Trace element variation in speleothem aragonite: implications for palaeoenvironmental reconstruction

Trace element variation in annually laminated stalagmites has the potential to record long-term, high-resolution palaeoclimatic data provided both (1) the relationship between trace element content in the carbonate and the climate, and (2) the mechanism of stalagmite growth, are understood. Here we report a secondary ion mass spectrometry (SIMS) study of an aragonitic speleothem (T7) from Cold Air Cave, Southern Africa, which achieves small (mum) spatial resolution and ppm precision. T7 has a series of fine visible laminae believed to be annual to subannual. This article establishes (1) the usefulness of SIMS in analyzing speleothem aragonite at the spatial resolution and precision relevant to climatic studies, (2) the distribution of trace elements in aragonite on micron scales and their relationship to models for speleothem growth and (3) the potential of aragonite speleothems to encode climatic data at subannual resolution. Complex reproducible oscillations in Sr/Ca and Ba/Ca are observed parallel to the growth direction of the speleothem. However, in addition to this variability, we observe trace element oscillations perpendicular to the growth direction and infer that aragonite crystallites in the speleothem are laterally, as well as vertically zoned. Stepscans parallel to the growth direction demonstrate complex zoning patterns from which simple climatic data are difficult to extract due to small-scale lateral heterogeneity in the aragonite. However, time-series analysis of the whole data set reveals regular cycles in Sr/Ca and Ba/Ca at a scale that matches the magnitude of the visible laminae. On this basis, it appears that within the complex geochemical variability, there resides a regular environmental control, possibly variation in water throughput, hence rainfall. (C) 2001 Elsevier Science B.V. All rights reserved.