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

在IODP 363考察期间，首次在西太平洋暖池中心的战略位置（地点U1490, 05°48.95′N, 142°39.27′E，水深2341 m，巴布亚新几内亚）恢复了扩展的未受干扰的中新世沉积物序列。这个富含碳酸盐和粘土的序列提供了一个理想的档案，可以监测太平洋水团结构的变化和经向翻转环流，通过地球气候变化的不同平均状态，从一个几乎无冰的世界过渡到一个永久的冰川世界。我们的项目将集中在~18 ~ 8.5 Ma区间，这段时间以几次基本的气候重组为标志，为研究在比现代更温暖的地球上辐射强迫变化、赤道到极地温度梯度变化和大气-海洋环流变化之间的关系提供了机会。此外，U1490遗址在~18 ~ 8.5 Ma区间具有分辨率极高的磁地层学特征，首次实现了高分辨率深水同位素旋回地层学与地磁极性时间尺度（GPTS）的直接对比，对验证和锚定新近纪时间尺度至关重要。结合严格限定的年代地层学，U1490底栖生物稳定同位素和碳酸盐积累记录将阐明太平洋环流主要重组的时间以及与低纬度和高纬度气候变化的联系。特别是，我们将检验以下假设：1)赤道至极温度梯度的变化强烈影响了深中层水团的相对形成和太平洋经向翻转的强度；(2)中新世中期气候转型期间，随着冰盖扩张，腐蚀性南源中深水团的扩张导致了太平洋和印度洋深处碳酸盐的枯竭和二氧化碳的储存；(3)印度尼西亚通流作为深海和中间水交换的管道发挥了关键作用，影响了印度洋的热量收支和海洋/大气耦合。在二期工程中，我们建议增加~18 ~ 13.7 Ma区间的样品间距，以充分解决倾角和短偏心旋回问题，从而建立可靠的天文调谐同位素地层。这对于理解中新世中期气候最佳期的开始和发展以及南极东部冰的逐步扩张过程中的环流变化具有重要意义。该区间的特征是较低（平均~1 cm/ kyr）和较强的波动沉降率，这是基于船上初步年龄模型所预测的。

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.