Elemental signals in karst: from soil to speleothem

Soils develop dynamically by reworking of their constituents and only rarely do they accrete upwards with time to preserve a direct record of their past state. However, underneath soils on limestones there are often caves containing calcareous deposits such as stalagmites (known generically as speleothems) that grow at a steady rate over thousands of years, recording past conditions. We have known for some time that humic substances derived from soils penetrate into caves and are preserved in speleothems where they can sometimes be detected by their fluorescence, often as distinct layers forming during periods of high water infiltration. At many temperate sites most infiltration occurs in the autumn season when soil moisture deficit is overcome, vegetation is dying back, and elements are not being as tightly cycled in the soil. Using very sensitive and high-resolution techniques we have discovered that enrichments in particular elements are found in such layers, including not only the nutrient phosphorus, but also a whole family of other elements including heavy metals, bromine and yttrium (which behaves like the rare earth elements). We also have evidence for preferential enrichments during periods of deforestation. In order to make appropriate use of these observations to develop tools for interpreting the past history of soils, vegetation and climate, a fundamental process understanding is required, yet there is virtually no published literature on the transport of members of the above family of elements in karstic systems. We suspect that organic colloids are important agents of transport, but there are likely to be effects related to preferential retention of colloids or elements within the karstic aquifer. Also, just as plant scientists want to know about the bioavailability of elements in soils, so speleothem workers need to know whether colloidally transported elements could desorb and be incorporated as free ions, or whether they remain attached to colloids. A recently developed technique (diffusive gradients in thin films, DGT) can be used to establish element availability, but it has never been used in such a context and so we propose to do so. Finally a comparison with stalagmite composition will allow us to understand which elements to target in future researches on past environments. The proposed project complements a range of work in our research groups on the characterization of colloids, metal specidation, the chemistry of stalagmites, and experimental stalagmite growth, and will give us a better basis both for our palaeoenvironmental research and for future applications of DGT.