Oxygen decline in the ocean: Implications for oceanic N2O production and the atmosphere

N2 O是对流层中重要的温室气体，其每摩尔的全球变暖潜势约为CO2的300倍。一氧化二氮在大气中的存在时间也足够长，足以使其在迁移到平流层并在平流层中发生消耗臭氧的光化学反应后幸存下来（de Bie等人，2002年）。海洋是大气中N2 O的重要来源，约占所有自然来源的35%。N2 O的微生物产生（通过NH 4+氧化、硝化-反硝化、反硝化）在很大程度上受溶解氧（DO）浓度的调节。低于某个但定义不明确的DO阈值，N2 O的产生将大幅增加。越来越多的证据表明，海洋中的溶解氧浓度正在下降;鉴于一氧化二氮对大气的重要性，这引起了人们的关注，因为这无疑将导致海洋一氧化二氮的产生和向大气的排放增加。在此之前，我们必须1）确定海洋中目前的N2 O条件（即N2 O分布，N2 O生产和生产途径），以及2）确定N2 O条件在未来不同的DO情景下将如何变化。为了解决第一点，工作将在东北部热带大西洋进行，其中DO浓度更具有当代海洋的特点。为了解决第二点，我们将在两个极端的氧气最低区（热带南太平洋东部和热带大西洋东北部的低溶解氧涡流）进行工作，其中溶解氧浓度远低于当今海洋的大部分地区。在我们的每个研究区域，我们将测量1）N2 O浓度，2）N2 O的del 15 N，del 18 O和15 N站点偏好特征，以确定不同的N2 O生产途径的相对重要性散装N2 O池，和3）N2 O生产率通过每个不同的生产途径使用15 N示踪技术。还将进行分子分析，以表征和量化不同的N2 O生产生物。当然，如果我们要准确地评估由于溶解氧浓度降低而导致海洋中N2 O产量增加对大气的影响，那么我们必须更全面地了解影响N2 O海-气通量的因素。为了解决这个问题，我们将使用涡度协方差技术直接测量N2 O通量从海洋和比较这些一系列的物理，化学和生物参数。

N2O is an important greenhouse gas in the troposphere and has a per mol global warming potential ~300 times that of CO2. The atmospheric lifetime of N2O is also sufficiently long enough for it to survive transport to the stratosphere where it undergoes ozone depleting photochemical reactions (de Bie et al. 2002). The ocean is a significant source of N2O to the atmosphere, accounting for ~35% of all natural sources. The microbial production of N2O, (via NH4+ oxidation, nitrifier-denitrification, denitrification), is largely regulated by dissolved oxygen (DO) concentrations. Below a certain but poorly defined DO threshold, N2O production will increase substantially. There is increasing evidence that DO concentrations in the ocean are decreasing; given the atmospheric importance of N2O, this raises cause for concern as it will undoubtedly lead to an increase in oceanic N2O production and emissions to the atmosphere. Before this happens it is critical that we 1) identify the current N2O conditions (i.e. N2O distributions, N2O production and the production pathways) in the ocean, and 2) determine how N2O conditions will change under different future DO scenarios. To address the first point, work will be conducted in the north eastern tropical Atlantic where DO concentrations are more characteristic of much of the contemporary ocean. To address the second point, we will conduct work in two extreme oxygen minimum zones (the eastern tropical South Pacific and a low DO eddy in the north eastern tropical Atlantic) where DO concentrations are much lower than in much of the present day ocean. In each of our study regions we will measure 1) N2O concentrations, 2) del15N, del18O and 15N site preference signatures of N2O to determine the relative importance of the different N2O producing pathways to bulk N2O pools, and 3) N2O production rates via each of the different production pathways using 15N tracer techniques. Molecular analyses will also be conducted to characterize and quantify the different N2O producing organisms. Of course, if we are to accurately assess the impact that increasing N2O production in the ocean, as a result of decreasing DO concentrations, will have for the atmosphere, it is critical that we develop a more comprehensive understanding of the factors that influence sea-to-air fluxes of N2O. To address this, we will use the eddy covariance technique to directly measure N2O fluxes from the ocean and compare these to a range of physical, chemical and biological parameters.