHP markers with other geochemical and isotopic evidence and modelling. This integrated approach will allow us to fully evaluate the existence of previously unrecognised methane emission events and subsequent methane oxidation in the Quaternary ocean. Combined with coupled atmosphere-land-ocean modelling the data will allow us to address the relevance of the target process in oxic settings and others which are approaching oxygen-free conditions. This will then provide the first quantitative estimates on greenhouse gas volumes emitted by the emission events and explore climate effects.