| Summary: | ABSTRACT This paper presents high-resolution records of 13 C/ 12 C ratios of organic matter from two loess sequences in northwestern Europe. Our analysis is the first attempt to use organic matter δ 13 C as a record of the response of vegetation to climatic variations in an area where climatic changes were not strong enough to induce a radical change in vegetation cover. Over the last climatic cycle, the vegetation of the Rhine Valley showed a strong predominance of C3 plants. Thus, the small δ 13 C variations, with an amplitude of only 1.5‰ to 3‰, are interpreted as corresponding to fluctuations in water supply and atmospheric CO 2 concentration variations rather than to the ratio of C4 to C3 vegetation. Furthermore, loess sequences accumulated at high rates and allow high correlation with climatic proxy data, like the Greenland Ice Sheet Project 2 (GISP2) δ 18 O and the variations in CO 2 concentration recorded in the Vostok ice core. The δ 13 C constitutes a reliable and complementary proxy to study small climatic stresses endured by vegetation during the Weichselian in northwestern Europe. Moreover, by using absolute age control and correlations between global (Vostok-CO 2 ) or semi-global (GISP2-δ 18 O) climate effects, δ 13 C values of organic matter in loess sequences offer a new tool to establish a refined chronology in continental sequences. analyzer (Carlo Erba NA 1500) and are expressed in weight percent, with a relative precision of 1 wt%. Carbon isotope ratios, obtained from a VG Optima mass spectrometer, are expressed as δ 13 C [δ 13 C = ( R sample /R std -1 ) × 1000 (‰) where R = 13 C/ 12 C, and the standard (std) is the Peedee belemnite (PDB)] in per mil with an absolute precision of 0.1‰. Organic carbon content records in Achenheim and Nußloch are reported in Carbon isotope fractionation by plants depends also on the atmospheric CO 2 supply. When the CO 2 concentration ([CO 2 ]) is high, a large frac- 584 GEOLOGY, July 1998 tionation is observed. But if [CO 2 ] is low, growth is limited, and ...
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