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.
Highlights
Arctic lakes are sources of methane within the global carbon cycle (Bastviken et al, 2004)
Our primary goal was to contribute towards the understanding of the sedimentary signature of methane production and oxidation in thermokarst lakes using diploptene δ13C values as a proxy for the occurrence of methane oxidising bacteria (MOB)
Diploptene was present in almost all samples, and the δ13C values were low, suggesting the presence of MOB in three zones with differing levels of methane ebullition emissions rates
Summary
Arctic lakes are sources of methane within the global carbon cycle (Bastviken et al, 2004). Thermokarst and thermokarst-affected lakes (those formed and/or influenced by thaw and collapse of ice-rich ground) are recognised as important but variable past and present sources of methane flux to the atmosphere (Shirokova et al, 2012; Walter et al, 2006, 2007a, 2008; Wik et al, 2013). Davies et al.: Diploptene δ13C values from lake sediments (i.e. Holocene) variations in lake-derived methane flux to the atmosphere and changes in emissions during discrete climatic events in the past are generally not well understood (but see Walter Anthony et al, 2014; Walter et al, 2007b). Understanding methane activity in lakes over recent (e.g. decadal/centennial) and longer (millennial) time periods and its relationship with forcing factors (e.g. temperature) could provide useful constraints for the projection of future fluxes with arctic warming
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