Diploptene <i>δ</i><sup>13</sup>C values from contemporary thermokarst lake sediments show complex spatial variation

Abstract

Abstract. Cryospheric changes in northern high latitudes are linked to significant greenhouse gas flux to the atmosphere, for example, methane that originates from organic matter decomposition in thermokarst lakes. The set of pathways that link methane production in sediments, via oxidation in the lake system, to the flux of residual methane to the atmosphere is complex and exhibits temporal and spatial variation. The isotopic signal of bacterial biomarkers (hopanoids, e.g. diploptene) in sediments has been used to identify contemporary ocean-floor methane seeps and, in the geological record, periods of enhanced methane production (e.g. the PETM). The biomarker approach could potentially be used to assess temporal changes in lake emissions through the Holocene via the sedimentary biomarker record. However, there are no data on the consistency of the signal of isotopic depletion in relation to source or on the amount of noise (unexplained variation) in biomarker values from modern lake sediments. We assessed methane oxidation as represented by the isotopic signal of biomarkers from methane oxidising bacteria (MOB) in multiple surface sediment samples in three distinct areas known to emit varying levels of methane in two shallow Alaskan thermokarst lakes. Diploptene was present and had δ13C values lower than −38 ‰ in all sediments analysed, suggesting methane oxidation was widespread. However, there was considerable variation in δ13C values within each area. The most 13C-depleted diploptene was found in an area of high methane ebullition in Ace Lake (diploptene δ13C values between −68.2 and −50.1 ‰). In contrast, significantly higher diploptene δ13C values (between −42.9 and −38.8 ‰) were found in an area of methane ebullition in Smith Lake. δ13C values of diploptene between −56.8 and −46.9 ‰ were found in the centre of Smith Lake, where ebullition rates are low but diffusive methane efflux occurs. The small-scale heterogeneity of the samples may reflect patchy distribution of substrate and/or MOB within the sediments. The two ebullition areas differ in age and type of organic carbon substrate, which may affect methane production, transport, and subsequent oxidation. Given the high amount of variation in surface samples, a more extensive calibration of modern sediment properties, within and among lakes, is required before down-core records of hopanoid isotopic signatures are developed.

Microbial lipid signatures in Arctic deltaic sediments - insights into methane cycling and climate variability

“…MOB produce hopanoids that are depleted in 13 C compared to lipids produced by photosynthetic autotrophs (e.g., sterols, n-alkanes; Pancost et al, 2000). In agreement with this observation, we find large amount of diploptene (17β(H), 21β(H)-hop-22(29)-ene) in MD-1 (high closure) and UD-4 sediments (low closure) with δ 13 C values of -51.6 ± 2.5‰ and -56.6 ± 1.8‰, respectively, comparable with values reported for Alaskan Arctic thermokarst lakes (Davies et al, 2016). Hence, it seems that the oxidation of methane occurs aerobically and is primarily mediated by bacteria, but further studies that target the methanotrophic archaea (using specific primers or biomarkers) are needed.…”

Microbial Signatures in Deep CO2-Saturated Miocene Sediments of the Active Hartoušov Mofette System (NW Czech Republic)

“…In contrast, the shift to a domination of Hop-17(21)-ene and the decrease of the δ 13 C hopanoid signal to values ranging between −50 and −60%0 in the Cypris Fm. ( Figures 3B,C ) illustrates that planktonic freshwater bacteria or benthic aerobic methanotrophs dwelling on microbially-derived methane at the water-sediment interface might have formed a significant part of the source bacteria for the deposited hopanoids (Nealson, 1997 ; Whiticar, 1999 ; Summons et al, 2006 ; Naeher et al, 2014 ; Davies et al, 2016 ; Hoefs, 2018 ).…”

δ13C values of bacterial hopanoids and leaf waxes as tracers for methanotrophy in peatlands

“…-up to -38‰;van Winden et al, 2010;Zheng et al, 2014). Low C30 hopene δ 13 C values (up to -60‰) have also been identified in lacustrine settings (Davies et al, 2015;Naeher et al, 2014), indicating incorporation of isotopically light CH4 into the bacterial community. As C30 hopenes are produced by a variety of organisms (including methanotrophs; e.g.…”