Multi-proxy constraints on the climatic significance of trace element records from a New Zealand speleothem

Abstract

Trace element concentrations and uranium isotope ratios are reported for a speleothem from the South Island of New Zealand. TIMS uranium-series dating of the speleothem indicates it to have grown from 31 000 years ago to the present, providing a continuous record of the last deglaciation. Trace element abundances measured in this speleothem using ICP-MS are found to exhibit strong temporal relationships with δ 13C and other measured variables, and with the environmental changes inferred from them. Strontium and barium concentrations are positively correlated with inferred changes in the productivity and extent of vegetation cover above the cave. Magnesium concentration appears to have responded to changes in groundwater residence time, assumed to have an inverse relationship with effective meteoric precipitation above the cave. The degree of 234U/ 238U disequilibrium also appears to have varied in response to hydrological changes, and together with the magnesium data implies a post-glacial increase in regional effective precipitation to have culminated ca. 13 000 cal yr B.P., some 2000 years after a dramatic post-glacial increase in forest extent previously inferred for the region. This increase in forest extent is thus unlikely to have been caused by a significant increase in precipitation, and is assumed to have been driven predominantly by a rapid increase in regional temperature, centred on 15 000 calendar years before present.