Stable oxygen isotopes in fossil chironomids: development of a new proxy for past climate change

Abstract

Climate is changing rapidly. The Intergovernmental Panel on Climate Change (IPCC) recently published a comprehensive report stating that the current climate warming is unequivocal. It assessed the possible causes and magnitude of future climate warming as well as the expected consequences. Many ecosystems are likely to be affected by changing environmental conditions. In order to obtain a better insight into naturally and anthropogenically forced climate variability and possible impacts of future environmental change on ecosystems we need to understand past climate variations and their effects on the biosphere. A better understanding of past and future climate change requires development of proxies that have the potential to provide long-term palaeoclimate records as instrumental measurements of climatic variables have often started after the onset of anthropogenically-induced warming and, hence, are often too short to assess natural variability or long-term trends. Therefore, natural archives need to be consulted when reconstructing environments older than ~100 years. The main aim of this thesis was to develop the use of stable oxygen isotopes in chironomids as a proxy for past changes in lake water ?18O and, therefore, for past climate change. As chironomids have previously been used to successfully reconstruct past temperatures based on modern species-environment relationships, this approach would provide an opportunity to use fossil remains of one organism group for two independent climate reconstructions. The use of chironomids to obtain ?18O records would eventually allow stable oxygen isotope records to be obtained from lakes where ?18O records based on other organism groups are difficult to develop, e.g. lakes on acid bedrock or lakes which are characterized by a significant inwash of terrestrial silica or cellulose. In a first step to achieve this aim, the effect of chemical pretreatments that are commonly applied in palaeoecology, on ?18O, chemical composition, and morphology of chironomid head capsules was explored. In a second step, ?18O analysis was performed on subfossil chironomid head capsules and ephippia from cladocerans isolated from surface sediments of thirty deep and stratified lakes along a transect ranging from northern Sweden to southern Italy. The results indicated that chironomid ?18O was strongly and positively related to lake water and precipitation ?18O. In a third step, stable oxygen isotopes in fossil chironomids from late glacial and early Holocene sediments from Rotsee (Switzerland) were compared to bulk carbonate ?18O from the same sediment core to examine whether chironomid ?18O can track centennial- to millennial-scale changes in past lake water ?18O. Carbonate particles adhering to chironomid remains resulted in a masking of the signal, leading to a poor correlation between chironomid and bulk carbonate ?18O. Therefore, a pretreatment to remove contaminating carbonates in a pH-neutral environment was applied to the head capsules. Eventually, a reliable late glacial and early Holocene chironomid ?18O record was obtained that agreed well with the one obtained from bulk carbonates, which allowed for inferences of past lake water ?18O and climatic variability.