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

全球海洋的综合热含量（OHC）是了解地球能量平衡的一个基本气候变量。海洋热含量与高纬度过程密切相关，高纬度过程调节着海气热和辐射气体的通量，并控制着深水形成的速率。为了定量解决过去热含量的变化，最近开发了一个新的全球平均海洋温度（MOT）的冰芯代理。该MOT代理采用对极地冰中捕获的全球混合良好的大气稀有气体的高精度测量，这些气体对全球海洋变暖或变冷高度敏感（Baggenstos等，2019;Bereiter， Shackleton等，2018；Shackleton等，2019,2020）。惰性气体是大气、海洋和冰冻圈之间物理相互作用的有力示踪剂，因为它们的化学和生物惰性，缺乏长期的汇和源，并且在大气中的空间分布均匀。对流层中氪（Kr）和氙（Xe）混合比的变化在数量上与MOT有关，因为温度对这些气体在海水中的溶解度有很强的控制。也就是说，随着整个海洋变暖，Kr和Xe的溶解度降低，这导致这些溶解气体从全球海洋中净脱气，从而增加了它们在大气中的浓度。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的比值可以用来约束过去的MOT。利用对古代大气的多个极地冰芯档案的测量，在最近的几项研究中定量地重建了过去25000年来MOT的变化（因此也重建了热含量的变化）。过去大气Xe/N2和Kr/N2到MOT的定量转换不仅依赖于这些气体在水中的溶解度函数，还依赖于过去全球海洋体积、盐度、海平面压力的变化以及全球海洋中Xe、Kr和N2的饱和状态。在现代海洋中，在全球深海中，Kr和Xe在深度上系统性地处于几个百分比的不饱和状态，而N2更接近溶解度平衡（Hamme等人，2017；Loose等人，2016;Loose & Jenkins, 2014； Nicholson等人，2016；Seltzer等人，2019）。有充分证据表明，现代海洋中重惰性气体的欠饱和被认为是由全球海洋环流和高纬度过程的复杂作用造成的，如冬季高纬度地表水冷却速率、海冰范围、冰川融水输入的变化，以及冬季风暴强度驱动不同程度的扩散与气泡媒介的海气交换。在末次盛冰期（LGM）期间，Kr和Xe的不饱和程度目前仍是一个完全开放的问题，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冰芯MOT重建的重要性，需要对过去气候状态下全球海洋中的这些气体进行模拟。我们建议使用一套数值模式实验，包括平衡和单强迫（例如，孤立海冰、海洋环流、海气交换动力学的影响），来估计过去海洋的Kr、Xe和N2饱和状态，特别强调LGM和末次消冰期间的突变变暖时期。这不仅使我们能够通过对一个目前不受约束但重要的变量（Deq）的首次估计来完善现有的极地冰芯稀有气体MOT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工业化前、冰川时期和未来气候中高纬度冰-海洋-大气相互作用的关系。

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.