Stable carbon isotopes of invertebrate remains: do they reveal past methane release from lakes?

Abstract

Lakes are a source of methane, an important greenhouse gas in the atmosphere. In order to understand increasing methane emissions in the present, it is important to study the variations of methane release during past periods of climate change. However, records of methane release from lakes over time scales longer than a few years are extremely rare. In this thesis a method is explored to reconstruct past methane availability in lakes based on the stable carbon isotope composition (delta 13C) of aquatic invertebrate remains. Methane-derived 13C-depleted carbon can be an alternative carbon source to plant-derived material for invertebrates in the sediment and water column of lakes, that can lead to markedly depleted delta 13C values in invertebrate tissues. Using culturing experiments, it was demonstrated that methane-derived carbon is incorporated into chironomid head capsules. Also, it was shown that chemical pre-treatments that are commonly used for sediment processing do not have a noticeable effect on delta 13C values of chironomid remains and that a minimum of 20 microgram chitinous material is required for reproducible delta 13C measurements. In study lakes in Sweden and Siberia, delta 13C values of chitinous remains of several invertebrate taxa in surface sediments were negatively correlated with diffusive methane fluxes. Strong and significant correlations are observed between diffusive methane fluxes and delta 13C of Chironomini and Daphnia remains. Similar correlations were also observed within two Swedish lakes, in which surface sediment cores were obtained along depth transects. Delta 13C values of Chironomini and Daphnia were lower in sections of the lake basin in which the sediments had higher methane release rates. This suggests that delta 13C in remains of several invertebrates, especially Chironomini and Daphnia can be indicative of methane availability in lakes. In contrast, Orthocladiinae and Ephemeroptera, always had relatively high delta 13C values, indicating a plant-derived carbon source. In a sediment record from a Siberian thermokarst lake, delta 13C values of the remains of Chironomini and Daphnia showed relatively large negative delta 13C shifts in warm periods (AD 850-1150 and since AD 1970) compared with delta 13C values of Orthocladiinae remains and bulk sediment. Based on the observed correlations between methane fluxes and delta 13C values of Chironomini and Daphnia in surface sediment samples, this suggests that more methane-derived carbon was available in warm periods compared with colder periods. The research in this thesis demonstrates the potential to use taxon-specific stable carbon isotopes analysis of invertebrate remains for qualitative reconstructions of methane availability in lakes. This new method can lead to a better understanding of lake methane emissions in relation to climate change