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/Ca and Sr/Ca display a well-defined anticorrelation, while Sr/Ca and Ba/Ca are positively correlated. Imaging of elemental ratios in two dimensions reveals that the high frequency signal is well preserved and laterally continuous across the sample. TIMS 230Th ages and annual ultraviolet luminescence banding provide chronological control and demonstrate that the high frequency cyclicity is annual. Trace element variations in speleothem calcite are a reflection of hydrochemical processes in the unsaturated zone overlying the cave (supply effects) and partitioning at the water–calcite interface. Mg partitioning between water and calcite is temperature dependent. Calculations suggest that the annual 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 time in the unsaturated zone (which reflect effective precipitation) provide a more probable explanation for the observed annual oscillations and also explain the inverse relationship between Mg/Ca and Sr/Ca. This record suggests that trace elements in speleothems have the potential to provide the high resolution insights into interannual palaeoclimatic variability required for assessment of general circulation models.