Quantitative reconstruction of late Quaternary climate in monsoonal East Asia: Validation and application of tetraether membrane lipid proxies and clumped isotope thermometry
Authors:   , Guo Jingjing 
Published: August 2024

The East Asian Monsoon (EAM) is one of the strongest monsoon systems and controls the water supply to over 20% of the world’s population. Theoretically, the EAM will strengthen in response to ongoing global warming, however, instrumental data indicates that the EAM has been weakening and becoming more erratic since the 1970s. The Chinese Loess Plateau (CLP) with continuous alternating paleosol and loess layers is considered as one of the best terrestrial archives of past EAM climate. The absolute temperature plays a key role in the EAM strength. This thesis aims to quantitatively reconstruct late Quaternary climate variations in the EAM region by using two independent tools, i.e., branched glycerol dialkyl glycerol tetraethers (brGDGTs), which are membrane-spanning lipids of temperature-sensitive soil bacteria, and carbonate clumped-isotope thermometry (D47) using land snail shells stored in loess. These two paleothermometers are applied to a loess-paleosol sequence from Yuanbao section, located on the western edge of the CLP. Prior to application of the brGDGT-based paleothermometer, the distribution of brGDGTs in soils along an aridity transect was investigated to better understand the reliability of brGDGTs as paleothermometer in semi-arid to arid regions (Chapter 2). The impacts of embedding resin used to fix snail shells for high-resolution sampling on stable and clumped isotope measurements were also tested (Chapter 3). The two independent temperature proxies show similar trends and comparable absolute temperatures over the past 21 kyr (Chapter 4). Similar reconstructed temperatures for the last glacial maximum (LGM) and the late Holocene suggest that soil moisture availability reshapes the reconstructed land surface temperatures through differences in the heat capacity of dry and wet soils. Additionally, comparison of reconstructed temperatures with climate model simulations indicates that the brGDGT-based records are more sensitive to orbital forcing than to greenhouse gas and ice sheet forcings (Chapter 5). This thesis also found that the degree of cyclization of brGDGTs can be used to quantify the amount of monsoon precipitation on the CLP, which shows precession and obliquity cycles over the past 130 kyr (Chapter 6). However, the brGDGT-based temperature records fail to capture the glacial-interglacial variations prior to the last deglaciation. Thus, a thorough evaluation of brGDGT distributions is essential before interpreting brGDGT-based proxies in a paleoclimatic context (Chapter 7). The sub-annual temperature records based on D47 of land snail fossils reflect the land surface temperature during their growing season, and indicates that the LGM was ~7 °C colder than present day. In Chapter 9, the 21-kyr land surface temperature record based on D47 of snail shells was extended to 75 kyr. While the T47 record shows similar trends as the brGDGT-based record presented in Chapter 7, T47 consistently records temperatures around 8 °C higher than those based on brGDGTs in the interval from 75 to 21 ka. This discrepancy may arise from differing growing seasons for snails and soil bacteria.