NSFGEO-NERC: Understanding the Drivers of Inert Gas Saturation to Better Constrain Ice Core-Derived Records of Past Mean Ocean Temperature

NSFGEO-NERC：了解惰性气体饱和的驱动因素，以更好地限制冰芯记录的过去平均海洋温度

• 批准号: NE/W007258/1
• 负责人: Samar Khatiwala
• 金额: $ 13.22万
• 依托单位: University of Oxford
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2021
• 资助国家: 英国
• 起止时间: 2021 至 无数据
• 项目状态: 未结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FW007258%2F1
• 关键词: NSFGEO; NERC; Understanding; Drivers; Inert

The integrated heat content of the global ocean (OHC) is a fundamental climate variable for understanding Earth's energy balance. The OHC is intimately tied to high-latitude processes, which regulate air-sea fluxes of heat and radiative gases and control rates of deep-water formation. To quantitatively resolve past changes in OHC, a new ice core proxy for global mean ocean temperature (MOT) has recently been developed. This MOT proxy employs high-precision measurements of globally well-mixed atmospheric noble gases trapped in polar ice, which are highly sensitive to global ocean warming or cooling (Baggenstos et al., 2019; Bereiter, Shackleton, et al., 2018; Shackleton et al., 2019, 2020). Noble gases are powerful tracers of physical interaction between the atmosphere, ocean, and cryosphere due to their chemical and biological inertness, lack of long-term sinks and sources, and spatially uniform distribution in the atmosphere. Changes in krypton (Kr) and xenon (Xe) mixing ratios in the troposphere are quantitatively linked to the MOT due to the strong control of temperature on the solubility of these gases in seawater. That is, as the whole ocean warms, Kr and Xe solubilities decrease, which leads to net degassing of these dissolved gases from the global ocean and thereby increases their atmospheric concentrations. Because the heavy noble gases - Kr and Xe - have stronger solubility temperature dependences than nitrogen (N2), the ratios Xe/N2 and Kr/N2 measured in past atmospheric air bubbles trapped in ice cores can be used to constrain past MOT. Using measurements from multiple polar ice core archives of ancient atmospheric air, past changes in MOT (and therefore in OHC) over the past 25 thousand years have been quantitatively reconstructed in several recent studies.The quantitative translation of past atmospheric Xe/N2 and Kr/N2 to MOT relies not only on knowledge of the solubility functions of these gases in water, but also on past changes in global ocean volume, salinity, sea-level pressure and the saturation states of Xe, Kr, and N2 in the global ocean. In the modern ocean, Kr and Xe are systematically undersaturated at depth by several percent throughout the global deep ocean, whereas N2 is closer to solubility equilibrium (Hamme et al., 2017; Loose et al., 2016; Loose & Jenkins, 2014; Nicholson et al., 2016; Seltzer et al., 2019). The well documented undersaturation of heavy noble gases in the modern ocean is thought to result from a complex function of global ocean circulation and high- latitude processes, such as changes in the wintertime cooling rates of high-latitude surface waters, sea-ice extent, glacial meltwater input, and wintertime storm intensities driving variable degrees of diffusive versus bubble-mediated air-sea gas exchange. The degree to which Kr and Xe may have been undersaturated during the last glacial maximum (LGM) presently remains an entirely open question, yet one that is essential for reconstructing past MOT. To quantify the importance of past changes in undersaturation of inert gases in the deep ocean for ice core MOT reconstruction, there is a need for simulation of these gases in the global ocean under past climate states. We propose to use a suite of numerical model experiments, both equilibrium and single- forcing (e.g., isolating the effects of sea ice, ocean circulation, air-sea gas exchange dynamics), to estimate the Kr, Xe, and N2 saturation states of the past ocean, with particular emphasis on the LGM and periods of abrupt warming during the last deglaciation. This will not only allow us to refine existing polar ice core noble gas records of MOT by producing the first estimates of a presently unconstrained but important variable (Deq), but it will also enable better understanding of the physical drivers of undersaturation and their relationship to high-latitude ice-ocean-atmosphere interaction in preindustrial, glacial, and future climates.

全球海洋的综合​​热含量（OHC）是了解地球能量平衡的基本气候变量。 OHC 与高纬度过程密切相关，高纬度过程调节海气通量和辐射气体，并控制深水形成的速率。为了定量解决 OHC 过去的变化，最近开发了一种新的全球平均海洋温度（MOT）冰芯代理。该 MOT 代理采用高精度测量极地冰中捕获的全球混合良好的大气稀有气体，这些气体对全球海洋变暖或变冷高度敏感（Baggenstos 等人，2019 年；Bereiter、Shackleton 等人，2018 年；Shackleton等，2019、2020）。稀有气体由于其化学和生物惰性、缺乏长期的汇和源以及在大气中的空间均匀分布，是大气、海洋和冰冻圈之间物理相互作用的有力示踪剂。由于温度对这些气体在海水中溶解度的强烈控制，对流层中氪 (Kr) 和氙 (Xe) 混合比的变化与 MOT 定量相关。也就是说，随着整个海洋变暖，氪和氙的溶解度降低，这导致这些溶解气体从全球海洋中净脱气，从而增加了它们的大气浓度。由于重惰性气体 - Kr 和 Xe - 比氮 (N2) 具有更强的溶解度温度依赖性，因此在冰芯中捕获的过去大气气泡中测量的 Xe/N2 和 Kr/N2 比率可用于约束过去的 MOT。最近的几项研究利用多个极地冰芯古代大气空气档案的测量结果，定量重建了过去 25000 年 MOT（以及 OHC）的变化。过去大气 Xe/N2 和 Kr/ 的定量转换N2 到 MOT 不仅依赖于这些气体在水中的溶解度函数的知识，还依赖于过去全球海洋体积、盐度、海平面压力以及全球海洋中 Xe、Kr 和 N2 的饱和状态的变化。在现代海洋中，整个全球深海中，Kr 和 Xe 在深度上系统性地欠饱和几个百分点，而 N2 更接近溶解度平衡（Hamme 等人，2017 年；Loose 等人，2016 年；Loose & Jenkins，2014 年；尼科尔森等人，2016；塞尔策等人，2019）。现代海洋中重惰性气体的饱和度有据可查，被认为是全球海洋环流和高纬度过程的复杂作用的结果，例如高纬度地表水的冬季冷却速率、海冰范围的变化，冰川融水输入和冬季风暴强度驱动不同程度的扩散与气泡介导的空气-海洋气体交换。在末次盛冰期（LGM）期间，Kr 和 Xe 的不饱和程度目前仍然是一个完全悬而未决的问题，但这对于重建过去的 MOT 至关重要。为了量化过去深海惰性气体欠饱和变化对于冰芯 MOT 重建的重要性，需要模拟过去气候状态下全球海洋中的这些气体。我们建议使用一套数值模型实验，包括平衡和单强迫（例如，隔离海冰、海洋环流、海气交换动力学的影响），来估计 Kr、Xe 和 N2 饱和状态过去的海洋，特别强调末次盛冰期和末次冰消期期间的突然变暖时期。这不仅使我们能够通过对目前不受约束但重要的变量（Deq）进行首次估计来完善现有的极地冰芯稀有气体记录，而且还将使我们能够更好地理解欠饱和的物理驱动因素及其与饱和度的关系。工业化前、冰川时期和未来气候中的高纬度冰-海洋-大气相互作用。