UKIODP Moratorium Award - IODP Expedition 361, Southern African Climates - Ian Hall

南非气候（SAFARI）考察队361将调查上新世/更新世期间气候与阿古拉斯洋流之间的相互作用。南印度洋和莫桑比克海峡的六个钻探点将确定(1)阿古拉斯洋流的敏感性，(2)印度洋-大西洋门户环流的动力学，以及(3)在过去5 Ma的主要海洋和气候重组期间，阿古拉斯泄漏与大西洋经向翻转环流（AMOC）变化之间的联系。阿古拉斯流是南半球最强的西部边界流，从热带印度洋沿东非边缘输送约70西沃特的温暖咸水到非洲的尖端。与大气的热量和水分交换影响着南部非洲的气候，包括个别天气系统，如该地区的温带气旋形成和降雨模式。最近的海洋模式和古海洋学数据进一步指出，在晚更新世期间，阿古拉斯流在控制AMOC的强度和模式方面发挥了潜在的作用。含盐的阿古拉斯水向南大西洋的溢出刺激了浮力异常，这是整个盆地AMOC的控制机制，对北大西洋和北半球气候的对流活动有影响。远征361的目标是将这项工作扩展到上新世/更新世期间的主要海洋和气候重构时期，以评估阿古拉斯海流和随后的（洋间）海洋热盐运输在塑造区域和全球尺度的海洋和气候发展中所起的作用。在这个暂停建议中，将特别关注中更新世过渡（MPT），大约在距今（BP）之前的1250万年和70万年（kyr），这是更新世气候历史的一个关键时期。在此期间，冰期-间冰期旋回的周期从40千基尔的节奏转变为频率更低、强度更大、不对称的100千基尔模式。这种转变是在轨道强迫没有显著变化的情况下发生的，这表明气候系统的内部反馈在导致这一重大气候重组中发挥了相当大的作用。大多数关于MPT起源的假设都援引了对长期冷却的反应，这种冷却是由大气二氧化碳分压降低所驱动的。过去800 kyr的冰芯记录表明冰期-间冰期二氧化碳分压波动为~ 100ppm。在过去的三十年里，人们提出了各种碳封存和深海储存机制。其中一种机制是，由于沙尘通量增加，南大洋亚南极区预先形成的营养物质利用效率提高。在这个高营养、低叶绿素（HNLC）地区的铁限制条件下，冰川尘埃施肥可能推动了初级生产力的增加和海洋表面碳输出的增加。考虑到这一机制作为晚更新世二氧化碳分压变化的驱动因素的潜力，可以想象，在整个MPT中可能发生了类似的过程。我将使用来自Agulhas高原361站点的材料来重建横跨MPT的亚南极区的出口生产。这将使我能够研究软组织生物泵对MPT开始时观察到的铁沉积增加的响应，并最终有助于我们理解导致MPT后世界冰川二氧化碳分压降低的过程。

The South African Climates (SAFARI) Expedition 361 will investigate the interaction between climate and the Agulhas Current during the Pliocene/Pleistocene. Six drill sites in the southern Indian Ocean and Mozambique Channel will determine (1) the sensitivity of the Agulhas Current, (2) the dynamics of Indian-Atlantic gateway circulation, and (3) the connection between Agulhas Leakage and shifts of the Atlantic Meridional Overturning Circulation (AMOC) during major ocean and climate reorganizations of the past 5 Ma. The Agulhas Current is the strongest western boundary current in the Southern Hemisphere, transporting some 70 Sv of warm and saline surface waters from the tropical Indian Ocean along the East African margin to the tip of Africa. Exchanges of heat and moisture with the atmosphere influence southern African climates, including individual weather systems such as extratropical cyclone formation in the region and rain-fall patterns. Recent ocean models and paleoceanographic data further point at a potential role of the Agulhas Current in controlling the strength and mode of the AMOC during the Late Pleistocene. Spillage of saline Agulhas water into the South Atlantic stimulates buoyancy anomalies that act as a control mechanism on the basin-wide AMOC, with implications for convective activity in the North Atlantic and Northern Hemisphere climate. Expedition 361 aims to extend this work to periods of major ocean and climate restructuring during the Pliocene/Pleistocene to assess the role that the Agulhas Current and ensuing (interocean) marine heat and salt transports have played in shaping the regional- and global-scale ocean and climate development.Within this moratorium proposal a specific focus will be on the The Middle Pleistocene Transition (MPT), circa 1250 and 700 thousand years (kyr) before present (BP) which represents a key interval of Pleistocene climate history. During this time the periodicity of glacial-interglacial cycles altered from a 40 kyr rhythm to a less frequent, more intense and asymmetric 100 kyr pattern. This shift occurred in the absence of significant changes in orbital forcing, suggesting that internal feedbacks in the climate system played a considerable role in bringing about this major climatic reorganisation. Most hypotheses for the origin of the MPT invoke a response to a long-term cooling, driven by decreasing atmospheric pCO2. Ice core records extending over the past 800 kyr indicate glacial-interglacial pCO2 fluctuations of ~100 ppm. Various mechanisms of carbon sequestration and deep ocean storage have been proposed over the past three decades. One of these mechanisms invokes an increase in the efficiency of preformed nutrients utilisation in the Subantarctic Zone of Southern Ocean as a result of increased dust flux. Glacial dust fertilisation in the iron-limited conditions of this High Nutrient, Low Chlorophyl (HNLC) region, could have driven an increase in primary productivity and increased export of carbon from the surface ocean. Given the potential of this mechanism as a driver of Late Pleistocene pCO2 variability, it is conceivable that a similar process may have operated across the MPT. I will use use material from Exp 361 Site on the Agulhas Plateau to reconstruct export production in the Subantarctic Zone across the MPT. This will allow me to investigate the response of the soft-tissue biological pump to the increased iron deposition observed during the onset of the MPT and will ultimately contribute to our understanding of the processes that resulted in lower glacial pCO2 in the post-MPT world.