Collaborative Research: U.S. GEOTRACES Pacific Meridional Transect: Thorium-232, Thorium-231 and Protactinium-231 as tracers of trace element supply and removal

国际GEOTRACES项目的目标是了解海洋中微量化学元素及其同位素的分布。许多微量元素对生命至关重要，它们在海水中的极低浓度被认为限制了大部分海洋的生物生产力（生育力）。这一限制反过来又在一定程度上限制了海洋生态系统所能支持的渔业水平，以及海洋从大气中吸收二氧化碳的能力。虽然这些微量元素微量营养素的重要性已得到公认，但它们在海洋分布的许多基本特征仍不为人所知。其他元素和同位素的测量可用于了解影响微量营养元素分布的过程。在这方面特别重要的两种天然存在的放射性同位素是钍-230和镤-231，它们已被指定为GEOTRACES测量的关键参数。该项目将侧重于测量这两种同位素，以便提供有关向太平洋供应铁的过程以及这些过程的速率的关键信息。该项目将支持一名早期职业研究者、一名博士后研究员以及本科生和研究生。将沿着阿拉斯加和塔希提岛之间的南北样带收集海水和悬浮颗粒物质的样本，以检查提供和去除微量元素的过程。将对这些样品进行天然放射性核素钍-232、钍-230和镤-231的分析。沿样带收集的气溶胶样本和沉积物也将被分析。该项目与其他参与考察的研究人员合作，将：1)解释钍-232和钍-230的分布，以量化由灰尘输送的铁和其他微量元素的供应，以及阿拉斯加边缘火山沉积物中化学反应提供的微量元素；2)确定主要颗粒相和颗粒微量元素在整个水柱中的下沉通量，以量化其从海洋中去除的速度；3)比较阿拉斯加至塔希提横断面不同环境下的微量元素去除速度，以确定调节海洋中微量元素去除速度的关键物理、化学和生物因素，如粉尘供应和生物生产力。

The goal of the international GEOTRACES program is to understand the distributions of trace chemical elements and their isotopes in the oceans. Many trace elements are essential for life and their extremely low concentrations in seawater are thought to limit biological productivity (fertility) throughout much of the ocean. This limitation, in turn, partially constrains the level of fisheries that can be supported by marine ecosystems as well as the ocean's capacity to absorb carbon dioxide from the atmosphere. Whereas the importance of these trace-element micronutrients is well established, many basic features of their ocean distribution remain unknown. Measurements of other elements and isotopes can be used to understand the processes that influence the distributions of the micronutrient elements. Two naturally-occurring radioactive isotopes that are particularly important in this regard are thorium-230 and protactinium-231, which have been designated as key parameters to be measured as part of GEOTRACES. This project will focus on the measurement of these two isotopes in order to provide critical information about the processes that supply iron to the Pacific Ocean, as well as the rates of those processes. The project will support an early career investigator, a postdoctoral researcher, and students at the undergraduate and graduate levels.Samples of seawater and of suspended particulate material will be collected along a north-south transect between Alaska and Tahiti to examine the processes that supply and remove trace elements. These samples will be analyzed for naturally-occurring radionuclides thorium-232, thorium-230 and protactinium-231. Aerosol samples and sediments collected along the transect will be analyzed as well. In collaboration with other investigators involved in the expedition, this project will:1) Interpret the distributions of thorium-232 and thorium-230 to quantify the supply of iron and other trace elements delivered by dust as well as the trace elements supplied by chemical reactions in volcanic sediments along the Alaskan margin, 2) Determine the sinking flux of major particulate phases and of particulate trace elements throughout the water column to quantify their rate of removal from the ocean, and 3) Compare the rate of trace element removal among contrasting environmental regimes to be encountered along the Alaska to Tahiti transect to identify the key physical, chemical and biological factors, such as dust supply and biological productivity, that regulate the rate of trace element removal from the ocean.