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

在上新世-更新世(约5.0 Ma末期)，地球气候从温暖和相对稳定的状态过渡到寒冷条件，其特点是冰川/间冰期循环放大，北半球形成广泛的冰盖。最近有人假设，这种全球气候变化的原因，将一个温暖的星球推向冰室条件，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。虽然一些研究表明，极地海洋(北太平洋和南大洋)的同年层结在减少深海向大气排放二氧化碳方面发挥了主要作用，但低纬高生产区变化的影响尚不清楚。这一点特别令人感兴趣，因为最近的工作表明，控制生物碳泵的主要过程之一--海洋氮循环的主要变化主要发生在热带和亚热带海洋。为了研究海洋环流、大气二氧化碳和全球变冷之间可能的气候反馈机制，以及过去在更新世高生产区营养条件的变化，我们的目标是建立δ15N和氯离子的δ15N，并结合低纬上升流区的烷酮和温度记录。这些区域可能对全球海洋氮清单以及大气-海洋二氧化碳交换非常敏感，并对其产生很大影响。研究毛里塔尼亚、纳米比亚、秘鲁和赤道东太平洋东部边界流内的沿海上升流系统，将使我们能够确定营养丰富的水团平流和养分利用的变化，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水平变化的估计，重建过去的营养条件，再加上过去的循环和生产力的其他替代记录，应能揭示低纬高生产区的生物地球化学循环在更新世-更新世期间对全球变冷的作用。

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.