Deep Ocean Circulation and Carbon Cycle Links During the Quaternary

这项提议的总体重点是重建全球深海环流和过去150万年来冰川/间冰期循环期间的碳循环变化。在此期间，北半球冰川活动加剧，冰期周期从40KYR转变为100KYR。这些变化的原因尚不清楚，但很可能是由气候系统内部的过程引发的。我们提出的研究将更好地理解过去海洋环流的机制及其与深海碳储存的联系。我们的重点是太平洋，这是体积最大的深海，以及来自大西洋的低营养水域流入其中。深海太平洋的巨大体积意味着其环流和碳含量的变化会影响大气中的二氧化碳水平。我们选择了可以在海洋环流和气候变化的多个指标之间进行比较的核心。它们沿着一条进入太平洋深处的主要深水流动路径的分布，以及水柱中的不同深度，将使我们能够重建有多少来自大西洋的营养物质和贫碳水进入太平洋。我们将测量已经有多个古海洋学替代记录(底层水温、冰量、海底碳同位素、可分类泥沙等)的岩芯上的水团示踪剂钕(ND)同位素。这是首次将所有这些地球化学工具应用于相同的岩心和相同年龄的样品，因此我们将能够直接比较这些不同示踪剂的结果，无论是在时间上还是空间上。其中一个岩芯，即ODP1123站，记录了150万年前的海洋状况，具有良好的年代学，而查塔姆隆起岩芯是从中等深度到深海深度的岩芯横断面。这些核心将使我们能够从时间和垂直两个角度了解过去的变化。将西南太平洋的ND同位素记录与大西洋和印度洋的记录进行比较，可以提供有关全球尺度温盐环流变化的新信息。结合使用ND同位素和海底有孔虫碳同位素替代物，可以将影响这两个替代物的海洋环流的作用从只影响碳同位素的全球碳循环变化中分离出来。我们在这个建议中展示的初步数据表明，有孔虫覆盖物的ND同位素组成是过去数百万年来深水化学的有力档案。我们现在可以观察到，海洋中的许多岩心具有ND同位素和海底有孔虫碳同位素，它们与深海环流的变化在很长的路径上是一致的，尽管在一些地点和时间段存在与这一行为脱钩的现象，包括在西南太平洋。拥有这两个指标的完整可比记录将极大地促进我们对水团如何通过深海传播的理解，并对海洋环流如何对人为气候变化做出反应进行建模。深水来源和流动还将受到海洋模型的限制，该模型可以利用多种化学替代物的分布来推断流动路径，并量化深水形成区的贡献以及边界处的输入。该模型可以用来拟合所有的古数据，而不是简单地进行情景模拟，并且可以在不同的时间段产生和检验全球范围内的Nd同位素分布。该模型对于古海洋学研究来说是高分辨率的，但它的计算效率仍然足够高，可以运行数千年。这种采用多代理方法的数据/模型协作将改变我们对深海水团来源、结构和全球温盐环流中关键位置的碳储存的理解。

The overall focus of this proposal is to reconstruct global deep-ocean circulation and the carbon cycle changes during glacial/interglacial cycles over the last 1.5 million years. During that time period northern hemisphere glaciation increased in intensity and the periodicity of ice ages shifted from a 40 kyr to a 100 kyr cycle. The causes for these changes are unknown, but were likely triggered by processes internal to the climate system. The research we propose will develop a better understanding of the mechanisms of past ocean circulation and its link to storage of carbon in the deep ocean. We focus on the Pacific, the largest deep ocean by volume, and the flow of lower nutrient Atlantic-sourced waters into it. The large volume of the deep Pacific means that changes in its circulation and carbon contents affect atmospheric carbon dioxide levels. We have chosen cores that allow comparison between multiple proxies of ocean circulation and climate variation. Their distribution along a major deep-water flowpath into the deep Pacific Ocean, and different depths in the water column, will allow us to reconstruct how much Atlantic-sourced nutrient and carbon poor water entered the Pacific.We will measure neodymium (Nd) isotopes as a tracer of water mass on cores which already have multiple palaeoceanographic proxy records (bottom-water temperature, ice volume, benthic carbon isotopes, sortable silt, and others). This is the first time that all of these geochemical tools will be applied on the same cores and the samples of the same age, so we will be able to directly compare the results of these different tracers, both temporally and spatially. One core, ODP Site 1123, records ocean conditions going back 1.5 million years, with a well-developed chronology, while the Chatham Rise cores are a depth transect of cores from intermediate to abyssal depths. These cores will allow us to understand past changes from both temporal and vertical perspectives. Comparing Nd isotope records from the south-western Pacific to records from the Atlantic and Indian Oceans, can provide new information about changes in global-scale thermohaline circulation.The combined use of Nd isotopes and benthic foraminiferal carbon isotope proxies can be used to deconvolve the role of ocean circulation, which affects both proxies, from global carbon cycling changes, which affect only carbon isotopes. The preliminary data we show in this proposal illustrate that the Nd isotopic composition of foraminiferal coatings is a robust archive of deep-water chemistry during the past few million years. We can now observe that many cores in the ocean have Nd isotope and benthic foraminiferal carbon isotopes which are compatible with changes in deep-water circulation on long pathlengths, though decoupling from this behaviour exists at some sites and time periods, including in the south-western Pacific. Having complete comparable records for both proxies will greatly advance our understanding of how water masses propagate through the deep ocean with implications for modelling how ocean circulation may react to anthropogenic climate change. Deep-water source and flow will also be constrained by an ocean model, which can use the distribution of multiple chemical proxies to deduce flowpaths and quantify the contributions of deep-water formation areas, as well as inputs at boundaries. The model can be used to fit all the palaeo-data, rather than simply doing scenario simulations, and global distributions of Nd isotopes can be produced and tested for different time periods. The model is high resolution for a palaeoceanographic study, but it is still computationally efficient enough to be run for many thousands of years. This data/model collaboration with a multi-proxy approach will transform our understanding of deep-ocean water-mass sourcing and structure and carbon storage at a key location in the global thermohaline circulation.