Synchrotron X-ray distinction of seasonal hydrological and temperature patterns in speleothem carbonate

Wynn, Peter and Fairchild, Ian J. and Spötl, Christoph and Hartland, Adam and Mattey, Dave and Fayard, Barbara and Cotte, Marine (2014) Synchrotron X-ray distinction of seasonal hydrological and temperature patterns in speleothem carbonate. Environmental Chemistry, 11 (1). pp. 28-36. ISSN 1448-2517

Abstract

Synchrotron micro-X-ray fluorescence (µXRF) spectrometry is used to reveal trace element patterns within speleothem calcite at the sub-annual scale and provide one of the first calibrations to prevailing meteorological conditions. Mapping of Zn and SO42– within speleothem calcite was performed at the European Synchrotron Radiation Facility over three annual cycles (1977–1979). Peaks in µXRF Zn concentrations occur on an annual basis, although banding of lower XRF intensity reveals multiple events at the sub-annual scale. The delivery of Zn to the speleothem was found to be dependent upon the presence of a water excess, the condition of any overlying snowpack and the pH of the soil solution as controlled by microbial activity. This generated a pattern of Zn event laminae that documented increasing concentrations from winter through to the following autumn and complies with existing models inferring surface-active trace metals are delivered to the point of speleothem growth in association with natural organic matter (referred to as NOM–metal complexes). Minimum and maximum concentrations of speleothem SO42– coincide with winter and summer respectively, in contrast to the near constant SO42– concentrations of the drip water. Fluctuations in speleothem SO42– levels closely follow changes in cave external temperatures, thereby validating existing models of sulfate incorporation into carbonate minerals thought to be driven by cave ventilation dynamics and internal cave atmospheric pCO2 (partial pressure). At the current resolution of analysis, this represents some of the first evidence linking event-based meteorological (temperature and precipitation) records to the trace element content of speleothem calcite, building towards reconstruction of indices of climatic change beyond the instrumental period.