Collaborative Research: Novel constraints on air-sea gas exchange and deep ocean ventilation from high-precision noble gas isotope measurements in seawater

这项拟议的工作结合了化学海洋学、海洋模型和固体地球地球化学领域，以开发溶解在海水中的重惰性气体的稳定同位素组成，作为海-气交换的新物理示踪剂。由于稀有气体在化学上是惰性的，因此是量化物理过程的理想工具。由于Ar(Ar)、Kr(Kr)和Xe(Xe)同位素比值具有不同的溶解度和扩散比，正如最近在实验室实验中量化的那样，它们通过增加具有独特灵敏度的新限制条件来补充现有的海水中大量惰性气体测量。对惰性气体海气交换的精确限制，对于适当量化生物地球化学重要气体(如二氧化碳和氧气)以及通风年龄示踪剂(如六氟化硫和氯氟化碳)的生产、消费和物理运输至关重要。此外，与惰性气体观测相比，全球环流模型(GCM)通常低估了深海的通气量。将这些新的同位素约束引入模型模拟将有助于确定与深水形成有关的物理过程，这些物理过程需要在未来的GCM开发中进行改进。由于颠覆的环流与对未来气候的预测密切相关，因为辐射气体和热量都被输送到深海，所以国际上对改进未来的模型预测有着广泛的兴趣。因此，在现有的海水惰性气体研究基础上增加高精度的惰性气体同位素测量，将为海洋生物地球化学和物理海洋学领域提供有价值的新制约。研究生和博士后学者的教育和培训将有助于美国的人力资源基础。这项拟议的工作将开发海水中高精度的Ar、Kr和Xe稳定同位素比值，作为新的海洋示踪剂。与2018年的试点研究一起，拟议的测量代表着海水中第一次高精度的Kr和Xe同位素比率分析。该项目的一个关键目标是检验在整个深海观察到的Ar、Kr和Xe不饱和的两个具体假设：(1)在深水形成期间，由于没有足够的平衡时间而导致表层海洋吸收气体的快速冷却；(2)冰川融化导致的次表层冷却，导致困在冰中的气泡溶解。虽然这两个不相互排斥的过程产生了类似的重惰性气体不饱和模式，但这些气体的同位素比率很适合区分每个过程的相对重要性。具体地说，理论预测表明，与快速吸收表面海洋气体相关的动力学分馏导致重-轻同位素比率下降，但由于重力浓缩的冰川融水的输入，这些比率将增加。其他目标包括：(A)将观测结果与模型模拟进行比较，以确定深水形成过程的GCM表示的成功和不足之处；(B)从SIO码头进行为期一年的表面-海洋观测，以测试与气泡注入和上升流有关的同位素分馏模型，以及对生物地球化学重要气体饱和的影响。这项工作将改进最近的溶解惰性气体同位素分析方法，通过增加样品大小和精炼净化技术来实现60%的精确度改进。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

The proposed work brings together the fields of chemical oceanography, ocean modeling, and solid Earth geochemistry to develop the stable isotope composition of heavy noble gases dissolved in seawater as novel physical tracers of air-sea gas exchange. Noble gases represent ideal tools for quantifying physical processes due to the fact that they are chemically inert. Because argon (Ar), krypton (Kr), and xenon (Xe) isotope ratios have distinct solubility and diffusivity ratios, as recently quantified in laboratory experiments, they complement existing bulk noble gas measurements in seawater by adding new constraints with unique sensitivities. Precise constraints on air-sea exchange of inert gases are paramount to properly quantifying production, consumption, and physical transport of biogeochemically important gases (such as carbon dioxide and oxygen) as well as ventilation age tracers (such as sulfur hexafluoride and CFCs). Additionally, global circulation models (GCMs) routinely underestimate deep-ocean ventilation compared to noble gas observations. Introducing these new isotopic constraints into model simulations will help identify physical processes related to deep-water formation that require improvement in future GCM development. Because the overturning circulation is closely tied to projections of future climate, by both the transports of radiative gases and heat into the deep ocean, there is broad international interest in improving future model projections. Therefore, adding high-precision noble gas isotope measurements to the existing body of research on inert gases in seawater will provide valuable new constraints for both the marine biogeochemistry and physical oceanography communities. Education and training of a graduate student and postdoctoral scholar will contribute to the human resource base of the United States. The proposed work will develop high-precision Ar, Kr, and Xe stable isotope ratios in seawater as new oceanographic tracers. Along with a 2018 pilot study, the proposed measurements represent the first high- precision Kr and Xe isotope ratio analyses in seawater. A key goal of this project is to test two specific hypotheses for the observed undersaturation of Ar, Kr, and Xe throughout the deep ocean: (1) rapid cooling-induced gas uptake by the surface ocean during deep-water formation with insufficient time for equilibration before sinking, or (2) subsurface cooling caused by melting of glacial ice, leading to the dissolution of air bubbles trapped in ice. Whereas both of these non-mutually exclusive processes produce similar patterns of heavy noble gas undersaturation, the isotope ratios of these gases are well suited to distinguish the relative importance of each process. Specifically, theoretical predictions suggest a decrease in heavy-to-light isotope ratios from the kinetic fractionation associated with rapid surface ocean gas uptake, but an increase in these ratios from the input of gravitationally enriched glacial meltwater. Other goals include: (a) comparing observations to model simulations to identify successes and shortcomings of GCM representations of deep-water formation processes, and (b) a year-long time series of surface-ocean observations from the SIO pier to test models of isotopic fractionation associated with bubble injection and upwelling, with implications for the saturation of biogeochemically important gases. This work will improve upon a recent method for dissolved noble gas isotopic analysis by increasing sample sizes and refining purification techniques to achieve a 60% improvement in precision.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.