Climate influence on geochemistry parameters of waters from Santana–Pérolas cave system, Brazil

Abstract

A four-year study of water geochemistry and hydrology was performed in a relatively deep cave system (overlying bedrock thickness varies from 100 to 300 m) as part of two monitoring programs, from June 1990 to February 1992 and from March 2000 to March 2002. The pH, saturation index for calcite, Ca and SO 4 concentrations, and elemental ratios of Mg/Ca and Sr/Ca were measured in surface, well and drip waters throughout the system. Despite local hydrological and geological differences among sampling sites, the monitoring revealed significant synchronous intersite variations in these parameters that are related to seasonal changes in rainfall recharge, suggesting that element ratios of speleothems formed in deep caves are capable of recording short-term climate variations. Groundwater residence time appears to be the main factor affecting the water composition in the epikarst. Epikarstic waters are always undersaturated with respect to calcite but both saturation index (S.I.) and dissolved solids content increase substantially during drier periods because of longer residence times and longer interaction between meteoric water and limestone. By contrast, results from cave drips and rimstone pools indicate constant supersaturation for calcite and demonstrate that a major control on trace element ratios of waters in the deep vadose zone is the degree of prior calcite precipitation. This mechanism is more effective during drier periods when higher Mg/Ca and Sr/Ca values are observed in all drip and pool sites. However, variations do occur independently of the general trend of drip discharge, which suggest non-linear features in cave seepage water geochemistry. In addition, synchronous variations in SO 4 and Cl concentration indicate high connectivity between different water flow pathways characterized by similar response to interseasonal changes in vadose water level. Fluctuations in trace element ratios of cave streams appear to reflect increased contribution of waters flushed from the vadose fissure aquifer during very wet periods by a piston flow mechanism. Flushing episodes are also responsible for maintaining more positive saturation indices in streams even during periods of high river discharge. Our results suggest that trace elements are a potential proxy for past rainfall changes but they also reveal different scenarios for interpreting trace elements ratios of speleothems and freshwater tufa deposited in a deep cave systems located in tropical humid areas.