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

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.