Changes in Pacific Meridional Overturning Circulation across the Miocene Climate Optimum and the ensuing transition to a permanently glaciated Earth

点击翻译按钮获取中文摘要

During IODP Expedition 363, an extended undisturbed Miocene sediment succession was retrieved for the first time in a strategic location at the heart of the Western Pacific Warm Pool (Site U1490, 05°48.95ʹN, 142°39.27ʹE in 2341 m water depth, off Papua New Guinea). This carbonate- and clay-rich sequence provides an ideal archive to monitor changes in Pacific water mass structure and meridional overturning circulation through different mean states of Earth’s climate variability during the transition from an almost ice-free to a permanently glaciated world. Our project will focus on the interval ~18 to 8.5 Ma, which was marked by several fundamental climate re-organizations, offering the opportunity to investigate the relationships between changes in radiative forcing, variations in the Equator-to-pole temperature gradient and shifts in atmosphere-ocean circulation on a warmer-than-modern Earth. In addition, the interval ~18 to 8.5 Ma is characterized by an exceptionally well-resolved magnetostratigraphy at Site U1490, which allows for the first time a direct correlation of high-resolution deep water isotope cyclostratigraphy to the Geomagnetic Polarity Time Scale (GPTS) and is crucial for verifying and anchoring the Neogene Timescale. In combination with a well-constrained chronostratigraphy, the U1490 benthic stable isotopes and carbonate accumulation records will shed light on the timing of major re-organizations in Pacific circulation and linkages to low- and high-latitude climate change. In particular, we will test the hypotheses that 1) changes in the Equator-to-pole temperature gradient strongly influenced the relative formation of deep-intermediate water masses and the strength of Pacific meridional overturning; 2) the expansion of corrosive southern sourced intermediate-deep water masses following ice sheet expansion during the middle Miocene Climatic Transition contributed to carbonate depletion and CO2 storage in the deep Pacific and Indian Oceans and 3) the Indonesian Throughflow played a key role as a conduit of deep and intermediate water exchange, influencing the heat budget and ocean/atmosphere coupling of the Indian Ocean. In Phase 2 of this project, we propose to increase the sample spacing in the ~18 to 13.7 Ma interval to fully resolve obliquity and short eccentricity cycles in order to develop a reliable astronomically-tuned isotope stratigraphy. This will be crucial to understand circulation changes during the onset and development of the middle Miocene Climatic Optimum and stepwise East Antarctic ice expansion. This interval is characterized by lower (mean ~1 cm/ kyr) and more strongly fluctuating sedimentation rates than originally anticipated, based on the shipboard preliminary age model.