Plio-Pleistocene climate change and N-C cycles in low-latitude upwelling systems

在上新世-更新世（约最后~5.0 Ma）期间，地球气候由温暖和相对稳定状态过渡到寒冷状态，其特征是北半球冰期/间冰期旋回的扩大和大范围冰盖的形成。最近有一种假设认为，这种全球气候变化的原因——将一个温暖的星球推向“冰窖”状态——与大气中二氧化碳的逐渐减少密切相关。然而，这种减少的原因仍然不确定。虽然一些研究提出，极地海洋（北太平洋和南大洋）的同期分层在减少从深海向大气释放的二氧化碳方面发挥了主要作用，但低纬度高生产力地区变化的影响仍不清楚。这是特别有趣的，因为最近的工作表明，海洋氮循环的主要变化，控制生物碳泵的主要过程之一，主要发生在热带和亚热带海洋。为了探讨上新世—更新世期间海洋环流、大气CO2和全球变冷与高生产力地区过去营养条件变化之间可能的气候反馈机制，我们将结合低纬上升流地区的烯烃δ13C和温度记录建立δ15N和氯。这些区域可能对全球海洋氮库存以及大气-海洋二氧化碳交换非常敏感并具有很大影响。调查毛里塔尼亚、纳米比亚、秘鲁和东赤道太平洋东部边界洋流内的沿海上升流系统，将使我们能够确定富含营养物质的水团平流和营养物质利用的变化，以及它们与热带和亚热带海洋地区反硝化和固氮过程的平衡。结合对过去地表水二氧化碳分压水平变化的估计，对过去养分条件的重建，再加上过去环流和生产力的其他替代记录，应该能够揭示低纬度高生产力地区的生物地球化学循环在上新世-更新世期间全球变冷中的作用。

During the Pliocene-Pleistocene (about the last ~5.0 Ma), the Earth’s climate transitioned from a warm and relative stable state towards cold conditions marked by amplified glacial/interglacial cycles and formation of widespread ice-sheets in the Northern Hemisphere. It has been recently hypothesized that the causes of this global climate switch, driving a warm planet to “icehouse” conditions, were intimately tied to a progressive decline in atmospheric CO2. However, the reasons of this diminution are still uncertain. While some studies proposed that the coeval stratification of the polar oceans (North Pacific and Southern Ocean) has played a preponderant role in reducing CO2 release from the deep ocean to the atmosphere, the effects of changes in low-latitude high productive areas remain unclear. This is particularly intriguing since recent work indicated that major changes in the oceanic nitrogen cycle, one of the main processes controlling the biological carbon pump, mainly occurred in the tropical and subtropical oceans. To investigate possible climatic feedback mechanisms between ocean circulation, atmospheric CO2, and global cooling with past changes of nutrient conditions in high productive areas during the Plio-Pleistocene, we aim to establish δ15N and of chlorines in combination with alkenone δ13C and temperature records in low-latitude upwelling regions. These regions may have been very sensitive to and highly influential on the global ocean nitrogen inventory as well as for the atmosphere-ocean CO2 exchange. Investigating the coastal upwelling systems within eastern boundary currents off Mauritania, Namibia, Peru, and in the Eastern Equatorial Pacific will allow us to determine variations in the advection of nutrient-rich water masses and in nutrient utilisation as well as their balance with the processes of denitrification and nitrogen fixation in tropical and subtropical oceanic areas. Together with estimation of past changes in surface-water PCO2 levels the reconstructions of past nutrient conditions, coupled with other proxy records for past circulation and productivity, should unravel the role of biogeochemical cycling in low latitude high productive regions for global cooling during the Plio-Pleistocene.