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

合作研究：海水中高精度稀有气体同位素测量对海气交换和深海通风的新限制

• 批准号: 1924394
• 负责人: Jeffrey Severinghaus
• 金额: $ 36.99万
• 依托单位: University of California-San Diego Scripps Inst of Oceanography
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2019
• 资助国家: 美国
• 起止时间: 2019-09-01 至 2022-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1924394&HistoricalAwards=false
• 关键词: Collaborative; Research; Novel; constraints; air

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.

本研究将化学海洋学、海洋模拟和固体地球地球化学等领域结合起来，开发溶解在海水中的重惰性气体的稳定同位素组成，作为海气交换的新型物理示踪剂。惰性气体由于其化学惰性，是量化物理过程的理想工具。由于氩（Ar）、氪（Kr）和氙（Xe）同位素比率具有不同的溶解度和扩散率，最近在实验室实验中进行了量化，它们通过添加具有独特灵敏度的新约束来补充现有的海水中大量惰性气体测量。对惰性气体的海气交换的精确限制对于正确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（如六氟化硫和氟氯化碳）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，全球环流模式（GCMs）通常低估了深海通风。在模型模拟中引入这些新的同位素约束将有助于确定与深水地层相关的物理过程，这些过程需要在未来的GCM开发中得到改进。由于翻转环流通过向深海输送辐射气体和热量与对未来气候的预测密切相关，因此国际社会对改进未来模式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约束。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。提出的工作将在海水中开发高精度的Ar， Kr和Xe稳定同位素比率，作为新的海洋示踪剂。与2018年的一项试点研究一起，拟议的测量结果代表了海水中首次高精度的Kr和Xe同位素比率分析。该项目的一个关键目标是验证两个特定的假设，以解释在整个深海中观测到的Ar、Kr和Xe的不饱和：(1)在深水形成过程中，表层海洋快速冷却导致气体吸收，在下沉之前没有足够的时间进行平衡，或者(2)冰川融化引起的地下冷却，导致被困在冰中的气泡溶解。虽然这两种非互斥过程都产生类似的重惰性气体欠饱和模式，但这些气体的同位素比率很适合区分每种过程的相对重要性。具体地说，理论预测表明，与表面海洋气体快速吸收相关的动力学分馏使重、轻同位素比值降低，但重力富集的冰川融水的输入使这些比值增加。其他目标包括：(a)将观测结果与模式模拟进行比较，以确定深水形成过程的GCM表示的成功和不足；(b)从SIO码头进行为期一年的海洋表面观测序列，以测试与气泡注入和上升流相关的同位素分馏模型，并对生物地球化学重要气体的饱和度产生影响。这项工作将改进最近的溶解惰性气体同位素分析方法，通过增加样本量和精炼净化技术，使精度提高60%。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。