A discontinuous climatic record from 187 to 74 ka from a speleothem of the Clamouse Cave (south of France)

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Abstract

This study presents a continental paleoclimatic record in the south of France, based on δ18O, δ13C and the 234U/238U activity ratio, dated by the 230Th/234U disequilibrium method with thermal ionization mass spectrometry. A stalagmite (Cla4) from the Clamouse Cave offers a discontinuous stable isotopic record between 189 ka and 74 ka which covers marine isotope stages (MIS) 7, 6.4, 5.5, 5.3 and 5.1. The growth phases of the Cla4 stalagmite correspond to high sea level stages, except during MIS 6.4 (169–162 ka). All the growth phases of Cla4 correspond to humid periods, corresponding to the sapropel events observed in the eastern Mediterranean basin. Thus, the influence of a strong hydrological activity in the eastern Mediterranean basin during these periods prevailed as far west as the south of France. Because the karstic system studied strongly buffers the isotopic composition of water, isotopic variations of the calcite deposited in the cave represent mainly global and large-scale environmental variations when isotopic equilibrium conditions prevailed for calcite crystallization. Sub-stage 5.3, the end of 5.5 and MIS 7 were colder by about 4–6°C (calculated temperature) compared to present-day temperature while the growth phases of sub-stages 5.1 and the beginning of 5.5 reflect environmental conditions close to present ones. The δ18O and δ13C values of cave deposits of the sub-stage 6.4 are: (1) strongly marked by kinetic fractionation processes such as evaporation due to moisture deficit within the cave atmosphere during the first step of this growth phase and (2) related to higher humidity due to a second period of enhanced rainfall during the second step of growth. This study shows that even if calcite has not been deposited at isotopic equilibrium, its isotopic composition can give insights into the environmental conditions at the time of deposition.

Introduction

A detailed knowledge of the timing of past climatic changes in continental records is essential for a comparison with the marine [1], [2] or ice [3], [4] core records because of their close relationships, and for global reconstructions of climate variability. Speleothems provide reliable paleoenvironmental records as numerous climatic proxies may be investigated such as oxygen and carbon stable isotope ratios, petrographic analyses, fluorescence banding, trace elements concentrations or EPR (Einstein–Podolsky–Rosen) spectrometry and reflectance [5], [6], [7], [8], [9], [10]. Speleothems are barely affected by erosion or other post-depositional processes, and they will also complement the continental record based on lake sediments or pollen sequences. Moreover, because of the generally closed system conditions for the U–Th system, they can be precisely dated back to ca. 500 ka by the 230Th/234U disequilibrium method [11] with thermal ionization mass spectrometry (TIMS).

We investigate the patterns of growth and of the stable carbon and oxygen isotope changes of the Cla4 stalagmite (Clamouse Cave, south of France). The present-day isotopic compositions of rain, seepage water and calcite were measured in order to determine the isotopic equilibrium state of the modern chemical system, which will be compared to the past isotopic changes in the Cla4 speleothem. The climatic changes in Cla4 during the glacial marine isotope stage (MIS) 6.4 will be particularly discussed. The Cla4 climatic record will be compared to North Atlantic and Mediterranean marine records, and to their continental margins [12], [13].

Section snippets

Site and speleothem description

The studied stalagmite (Cla4) was collected from Clamouse Cave, near the Mediterranean Sea (Fig. 1). The Clamouse area is close to the southern province of the Larzac Plateau, a large karstic plateau elevated 700–900 m asl.

The cave is a complex karstic system, developed in upper Jurassic dolomites and limestones. The 3500 m long explored galleries are arranged in three major levels in relation to the deepening of the Herault River [14]. The lowest level (35–60 m asl) is still active with a

Sampling strategy

The stalagmite was cut longitudinally and one half was used for sampling. Pieces of 20×3×5 mm were cut parallel to growth layers to provide samples for Th/U dating. When visible, a hiatus is characterized by a thin white or colored clay layer separating two zones of calcite more or less colored or with different crystal fabrics. Fifteen samples were preferentially cut on both sides of visible growth hiatuses in order to precisely determine their duration (Fig. 2). Additional samples (83) from

U–Th dating

Table 1 gives analytical results, U contents, age estimates and measured 234U/238U activity ratios for each of the 15 samples collected above and below the visible hiatuses. Correction for detrital contamination was not required because the 230Th/232Th activity ratios are greater than 400 for all samples. The deposition of this stalagmite started before 189.00 ka and ended after 74.54 ka (Table 1). The lack of age inversion for all samples (within the 2σ error for the growth phase of ∼189 ka)

Discussion

The Cla4 stalagmite grew discontinuously over the 190–75 ka interval that encompassed MIS 5, 6 and the end of MIS 7 [2]. The Cla4 growth phases will be first compared to the precisely U/Th-dated high sea stands [31], [32], [33], [34], [35], [36], [37] and to reconstructed sea levels from the orbitally tuned marine isotopic records [1], [2]. Then, the Cla4 paleodata will be compared: (1) to Mediterranean continental [32] and marine [33] records, and (2) to the North Atlantic marine records [34],

Conclusion

The U/Th dating of stalagmite Cla4 from the Clamouse Cave in southern France shows that the Cla4 stalagmite grew discontinuously during the warm and humid phases (MIS 5.1, 5.3, 5.5 and 7), as well as during a brief period during the glacial stage MIS 6. During MIS 5, the temperature oscillated with an amplitude of ∼4°C, and could have decreased by ∼6°C at the end of sub-stage 5.5. The growth rates of Cla4 vary by a factor of ∼100 independently of these warmer/colder climatic conditions, and

Acknowledgements

This study was supported by the Centre National de la Recherche Scientifique et le Commissariat à l’Energie Atomique, UMR 1572. The Cla4 stalagmite from Clamouse Cave was collected with the permission and the help of P. Dubois. We wish to thank C. Vertonghen, M. Bakalowicz and S. Van-Exter for their active collaboration in the field. Thanks also to M. Stievenard and O. Cattani for the stable isotope measurements on water. Thanks are also due to L. Turpin for his helpful discussions, as well as

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