International Geologiical Congress - Oslo 2008


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GSM-03 Karst as a global phenomenon - a tribute to Derek Ford and Paul Williams - Part 2


Stable isotopes in spelothems - From wiggly lines to climate gradients


Tim Atkinson, University College London (United Kingdom)
Peter J Rowe, University of East Anglia (United Kingdom)
Tony Osborn, University College London (United Kingdom)
Helen Blyth, Red Rose Cave & Pothole Club (United Kingdom)
Sarah White, University College London (United Kingdom)
Phil Hopley, University College London (United Kingdom)
David Richards, University of Bristol (United Kingdom)
Dirk Hoffmann, University of Bristol (United Kingdom)
Peter L Smart, University of Bristol (United Kingdom)


The idea that speleothems hold a stable isotope record of past climate was first put forward by A Wilson and C Hendy in New Zealand in 1968, and was developed when DC Ford and co-workers in Canada combined isotopic analysis with U-Th dating, and began producing isotope records for North America and Europe. From small beginnings, speleothem palaeo-climatology has grown logistically and now involves research groups all over the world. Nevertheless, problems remain. Very early, Hendy devised the eponymous test for equilibrium of d18O between water and calcite in stalagmites. Relatively few specimens exhibit the definite lack of correlation between d13C and d18O along visible laminae that the test requires, while many show strong correlations along the growth axis, suggesting that non-equilibrium processes may control both isotopes. A recent study of three coeval Holocene speleothems deposited a few metres apart in Lancaster Hole, NW England demonstrates independent fluctuations of up to 1.5 o/oo in d18O and 5o/oo in d13C, despite all three being subject to identical histories of temperature and drip-water d18O. This idiosyncratic behaviour is best accounted for by invoking rapid degassing of CO2 from the parent solution as a control, both on the correlation between isotopes for a single specimen, and on the offsets between specimens. Such an explanation is consistent with published studies of drip hydrology and dye tracing of infiltration waters, which indicate large differences in drips' responses to rainfall and drought, combined with a high degree of isotopic mixing. Thus, d13C records may best be interpreted in terms of degassing time, a surrogate for drip rate, whereas d18Ocalcite reflects a combination of factors - d18O in parent water and annual precipitation, degassing time, and cave temperature. Published records from Israel and China, the latter showing correlations of d18Ocalcite between caves hundreds of km apart suggest that these controls may act coherently, responding to rainfall amount in seasonally wet-dry climate systems. In humid Europe, the d18Ocalcite anomaly during the cold Younger Dryas period shows a gradient in amplitude that is harder to interpret, from -2o/oo in the Austrian Alps, -1o/oo in Belgium, and +1o/oo in NW England. Fluid inclusions in speleothem provide an archive for parent water composition and are a close proxy for palaeo-rainfall d18O. From study of a flowstone from Lancaster Hole, deposited episodically over the last glacial-interglacial cycle, we have estimated the changing isotopic gradient between Britain and Greenland and the long-term temporal relationship between d18O in meteoric water and mean annual temperature. These relationships can be compared quite directly with output from large-scale climate models, suggesting that there is potential for more systematic model-data comparisons, with speleothem studies focused on areas identified by modellers as particularly sensitive.


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