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The geochemical signature of many speleothems used for reconstruction of past continental climates is affected by kinetic isotope fractionation. This limits quantitative paleoclimate reconstruction and, in cases where the kinetic fractionation varies with time, also affects relative paleoclimate interpretations. In carbonate archive research, clumped isotope thermometry is typically used as proxy for absolute temperatures. In the case of speleothems, however, clumped isotopes provide a sensitive indicator for disequilibrium effects. The extent of kinetic fractionation co-varies in Δ47 and δ18O so that it can be used to account for disequilibrium in δ18O and to extract the past drip-water composition. Here we apply this approach to stalagmites from Bunker Cave (Germany) and calculate drip-water δ18Ow values for the Eemian, MIS3, and the Holocene, relying on independent temperature estimates and accounting for disequilibrium. Applying the co-variation method to modern calcite precipitates yields drip-water δ18Ow values in agreement with modern cave drip-water δ18Ow of −7.9 ± 0.3‰, despite large and variable disequilibrium effects in both calcite δ18Oc and Δ47. Reconstructed paleo-drip-water δ18Ow values are lower during colder periods (e.g., MIS3: −8.6 ± 0.4‰ and the early Holocene at 11 ka: −9.7 ± 0.2‰) and show higher values during warmer climatic periods (e.g., the Eemian: −7.6 ± 0.2‰ and the Holocene Climatic Optimum: −7.2 ± 0.3‰). This new approach offers a unique possibility for quantitative climate reconstruction including the assessment of past hydrological conditions while accounting for disequilibrium effects.

Poster; International audience

The stable oxygen isotope parameter δ18O remains the most widely utilised speleothem proxy for past climate reconstructions. Uncertainty can be introduced into stalagmite δ18O from a number of factors, one of which is the heterogeneity of groundwater flow in karstified aquifers. Here, we present a lumped parameter hydrological model, KarstFor, which is capable of generating monthly simulations of surface water – ground water – stalagmite δ18O for more than thousand-year time periods. Using a variety of climate input series, we use this model for the first time to compare observational with modelled (pseudoproxy) stalagmite δ18O series for a site at Assynt, NW Scotland, where our knowledge of δ18O systematics is relatively well understood. The use of forward modelling allows us to quantify the relative contributions of climate, peat and karst hydrology, and disequilibrium effects in stalagmite δ18O, from which we can identify potential stalagmite δ18O responses to climate variability. Comparison of the modelled and actual stalagmite δ18O for two stalagmites from the site demonstrates that, for the period of overlapping growth, the two series do not correlate with one another, but forward modelling demonstrates that this falls within the range explicable by differences in flow routing to the stalagmites. Pseudoproxy δ18O stalagmite series highlight the potential significance of peat hydrology in controlling stalagmite δ18O over the last 1000 yr at this site.

Poster; International audience

Grotta Savi, which opens on the flanks of Monte Stena in Rosandra Valley (Carso Triestino, Italy), consists in a network of sub-horizontal galleries developed from 300 to 350 meters a.s.l., and a total extension of >4 km. SV1 stalagmite, which comes from the inner part of the cave, is the first European speleothem that spans continuously the last 17 ka. Eighteen MC-ICPMS U/Th method datings were obtained from the 27 cm tall stalagmite, and 345 stable isotope analyses were carried out along its growth axis (mean resolution: 50±43 yrs). The stalagmite began to growth at 16.8±0.51 ka, and the axial growth rate was very low (below 10 µm/yr) until the end of the Younger Dryas From ~10.67 to ~7.59 ka a faster axial growth rate was recorded (30 to 44 µm/yr). The δ18O record reflects the amount of water that fed the stalagmite, which depends on mean annual rainfall amount and by the evapotranspiration at the surface. The δ18O record is positively correlated with the same record from Soreq Cave in Israel (BAR-MATTHEWS ET AL., 1997), and inversely with the 18O record of GISP2 ice core (GROOTES ET AL., 1993). For the more recent part of SV1, we observed a positive correlation between δ18O-calcite and the reconstructed mean annual temperature for the last 500 years in the Alps (LUTERBACHER ET AL. 2004). The strong temperature increase between 1800 AD and today is replicated by the positive δ18O trend in SV1.

International audience