Speleothems as indicators of wet and dry periods

Calcareous speleothems provide a record of dripwater composition which in turn is a function of climatic conditions. The historical focus of speleothem palaeoclimate studies has been on the derivation of palaeotemperatures through oxygen isotope studies. However, it is now realized that water availability is a more generally important control on their characteristics. Growth rate and growth morphology in principle should give rise to recognizable changes at low flow. However, accidental plumbing effects during aquifer evolution, can also lead to variations in water supply and it is not easy to distinguish these effects. In areas where there is a strong amount effect on the delta O-18 composition of atmospheric precipitation, the speleothem d18O composition can be a direct (and inverse) function of rainfall. High-resolution methods are now available to distinguish the composition and relative abundance of winter and summer precipitation in speleothems which formed from drips of seasonally-varying composition. Two seasonally-varying processes can be responsible for significant geochemical effects during the year. Seasonal (normally summer) dryness enhances CO2-degassing which leads to elevated delta C-13, Mg/Ca and Sr/Ca in dripwaters, characteristics which are transferred to speleothems. The same effects can arise by enhanced degassing at low PCO2. High-resolution analysis can distinguish the seasonal processes and, where conducted at several time intervals, allows a more confident interpretation of longer-term records.