Spatial variability of diploptene δ¹³C values in thermokarst lakes: the potential to analyse the complexity of lacustrine methane cycling

Abstract

Abstract. Cryospheric changes in northern high latitudes are linked to significant greenhouse gas flux to the atmosphere, including methane release that originates from organic matter decomposition in thermokarst lakes. The connections between methane production in sediments, transport pathways and oxidation are not well understood and this has implications for any attempts to reconstruct methane production from sedimentary archives. We assessed methane oxidation as represented by methane oxidising bacteria across the surface sediments of two interior Alaska thermokarst lakes in relation to methane emissions via ebullition (bubbling). The bacterial biomarker diploptene was present and had low δ13C values (lower than −38 ‰) in all sediments analysed, suggesting methane oxidation was widespread. The most δ13C-depleted diploptene was found in the area of highest methane ebullition emissions in Ace Lake (δ13C diplotene values between −68.2 and −50.1 ‰), suggesting a positive link between methane production, oxidation, and emission in this area. In contrast, significantly less depleted diploptene δ13C values (between −42.9 and −38.8 ‰) were found in the area of highest methane ebullition emissions in Smith Lake. Lower δ13C values of diploptene were found in the central area of Smith Lake (between −56.8 and −46.9 ‰), where methane ebullition rates are low but methane diffusion appears high. Using δ13C-diplotene as a proxy for methane oxidation activity, we suggest the observed differences in methane oxidation levels among sites within the two lakes could be linked to differences in source area of methane production (e.g. age and type of organic carbon) and bathymetry as it relates to varying oxycline depths and changing pressure gradients. As a result, methane oxidation is highly lake-dependent. The diploptene δ13C values also highlight strong within-lake variability, implying that single-value, down-core records of hopanoid isotopic signatures are not secure indicators of changing methane flux at the whole-lake scale.