Collaborative Research: GEOTRACES Atlantic Section Nitrate Isotope Measurements

氮是海洋中浮游生物普遍需要的两种主要营养物质之一，其可用性可以影响海洋的生态，生产力和碳循环。虽然海洋中固定氮的循环在某种意义上是其他营养循环的象征，但它的独特之处还在于其最大的输入（氮固定）和输出（反硝化）是生物介导的，这使得海洋氮预算容易受到复杂的生物反馈的影响。因此，它提供了一个平台，可以提出全球地球化学循环的核心问题之一：个体生物和群体的行为如何与物理化学条件共同产生一个数十亿年来一直适合居住的全球地球表面环境？海洋固定N输入/输出预算的主导条款的特点很差，我们把注意力集中在这里的N固定。开发强大的估计全球固氮率和分布从“直接”船上测量固氮活性是复杂的固有的空间和时间的变化，这种生物介导的通量。因此，地球化学方法来估计氮固定投入已经走到了前列。目前，硝酸盐稳定同位素的测量，这可以提供一个综合的估计氮固定在区域或流域尺度上，是稀疏的大西洋，主要集中在马尾藻海。GEOTRACES计划提供了一个平台，通过对流域规模模式的说明，将这些数据置于更广泛的背景下。在这个项目中，来自普林斯顿大学、布朗大学和伍兹霍尔海洋研究所的研究人员将测量作为GEOTRACES北大西洋部分一部分收集的海水和大气样本中硝酸盐的d15 N。硝酸盐d15 N是GEOTRACES的“核心参数”，将补充其他测量，并将本身提供重要的限制海洋过程，包括氮固定，横向硝酸盐运输，低纬度N循环，北非上升流地区对整个盆地的营养通量的影响，以及固定N与地中海的交换。除了产生这种特定的过程相关的见解，这项工作将提供一个第一个跨流域的意见，硝酸盐同位素在内部，因此将有助于简单地表征不同的内部水体，包括模式沃茨，南极中层水，地中海中层水，较低和较高的北大西洋深水，南极底层水的同位素信号。最后，大气硝酸盐沉积的同位素特征将告知我们的N同位素预算和北大西洋的同位素梯度的理解。综合起来，这些测量结果将有助于深入了解现代海洋地球化学过程，并为现代物理海洋学和古海洋学应用提供一级背景信息。作为后者的一个例子，大西洋沉积物的研究试图重建过去的变化率的N固定，基于现代发现，N固定似乎降低了马尾藻海的温跃层硝酸盐的d15 N。这项古海洋学工作的进展依赖于现代大西洋硝酸盐d15 N的更完整的图像。更广泛的影响：拟议的研究的更广泛的影响包括博士后研究人员的指导和包括本科生在国家的最先进的研究。该项目还将为北大西洋提供高质量的硝酸盐同位素数据集，供更广泛的社区使用。

Nitrogen is one of the two major nutrients required universally by plankton in the ocean, and its availability can affect the ocean's ecology, productivity, and carbon cycle. While the cycling of fixed N in the ocean is in one sense emblematic of other nutrient cycles, it is also unique in that its largest input (N fixation) and output (denitrification) are biologically mediated, which renders the ocean N budget susceptible to complex biological feedbacks. It thus provides a platform for asking one of the core questions of global biogeochemical cycles: How is it that the actions of individual organisms and groups conspire with physicochemical conditions to produce a global Earth surface environment that has been continuously habitable for billions of years? The dominant terms in the oceanic fixed N input/output budget are poorly characterized, and we focus our attention here on N fixation. Developing robust estimates of the global rate and distribution of N fixation from "direct" shipboard measurements of N fixing activity is complicated by the inherent spatial and temporal variability of this biologically mediated flux. Thus, geochemical approaches for estimating N fixation inputs have come to the forefront. Currently, nitrate stable isotope measurements, which could provide an integrative estimate of N fixation on a regional or basin scale, are sparse in the Atlantic, being focused primarily in the Sargasso Sea. The GEOTRACES program provides a platform to put these data into a broader context through the illumination of basin-scale patterns. In this project researchers from Princeton University, Brown University, and the Woods Hole Oceanographic Institution will measure the d15N of nitrate in seawater and atmospheric samples collected as part of the GEOTRACES North Atlantic Section. Nitrate d15N is a GEOTRACES "core parameter" that will complement other measurements and will by itself provide important constraints on the oceanographic processes, including N fixation, lateral nitrate transport, low latitude N cycling, the effect of the North African upwelling regions on nutrient fluxes across the basin, and the exchange of fixed N with the Mediterranean. In addition to yielding such specific process-related insights, this work will provide one of the first cross-basin views of nitrate isotopes in the interior and will thus help to simply characterize the isotope signals of different interior water masses, including the Mode Waters, Antarctic Intermediate Water, Mediterranean Intermediate Water, Lower and Upper North Atlantic Deep Water, and Antarctic Bottom Water. Finally, the isotopic characterization of atmospheric nitrate deposition will inform our understanding of the N isotope budget and isotopic gradients of the North Atlantic. Combined, these measurements will yield insight into modern biogeochemical processes and will also provide first order background information for both modern physical oceanographic and paleoceanographic applications. As an example of the latter, studies of Atlantic sediments seek to reconstruct past changes in the rate of N fixation, based on the modern finding that N fixation appears to lower the d15N of thermocline nitrate in the Sargasso Sea. Progress in this paleoceanographic work relies on a more complete picture of nitrate d15N in the modern Atlantic. Broader impacts: The broader impacts of the proposed study include the mentoring of a postdoctoral investigator and the inclusion of undergraduates in state-of-the-art research. The project will also provide a high-quality nitrate isotope data set for the North Atlantic for use by the broader community.