Comment on cp-2023-32

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Knowledge of Holocene temperature changes is crucial for addressing the problem of the discrepancy between Holocene proxy temperature reconstructions and climate model simulations. The complex spatiotemporal pattern of temperature variations on the Tibetan Plateau (TP) further complicates the study of Holocene continental climate change. The discrepancy between model-based and proxy-based Holocene temperature reconstructions possibly results from the seasonal biases and environmental ambiguities of the proxies. Quantitative temperature reconstructions using different proxies from the same sediment core can provide an effective means of evaluating different proxies; however, this approach is unusual in terrestrial environments. Here, we present an ice-free-season temperature record for the past 15 ka from a shallow, freshwater lake on the eastern TP, based on brGDGTs (branched glycerol dialkyl glycerol tetraethers). This record shows that the Holocene Thermal Maximum lags the pollen-based July temperature recorded in the same sediment core. We conclude that the mismatch between the brGDGTs-based and pollen-based temperatures is primarily the result of seasonal variations in solar irradiance. The overall pattern of temperature changes is supported by other summer temperature records, and the Younger Dryas cold event and the B&oslash;lling&ndash;Aller&oslash;d warm period are also detected. A generally warm period occurred during 8&ndash;3.5 ka, followed cooling in the late Holocene. Our findings have implications for understanding the seasonal signal of brGDGTs in shallow lakes, and provide critical data for confirming the occurrence of seasonal biases in different proxies from high-elevation lakes. To further investigate the significance of the brGDGTs and temperature patterns on the TP, we reviewed previously published brGDGTs-based Holocene temperature records across the TP. The results demonstrate that brGDGTs can record both annual mean temperature and a warm-biased temperature, and that both show a gradual warming trend during the Holocene with relatively cooler conditions during the middle Holocene, and a cooling trend during the middle to late Holocene. We analyzed the possible reasons for the diverse brGDGTs records on TP and emphasize the importance of considering lake conditions and modern investigations of brGDGTs in lacustrine systems when using brGDGTs to reconstruct paleoenvironmental conditions.