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

在过去的几十年里，由于全球变暖，海洋中的氧含量已经减少了。这导致了高度缺氧的海洋区域的扩张。这种不断扩大的氧气最小带（OMZs）在东太平洋普遍存在，这是高生物生产力和随后的有机物再矿化的结果，这是由富含营养的水上涌驱动的。为了判断增强的缺氧是在过去自然变率的范围内还是之外，唯一的途径是沉积记录。然而，通常用于重建过去缺氧强度和模式的地球化学指标产生了有争议的结果，特别是在开阔的海洋环境中。在这里，我们建议应用最近在秘鲁OMZs和瓜伊马斯盆地的现代环境研究中获得的关于缺氧条件代理的功能和影响因素的新知识来重建过去300万年的缺氧条件模式的演变。我们将利用ODP Site 680（后来被重新钻探为IODP Site 1228）的沉积物，这些沉积物来自秘鲁陆架沿海上升流OMZ的现代中心（250米水深），而DSDP Site 479则来自更深的Guaymas盆地OMZ中心（750米水深），这是北太平洋OMZ的一部分。这两个地点的缺氧沉积物覆盖了从现在到上新世晚期的时期。680遗址显示更新世间冰期有机碳含量具有明显的冰期/间冰期旋回性，最高可达15%，而479遗址较深，有机碳含量较为均匀，接近3%。为了确定过去不同缺氧模式的长期和短期变化，我们将采用一系列代理指标，包括通过沉积物顺序浸出获得的不同铁池，稳定的Mo同位素，氧化还原敏感金属浓度和大块沉积物n同位素组成。基于最近为各自的现代环境校准的这套代理，将有可能重建过去缺氧条件模式的可变性及其潜在的内部和外部气候驱动因素。此外，对这两个地点重建的海水钼同位素组成进行比较，将有助于约束过去全球海水钼同位素组成是否存在长期变化，以及这些变化是否影响了过去300万年中氧合和氧化还原条件的重建。

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.