Biogeochemical Flows of Nitrogen in a Tropical Upwelling System (A CARIACO Time Series Project)

海洋氮循环在控制海洋生产力和碳等其他元素的通量方面起着至关重要的作用。重建这一重要循环如何随时间变化的能力在很大程度上取决于沉积物中的氮同位素记录，但在有机氮的形成和转移过程中有几个过程可能会使这一分析复杂化。由国家科学基金会支持的加勒比海洋时间序列站详细记录了委内瑞拉附近加勒比南部这一地区的季节性上升流、营养物质、表层生产力和出口通量之间的关系。在这项研究中，哥伦比亚大学拉蒙特·多尔蒂地球天文台的研究人员将调查表层生态的几个方面，这些方面可能会影响有机物质中的氮同位素特征，包括氮的吸收及其营养和垂直转移。除了对Cariaco盆地的化学进行了充分的研究外，这项工作还基于一项为期两年的初步研究，该研究表明，中大型浮游动物中氮(15N)的同位素特征在冬季和夏季上升流期间增加，并类似于转移到下面沉积物圈闭的物质的同位素特征。观察到的关联表明，对上升流的响应增加的产量是营养交换和出口通量中的一个关键事件。尽管这种双重作用是新产量的标志之一，但Cariaco时间序列提供了一个不同寻常的好机会来评估可能控制所观察到的季节性同位素变化的两个主要过程：1)由于浮游植物群落的变化，无机氮同化过程中的可变分馏；以及2)由于经历的营养转移数量的变化，有机质的可变同位素浓缩。在委内瑞拉和其他科学家的合作下，将对纳米和微型浮游生物大小的类别以及硝酸盐中氮的同位素值进行明确的同位素采样，以便有可能检验这些假设。该项目还将评估通过厌氧氨氧化(Anammox)途径去除结合氮的可能性。作为这项研究的次要目标，还将评估有机碳(13C)同位素丰度的变化。在更广泛的影响方面，该项目将与美国和委内瑞拉的科学家合作，并将在本学年和拉蒙特-多尔蒂大学的本科生暑期研究体验(REU)项目期间为几名本科生的培训提供支持。它还将扩大现有的加勒比时间序列网站(http://www.imars.usf.edu/CAR/index.html))，提供更好的教育途径获取数据，并指导学生在课堂上开展活动。

ABSTRACTOCE-063609The marine nitrogen cycle plays a critical role in controlling ocean productivity and the fluxes of other elements such as carbon. The ability to reconstruct how this important cycle has changed through time depends largely on nitrogen isotopic records in sediments, but there are several processes during the formation and transfer of organic nitrogen that can complicate this analysis. The NSF supported, CARIACO ocean time-series station has provided a detailed record of the relationships among seasonal upwelling, nutrients, surface productivity, and export flux in this region of the southern Caribbean off Venezuela. In this study, researchers at the Lamont Doherty Earth Observatory of Columbia University will investigate several aspects of surface layer ecology that could impact the nitrogen isotopic signature in organic material including nitrogen uptake and its trophic and vertical transfer. In addition to the well studied chemistry of the Cariaco Basin, this work is based on a preliminary, two-year study that indicated that the isotopic signature of nitrogen (15N) in the meso- and macrozooplankton increased in the winter and summer upwelling periods and were similar to the isotopic signatures of the material transferred to underlying sediment traps. The observed associations suggest that the elevated production in response to upwelling is a key event in both trophic exchange as well as in export flux. Although this dual role is one of the hallmarks of new production, the Cariaco time series provides an unusually good opportunity to evaluate two major processes that may control the observed seasonal isotopic variations; 1) Variable fractionation during inorganic nitrogen assimilation due to community changes in the phytoplankton; and 2) Variable isotopic enrichment of organic mater due to changes in the number of trophic transfers it undergoes. With the collaboration of Venezuelan and other scientists, explicit isotopic sampling of the nano- and microplankton size classes as well as of the isotopic value of nitrogen in nitrate will be done so that it will be possible to test these hypotheses. This project will also evaluate the possible removal of combined nitrogen by the anoxic ammonium oxidation (anammox) pathway. As a secondary goal of this study, the changes in the isotopic abundance of organic carbon (13C) also will be evaluated. In terms of broader impacts, the project will feature collaboration with both US and Venezuelan scientists and will provide support for the training of several undergraduate students both during the school year and during the summer research experiences for undergraduates (REU) program at Lamont-Doherty. It will also augment the existing CARIACO Time Series web site (http://www.imars.usf.edu/CAR/index.html) by providing better educational access to the data as well as directed student activities for classroom use.