Multiproxy biomarker, isotopic and pollen reconstructions of the middle to late Holocene paleoclimate of the Loess Plateau in centre China

The Asian monsoon is a key component of the earth's climate system that directly affects the livelihood of 50 million people on the loess plateau of central China. At the far edge of monsoonal influence, this region is especially vulnerable to future changes in temperature and evaporation / precipitation. Therefore, paleoclimatic information on the natural sensitivity of the region to changes in monsoon driven aridity are crucial. Despite the need for multiproxy records of Holocene climate from this region, reconstructions are rare, because of the low resolution of loess deposits and the scarcity of other paleoclimate archives (e.g. natural lakes, speleothems). Here we present multiple proxy records from Tianchi lake, one of the few nature lakes on the loess plateau and central China. The chronology is well constrained by a high-resolution (20 AMS 14C dates) radiocarbon age-model, spanning the past 6200 years. Here we present pollen, Glycerol dibiphytanyl glycerol tetraethers (GDGTs), lake macrophyte and higher plant-wax biomarkers to reconstruct regional climate change during the middle to late Holocene. Evidence from pollen data suggest that deciduous trees decreased from 6200 cal yr BP and then more rapidly from 1000 yr BP. Modern and downcore molecular distribution patterns of n-alkanes and n-alkanoic acids, especially n-alkane Paq values, suggest increasing relative abundance of macrophytes over this time, which we interpret (based on lake morphology) as decreasing lake-level. Using the recent Sun et al (2011) regional calibration we derive mean annual GDGT based temperatures (MBT/CBT-MATs) with reasonable ranges. Our temperature reconstruction closely correlates on millennial to centennial timescales with the independent D/H measurements on C28 fatty acid methyl esters (C28 FAMEs), whose signal is assumed to derive primarily from terrestrial plant waxes and the δD values to reflect local changes in relative humidity. Comparisons of our independent GDGT temperatures and plant-wax hydrogen isotopic records with stalagmite δ18O records from the monsoon region and NH summer insolation suggests strongly that our record reflects regional changes in monsoon strength forced by NH summer insolation. Superimposed on the longer-term insolation driven changes are centennial scale variations, recorded by both the independent reconstructions of relative humidity (C28 FAME δD) and temperature (MBT/CBT-MAT). In the most recent 1000yr, and especially the last 500yr of the record, the lake sediments record significant changes in many parameters, magnetic susceptibility, rapid increases in herbaceous pollen and decreases in deciduous trees, changes in biomarker distributions and isotopes. This is coeval with documentary records of increasing local population density and infers historical human impact on the catchment.<br />
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