Pliocene to Pleistocene reconstruction of regional versus global ocean oxygenation from eastern Pacific anoxic sediments

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Over the past decades the oxygen content of the ocean has decreased as a consequence of global warming. This has resulted in the expansion of oceanic regions that are highly deficient in oxygen. Such expanding oxygen minimum zones (OMZs) prevail in the eastern Pacific Ocean as a consequence of high biological productivity and subsequent remineralisation of organic matter, which is driven by upwelling of nutrient-rich waters. In order to judge if enhanced anoxia is within or outside the range of the natural variability of the past, the only access is offered by the sedimentary record. The geochemical proxies commonly used to reconstruct the strength and modes of past anoxia, however, yield controversial results, in particular in open ocean settings. Here we propose to apply new knowledge on the functioning and influencing factors of proxies for anoxic conditions obtained during recent studies of modern settings in the OMZs off Peru and in the Guaymas Basin to reconstruct the evolution of the modes of anoxic conditions over the past 3 million years. We will utilize sediments of ODP Site 680 (later redrilled as IODP Site 1228) from the present day centre of the Peruvian shelf coastal upwelling OMZ (250 m water depth) and DSDP Site 479 from the centre of the deeper Guaymas Basin OMZ (750 m water depth), which is part of the large North Pacific OMZ. The anoxic sediments of both locations cover the time period from the present to the late Pliocene. Site 680 shows a pronounced glacial/interglacial cyclicity of organic carbon contents reaching up to 15% during interglacials of the Pleistocene, whereas deeper Site 479 shows more uniform organic carbon contents near 3%. In order to determine long and short term changes of different modes of anoxia in the past we will apply a combination of proxies including different Fe pools obtained through sequential leaching of the sediments, stable Mo isotopes, redox sensitive metal concentrations and bulk sediment N-isotope compositions. Based on this suite of proxies recently calibrated for the respective modern settings it will be possible to reconstruct the variability of the modes of anoxic conditions and their potential internal and external climatic drivers in the past. In addition, comparison between the reconstructed seawater Mo isotope compositions at the two sites will allow to constrain if there were secular changes of past global seawater molybdenum isotope composition and if these changes impacted reconstructions of oxygenation and redox conditions over the past 3 million years.