Collaborative Research: GEOTRACES Atlantic Section Nitrate Isotope Measurements

氮是海洋中浮游生物普遍需要的两种营养物质之一，它的可获得性会影响海洋的生态、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(N固定)和输出(反硝化)是由生物调节的，这使得海洋N的收支容易受到复杂的生物反馈的影响。因此，它为提出全球生物地球化学循环的核心问题之一提供了一个平台：单个生物体和群体的活动是如何与物理化学条件相结合，产生数十亿年来持续宜居的全球地球表面环境的？海洋固定N投入/产出预算中的主要项没有得到很好的刻画，这里我们把注意力集中在N固定上。从船上对固氮活性的“直接”测量得出全球固氮速率和分布的可靠估计，由于这种生物介导的通量固有的时空变异性而变得复杂。因此，估算氮固定输入的地球化学方法已经走到了前列。目前，硝酸盐稳定同位素测量在大西洋很稀少，主要集中在马尾藻海，可以对区域或盆地尺度上的氮固定提供综合估计。GEOTRACES计划提供了一个平台，通过阐明盆地尺度的模式，将这些数据放在更广泛的背景下。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北大西洋部分的一部分采集的海水和大气样本中硝酸盐的d15N。硝酸盐d15N是GEOTRACES的一个“核心参数”，它将补充其他测量，本身将对海洋过程提供重要的制约，包括氮的固定、硝酸盐的横向输送、低纬氮的循环、北非上升流区域对整个盆地营养物质通量的影响，以及固定氮与地中海的交换。除了产生这种与过程有关的具体见解外，这项工作还将提供关于内陆硝酸盐同位素的第一批跨盆地观点之一，从而有助于简单地描述不同内陆水团的同位素信号，包括模式水、南极中间水、地中海中间水、北大西洋下部和上部深水以及南极底层水。最后，大气硝酸盐沉积的同位素特征将有助于我们了解北大西洋的N同位素收支和同位素梯度。综合起来，这些测量将对现代生物地球化学过程有深入的了解，还将为现代物理海洋学和古海洋学应用提供一流的背景信息。作为后者的一个例子，对大西洋沉积物的研究试图重建过去固氮速率的变化，基于现代发现，固氮似乎降低了马尾藻海温跃层硝酸盐的d15N。这项古海洋学工作的进展有赖于对现代大西洋中硝酸盐d15N的更完整的了解。更广泛的影响：拟议研究的更广泛影响包括指导博士后研究员和将本科生纳入最先进的研究。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.