菲律宾海盆底层水体性质对中更新世气候转型的响应机制

中更新世气候转型(MPT)是地球气候系统内外部多个因素共同作用的结果，而海洋深部通风和碳循环变化是关键环节。北太平洋深层环流是全球大洋温盐循环的重要组成，同时北太平洋深海呼吸碳库变化可能对MPT前后冰期-间冰期旋回周期性转变起到至关重要的作用。北太平洋深水环流研究主要限制在零星分布的海山/高原上，对更新世以来北太平洋底层水体演化历史的了解不足。本申请拟选择菲律宾海盆柱状沉积物(5000~5500m)和海水-孔隙水样品为研究对象，以“现代菲律宾海盆海水-沉积物元素循环规律研究→更新世以来菲律宾海盆底层水体性质和通风状态→菲律宾海盆底层水对中更新世气候转型事件响应机制”为主线，充分注重“现代过程与地质记录相结合，机制解释与模型验证相结合”，认识更新世以来菲律宾海底层水体的来源、通风和影响，揭示菲律宾海盆底层水体变化对中更新世气候转型事件的响应过程，阐明大洋深层环流变化的内在联系和相互作用。

The mid-Pleistocene transition (MPT) is widely recognized as a shift in paleoclimatic periodicity from 41- to 100-kyr cycles, which largely reflects integrated changes in global ice volume, sea level, and ocean temperature from the marine realm. Hypotheses to explain what caused the MPT include nonlinear climate responses to Milankovitch forcing, cooling of sea-surface temperatures and increased sea-ice cover, changes in THC vigor inferred from benthic foraminiferal carbon isotopes, long-term or sudden decrease of atmospheric partial pressure of CO2(pCO2), and stabilization of ice sheets from loss of underlying regolith. A point of consensus is that decreased pCO2 was important for enhancing cooling and facilitating ice-sheet growth, and modeling studies conclude that pCO2 reduction during glacial periods may be the result of decreased ocean ventilation, increased carbonate alkalinity, terrestrial carbon storage, and increased marine productivity and that the deep ocean’s storage capacity would be enhanced by a weaker THC. The abyssal North Pacific Ocean’s large volume, depth, and terminal position on the deep oceanic conveyor make it a candidate site for deep carbon trapping as postulated by climate theory to explain the massive glacial drawdown of atmospheric CO2. The major abyssal basins of the North Pacific (such as the Philippines Sea) have depths of 5500–6500 m, far below the modern and glacial calcite compensation depths (CCD) presently located at ~4500m. Carbonate-based proxy reconstructions of the deep North Pacific circulation are therefore restricted to isolated seamounts and submarine plateaus towering more than 2000m above the abyssal basins. To our knowledge, carbonate-free basin sediments have not yet been purposely studied in the context of deep-bottom water reconstruction. In this proposal, based on redox-sensitive trace metals of bulk sediments, and REEs and Sr-Nd isotopes in Fe-Mn oxyhydroxides from two cores in the Philippines Sea, together with the seawater-porewater analysis, we try to (1) reveal the elemental cycles in the seawater-porewater-sediment systems; (2) reconstruct the bottom water properties and discuss the potential shifts of the sources and ventilation history; and (3) present a detailed view on how the Philippines Sea bottom water properties response to the mid-Pleistocene transition. In combination with the results from other areas and test by models, we constrain the link between circulation changes in the North Pacific and climate change events, as well as the temporal relationship to variations in the Atlantic Meridional Overturning Circulation (AMOC), and then test the Atlantic-Pacific inter-basin seesaw theory.