Collaborative Research: Pacific Ocean stratification since the last ice age: New constraints from benthic foraminifera

海洋环流被认为在从上一个冰河时代到现代间冰期（地球最近的“冰川消退”）的过渡中发挥了关键作用。然而，缺乏对冰川消融的机制解释是我们对海洋-气候系统理解的一个主要限制。改变全球气温的一种方法是改变大气中二氧化碳的含量，越来越多的证据表明，上一次冰期消退的变暖至少有一部分是由海洋向大气中释放二氧化碳造成的。海洋密度结构和环流的根本变化可能导致这样的碳释放，但很少有关于海水物理性质的脱冰记录，特别是来自最大的海洋盆地太平洋的记录。在该项目中，将利用海洋沉积物岩心的地球化学证据，建立西南太平洋海水温度和其他物理性质的多个高分辨率记录。这项合作工作将加强参与大学之间的联系，并为科学领域的研究生和本科生提供实践培训。该项目的海洋记录将从上一个冰河时代一直延伸到现在，提供海水特性的详细历史，这将阐明海洋在全球气候变化中的作用。较冷的温度和较高的太平洋深海盐度可能都导致了末次冰期海水密度的增加。而中、中深度冰洋的温度值和垂直温度结构则不受约束。海水温度和通风在这些关键深度范围内变化的时间也不清楚。确定海洋密度结构可以帮助限制过去的环流，而确定密度的第一步是确定温度。利用~1100、1600和2500 m水深的3个沉积物岩心，以~250年的分辨率重建了西南太平洋近3万年的海水温度和海水（delta18OSW）氧同位素组成。高沉积速率和完善的地层关联的tephra年代学使得来自新西兰丰盛湾的目标沉积物岩心对于评估该地区的快速变化特别有价值。利用底栖有孔虫方解石（Uvigerina）的Mg/Ca重建海水温度，并结合同一物种底栖方解石的delta18O计算delta18OSW。至少，delta18OSW作为一种保守的水团示踪剂，限制了海洋内部的混合。这种性质也可以用来估计盐度的相对变化，特别是随着对delta18OSW与盐度之间过去关系的更多估计（例如，从孔隙水研究中）的出现，最终使我们更接近于量化密度。这些记录将追踪最后一次冰期终止期间海洋变暖的模式，从而使有关海洋去分层的假设得到检验。一套50个岩心顶部也将用于地面真相和改进Mg/Ca温度代理，如果合适的话，提供特定区域的温度灵敏度。该项目的结果还可以提高研究界描述冰川循环的能力。例如，冰川温度剖面可以帮助约束末次冰期最大状态估计的解，这对南大洋结构非常敏感。

Ocean circulation is thought to have played a key role in the transition from the last ice age to the modern interglacial (Earth's most recent "deglaciation"). However, the lack of a mechanistic explanation for the deglaciation represents a major limitation in our understanding of the ocean-climate system. One way to change global temperature is to alter the amount of CO2 in the atmosphere, and there is mounting evidence that at least part of the last deglacial warming was caused by the release of CO2 from the ocean into the atmosphere. A fundamental shift in the ocean's density structure and circulation could have caused such a carbon release, but there are very few deglacial records of physical seawater properties, particularly from the Pacific Ocean, the largest ocean basin. During the course of this project, multiple high-resolution records of seawater temperature and other physical properties from the Southwest Pacific Ocean will be established using geochemical evidence from marine sediment cores. This collaborative work will strengthen ties between the participating universities and provide practical training for graduate and undergraduate students in the sciences. The project's marine records will extend from the last ice age through the present, providing a detailed history of seawater properties that will shed light on the ocean's role in global climate change. Colder temperatures and higher salinities of abyssal waters in the Pacific likely both contributed to greater seawater density during the last glacial period. However, temperature values and the vertical temperature structure of the glacial ocean across middle to intermediate depths are unconstrained. The timing of changes in seawater temperature and ventilation across these key depth ranges are also unclear. Determining ocean density structure can help constrain past circulation, and a first step in establishing density is to determine temperature. Three sediment cores (~1100, 1600, and 2500 m water depth) will be used to reconstruct seawater temperature and oxygen isotope composition of seawater (delta18OSW) for the past 30,000 years in the southwest Pacific Ocean at ~250 year resolution. High sediment accumulation rates and well-established, stratigraphically-linked tephra chronologies make the targeted sediment cores from New Zealand's Bay of Plenty particularly valuable for assessing rapid changes in this region. Seawater temperature will be reconstructed using Mg/Ca of benthic foraminiferal calcite (Uvigerina) and then combined with delta18O of benthic calcite from the same species to calculate delta18OSW. At a minimum, delta18OSW acts as a conservative water mass tracer, placing constraints on mixing in the ocean interior. This property may also be used to estimate relative changes in salinity, particularly as more estimates of the past relationship between delta18OSW and salinity become available (e.g., from pore-water studies) ultimately bringing us closer to quantifying density. These records will track the pattern of oceanic warming during the last glacial termination, allowing hypotheses regarding ocean de-stratification to be tested. A suite of 50 core-tops will also be used for ground-truthing and refining the Mg/Ca temperature proxy, providing a region-specific temperature sensitivity if appropriate. The project's results could also improve the research community's ability to characterize glacial circulation. For example, glacial temperature profiles could help constrain the solution of last glacial maximum state estimates, which are very sensitive to Southern Ocean structure.