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

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 (δ¹⁸O_precip) are simulated. The model-simulated wettest period and the corresponding period of maximum depletion of δ¹⁸O_precip in the SA region is the 8 ka period. We find that strengthened circulation, increased convection, and precipitation led to the δ¹⁸O_precip depletion in the SA region. In the tropical Indian monsoon region, where convective precipitation is dominant, the δ¹⁸O_precip values are inversely correlated with the local convection and the amount of precipitation. The δ¹⁸O_precip 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 δ¹⁸O_precip 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.