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

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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.