Controls on the geochemistry of speleothem-forming karstic drip waters

Research was performed at Crag Cave, Castleisland, southwest Ireland, and PB Cave, Castleton, Derbyshire, in order to determine the main factors responsible for modifying rainwater geochemistry during flow through soil and karstic aquifer zones. Monitoring was performed on a daily basis in summer and winter at Crag Cave, and on a monthly basis over one year at PB Cave. At both sites, biannual peaks in karst system Ca2+ concentrations occurred due to: (i) promotion of microbial C02 production by increased summer temperatures, and (ii) retardation of gaseous exchange by ponding of elevated winter rainfall input leading to an unseasonable build up in soil zone C02. Therefore, speleothems at both sites may form biannual bands in hydrological years subject to elevated winter rainfall input. In addition to variations in carbonate weathering due to fluctuations in C02 levels, cation yields in Crag Cave matrix soil water were controlled by dolomite dissolution (Mg2+), plant uptake (K+), and evapotranspiration balanced by enhanced winter marine aerosol input (Na+). Strontium isotope analysis indicates that sr 2+ was derived from a 50:50 silicate/carbonate mixture, whilst the relatively light 313C signal was related to direct evolution of C02 into the aqueous phase in waterlogged pores. Within the Crag Cave aquifer variations in karst water geochemistry were controlled by dilution, flow switching, prior precipitation of calcite and dolomite dissolution along the flow path. Strontium isotope analysis indicates that dissolution in the aquifer dominated, with sr2+ being sourced from a 25:75 silicate/carbonate mixture. Light karst water 313C values were constrained by the supply of light soil gas to the aquifer. Elevation in the Mg/Ca and Sr/Ca ratios in the Crag Cave speleothem record compared to present day analogues indicates that the former Holocene climate was drier, whilst heavier 87Sr/86Sr ratios and o13C values suggest variation in soil hydrology over time