Orbitally driven evolution of Asian monsoon and stable water isotope ratios during the Holocene: Isotope-enabled climate model simulations and proxy data comparisons

Abstract The variation of stable water isotopes (δ18O, δD) in climate archives is an important proxy to understand the evolution of South Asian monsoon (SA) precipitation over the Holocene. In this study, using an isotope-enabled climate-model, we examine the responses of water isotopes in precipitation over the SA region to orbital changes in the early to late Holocene (8 ka to 0 ka). Precipitation is enhanced during 8 ka to 4 ka in response to increased summertime insolation, and correspondingly, larger magnitudes of negative water isotope ratios in precipitation (δ18Oprecip) are simulated. The model-simulated wettest period and the corresponding period of maximum depletion of δ18Oprecip in the SA region is the 8 ka period. We find that strengthened circulation, increased convection, and precipitation led to the δ18Oprecip depletion in the SA region. In the tropical Indian monsoon region, where convective precipitation is dominant, the δ18Oprecip values are inversely correlated with the local convection and the amount of precipitation. The δ18Oprecip values in the East Asian Summer Monsoon (EASM) region are not well-correlated with local precipitation, likely due to the enhanced convection and depletion of vapor in upstream areas and mixed precipitation types in the region. The modeled δ18Oprecip values are evaluated against δ18O inferred from proxy archives. Proxy data and model-simulated isotope ratios in precipitation agree on the larger magnitude of negative isotopic values during the 4 ka, 6 ka, and 8 ka periods relative to the present, although model-simulated enrichment in the 2 ka period is inconsistent with some proxy-records.