Quantifying Upper Ocean Export and Remineralization of Bioactive and Particle Reactive Trace Elements along the US GEOTRACES Tahiti to Alaska Transect

国际GEOTRACES计划的目标是了解微量化学元素及其同位素在海洋中的分布。GEOTRACES的一个核心方法是协调考察，测量海洋中关键梯度的多种微量元素和同位素（TEI），如生产力、氧化还原/氧、边界清除强度和热液活动的变化。然而，痕量元素分布不足以满足GEOTRACES的目标，即“确定控制海洋中TEI分布的过程和量化通量，并确定这些变化的分布对环境条件的敏感性。“测量过程的速率需要使用其他测量方法，例如天然存在的放射性同位素。该项目涉及在太平洋从阿拉斯加到塔希提岛的美国GEOTRACES考察中测量钍元素的两种同位素。钍有一种倾向，成为与海洋中的粒子，因此它可以用来衡量其他元素的下沉出海洋表面。半衰期较短的钍同位素可用于跟踪发生在更快时间尺度（周或季节）上的过程，例如与海洋表面沃茨中的生物有关的过程，而半衰期较长的同位素可用于研究发生在更长时间尺度上的过程，如海洋环流。该项目将利用半衰期分别为24.1天和1.9年的钍-234和钍-228的测量结果来估计钍和其他元素在上层海洋的下沉速度，该提议分为四项主要活动：1）利用短寿命的钍-234来估计粒子输出和再矿化速度; 2）Th-234与主要生物相（例如，碳、氮、生物硅）和微量元素（如铁、锰等）导出其他主要和微量元素的通量剖面; 3）使用Th-228扩展时间/深度的输出和再矿化率; 4）检查#1-3中的变化如何与颗粒浓度和类型、浮游生物群落变化、净群落生产、气溶胶输入/尘埃、物理强迫（例如，上升流）、热液羽流和沿该GEOTRACES样带的其他沿着特征的预期梯度相关。总Th-234：U-238的剖面提供了Th-234通量与深度的定量估计，其沿着与Th-234和TEI的平行粒度分级颗粒剖面允许我们凭经验计算任何TEI的净颗粒输出的垂直剖面。通过高分辨率的垂直和空间采样，Th-234的应用可以扩展到海洋表层净清除区以下，包括对上层几百米的颗粒输出和再矿化的量化。根据先前的研究，Th-234通量模型需要根据物理传输进行调整，特别是赤道上涌。选定的Th-228剖面可以进一步扩展这些通量和矿化估计值，使其具有更深的深度和更长的时间尺度。该项目将支持一个博士学位。学生.

The goal of the international GEOTRACES program is to understand the distributions of trace chemical elements and their isotopes in the oceans. A core approach of GEOTRACES involves coordinated expeditions that measure multiple trace elements and isotopes (TEIs) across key gradients in the ocean, such as changes in productivity, redox/oxygen, boundary scavenging intensity, and hydrothermal activity. However, trace elemental distributions are not enough to meet the GEOTRACES goal to "identify processes and quantify fluxes that control the distribution of TEIs in the ocean and to establish the sensitivity of these changing distributions to environmental conditions." To measure rates of processes requires the use of other measurements, such as natural ly occurring radioactive isotopes. This project involves the measurement of two isotopes of the element thorium on a U.S. GEOTRACES expedition in the Pacific Ocean, from Alaska to Tahiti. Thorium has a tendency to become associated with particles in the oceans, and therefore it can be used to measure the sinking of other elements out of the ocean surface. Thorium isotopes with short half-lives can be used to track processes that occur on faster timescales (weeks or seasons), such as those related to biology in the surface waters of the ocean, while isotopes with longer half-lives are useful for studying processes that occur on longer time scales like that of ocean circulation. This project would use measurements of thorium-234 and thorium-228, with half-lives of 24.1 days and 1.9 years, respectively, to estimate sinking rates of thorium and other elements in the upper ocean.The proposal is broken down into four main activities: 1) Use short-lived Th-234 to estimate particle export and remineralization rates; 2) use ratios of Th-234 to major biogenic phases (e.g., carbon, nitrogen, biogenic silica) and trace elements (e.g. iron, manganese, etc.) to derive flux profiles of other major and trace elements; 3) extend the export and remineralization rates in time/depth using Th-228; 4) examine how changes in #1-3 are related to expected gradients in particle concentration and type, plankton community shifts, net community production, aerosol inputs/dust, physical forcing (e.g. upwelling), hydrothermal plumes, and other features along this GEOTRACES transect. Profiles of total Th-234:U-238 provide quantitative estimates of Th-234 flux vs. depth, which along with parallel size-fractionated particulate profiles of Th-234 and TEIs allow us to empirically calculate the vertical profile of net particle export for any TEI. With high resolution vertical and spatial sampling, the application of Th-234 can be extended below the zone of net removal in the surface ocean to include quantification of particle export and remineralization in the upper several hundred meters. Based upon prior studies, the Th-234 flux model will need to be adjusted for physical transport, especially upwelling across the equator. Selected Th-228 profiles can extend these flux and remineralization estimates even further with depth and over longer time scales. The project will support a Ph.D. student.