Evidence for past variations in methane availability in a Siberian thermokarst lake based on δ¹³c of chitinous invertebrate remains

Understanding past methane dynamics in arcticwetlands and lakes is crucial for estimating future methane release. Methane fluxes from lake ecosystems have increasingly been studied, yet only few reconstructions of past methane emissions from lakes are available. In this study,we develop an approach to assess changes in methane availability in lakes based on δ¹³c of chitinous invertebrate remains and apply this to a sediment record from a Siberian thermokarst lake. Diffusive methane fluxes from the surface of ten newly sampled Siberian lakes and seven previously studied Swedish lakes were compared to taxon-specific δ¹³c values of invertebrate remains from lake surface sediments to investigate whether these invertebrates assimilated ¹³C-depleted carbon typical for methane. Remains of chironomid larvae of the tribe Orthocladiinae that, in the study lakes, mainly assimilate plant-derived carbon had higher δ¹³c than other invertebrate groups. δ¹³c of other invertebrates such as several chironomid groups (Chironomus, Chironomini, Tanytarsini, and Tanypodinae), cladocerans (Daphnia), and ostracods were generally lower. δ¹³c of Chironomini and Daphnia, and to a lesser extent Tanytarsini was variable in the lakes and lower at sites with higher diffusive methane fluxes. δ¹³c of Chironomini, Tanytarsini, and Daphnia were correlated significantly with diffusive methane flux in the combined Siberian and Swedish dataset (r1=-0.72, p = 0.001, r = 0.53, p = 0.03, and r1\4;0.81, p < 0.001, respectively), suggesting that δ¹³c in these invertebrates was affected by methane availability. In a second step, we measured δ¹³c of invertebrate remains from a sediment record of Lake S1, a shallow thermokarst lake in northeast Siberia. In this record, covering the past ca 1000 years, δ¹³c of taxa most sensitive to methane availability (Chironomini, Tanytarsini, and Daphnia) was lowest in sediments deposited from ca AD 1250 to ca AD 1500, and after AD 1970, coinciding with warmer climate as indicated by an independent local temperature record. As a consequence the offset in δ¹³c between methane-sensitive taxa and bulk organic matterwas higher in these sections than in other parts of the core. In contrast, δ¹³c of other invertebrate taxa did not show this trend. Our results suggest higher methane availability in the study lake during warmer periods and that thermokarst lakes can respond dynamically in their methane output to changing environmental conditions.