Pliocene palaeoclimate off SE Africa: Insights from IODP Expedition 361

有人提出，东非在300-500万年前经历了强烈的干旱化，这是通过印度尼西亚海道进入印度洋的水源分配发生变化的结果[deMenocal，1995；Cane和Molnar，2001]。海洋环流模式的结果表明，大约5百万年前，新几内亚向北移动，可能将流经印度尼西亚的水流来源从温暖的南太平洋转向相对较冷的北太平洋水域。在现代气候中，东非降雨量和印度洋海温之间有很强的联系，温度越高降雨量越多[Goddard和Graham，1999]。假设现代海洋循环系统在上新世以类似的方式运作，新几内亚在构造驱动下向北移动的后果将导致印度洋海面温度下降，最终导致东非降雨量减少。为什么研究东非/东南部非洲在这段时间内的气候变化很重要？我们都来自非洲。我们知道，非洲人的进化和东非人属的出现的主要步骤发生在上新世[Bobe和Behrensmeyer，2004]。这些事态发展似乎发生了重大的全球和区域气候变化，特别是在2.8 Ma附近的非洲转向更干旱、更开放的环境条件[deMenocal，1995]。东非从潮湿到更加干旱的状况主要与北半球冰川周期开始相关的北大西洋海面寒冷温度的遥远强迫有关[deMenocal，1995]。然而，在这里，我们想要测试的是，由于流经印度尼西亚群岛的水的变化，西南印度洋温度的下降是否推动了东非和东南部非洲降雨量的变化。

It has been suggested that East Africa has undergone strong aridification between 3-5 million years ago as a consequence of changes in the source water distribution through the Indonesian seaway into the Indian Ocean [deMenocal, 1995; Cane and Molnar, 2001]. Results from an ocean circulation model showed that the northward displacement of New Guinea, about 5 Myr ago, may have switched the source of flow through Indonesia from warm South Pacifc to relatively cold North Pacifc waters. In the modern climate there is a strong link between east African rainfall and Indian Ocean SST, with warmer temperatures associated with more rain [Goddard and Graham, 1999]. Assuming that the modern ocean circulation system worked in a similar way in the Pliocene, the consequence of a tectonically driven northward move of New Guinea would have caused a reduction in sea surface temperatures in the Indian Ocean ultimately leading to reduced rainfall over eastern Africa. Why is it important to study climate changes in East/southeast Africa over that time period? We all come from Africa. We know that major steps in the evolution of African hominids and the emergence of the genus Homo in East Africa occurred in the Pliocene [Bobe and Behrensmeyer, 2004]. These developments seem to fall into times of major global and regional climatic changes especially with shifts to more arid, open environmental conditions in Africa near 2.8 Ma [deMenocal, 1995]. The change from humid to more arid conditions in East Africa has been linked mainly to remote forcing by cold North Atlantic sea-surface temperatures associated with the onset of Northern Hemisphere glacial cycles[deMenocal, 1995]. Here, however, we want to test if instead a cooling in the southwest Indian Ocean temperature due to a change of water through the archipelago of Indonesia drove changes in East and southeast Africa rainfall variability.