The systematics of oxygen isotopes in chironomids (Insecta: Diptera) : a tool for reconstructing past climate

• Main Author: Lombino, A. G.
• Published: University College London (University of London) 2015
• Subjects: 550
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.654622

Oxygen isotope ratios have become an indispensable tool in elucidating past climates. In recent years the chitinous remains of chironomid larvae (Insecta: Diptera: Chironomidae), which are abundant in most lacustrine sediments, have received increasing attention as a proxy for reconstructing the oxygen isotope composition of past lake waters (δ18Olakewater). The interpretation of stratigraphic changes in the oxygen isotopic composition of chironomid remains (δ18Ochironomid) is underpinned by the fundamental assumption that δ18Ochironomid is primarily reflective of δ18Olakewater, which itself is intimately associated with climate. In this investigation a series of laboratory and field-based calibration studies were conducted, with the aim of contributing to the development of δ18Ochironomid as a tool in palaeoclimate reconstructions. All analyses were performed on purified chironomid remains using a high temperature conversion elemental analyser (TC/EA) coupled, via a ConFlo III open split interface unit, to a Delta V Advantage isotope ratio mass spectrometer (IRMS). Compositional heterogeneity and exogenous contamination are known to have a deleterious influence on δ18O determinations from chitinous remains, including chironomids. In order to produce meaningful δ18Ochironomid measurements, non-amino polysaccharide impurities present in chironomid samples were eliminated through a series of liquid solvent-based extractions (2:1 DCM: MeOH, 0.25M HCl, 0.25M NaOH) performed at 20°C for 24 hours. The chosen reaction conditions were based on the results of a systematic study evaluating the influence of chemical pre-treatments on the δ18O of contemporary chironomid head capsules isolated from commercially grown larvae. To date the absence of a standardised pre-treatment procedure has hindered inter-laboratory comparisons, therefore it is recommended that analysts employ a similar protocol in the future. Laboratory and field-based calibration studies indicate that the interpretation of δ18Ochironomid is not straightforward, with signals influenced to varying degrees by δ18Olakewater, temperature and secondary factors (e.g. diet). The findings of these calibration studies were used to construct a novel chironomid-carbonate palaeothermometer, which was applied to δ18Ochironomid and δ18Obulk_carbonate records obtained from a Late-glacial sediment sequence (Hawes Water, UK). Although the general climate trends reported by this approach are generally in good agreement with other palaeoclimate reconstructions from the region, absolute temperature estimates were unrealistically low (ranging between −9 and +11°C). Based on the limited available data the spurious temperature estimates were attributed to diagenetic alteration. The strong correlation observed between the two independent δ18O records indicates that diagenetic alterations are likely to have retained some of the original isotopic signature. Despite the efforts of this investigation it is clear that this approach remains in its infancy, with further extensive calibration studies necessary. However, the results presented in this thesis demonstrate that in suitable limnological settings δ18Ochironomid has great potential for elucidating past climates.