Evidence for past variations in methane availability in a Siberian thermokarst lake based on b13C of chitinous invertebrate remains

Abstract

Understanding past methane dynamics in arctic wetlands 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 ?13C 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 ?13C 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 ?13C than other invertebrate groups. ?13C of other invertebrates such as several chironomid groups (Chironomus, Chironomini, Tanytarsini, and Tanypodinae), cladocerans (Daphnia), and ostracods were generally lower. ?13C of Chironomini and Daphnia, and to a lesser extent Tanytarsini was variable in the lakes and lower at sites with higher diffusive methane fluxes. ?13C of Chironomini, Tanytarsini, and Daphnia were correlated significantly with diffusive methane flux in the combined Siberian and Swedish dataset (r = ?0.72, p = 0.001, r = ?0.53, p = 0.03, and r = ?0.81, p < 0.001, respectively), suggesting that ?13C in these invertebrates was affected by methane availability. In a second step, we measured ?13C 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, ?13C 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 ?13C between methane-sensitive taxa and bulk organic matter was higher in these sections than in other parts of the core. In contrast, ?13C 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.

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