Editorial preface to special issue: Recent advances in Indian Ocean paleoceanography and paleoclimate

This Special Issue on Recent advances in Indian Ocean Paleoceanography and Paleoclimate comprises thirteen research articles. Contributions variously discuss Neogene oceanographic evolution of the tropical Indian Ocean in relation to global climate change, oxygenation, acidification and its effects on calcifying phytoplankton, variability of the Indian-Australian monsoon and monsoon-response to external forcing and internal feedbacks. Some key findings are as follows: Hyperthermal events at eccentricity maxima during the Miocene Climatic Optimum led to the acidification of poorly ventilated deep waters and carbonate dissolution in the equatorial Indian Ocean. During the Middle Miocene Climatic Transition, intensification of deep ocean ventilation at low-latitudes following Antarctic ice sheet expansion prevented re-establishment of hyperthermal conditions after similar to 13.1 Ma. Bottom water conditions in the eastern Arabian Sea remained oxic during the late Miocene-Pliocene, but shifted to suboxic during the Pleistocene, most likely due to the combined effect of decreasing deep-water ventilation and enhanced monsoon circulation. During this interval, changes in monsoonal winds and the inter-ocean transfer of water masses through the Indonesian Throughflow strongly affected the surface-subsurface dynamics of the tropical Indian Ocean. Shoaling of the thermocline in the tropical eastern Indian Ocean occurred between 6 and 3.4 Ma in response to intensification of the Indian summer monsoon. Enhanced Indonesian Throughflow resulted in intensification of cross-equatorial sea surface temperature and pressure gradients and increased summer monsoon precipitation at 2.95 Ma, as inferred from Arabian Sea proxy records. Eastward flowing nutrient-rich, saltier water influenced the surface hydrography of the tropical eastern Indian Ocean between 2.7 and 1.2 Ma. Variations in Indian monsoon precipitation and wind intensity acting in concert with Northern Hemisphere climatic oscillations drove millennial- to centennial-scale changes in surface hydrography and productivity in the Bay of Bengal and Andaman Sea during the late Glacial-Holocene. Rapid intensification of Australian summer monsoon precipitation at the end of glacial terminations coincided with maxima in atmospheric CO2 and CH4 concentrations and in Antarctic temperature. Evidence presented in this special issue indicates that greenhouse gas concentrations and high latitude climate in conjunction with variations in the insolation gradient between the Southern Hemisphere tropics and subtropics modulated the strength of the tropical convection, the latitudinal migration of the Inter-Tropical Convergence Zone and the intensity of the Indian-Australian monsoon.