Evolution of Asian Monsoon and Stable Water Isotope Ratios During the Holocene

Abstract:

The variation of stable isotopes of water (δ¹⁸O, δD) in climate archives is an important proxy to understand the evolution of precipitation in the South Asian (SA) monsoon region. Various proxy records indicate spatial heterogeneity and asynchronous evolution of precipitation in the two major regimes of the SA monsoon, Indian Summer Monsoon (ISM) and East Asian Summer Monsoon (EASM), over the Holocene. We use IsoCAM, a water isotope-enabled atmospheric general circulation model, to simulate the climate responses and water isotopes in precipitation in the SA monsoon region to orbital changes in the early to the late Holocene (8 ka BP to 0 ka BP). Precipitation is enhanced during 8 ka to 4 ka BP 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 ISM region, where convective precipitation is dominant, the δ¹⁸O_precip values are inversely correlated with the local convection and the amount of precipitation (“amount effect”). The δ¹⁸O_precip values in the 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. Proxy data and model results are in agreement regarding the trend of more negative δ¹⁸O_precip during the 4 ka, 6 ka, and 8 ka periods relative to present, however, the model-simulated enrichment in the 2 ka period is inconsistent with some proxy-records. From the comparisons with the proxy records from the ISM and EASM regions, we find that our model can credibly simulate the orbitally-forced evolution of δ¹⁸O_precip values in the SA region during the Holocene. Our results encourage further proxy-model comparisons of both water isotopes and reconstructed precipitation for strengthening our capability to better understand past climate changes.