Novel concepts derived from microbial biomarkers in the Congo system : implications for continental methane cycling

Abstract

Methane is a climatically active gas and is a potential source of rapid global warming. Future climate change scenarios predict increased global temperatures, which, could destabilise large reservoirs of organic carbon currently locked in sediments and soils and further accelerate global warming. Comparable to the Arctic, the tropics store a large proportion of sedimentary carbon that is potentially highly vulnerable to climate change. However, the significance of tropical methane sinks in modifying methane emissions during past climate warm periods remains unresolved. This study focussed on determining the importance of aerobic methane oxidation (AMO) within the Congo during modern conditions and the Pleistocene. Bacteriohopanepolyols (BHPs), specifically aminobacteriohopane-31,32,33,34-tetrol (aminotetrol) and 35-aminobacteriohopane-30,31,32,33,34-pentol (aminopentol) are diagnostic molecular markers preserved in soils and sediments that can be used to trace AMO and, therefore, CH4 cycling within both modern and ancient systems (hereafter termed CH4 oxidation markers). In this project, BHP distributions were determined within modern samples from the Congo catchment, including; 22 soils, 6 Malebo pool wetlands and an estuarine sediment. To complement this work on modern systems, BHPs were also analysed within ancient sediments from the Congo fan (ODP 1075) dated to 2.5 Ma, including high resolution studies of marine isotope stages (MIS) 5, 11 and 13. Within ODP 1075, high concentrations of CH4 oxidation markers are observed with no strong down core degradation signature. The study presents the oldest reported occurrence of CH4 oxidation markers, to date, with these biomarkers detected in sediments dated to 2.5 Ma (226 meters composite depth). Similarly, high concentrations of aminotetrol and aminopentol are also observed within the modern Malebo pool wetland and estuarine sediment, suggesting these sites as likely sources of CH4 oxidation markers to the Congo Fan. High concentrations of CH4 oxidation markers were found during MIS 5, 11 and 13 within ODP 1075 sediments. The CH4 oxidation marker signature during MIS 5 and MIS 11 coincides with high global CH4 concentrations (EPICA Dome C), whereas MIS 13 was characterised by low atmospheric CH4 concentrations. The strong similarities in CH4 oxidation marker concentrations during all three interglacial suggests a non-linear response in BHP production/burial and global CH4. This iii disparity could be due to the displacement of main sediment supply to the Congo fan and/or the relative similarity of CH4 sources during these time intervals. Alternatively, these differences could also be due to a threshold behaviour between methanogenesis – methanotrophy and the synthesis of diagnostic BHPs. A long term reduction in the mean concentration of CH4 oxidation markers is observed between 1.7 Ma and 0.3 Ma, suggesting a long term trend towards greater continental aridity within the Congo, consistent with changes in vegetation zones. Significant uncertainty still remain about the response of tropical CH4 sources and sinks during global climate perturbations, however, this study emphasises the large potential of BHPs as powerful novel tracers for methane cycling, both on land and in the ocean and across all climate zones.