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

海洋环流被认为在从最后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，缺乏对冰川消退的机械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的二氧化碳含量，越来越多的证据表明，至少有一部分最后一次冰消期变暖是由海洋中的二氧化碳释放到大气中造成的。海洋密度结构和环流的根本性变化可能导致这种碳释放，但很少有冰消期海水物理性质的记录，特别是来自最大的海洋盆地太平洋。在该项目执行过程中，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特性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最后一个冰河时代一直延续到现在，提供海水属性的详细历史，这将揭示海洋在全球气候变化中的作用。太平洋深海沃茨温度较低，盐度较高，这两个因素都可能导致末次冰期海水密度较高。然而，温度值和垂直温度结构的冰川海洋跨越中到中深度是不受约束的。这些关键深度范围内海水温度和通风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。利用西南太平洋3个沉积物岩心（~1100、1600和2500 m水深），以~250年分辨率重建近30,000年来西南太平洋海水温度和氧同位素组成（δ 18 OSW）。高沉积物积累率和完善的，地层相关联的火山灰年表，从新西兰的丰盛湾的目标沉积物岩心特别有价值的评估在这一地区的快速变化。海水温度将重建底栖有孔虫方解石（Uvigerina）的Mg/Ca，然后结合来自同一物种的底栖方解石的δ 18 O来计算δ 18 OSW。至少，delta 18 OSW作为保守的水团示踪剂，限制了海洋内部的混合。该属性还可以用于估计盐度的相对变化，特别是当对delta 18 OSW和盐度之间的过去关系的更多估计变得可用时（例如，从孔隙水研究）最终使我们更接近量化密度。这些记录将追踪末次冰期结束时海洋变暖的模式，从而验证有关海洋分层的假设。一套50个核心顶部也将用于地面实况和完善镁/钙温度代理，提供一个区域特定的温度敏感性，如果合适的。该项目的结果还可以提高研究界描述冰川循环特征的能力。例如，冰川温度剖面可以帮助限制对末次冰期最大状态估计的解决方案，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.