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

在IODP考察期间，首次在西太平洋暖池中心的战略位置(U1490站，05°48.95ʹN，142°39.27ʹE，2341米水深，巴布亚新几内亚近海)恢复了延长的未受干扰的中新世沉积序列。这一富含碳酸盐和粘土的序列提供了一个理想的档案，用于监测太平洋水团结构的变化，以及在从几乎没有冰的世界向永久冰川世界过渡期间，通过地球气候变异性的不同平均状态监测太平洋水团结构和经向翻转环流的变化。我们的项目将集中在~18至8.5 Ma之间，这是以几次基本的气候重组为标志的，为研究辐射强迫变化、赤道至极地温度梯度变化和比现代地球上的大气-海洋环流变化之间的关系提供了机会。此外，在~18~8.5 Ma之间，U1490地磁地层学具有很好的分辨率，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，这对验证和锚定新近纪时间尺度至关重要。结合严格限制的年代地层学，U1490海底稳定同位素和碳酸盐堆积记录将揭示太平洋环流重大重组的时间以及与低纬和高纬度气候变化的联系。特别是，我们将检验以下假设：1)赤道至极地温度梯度的变化强烈影响到深-中水团的相对形成和太平洋经向翻转的强度；2)中中新世气候过渡期间，随着冰盖扩张而产生的腐蚀性南源中-深水团扩大，导致碳酸盐枯竭和二氧化碳在深海和印度洋的储存；3)印尼贯通流作为深、中水交换的通道发挥了关键作用，影响了印度洋的热收支和海/气耦合。在该项目的第二阶段，我们建议在~18~13.7 Ma的范围内增加样品间距，以充分解决倾角和短偏心旋回，以发展可靠的天文调谐同位素地层学。这对于了解中新世中期气候最佳时期的环流变化和东南极冰的逐步扩张是至关重要的。根据船上初步年龄模型，这段时间的特点是沉积速率比最初预期的要低(平均~1厘米/KYR)，波动更大。

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.