Speleothem record of the last 180 ka in Villars cave (SW France): Investigation of a large δ18O shift between MIS6 and MIS5
Introduction
Climate variations are driven by a complex combination of factors such as orbital parameters, interactions between atmospheric and ocean circulation, greenhouse gas effects, and system feedbacks. Physically deciphering these various influences is one of the main goals for climatologists.
The penultimate deglaciation (Termination II, or TII; a period between 135 and 125 ka before present) one of the closest termination to the present day interglacial, occurred under orbital parameters different from the more recent Termination I (TI). Thus, characterizing this deglaciation is of first importance in order to better constrain the influence of orbital parameters on major climatic variations. Several studies based on different archives cover the TII period but few provide quantitative climatic data. Antarctic records such as the Vostok ice core cover several climatic cycles but their temporal resolution is low and the associated age models suffer from significant uncertainties. Nevertheless, deuterium excess records from such core can be used to quantify local temperature variations, including during Termination II (Vimeux et al., 2002). At lower latitudes, studies of Chinese speleothems provide a record of East-Asian monsoon strength with a well constrained chronology but these studies do not provide temperature estimates (e.g. Yuan et al., 2004, Cheng et al., 2006). In Western Europe, several sea-surface temperature (SST) records covering this key period were recovered from the Alboran sea and the Iberian Margin (mean annual SST derived from the U37k’ alkenone index: (Shackleton et al., 2002; Martrat et al., 2004, Martrat et al., 2007)), the Bay of Biscay and offshore West Ireland (seasonal SST deduced from foraminifera assemblages (Kandiano et al., 2004, Sánchez Goñi et al., 2008)). Quantified paleo-climate reconstructions of continental settings for TII exist but they remain scarce and often entail large uncertainties. The longest high resolution and continuous continental record in the area of interest (Western Europe and Mediterranean basin) was obtained from speleothems in Soreq cave (Israel), and covers the last 177 ka. According to several studies involving fluid inclusion δD, clumped isotope analyses and nearby marine cores (SST estimates from δ18OG.ruber and alkenones), carbonate δ18O variations through time in Soreq Cave reflect a combination of source isotopic composition, temperature and amount effect variations (Matthews et al., 2000, Bar-Matthews et al., 2003, McGarry et al., 2004, Affek et al., 2008, Almogi-Labin et al., 2009). Among other important results, these studies estimates the glacial/interglacial temperature rise in the region at ∼7 °C (Affek et al., 2008). The Corchia cave in Italy provides speleothem records of the penultimate deglaciation with a well constrained U–Th chronology, allowing precise tuning with Iberian Margin marine records (Drysdale et al., 2005, Drysdale et al., 2009); these studies date the inception of TII at 141.0 ± 2.5 ka and abrupt warming of the deglaciation at 133 ± 2.5 ka (mid-point). Over continental Western Europe, only a few quantified climate reconstructions have been established for the last climate cycle. Among them, the Monticchio Maar multiproxy record from South Italy (Brauer et al., 2007, Allen and Huntley, 2009) has a well constrained chronology based on radiometric dating and varve counting, covering the last 132 ka. Alternatively, the Velay lake record from southern France provides quantified estimates of temperature and rainfall over the last four glacial cycles, although its chronology is not as well constrained as that of the Monticchio record, and is the closest to our study site (∼300 km east from Villars, (Beaulieu et al., 1991, Cheddadi et al., 2005)). Pollen assemblages recovered from marine cores around Western Europe also allow reconstructions of terrestrial climate (e.g. Combourieu-Nebout et al., 2002, Sánchez Goñi et al., 2005). In summary, there is currently very limited quantitative data covering the penultimate deglaciation in Western Europe.
We present here a speleothem stable isotope profile (δ13Cc and δ18Oc) recovered from a flowstone drill core in Villars cave (SW France), which provides a discontinuous record of the last 180 ka. The oxygen profile exhibits a striking −5‰ drop in calcite δ18O coeval with the penultimate deglaciation, similar to the Soreq cave record. The present study focuses on this major climatic transition, aiming to characterize it quantitatively through the use of various analytical techniques: measurements of calcite stable isotopic compositions (C–O), trace elements relative concentrations (e.g., Mg, Sr), stable isotopic compositions of fluid inclusions (H–O), and calcite clumped isotopes (Δ47). Combination of these methods enables us to estimate the amplitude of the temperature rise between the end of the penultimate glacial and the last interglacial optimum and to understand the sharp drop of the calcite δ18Oc values during Termination II. The consequences in terms of paleodripwater oxygen isotopic composition (δ18Ow) and the different controls on the oxygen isotopic composition of the calcite (δ18Oc) in Villars Cave will be discussed.
Section snippets
Villars cave site
Villars cave is located in south-west France, about 200 km inland from the Atlantic coast (45°30′N, 0°50′E, 175 masl; Fig. 1 ). The local climate is oceanic temperate with mild and humid winters. The surface atmospheric mean annual temperature is 12.1 ± 0.7 °C (Sept. 1984–Aug. 2007, data from Nontron meteorological station (Genty et al., 2006, Wainer et al., 2009)) with average temperatures of 6.4 ± 1.3 °C in the winter (January–March) and 18.8 ± 1.1 °C in the summer (July–September). Total
U–Th dating
In total, 42 samples were analyzed in different laboratories. In the early stages of this study, two assessment dates were measured at the LSCE (France) according to procedures described in (Plagnes et al., 2002). Eighteen samples were dated in the Geological Survey of Israel (GSI). Less than 2 g were ground to powder, dissolved in nitric acid and spiked with 229Th–236U. The samples were then loaded into columns containing 2 ml Bio-Rad AG 1X8 200–400 mesh resin in order to separate U fraction
Hendy test
The Hendy test is one of the most widely used tool to test whether calcite from a given stalagmite was deposited out of isotopic equilibrium (Hendy, 1971, Schwarcz, 1986). In this test both δ18Oc and δ13Cc are measured along an individual lamina, and usually plotted against the distance from the apex of the stalagmite.
Samples precipitated under thermodynamic equilibrium conditions are expected to display the following criteria: (a) no correlation between δ18Oc and δ13Cc along a single growth
Discussion: the Vil-car-1 oxygen isotopic shift
In previous speleothem records from Villars Cave, the amplitude of δ18Oc variations remained limited, in contrast to δ13Cc profiles, displaying striking climatic features (Genty et al., 2003, Genty et al., 2005, Wainer et al., 2009). This 5‰ amplitude of δ18Oc variation recorded by Vil-car-1 is unusually large but previous records from this cave only extended back as far as ∼80 ka, and thus covered neither Termination II nor the Last Interglacial. It is thus important to understand the physical
Conclusions
The Vil-car-1 record is the first speleothem record from Villars cave that extends back sufficiently (∼180 ka) to record the penultimate deglaciation: the transition between Late MIS 6 and the Last Interglacial “optimum” is prominently recorded as a ∼5‰ depletion event affecting calcite δ18O values.
Investigation of the potential causes for this isotopic depletion yielded several important results. The oxygen isotope composition of paleodripwater trapped in fluid inclusions before, during and
Acknowledgements
This work was funded by the CNRS through the ANR PICC program and CNRS INSU programs (ECLIPSE). A significant part of the U–Th dating was done at the GEOTOP laboratory and SEM observations were realised in collaboration with Eric Robin using JEOL JSM 840. We thank the Villars Cave owner, the Versaveau family, O. Mestre for original meteorological data, P. Braconnot and E. Mosquet for the model IPSL-CM4 outputs, F. Mansouri, H. Rebaubier, F. Dewilde for their help with the stable isotopes and
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