Stable Isotopes of Argon in Seawater: New Tracers of Deep Water Formation Processes

最近的研究表明，18度水(EDW)是北大西洋的副热带模式水，在建立副热带气旋的营养库方面可能起着重要作用。结果表明，EDW的平流在北大西洋副热带环流的季节性温跃层下方注入了一块低营养水楔形。此外，营养物质耗尽的水团的存在可能会抑制对垂直过程的生物反应，这些垂直过程使营养物质在整个副热带环流中向上流动。这就提出了一个重要的问题，即EDW如何以及何时耗尽营养物质，以及信号如何被输出到副热带气旋的其余部分：1)在水团形成期间，与无机营养物质的生物耗尽有关的时间尺度是什么？2)EDW在俯冲时的营养物质浓度是多少？(3)EDW的形成对下游营养库和垂直营养盐通量有何影响？在这个项目中，杜克大学的研究人员将通过测量EDW形成区域的营养物质来解决这些问题。幸运的是，美国国家科学基金会资助的一个实地项目--CLIVAR模式水动力学实验(CLIMODE)目前正在加紧进行，以全面分析EDW的形成、维护和破坏。尽管CLIMODE的观测计划由一套测量组成，但它目前缺乏任何生物地球化学成分。该项目将在CLIMODE观测方案中增加营养采样，以检验有关EDW对地下营养库的影响的假设。具体地说，调查人员将在计划中的水文调查中增加营养分析，并将在两个CLIMODE系泊设备上分别安装一个硝酸盐传感器。前者将在形成区创建冬季第一个营养物质横截面，后者将在副热带环流中提供前所未有的硝酸盐时间序列。这项拟议工作的总体目标是进一步了解副热带模式水的水平平流对次表层养分库的影响程度以及相应的营养物质对初级生产者的可用性。这样的理解是至关重要的，因为副热带环流作为碳汇的作用取决于营养物质的可用性，这些营养物质为生物泵提供燃料，从表层海洋中去除有机碳，并在溶解度泵所需的二氧化碳分压中保持有利的海-气梯度。此外，由于水的大量生产在年际和年代际时间尺度上是不同的，建立副热带模式水和养分有效性之间的联系有助于我们理解初级生产力是如何受到气候变化的影响的。就更广泛的影响而言，显然，更好地了解地下营养库是如何补充和减少的，将有助于理解海洋是如何充当碳汇的。该项目还将支持一名博士后研究生的研究。

ABSTRACTOCE-0526826Recent studies suggest that Eighteen Degree Water (EDW), the subtropical mode water of the North Atlantic, may play an important role in setting the nutrient reservoir of the subtropical gyre. The findings indicate that advection of EDW injects a wedge of low nutrient water beneath the seasonal thermocline in the North Atlantic subtropical gyre. Furthermore, the presence of the nutrient-depleted water mass may dampen the biological response to vertical processes that flux nutrients upwards throughout the subtropical gyre. This raises important questions regarding how and when the EDW is depleted in nutrients and how the signal is exported to the rest of the subtropical gyre: 1) What are the time scales associated with the biological depletion of inorganic nutrients during water mass formation? 2) What is the nutrient concentration of the EDW upon subduction? and 3) How does the formation of EDW impact the downstream nutrient reservoir and vertical nutrient fluxes? In this project, researchers at Duke University will address these questions with nutrient measurements in the formation region of the EDW. Fortuitously, an NSF-funded field program, CLIvar MOde water Dynamics Experiment (CLIMODE), is currently ramping up for a comprehensive analysis of the formation, maintenance and destruction of EDW. Though CLIMODE's observational program is comprised of a suite of measurements, it currently lacks any biogeochemical component. This project will provide for the addition of nutrient sampling to the CLIMODE observational program in order to test hypotheses concerning the impact of EDW on the subsurface nutrient reservoir. Specifically, the investigators will add nutrient analyses to the planned hydrographic surveys and will mount a nitrate sensor on each of two CLIMODE moorings. The former will create the first cross-section of nutrients during the winter in the formation region, and the latter will provide an unprecedented time series of nitrate within the subtropical gyre.The overall goal of this proposed work is to further the understanding of the degree to which the horizontal advection of a subtropical mode water impacts the subsurface nutrient reservoir and the corresponding availability of nutrients to primary producers. Such an understanding is crucial since the subtropical gyre's role as a carbon sink depends on the availability of nutrients to fuel the biological pump, remove organic carbon from the surface ocean, and maintain a favorable air-sea gradient in the partial pressure of carbon dioxide necessary for the solubility pump. Furthermore, since water mass production is known to vary on interannual and decadal time scales, establishing the link between subtropical mode waters and nutrient availability advances our understanding of how primary productivity is impacted by climatic variability. In terms of broader impacts, it is clear that an improved understanding of how the subsurface nutrient reservoir is replenished and diminished will aid the understanding of how the ocean acts as a carbon sink. The project will also support the research of a pre-doctoral graduate student.