Geochemistry of Eocene/Oligocene sediments of ODP Site 177-1090@en

The Agulhas Ridge, off the tip of Africa between the Atlantic and Indian Oceans, is ideally located to capture the evolution of Paleogene-early Neogene circulation patterns associated with global cooling. Multiproxy records of productivity (biogenic barium (Baex), opal, CaCO3 mass accumulation rates (MARs)), nutrient and organic carbon burial (reactive phosphorus (Pr) MARs), and redox state of deep waters (U enrichment) from Ocean Drilling Program (ODP) Site 1090 reflect hydrographic shifts in this region between the middle Eocene and early Oligocene (~9–33 Ma). Several peaks in increased export productivity and burial of organic matter occurred within the late Eocene (~36.5, ~34, and ~33.7 Ma), which along with surface hydrologic conditions favoring opaline organisms over calcareous organisms could have aided in the draw down of pCO2 to a threshold level that facilitated large ice sheet development on Antarctica in the earliest Oligocene. Our multiproxy approach illustrates the importance of vertical as well as spatial hydrographic reorganization in amplifying or driving climatic cooling of the middle Eocene to early Oligocene by facilitating increases in the relative or absolute burial of organic carbon.阿古拉斯海岭（Agulhas Ridge）坐落于非洲南端大西洋与印度洋之间的海域，其地理位置得天独厚，可完整记录与全球变冷相关的古近纪至早新近纪环流演化过程。来自大洋钻探计划（Ocean Drilling Program, ODP）1090站位的多项代理指标记录，涵盖了生产力指标（生源钡（Baex）、蛋白石、碳酸钙质量堆积速率（mass accumulation rates, MARs））、营养盐与有机碳埋藏指标（活性磷（Pr）质量堆积速率）以及深部水体氧化还原状态指标（铀富集量），可反映该区域在中始新世至早渐新世（约9~33 Ma）期间的水文变化。晚始新世时期（约36.5 Ma、34 Ma及33.7 Ma）出现多期输出生产力提升与有机碳埋藏峰值；结合表层水文条件更有利于硅质生物而非钙质生物繁衍的特征，这些事件可助力大气二氧化碳分压（pCO2）降至临界阈值，为早渐新世初期南极大陆大型冰盖的形成创造了条件。本研究采用的多代理指标方法表明，通过提升有机碳埋藏的相对或绝对总量，垂直与空间尺度的水文重构过程能够加剧或驱动中始新世至早渐新世的全球气候变冷，其重要性不言而喻。