ANNUAL TRACE-ELEMENT VARIATIONS IN A HOLOCENE SPELEOTHEM

High resolution secondary ionization mass spectrometry (SIMS) analysis along the growth axis of a Holocene speleothem from northern Scotland reveals high frequency oscillations and longer term trends in Mg/Ca, Sr/Ca and Ba/Ca. At the scale of the high frequency oscillations, Mg/C a and Sr/Ca display a well-defined anticorrelation, while Sr/Ca and Ba /Ca are positively correlated. Imaging of elemental ratios in two dime nsions reveals that the high frequency signal is well preserved and la terally continuous across the sample. TIMS Th-230 ages and annual ultr aviolet luminescence banding provide chronological control and demonst rate that the high frequency cyclicity is annual. Trace element Variat ions in speleothem calcite are a reflection of hydrochemical processes in the unsaturated zone overlying the cave (supply effects) and parti tioning at the water-calcite interface. Mg partitioning between water and calcite is temperature dependent. Calculations suggest that the an nual Mg/Ca oscillations may be caused by seasonal temperature changes. However, the longer timescale variations in Mg/Ca are too great to be explained by this mechanism. Seasonal variations in water residence t ime in the unsaturated zone (which reflect effective precipitation) pr ovide a more probable explanation for the observed annual oscillations and also explain the inverse relationship between Mg/Ca and Sr/Ca. Th is record suggests that trace elements in speleothems have the potenti al to provide the high resolution insights into interannual palaeoclim atic variability required for assessment of general circulation models . (C) 1998 Elsevier Science B.V.