Abstract
Methane (CH4) 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 1.2Ma. 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 −30‰ to −40‰ 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.
Highlights
Methane (CH4) is a potent greenhouse gas (GHG) estimated to currently contribute about 20% to total global atmospheric radiative forcing (IPCC, 2007)
We confirm that aminopentol (III) is present in 120 of the 122 sediments analysed, as are the other aminoBHPs aminotriol (IV) and aminotetrol (V; Fig. 2) together with 2 other minor BHPs related to aminopentol (IIIa and IIIb; Table S1), up to 115 metres below sea floor and an estimated age of ca. 1.2 Ma based on the age model of Jahn et al (2005)
This study confirms molecular evidence for aerobic methane oxidation (AMO), in the form of specific aminoBHPs produced by aerobic methanotrophs, in marine sediments from the Congo deep sea fan (ODP Site 1075) dating back to 1.2 Ma
Summary
Methane (CH4) is a potent greenhouse gas (GHG) estimated to currently contribute about 20% to total global atmospheric radiative forcing (IPCC, 2007). The different pathways of biochemical cycling of methane, which exert a primary control on atmospheric and marine CH4 concentrations through its production and microbiological consumption, remain poorly constrained. Changes in the strength of tropical methane sources and sinks (i.e. wetlands, atmospheric oxidation), have been shown to exert a significant control on the atmospheric methane budget over the last 800,000 years Blunier et al, 1995; Loulergue et al, 2008; Singarayer et al, 2011) Such changes were modulated by fluctuations in the hydrological cycle and the extent of periglacial wetlands (Loulergue et al, 2008)
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