Modern stalagmite δ18O: Instrumental calibration and forward modelling

The delta O-18 isotopic composition of stalagmite calcite frequently contains a climate signal, but one that is difficult to interpret in terms of either rainfall or temperature. Over glacial-interglacial time periods and decadal sampling resolution, stalagmite delta O-18 clearly contains a 'first order' climate signal where the magnitude of changes in temperature and ocean/atmospheric circulation dominate the delta O-18 signature. However, at annual-biennial resolution sampling, and within interglacial periods, the magnitude of climate changes is smaller, and variability in delta O-18 introduced by soil, karst groundwater and cave processes can introduce considerable uncertainty into the climatic interpretation of stalagmite delta O-18 records. Here, two approaches that can quantify the climate signal contained with stalagmite delta O-18 are discussed. Firstly, linear regression based approaches are reviewed, which correlate stalagmite delta O-18 with instrumental climate parameters such as temperature and rainfall. The advantages and disadvantages of complex linear regression approaches that attempt to account for groundwater mixing within the karst aquifer are discussed. Secondly, a forward modelling approach is introduced, where stalagmite delta O-18 is modelled from rainfall delta O-18, surface climate parameters and a simple karst hydrology model. Using a case study from Gibraltar, this latter approach demonstrates that between stalagmite variability in delta O-18 of the order of 1 parts per thousand can be explained solely by differences in karst hydrology. Forward modelling suggests that a similar variability in delta O-18 might be observed between stalagmites within a cave, or between caves within a homogenous climate region, and highlights the difficulty in attempting to use delta O-18 as a paleo-thermometer. (C) 2009 Elsevier BM. All rights reserved.