Long-term elemental trends in drip waters from monitoring Bunker Cave: New insights for past precipitation variability

Abstract

A seven-year (2006–2013) monitoring was conducted in Bunker Cave, northwest Germany. The monitoring programme incorporates cave air temperature, the amount of precipitation, drip rates at five drip sites (TS 1, 2, 3, 5, 8) in the cave, and the analyses of cation (Ca2+, Mg2+, Sr2+, Na+, K+) and anion (HCO3−, NO3−, SO42−, Cl−) concentrations of rain, soil, and drip waters with a monthly (2006–2011) and bimonthly (2012−2013) interval. Further, the Ca, Mg, and Sr concentrations of the two host rock components, limestone and dolomite, were analysed.The analysis of rain, soil, and drip waters allowed to distinguish between geogenic and anthropogenic influences on the hydrochemistry of the cave. The limestone is the source of Ca2+, Mg2+, Sr2+, and HCO3− in all drip waters, as well as SO42− from pyrite dissolution from the limestone. Chloride and NO3− were inserted by anthropogenic sources such as aerosols from road salt, fertilisers, and industry. Drip site TS 1 shows a strong seasonal drip rate, which is simultaneous with a seasonal variation in ion concentrations (Ca2+, Mg2+, Sr2+, HCO3−, SO42−), induced by younger water transported by fracture flow during episodes of fast drip rates. Drip water at drip site TS 5 is recharged by water which probably dissolves prior calcite precipitation (PCP), resulting in low and constant Mg2+ and Sr2+ concentrations. Drip water at drip sites TS 2, 3, and 8 are mainly influenced by PCP, whereas drip waters at TS 2 and 8 are additionally influenced by incongruent dolomite dissolution indicated by their increasing Mg2+ and stable Sr2+ concentrations. Furthermore, Cl− and SO42− concentrations show increasing and decreasing long-term trends for these three PCP-influenced drip waters at sites TS 2, 3, and 8, respectively.The decreasing pattern in infiltration/precipitation from 2000 to 2013 is not very pronounced; however, the continuous draining of the aquifer is visible in the decreasing drip rates over the seven-year monitoring period. This is reflected in the long-term increasing trends of Mg/Ca, Sr/Ca ratios induced by PCP, and Cl− concentrations, as well as in the decreasing long-term trend of the SO42− concentrations in drip waters of drip sites TS 2, 3, and 8.For a better understanding of the cave system with its complex processes, a multi-element-proxy long-term cave monitoring of rain, soil, and drip waters appears to be a reasonable step to be followed by the interpretation of the element proxies of speleothems. This monitoring emphasizes Mg/Ca and Sr/Ca ratios, as well as Cl− and in particular SO42− concentrations as potential past infiltration/precipitation proxies. Mg/Ca and Sr/Ca ratios are the strongest proxies to reconstruct infiltration/precipitation of measured elements in stalagmites, in cases Sr/Ca is not overprinted by growth rate influences. In addition, the sulphur concentration in speleothems may be an infiltration/precipitation proxy in the case of a geogenic source from the host rock.