Hydrogen isotope records of long-chain alkanes from the monsoon boundary zone over the last 2400 years in North China

Abstract

The East Asian Summer Monsoon (EASM) plays a key role in the climate dynamics of East Asia, affecting the livelihood of millions of people by contributing to agriculture and water resources. The monsoon boundary zone (MBZ) is particularly sensitive to shifts in climate patterns. However, the complexity of water vapor sources in the region makes it difficult to understand precipitation processes. To address this issue, the records of lipid biomarker abundances and leaf wax stable hydrogen isotope composition of C31–alkanes (δDwax-C31) from a 238 cm sediment core extracted in Lake Dali, located at the northern edge of the EASM, were sampled and examined by 2 cm per sample. Based on analyses of the biogenic molecular indicators(biomarkers), precipitation isotopes, and other indicators of the evolution of the climate environment, we have reconstructed precipitation change history over the past 2400 years. The results show the decrease in alkanes concentrations, along with high A.I. values in Lake Dali sediments indicate an arid trend over the past 2400 years, which have led to a decrease in basin productivity and the development of grasslands. The low temperatures greatly limit the growth of C4 vegetation in this cold region, so the long-chain alkanes in the lake sediments is mainly derived from the leaf wax lipids of C3 vegetation in the grassland. The increase of Paq and the reduction of ACL indicate that the decrease of grass vegetation inputs in the basin and the outbreak of aquatic vegetation. Notably, aquatic vegetation outbreaks may be due to reduced precipitation leading to shrinking lakes and increased nutrients in the lakes. By comparing with the biogenic indicators of Lake Dali and other regional records, the δDwax-C31 value can be used as a proxy for the regional precipitation, with a gradual decrease from about −240 ‰ to about −190 ‰, which indicates a general decreasing trend in the precipitation in the last 2400 years. Specifically, in exception of some significant fluctuations on the centennial to millennial scale, δDwax-C31 value decrease by about 40 ‰ between 1800 and 1600 cal.a BP over a period of only 200 years. As we know, the diminishing solar radiation might lead to a weaker EASM and a reduction in precipitation. Thus, the decrease in regional temperature became the main reason for the anomalous negative precipitation isotope phenomenon due to the lower precipitation from 1800 to 1600 cal.a BP in the monsoon boundary zone (MBZ, this represents only the northern boundary). This study contributes to our understanding of δDwax-C31 within the MBZ and its significance as an indicator of rainfall processes. By analyzing this molecular proxy, the researchers gain insights into the past precipitation patterns and their implications for regional climate. Importantly, the study demonstrates the value of combining biogenic molecular indicators with geochemical indicators, as it leads to improved accuracy in reconstructing regional climate conditions. The findings shed light on long-term climate trends and provide a more comprehensive understanding of the historical impacts of climate change on the EASM region. Moreover, the study's contribution to interpreting the paleoclimate record is crucial for validating climate models used to project future climate scenarios. Overall, this research enhances our knowledge of regional climate dynamics and their implications, offering valuable insights for both historical analysis and future climate projections.