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，直接由微生物或化石碳的长期加热在沉积物中产生。众所周知，甲烷释放到海洋和大气中，在过去改变了全球气候和营养循环，特别是当被称为海洋天然气水合物的冰冻地下储层在短时间内大量释放时。随着全球变暖，不稳定的来源，如天然气水合物对海洋环境，气候，从而社会构成潜在的巨大威胁。因此，认识过去甲烷排放增加的机制至关重要。同样重要的是了解其来源和来自地下的路径，确定循环释放甲烷的过程，并量化随后的海洋-大气和地球化学反馈。这一研究项目提出了一项雄心勃勃的有机地球化学方案，以探索第四纪晚期过去100万年来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。迄今为止，目标过程很少受到关注，但可能在海洋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（特定的生物类藿烷，BHP）。我们最近分析了沉积记录（下至100米深度相当于约100万年）中的BHP，这些记录来自热带非洲刚果河前的一个巨大深海沉积扇。这些新数据将大量甲烷释放过程及其好氧微生物氧化的直接证据推回到地质学的过去;以前的研究表明类似的机制仅限于过去的45 ka。新的刚果风扇数据是令人鼓舞的，并提供了强有力的支持，以前未被承认的甲烷排放事件和有氧营业额在热带大西洋东部。在此基础上，我们将把我们的研究重点放在两个对比的沉积物记录，（1）刚果和亚马逊风扇，在整个研究期间有含氧水条件和（2）地中海。后者是众所周知的，其显着和快速的变化，在氧化和氧费（缺氧）之间交替，在过去的几百万年，往往甚至有毒（硫化物/富氧）的条件下，导致沉积物的形成异常丰富的有机碳，通常被称为腐泥。在这里，我们提出了一个多学科的方法，利用BHP标记与其他地球化学和同位素的证据和建模。这种综合的方法将使我们能够充分评估存在以前未被承认的甲烷排放事件和随后的甲烷氧化在第四纪海洋。结合大气-陆地-海洋耦合建模，这些数据将使我们能够解决目标过程在含氧环境和其他接近无氧条件下的相关性。然后，这将提供关于排放事件排放的温室气体量的第一个定量估计，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.