A paleoclimate record of the last 17,600 years in stalagmites from the B7 cave, Sauerland, Germany
Introduction
Fabrics and composition of calcite precipitates in caves (speleothems) are controlled by host rock parameters and in particular by the amount of surface precipitation, temperature and soil formation. Thus, speleothems are a valuable tool for reconstructing the continental climate evolution. Studies of Pleistocene calcite precipitates, e.g. from Devil's Hole in Nevada (Winograd et al., 1992), for European Holocene stalagmites (McDermott et al., 1999) as well as for Oman (Burns et al., 1998; Neff et al., 2001) and Israel speleothems (Ayalon et al., 1999; Bar-Matthews et al., 2000) have proved the usefulness of the method. The relatively fast growing stalagmites are well suited for paleoclimate research because their growth is understood after the studies of Dreybrodt and Franke (1987) and Dreybrodt (1999).
In this study sedimentological/petrographical and geochemical data for three Holocene stalagmites from B7-Cave (Sauerland, NW Germany, Fig. 1) at an elevation of 185 m above sea level (asl) are presented and interpreted in a paleoclimatic context (cf. Richter et al., 1997; Niggemann, 2000; Niggemann et al., 2000). In our experience, the understanding of the rhythmic fabrics of Holocene speleothems and recent precipitation mechanisms are basic prerequisites for the more difficult problem of interpreting of Pleistocene speleothems.
Section snippets
Geological setting and sampling
The Middle- to Upper Devonian carbonate formation of the Sauerland mountains (Northern Rhenish Massif) consist of biostromal accumulations at the external shelf of the Rhenoherzynikum (Burchette, 1981; Fig. 1). After the Upper Paleozoic tectonic deformation and subsequent uplift, these massive limestones have been exposed to repeated phases of erosion and karstification (Hammerschmidt et al., 1995; Niggemann, 1997). The latter authors consider most of the cave systems within the massive
Methods
To record the fabrics of the polished stalagmite halves, these halves were scanned and a PC program processing the scan files similar to the method of Genty and Quinif (1996) was developed. We used a program of K.H. Pielsticker with conventional hardware (PC, scanner) for analysing geological layered structures and transforming them into a scale of grey values (256 grey values, 0=white; 255=black). Thin sections (20–30 μm) of the stalagmite material were examined with a light microscope (Zeiss)
Fabrics
Light and dark layers are visible on the polished longitudinal sections of the stalagmites (Fig. 3). These light and dark rhythms can be digitized by the grey value technique. The light facies type (FT B) is mostly milky-white and partly brownish due to reflection of light by fluid inclusions. The dark facies type (FT A) is pure, owing its dark colour to the absorption of light. In thin sections under the microscope the clear layers appear lighter than the milky ones. In more or less every zone
General isotopic and microfacies trends
The recently accumulated calcite material of the actively growing and continuously dripping stalagmites in the B7 cave with a high rock covering consists of dark compact facies types. These dark facies types are in strong contrast to the white porous facies types in the central part of the stalagmites, containing fluid inclusions. It seems probable that the dark compact laminae develop preferentially during the wet winter times while the white porous laminae grow in dryer cave environments with
Outlook
More research concerning the relationship of precipitation, relative humidity and air circulation and consequently evaporation and CO2 degassing in the cave is needed to further improve the understanding of the direct climatic impact on the growth of speleothems. A special cave-climate program is underway (Pflitsch et al., 2000). There is also the need for monitoring the annual trend of the soil-CO2 carbon-isotopic composition. Such a program is planned for the near future. We are presently
Acknowledgements
We thank the German Research Foundation (Deutsche Forschungsgemeinschaft: SPP “Changes of the Geo-Bisophere during the last 15,000 years”) for financial support (Ri-216/15), Dipl.-Ing. K.H. Pielsticker for the grey value scans, H. Völkel for preparation and thin sections, R. Eichstädter for the TIMS measurements), G. Niggemann for precipitation sampling and the members of the Speleoclub Letmathe for supporting cave sampling.
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2015, Quaternary Science ReviewsCitation Excerpt :By doing the same exercise using the most recent empirical relationship based on a large set of cave studies that already take into account a “mean” disequilibrium inherently present in cave environment (Tremaine et al., 2011), a theoretical δ18Oc value of −6.85‰ is obtained that is even closer to the measured value meaning that δ18Oc precipitates in disequilibrium. It is interesting to note that in B7 cave in West Germany (Niggemann et al., 2003) located around 450 km northward from Milandre cave δ18O values of drip water (−8.4‰) and corresponding calcite (−6.3‰) are very similar to those observed in Milandre cave. The good agreement between the isotopic composition of drip and recent fluid inclusion water confirms that the cave signal is well preserved in speleothems.
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