Distribution Of Glycerol Dialkyl Glycerol Tetraether Lipids In Soils And Lakes: Implications For Paleoenvironmental Proxies

Glycerol dialkyl glycerol tetraethers(GDGTs) are increasingly popular biomarkers in the field of organic geochemistry. The GDGTs-based proxies(e.g. TEX86, MBT, CBT and BIT) have played important roles in paleoclimate studies, especially for paleotemperature reconstructions. Until now, however, our knowledge on the distribution of GDGTs in lakes and soils of north China is still limited, despite that the investigations of GDGTs(focused on the effect of temperature and p H on GDGTs) in terrestrial environments have been widely carried out worldwide. Here, we analyzed the distribution of GDGTs in soils of north China and in inland lakes on the northeastern Qinghai-Tibet Plateau using high performance liquid chromatography-atmospheric pressure chemical ionization-mass spectrometry(LC-APCI-MS), and investigated the relationships between GDGT distribution and environmental parameters. The aim of this Ph D thesis was to explore the potential of GDGTs as paleoclimate proxies, and to provide a scientific basis for the application of GDGTs in paleoclimatic studies in northwest China and the Qinghai-Tibet Plateau.(1) In soils, moisture plays an important role in controlling the distribution of GDGTs in north China. The Cyclization of Branched Tetraethers(CBT) correlates best with soil water content or mean annual precipitation in soils from arid-subhumid regions, indicating that CBT is mainly controlled by soil moisture in these soils, where water availability is a limiting factor for the producers of branched GDGTs(b GDGTs). Additionally, there is a positive correlation between soil water content and the BIT index for soils on the NE Qinghai-Tibetan plateau, suggesting that BIT might be used to trace soil moisture variations in soil/peat deposits.(2) Temperature and p H can also significantly affect the distribution of soil b GDGTs. In soils from the Chinese Loess Plateau to the northeast China, the inferred temperature by the MBT’/CBT proxy or Chinese b GDGT-SSM calibration(SSM: stepwise selection method) has(weak) negative correlations with MAAT. Instead, the new indices(MBT’5ME, Index 1) and calibrations(MATmr, MATmrs) based on the 5-methyl(or 5-methyl and 6-methyl isomers of) b GDGTs can yield temperature values that correlate positively with climatological or in-situ measured data. Moreover, the CBT’ index based on the 5-methyl and 6-methyl isomers of b GDGTs might be a better p H proxy as it correlates well with in situ soil p H over the whole p H range(4~9).Lake water depth, salinity and temperature might be all important factors controlling the distribution of GDGTs in lacustrine systems.(3) The TEX86 paleothermometer might be applicable for offshore sediments in Lake Qinghai, while b GDGTs-based proxies cannot be used to trace paleoenvironmental changes in terrestrial soils due to in situ b GDGT production in this lake.(4) The relative abundance of crenarchaeol(%cren) increased significantly with increasing water depth in core-top sediments of Lake Qinghai, and the %cren records in ancient sediments of Lake Qinghai and Lake Ckarkia are in agreement with inferred lake-level changes based on other proxies, indicating that %cren could be a useful lake-level proxy for paleohydrology studies.(5) ACE correlates significantly with water salinity in suspended particulate matter and surface sediments of lakes on the northeastern Qinghai-Tibetan Plateau, suggesting that ACE might be used to trace lake salinity changes in inland lakes.(6) We further extracted paleohydrology and paleotemperature signals of Lake Qinghai based on GDGT distribution in the Holocene section of core QH-2011, and found that the lake water was deeper and fresher during the late Holocene but shallower and more saline at the early Holocene. Moreover, we observed a strong anti-phase relationship between extracted TEX86 and %cren records during the 1~4 ka BP, supporting the concept emphasizing the importance of summer temperature in controlling evaporation and thus effective moisture for arid-semi humid regions.In summary, the results of this dissertation indicate that the distributions of GDGTs in north China are sensitive to variations of environmental parameters, and therefore, GDGTs is a powerful tool for paleoclimatic reconstructions.