Collaborative Research: Documenting N2 fixation in N deficient waters of the Eastern Tropical South Pacific

一些独立的地球化学和遥感证据表明，氮的固定可能与主要氧气最少区(OMZ)下游的地表水有关，特别是在热带南太平洋东部(ETSP)。然而，几乎没有直接证据支持这些推论。除了证实这些间接评估，记录ETSP中重要的氮固定将提供对两个长期争议的洞察：海洋N的收支是否如模型和古海洋数据所暗示的那样平衡，如果是，那么在空间上增加和移除N的过程如何，从而在时间上耦合？在这个项目中，南加州大学和迈阿密大学的研究人员将检验这样一种假设，即固定在ETSP中发生的面积速率等于或超过以前在更深入研究的地区所记录的速率，例如北大西洋亚热带的低营养水域。如果扩大到ETSP水域的表面积，每年可为全球海洋N预算增加10-50 Tg N的投入。他们将在连续两年的初夏和夏末在ETSP进行两次巡航，利用互补的生物和地球化学工具评估作为地表水新氮源的氮气固定的定量意义。氮气固定率将在两个时间/空间尺度上进行评估：每日/局部(瓶装15N2孵化和漂浮沉积物捕捉器)；以及季节/区域(使用系泊沉积物捕捉器和水柱TDNd15N的d15N预算)。这些估计提供了在两个夏季站址期间潜在的氮气固定的详细观察，预计这两个季节的比率最高，以及较低的预期氮气固定时间的延长观察。这些不同估计的组合将旨在确定该地区固定氮气是否有助于平衡海洋氮素收支。如果一切按计划进行，这项研究将确定在ETSP中固定氮气的数量重要性，以及这些以前未记录的速率是否有助于解决海洋氮素预算。其影响包括海洋N循环维持动态平衡的能力，从而在冰川/间冰期时间尺度上维持全球C循环。更广泛的影响：该项目将为博士后助理和研究生提供研究经验和机会。将有直接的互动和机会，在实验室和游轮上指导研究生。此外，该项目将例行公事地将本科生作为勤工俭学的学生，并参与南加州大学地球生物学REU计划。拟议的活动将与NSF资助的南加州大学和加州大学洛杉矶分校的COSEE-West计划的工作相结合。该中心包括一个由海洋研究人员、K-12教育工作者、教育中心和大洛杉矶地区的普通公众组成的网络。

Several independent lines of geochemical and remote sensing evidence suggest that dinitrogen (N2) fixation may be associated with surface waters downstream of major oxygen minimum zones (OMZs) and in particular in the Eastern Tropical South Pacific (ETSP). However, little direct evidence supports these inferences. Besides substantiating these indirect assessments, documenting significant N2 fixation in the ETSP would provide insight into two longstanding controversies: Is the marine N budget balanced, as implied by modeling and paleoceanographic data, and if so, how are the processes that add and remove N spatially, and thus temporally coupled? In this project researchers at the University of Southern California and the University of Miami will test the hypothesis that fixation occurs in the ETSP at areal rates that equal or exceed those previously documented in more well-studied regions such as the oligotrophic waters of the sub/tropical North Atlantic. If scaled to the surface area of ETSP waters, this could add an additional 10-50 Tg N per year of inputs to the global marine N budget. They will undertake two cruises in the ETSP during early and late summer in two consecutive years to assess the quantitative significance of N2 fixation as a source of new N to surface waters using complementary biological and geochemical tools. N2 fixation rates will be evaluated on two temporal/spatial scales: daily/local (bottle 15N2 incubations and floating sediment traps); and seasonal/regional (d15N budget using moored sediment traps and water column TDN d15N). These estimates provide detailed observations of potential N2 fixation during station occupation in two summer seasons, when rates are expected to be greatest, as well as prolonged observation over lower expected N2 fixation periods. A combination of these different estimates will aim to determine if N2 fixation in this region can help balance the marine N budget. If all goes as planned, this study will determine the quantitative importance of N2 fixation in the ETSP, and whether these previously undocumented rates can help resolve the marine N budget. Implications include the ability of the marine N cycle to maintain homeostasis, and thus the global C cycle on glacial/interglacial time scales. Broader Impacts: This project will provide research experience and opportunities for a postdoctoral associate as well as a graduate student. There will be direct interactions and opportunities for mentoring with graduate students in the laboratory and on cruises. Also, the project will routinely involve undergraduates as work-studies and participate in the USC Geobiology REU Program. The proposed activities will be integrated with the work of the NSF funded COSEE-West program to USC and UCLA. This Center involves a network of oceanographic researchers, K-12 educators, education centers and the general public in the greater Los Angeles area.