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T3
Chironomids as a proxy for reconstructing past lake water δ 18 O: a late glacial downcore
record from Hawes Water
A.Lombino 1 *, V. Jones 1 , J. Holmes 1 , T. Atkinson 1 , S.Brooks 2 & D. Gröcke 3
1 Department of Geography, University College London, Pearson Building, Gower Street, London
2 Department of Life Sciences, Natural History Museum, Cromwell Road, London
3 Department of Earth Sciences, Durham University, Durham, DH1 3LE
In recent years the remains of chironomid larvae have received increasing attention as a
proxy for reconstructing past lake water δ 18 O and climate. Preliminary studies have
demonstrated the existence a robust linear relationship between the oxygen isotope
composition of chironomid head capsules, which are ubiquitous in most lacustrine
sediments, lake water and mean annual temperature (MAT) in suitable lacustrine settings.
In this study a high-resolution (~2cm) chironomid δ 18 O record from a Late-Glacial
sediment sequence (Hawes Water) was produced, using a thermal combustion elemental
analyser (TC/EA) coupled to a Thermo Fisher Delta V isotope ratio mass spectrometer. In
order to produce meaningful δ 18 O determinations from chironomid remains, non-amino
polysaccharide impurities (both compositional and exogenous) were limited by series of
liquid reagent-based extractions. Sub-fossil chironomid samples were sequentially soaked
in solutions of DCM:MeOH (2:1), HCl (0.25M), NaOH (0.25M) for 24 hours at room
temperature. The δ 18 O of chironomid samples subjected to chemical pre-treatment
showed good agreement with the δ 18 Obulk carbonate (r 2 = 0.68) measured on the same
sediment core, successfully tracking well-known δ 18 O excursions during the Late-Glacial.
However, the amplitude and the onset of the high frequency isotope excursions differed
slightly in the two records mainly due to differential temperature effects on oxygen isotope
fractionation during the formation of carbonates and chironomid head capsules. A
palaeotemperature equation was then used to infer temperature changes throughout the
studied late-glacial period. The results of this study confirm that chironomid δ 18 O can
successfully track changes in lake water δ 18 O and that this proxy can become an
important tool in palaeothermometry, particularly in lakes where carbonate records are
incomplete or absent.
Keywords: chironomids; stable oxygen isotopes; Hawes Water; temperature