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Calcitic stalagmites from caves in the Sauerland, Germany, prove the existence of sub-Milankovitch cycles in precipitation during the last 6000 yr. The δ18O record dated with Th/U is interpreted as an indicator of paleohumidity. Spectral analysis of δ18O from 6000 a BP up to the recent top of a stalagmite from the Atta cave yields statistically significant peaks at 1450, 117, 64 and 57 a. Additionally we find a good correlation of the stalagmite’s δ18O and Δ14C from European tree rings. The 1450 a cycle in the stalagmite probably is analogous to the pervasive millennial scale climate cycle described by Bond et al. [Science 278 (1997) 1257–1266; 294 (2001) 2130–2136] derived from the amount of ice rafted debris in deep sediments from the North Atlantic. Our results suggest that the centennial to millennial shifts observed in the North Atlantic are accompanied by synchronous shifts of the climate in Northern and Central Europe, which most probably can be attributed to solar irradiation variations. [Copyright &y& Elsevier]

The last two decades have seen a considerable increase in studies using speleothems as archives of past climate variability. Caves under study are now monitored for a wide range of environmental parameters and results placed in context with speleothem data. The present study investigates trends from a seven year long monitoring of Bunker Cave, northwestern Germany, in order to assess the hydraulic response and transfer time of meteoric water from the surface to the cave. Rain-, soil-, and dripwater were collected from August 2006 to August 2013 at a monthly to bimonthly resolution and their oxygen and hydrogen isotope composition was measured. Furthermore, drip rates were quantified. Due to different drip characteristics, annual mean values were calculated for the drip rates of each drip site. Correlations of the annual mean drip rate of each site with precipitation and infiltration demonstrate that the annual infiltration, and thus the annual precipitation control the inter-annual drip-rate variability for all except one site. The hydraulic response is not delayed on an annual basis. All drip sites display identical long-term trends, which suggests a draining of a common karst reservoir over these seven years of monitoring. Correlations of soil- and dripwater monthly δ 18 O and δD values with atmospheric temperature data reveal water transfer times of 3 months to reach a depth of 40 cm (soilwater at site BW 2) and 4 months for 70 cm depth (soilwater at site BW 1). Finally, the water reaches the cave chambers (15 to 30 m below land surface) after ca. 2.5 years. Consequently, a temporal offset of 29 to 31 months (ca. 2.5 years) between the hydraulic response time (no time lag on annual basis) and the water transfer time (time lag of 29 to 31 months) was found, which is negligible with regard to Bunker Cave speleothems because of their slow growth rates. Here, proxies recording precipitation/infiltration and temperature are registered on a decadal scale. Variations in drip rate and thus precipitation and infiltration are recorded by δ 13 C and Mg/Ca ratios in speleothem calcite. Speleothem δ 18 O values reflect both temperature and precipitation signals due to drip rate-related fractionation processes. We document that long-term patterns in temperature and precipitation are recorded in dripwater patterns of Bunker Cave and that these are linked to the North Atlantic Oscillation (NAO). [ABSTRACT FROM AUTHOR]

Summary: Monitoring cave environments is important to understand processes in karst systems. If stalagmites from a specific cave are used as archives of past climate variability, a quantitative understanding of the soil–karst–speleothem system is crucial. The monitoring program performed in Bunker Cave (NW Germany), which includes monthly collection of climatological data as well as air and water samples from the cave and the overlying soil since 2006, is a prerequisite for the interpretation of speleothem data from the cave in terms of climate variability. The results show that Bunker Cave is a homogeneously ventilated cave with rather low pCO2 values of 580–1200ppmv, which lacks strong seasonal variations. The δ 18O value of cave drip water reflects the mean annual composition of the rain water, and the seasonal variability is strongly attenuated indicating a well-mixed karst aquifer. Hence, stalagmites from Bunker Cave should be well suited to record interannual/decadal climate trends. Seven drip sites in two cave chambers show three different types of discharge behaviour including slow seasonal drip, fast seasonal drip, and seepage-flow. The drip-rate patterns of most drip sites are consistent with the main infiltration events, taking into account a delay of several months. An instantaneous response to precipitation (i.e., piston-flow) is not observed for any drip site indicating a specific water capacity threshold in the soil/karst aquifer. Prior Calcite Precipitation (PCP) was also identified especially during times of low discharge. Though PCP is not as pronounced at Bunker Cave as in other caves, it probably has an influence on the Mg/Ca ratios of the stalagmites. [Copyright &y& Elsevier]