Variability in aerobic methane oxidation over the past 1.2Myrs recorded in microbial biomarker signatures from Congo fan sediments

Abstract

Methane (CH 4) is a strong greenhouse gas known to have perturbed global climate in the past, especially when released in large quantities over short time periods from continental or marine sources. It is therefore crucial to understand and, if possible, quantify the individual and combined response of these variable methane sources to natural climate variability. However, past changes in the stability of greenhouse gas reservoirs remain uncertain and poorly constrained by geological evidence. Here, we present a record from the Congo fan of a highly specific bacteriohopanepolyol (BHP) biomarker for aerobic methane oxidation (AMO), 35-aminobacteriohopane-30,31,32,33,34-pentol (aminopentol), that identifies discrete periods of increased AMO as far back as 1.2 Ma. Fluctuations in the concentration of aminopentol, and other 35-aminoBHPs, follow a pattern that correlates with late Quaternary glacial-interglacial climate cycles, with highest concentrations during warm periods. We discuss possible sources of aminopentol, and the methane consumed by the precursor methanotrophs, within the context of the Congo River setting, including supply of methane oxidation markers from terrestrial watersheds and/or marine sources (gas hydrate and/or deep subsurface gas reservoir). Compound-specific carbon isotope values of À30& to À40& for BHPs in ODP 1075 and strong similarities between the BHP signature of the core and surface sediments from the Congo estuary and floodplain wetlands from the interior of the Congo River Basin, support a methanotrophic and likely terrigenous origin of the 35-aminoBHPs found in the fan sediments. This new evidence supports a causal connection between marine sediment BHP records of tropical deep sea fans and wetland settings in the feeding river catchments, and thus tropical continental hydrology. Further research is needed to better constrain the different sources and pathways of methane emission. However, this study identifies the large potential of aminoBHPs, in particular aminopentol, to trace and, once better calibrated and understood, quantify past methane sources and fluxes from terrestrial and potentially also marine sources.

Bacteriohopanepolyols in tropical soils and sediments from the Congo River catchment area

“…5a). Lower diversity is reported for samples from the Congo fan (Handley et al, 2010;Talbot et al, 2014), with many of the methylated and pentose compounds below detection limit. Between 7 and 10 of the 12 BHPs in ODP 1075 are also found in the wetlands and soils, and are again a major component of all of the BHP profiles, representing > 90% of the total BHPs in the wetland and estuary samples (Fig.…”

“…Geographical locations of study site in the Congo, showing locations of 22 soil samples (circles), 6 floodplain wetland sediment samples (Malebo pool; triangle), the Congo estuary sediment sample (square) and ODP 1075 (star). The map is modified from Talbot et al (2014) and was generated using the planiglobe beta online plotting service (http://www.planiglobe.com).…”

“…Another group of diagnostic markers comprises those produced by aerobic methane oxidising bacteria (methanotrophs), including 35-aminobacteriohopane-31,32,33,34-tetrol (aminotetrol; 1e); 35-aminobacteriohopane-30,31,32,33,34-pentol (aminopentol; 1f), unsaturated aminopentol (4/5f) and aminopentol isomer (1f 0 ; e.g. Zhu et al, 2010;van Winden et al, 2012b;Berndmeyer et al, 2013;Talbot et al, 2014).…”

“…Studies of sediments from the Congo fan suggest terrigenous OC input as an important source of BHPs in these coastal marine sediments (Cooke et al, 2008b;Talbot et al, 2014). In this study, we have determined the BHP inventory of 22 soils and 6 wetland sediments (Malebo pool) from the Congo hinterland and 1 estuarine sediment from the mouth of the Congo River (Fig.…”

Rare bacteriohopanepolyols as markers for an autotrophic, intra-aerobic methanotroph

“…As BHPs have been specifically used as biomarkers for methanotrophs (e.g. Berndmeyer et al, 2013;Coolen et al, 2008;van Winden et al, 2012;Zhu et al, 2010;Talbot et al, 2014;Wagner et al, 2014), it is interesting to determine whether M. oxyfera is also capable of synthesizing BHPs and could form a potential source of BHPs, including 3-methyl BHPs, previously associated with aerobic metabolism, in anoxic environments.…”