Modern stalagmite δ18O: Instrumental calibration and forward modelling

The δ18O 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 δ18O clearly contains a ‘first order’ climate signal where the magnitude of changes in temperature and ocean/atmospheric circulation dominate the δ18O signature. However, at annual–biennial resolution sampling, and within interglacial periods, the magnitude of climate changes is smaller, and variability in δ18O introduced by soil, karst groundwater and cave processes can introduce considerable uncertainty into the climatic interpretation of stalagmite δ18O records. Here, two approaches that can quantify the climate signal contained with stalagmite δ18O are discussed. Firstly, linear regression based approaches are reviewed, which correlate stalagmite δ18O 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 δ18O is modelled from rainfall δ18O, surface climate parameters and a simple karst hydrology model. Using a case study from Gibraltar, this latter approach demonstrates that between stalagmite variability in δ18O of the order of 1‰ can be explained solely by differences in karst hydrology. Forward modelling suggests that a similar variability in δ18O might be observed between stalagmites within a cave, or between caves within a homogenous climate region, and highlights the difficulty in attempting to use δ18O as a paleo-thermometer.