Corroborated rainfall records from aragonitic stalagmites

Abstract

Sr and Ba levels of two proximal aragonite speleothems from Cold Air Cave, South Africa, are examined at μm resolution. A compositional map derived from parallel secondary ion mass spectrometry linescans indicates heterogeneity perpendicular to the stalagmite growth axis, precluding the use of single linescans as a climatic proxy technique. A ∼40-yr averaged trace element record for the cave produced from multiple parallel linescans on both stalagmites is compared with regional and local climatic (rainfall, temperature) data for 1955–1996. This period includes two extensive drought episodes. There is poor correlation between trace elements and annual mean temperature. Droughts correspond to minima in trace element ratios and peaks correspond to annual maxima in rainfall. The onset and termination of droughts are notable inflexions. However, in detail annual rainfall magnitude does not correlate directly with trace element ratios, indicating that kinetic factors, notably variations in speleothem growth rate, play a key role in trace element uptake by aragonite. Averaged long data sets constrained by chronological control have potential in the study of sub-decadal precipitation patterns.