Age models of ODP Leg 175 holes@en

A multiproxy approach including the use of stable isotopes, magnetic characterization analyses, and organic geochemistry has been adopted to consider factors such as productivity and terrigenous input over the past 1.5 m.y. at two areas off the western coast of Africa. These factors can, in turn, be used to consider variability in ocean circulation and upwelling in addition to changes in climate on the African continent. In particular, studies focused on the influence of glacial-interglacial cycles and evidence for the mid-Pleistocene revolution (MPR), a complex change in climate that occurred at ~1 Ma. A comparison of the records from the two areas drilled during Ocean Drilling Program Leg 175, the Congo Basin, at a latitude of 5°S (Holes 1076A and 1077A), and the Walvis Ridge, at 17°S (Hole 1081A), demonstrates that these sites are affected by different localized factors. The sites in the Congo Basin are strongly influenced by freshwater and sediment from the Congo River, whereas the site at the Walvis Ridge is located in the center of oceanic upwelling and contains a more marine signal. Evidence also suggests that the two sites responded differently to both long- and short-term climatic variations. In particular, the response at the Walvis Ridge to the MPR occurred over an extended period, from 1.1 to 0.8 Ma, and was associated with a change in the dominant source of terrigenous input to the site in conjunction with a change in the productivity signal. In the Congo Basin, the response to the MPR was more rapid, occurring between 0.9 and 0.8 Ma. During this period, the influence of the Congo River became significant. However, productivity records only began to respond toward the end of this interval, at 0.8 Ma.本研究采用多代用指标方法，结合稳定同位素（stable isotopes）、磁特征分析与有机地球化学分析手段，对非洲西海岸两处海域过去150万年间的海洋生产力与陆源输入等要素展开研究。上述代用指标可进一步用于反演非洲大陆气候变化之外的大洋环流与上升流变异性。其中，研究重点聚焦于冰期-间冰期旋回的影响，以及中更新世转型（mid-Pleistocene revolution, MPR）的相关证据——这是一场发生于约1 Ma的复杂气候变化事件。通过对比大洋钻探计划（Ocean Drilling Program）第175航次在两处位点获取的岩心记录：一处为南纬5°的刚果盆地（钻井孔1076A与1077A），另一处为南纬17°的沃尔维斯海岭（Walvis Ridge，钻井孔1081A），结果显示两处位点受不同局地因素影响。刚果盆地的钻井孔显著受刚果河携带的淡水与沉积物影响，而沃尔维斯海岭的钻井孔则位于大洋上升流中心，其信号更具海洋源性。研究证据同时表明，两处位点对长、短期气候变化的响应模式存在差异。具体而言，沃尔维斯海岭钻井孔对中更新世转型的响应持续时间较长，跨度为1.1至0.8 Ma，且与该位点陆源输入主要来源的转变及生产力信号的变化密切相关。而刚果盆地钻井孔对中更新世转型的响应则更为迅速，发生于0.9至0.8 Ma之间。在此期间，刚果河的影响显著增强，但生产力记录仅在该时段末期的0.8 Ma才开始出现响应。