The Marine Dissolved N2/Ar Ratio, A Tracer for Deep Ocean Denitrification?

硝酸盐在海洋碳循环中的作用及其相对较短的停留时间使得理解海洋氮循环至关重要；然而，目前没有足够的实验证据来准确地确定当前的通量。反硝化和固氮作用是海洋溶解无机氮的主要汇源。在这项研究中，华盛顿大学的一个研究小组将与维多利亚大学的同事合作，研究由反硝化引起的海水中N2/Ar比率的变化。先前的研究已经证明了这种示踪剂在太平洋和印度洋的氧气最低带的效用，但他们将调查观察到的该比率“背景”分布的变化。研究人员已经有未发表的数据表明，从大西洋到太平洋，在1000米以下的水域，N2/Ar比增加了约0.5%。如果假定这一增加是由深海沉积物中的反硝化作用引起的，则其反硝化作用约为80 Tg/年。这是估计的全球反硝化作用的很大一部分（在200至400 Tg/年之间），并且在使用全球沉积物成岩模型的深海沉积物反硝化作用的大部分未经检验的预测范围内。目前还不能明确地将观测到的深水柱N2/Ar增加归因于反硝化作用，因为它也可能是由南极深水形成过程引起的。研究人员将通过测量其他稀有气体比率（主要是Ne/Ar和Kr/Ar）来分离由于大气或冰水相互作用的物理过程引起的N2/Ar比率增加的部分，这些比率仅在海洋表面冷却和气泡过程中发生变化。他们将在有机遇船的战略位置测量N2/Ar、Ne/Ar和Kr/Ar的深层水柱剖面；拉布拉多海、北大西洋（百慕大时间序列地点）、德雷克海峡、马达加斯加南部的印度洋、夏威夷海洋时间序列地点的亚热带北太平洋以及p站的亚北极北太平洋对所有气体比率进行了初步测量，并进行了广泛的测试，以确定采样方法中的污染源。该提案涉及两个实验室的合作，以便能够对大量海洋地点进行采样，通过快速样本分析将大气污染降至最低，并通过实验室间校准创造最大的准确性。更广泛的影响：该项目将促进美国和加拿大之间的国际海洋科学合作。它将支持一位助理教授将她帮助开发的分析方法应用于海洋学中的一个重要问题的研究。华盛顿大学的一名博士候选人将接受化学海洋学领域的培训，使用气体比和同位素比质谱分析方法。

The role of nitrate in the ocean carbon cycle and its relatively short residence time make it crucial to understand the marine nitrogen cycle; however, there is currently insufficient experimental evidence to accurately determine present day fluxes. Denitrification and nitrogen fixation are the main sink and source for dissolved inorganic nitrogen in the sea. In this study a research team at the University of Washington will collaborate with colleagues at the University of Victoria to study changes in the N2/Ar ratio in seawater caused by denitrification. Previous research has demonstrated the utility of this tracer in the oxygen minimum zones of the Pacific and Indian Ocean, but they will investigate observed changes in the "background" distribution of the ratio. The investigators already have unpublished data that indicate the N2/Ar ratio increases by about 0.5 % from the Atlantic to Pacific Oceans in waters below 1000 meters. If this increase is assumed to be caused by denitrification in deep ocean sediments it amounts to roughly 80 Tg/yr of denitrification. This is a significant portion of estimated global denitrification (between 200 and 400 Tg/yr) and within the range of the largely untested predictions of deep-ocean sediment denitrification using global sediment diagenesis models. Presently it is not possible to unequivocally attribute the observed deep water column N2/Ar increase to denitrification because it could also be caused by deep-water formation processes in the Antarctic. The investigators will separate the fraction of the N2/Ar ratio increase due to the physical processes of atmosphere or ice-water interaction from that due to denitrification by measuring other noble gas ratios (primarily Ne/Ar and Kr/Ar) that change only in response to ocean surface cooling and bubble processes. They will measure deep water-column profiles of N2/Ar, Ne/Ar and Kr/Ar in strategically-located sites where there are ships of opportunity: the Labrador Sea, the North Atlantic at the Bermuda time-series site, the Drake Passage, the Indian Ocean south of Madagascar, the subtropical North Pacific at the Hawaii Ocean time-series site, and the subarctic North Pacific at Station P. Preliminary measurements of all of the gas ratios have been made, and extensive testing has been done to identify sources of contamination in the sampling methods. This proposal involves a two-laboratory collaboration to make it possible to sample a large number of ocean sites, minimize atmospheric contamination by rapid sample analysis, and create maximum accuracy through laboratory intercalibration. Broader Impacts: This project will promote international ocean science collaboration between the U.S.and Canada. It will support the research of an assistant professor to apply analytical methods that she has helped develop to an important problem in oceanography. A PhD candidate at the University of Washington will be trained in the area of chemical oceanography using analytical methods of gas ratio and isotope ratio mass spectrometry.