How dry was the Younger Dryas? Evidence from a coupled δ~2H-δ~(18)O biomarker paleohygrometer applied to the Gemundener Maar sediments, Western Eifel, Germany

摘要

Causes of the Late Glacial to Early Holocene transition phase and particularly the Younger Dryas period, i.e. the major last cold spell in central Europe during the Late Glacial, are considered to be keys for understanding rapid natural climate change in the past. The sediments from maar lakes in the Eifel, Germany, have turned out to be valuable archives for recording such paleoenvironmental changes.For this study, we investigated a Late Glacial to Early Holocene sediment core that was retrieved from the Gemundener Maar in the Western Eifel, Germany. We analysed the hydrogen (delta H-2) and oxygen (delta O-18) stable isotope composition of leaf-wax-derived lipid biomarkers (n-alkanes C-27 and C-29) and a hemicellulose-derived sugar biomarker (arabinose), respectively. Both delta H-2(n-)alkane and delta O-18(sugar) are suggested to reflect mainly leaf water of vegetation growing in the catchment of the Gemundener Maar. Leaf water reflects delta H-2 and delta O-18 of precipitation (primarily temperature-dependent) modified by evapotranspirative enrichment of leaf water due to transpiration. Based on the notion that the evapotranspirative enrichment depends primarily on relative humidity (RH), we apply a previously introduced "coupled delta H-2(n-alkane)-delta O-18(sugar) paleohygrometer approach" to reconstruct the deuterium excess of leaf water and in turn Late Glacial-Early Holocene RH changes from our Gemundener Maar record.Our results do not provide evidence for overall markedly dry climatic conditions having prevailed during the Younger Dryas. Rather, a two-phasing of the Younger Dryas is supported, with moderate wet conditions at the Allerod level during the first half and drier conditions during the second half of the Younger Dryas. Moreover, our results suggest that the amplitude of RH changes during the Early Holocene was more pronounced than during the Younger Dryas. This included the occurrence of a "Preboreal Humid Phase". One possible explanation for this unexpected finding could be that solar activity is a hitherto underestimated driver of central European RH changes in the past.