Biomarker Insights on Hydroclimate of the Indo-Pacific Warm Pool and Beyond During the Late Pleistocene

Warm sea surface temperatures (SSTs) and heavy rainfall are characteristic of the Indo-Pacific Warm Pool (IPWP), which is located under the ascending branch of the Indian and Pacific Walker Circulation cells. Perturbations in the Walker Circulation affect global patterns of precipitation, so understanding how this circulation responds to different climate states is useful. Paleoclimate proxies, such as precipitation isotopes or temperatures in the upper water column of the ocean, can help discern dynamic responses to external forcing mechanisms of the climate system. Tropical climate, including the IPWP, is generally thought to be dominated by the precession cycle's influence on seasonal incoming solar radiation over geological time scales; however, several recent studies suggest glacial-interglacial cycles exert an important control on climate variability in the Indo-Pacific region. The three studies presented here aim to clarify the glacial influence on IPWP circulation and compare the relative influences of the precession and glacial-interglacial cycles on tropical Indo-Pacific climate during the late Pleistocene. First, we compare precipitation isotope records from the IPWP with isotope-enabled simulations of the Last Glacial Maximum (LGM). We find that the glacial precipitation isotope signal in the IPWP is driven by a distinct spatial pattern of change in moisture transport: increased low-level moisture convergence results in depleted precipitation over the Pacific Ocean side of the IPWP, whereas increased divergence results in enriched precipitation over the Indian Ocean side. This pattern reflects a decoupling of the Indian (weaker) and Pacific (stronger) Walker Circulation cells during glacial conditions. Second, we use an empirical filter on five long precipitation isotope reconstructions from the IPWP and the Asian summer monsoon domains to the north to compare the competing orbital influences between regions. We find that glacial-interglacial shifts dominate the variability with a secondary precession component everywhere except eastern China. Third, we present four new climate reconstructions from the tropical western Indian Ocean and examine their spectral composition. We find a mix of precession and glacial-interglacial variability, suggesting that both cycles influence summer monsoon circulation across the Indian Ocean basin. Overall, it is clear that glacial-interglacial cycles play an important role in the tropical Indo-Pacific