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要旨:
Over the last glacial period, the climate of the Northern Hemisphere experienced numerous abrupt variations on millennial to centennial timescales known as Dansgaard-Oeschger events. These events, characterised by rapid warming at the beginning of interstadials and gradual cooling back to stadial conditions are best documented in Greenland ice cores and North Atlantic marine records, while their propagation onto the continents and potential feedbacks are less well documented. In this context, loess palaeosol sequences in central Europe are valuable archives, often recording these climatic changes in the form of brown soils and tundra gley horizons - indicating milder interstadial conditions - intercalated with primary loess deposits reflecting cold stadial conditions.
To reconstruct palaeoclimate changes at high resolution we use singular material from loess sediments: fossil earthworm calcite granules (ECG). ECGs, composed of aggregated sparite crystals formed in the calciferous earthworm glands, are secreted daily at the soil surfacemostly by Lumbricus species and experience limited vertical mixing within the loess sedimentary column. ECGs are thus an excellent terrestrial material for palaeoclimate reconstructions using stable isotopic geochemistry and radiocarbon dating. ECGs have been collected from two temporally overlapping loess-palaeosol sequences along an NNW-SSE transect in the Rhine River valley of western Germany. We present warm-season land-surface temperature and precipitation estimates at millennial timescales spanning ~ 45-22 cal kBP (late OIS 3 - OIS 2). We demonstrate that OIS 3-2 climate in the Rhine Valley was significantly cooler during the warm season and overall drier with annual precipitation reduced by up to 70%, compared to the present day. Interstadials were only slightly warmer (1-4°C) than stadial indicating strong attenuation compared to Greenland records. In combination with mesoscale wind and moisture transport modelling we can show that this region was dominated by westerlies and thereby inextricably linked to North Atlantic climate forcing.
The approach combining high-resolution age-depth modelling and geochemical proxy-based climate reconstruction can be readily adopted at other loess palaeosol sequences. We envisage a widespread application of this approach that would improve our understanding of regional variability over the European continent in response to North Atlantic climate changes over millennial to centennial timescales.
