Precipitation and temperature variability in Vietnam during Marine Isotope Stage 3 from terrestrial biomarkers

Southeast (SE) Asia is located in a transitional zone where hydroclimate is controlled by both the Indian and East Asian summer monsoon branches. Recent proxy-based studies and climate models suggest that the hydroclimate of SE Asia may be out of phase with neighboring regions, such as India and China. However, we lack sufficient proxy records to verify this postulation or to identify spatial and temporal variations. This study reconstructs both past temperatures and effective moisture in Central Vietnam during Marine Isotope Stage 3 (approximately 50,00 to 30,000 years BP) to determine how these two climate variables relate in the past. Terrestrial temperatures and precipitation are reconstructed using biomarkers (branched glycerol dialkyl glycerol tetraethers, brGDGT) and compound-specific isotope analyses (carbon and hydrogen-isotopic values of leaf wax n-acids, δ¹³C_wax, and δD_wax) from a buried peat deposit in the Central Highlands of Vietnam. The brGDGTs-derived annual temperatures range from 22.9 to 26.2°C and show a warming trend coincident with a weakening of summer insolation. A coincident and gradual enrichment of δ¹³C_wax from 47 to 33 kyr BP suggests a transition from C₃ to C₄ vegetation dominance. Such a response could signal an overall decrease in precipitation or a shift in the seasonality of precipitation. The δD_wax data, however, do not indicate an overall drying trend, which supports the idea that a shift in the seasonality of rainfall, along with higher annual temperatures, is driving the vegetation change. In addition, the δD_wax records may exhibit a trend opposite to a site in Thailand. We argue that the isotopic variability in the precipitation of Central Vietnam reflects the shift in moisture sources along with the shift in seasonality. In this case, an increase in amount of precipitation derived from the South China Sea in winter months is marked by rain enriched in δ²H-which could also be interpreted as a decrease in precipitation. The increase in rainfall during winter monsoon months (e.g. winter) in the Central Highlands of Vietnam does not appear to reach Thailand. We recommend that the precipitation proxies should be applied with knowledge of regional climate context and argue that better geographic representation of monsoonal climates is necessary to fully understand and model this critical climate system.