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

国际地学研究计划的目标是了解海洋中痕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，海水中的极低浓度被认为限制了整个海洋的生物生产力(肥力)。这一限制反过来又部分限制了海洋生态系统能够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营养素的重要性已得到充分证实，但其海洋分布的许多基本特征仍不为人所知。对其他元素和同位素的测量可以用来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自然产生的放射性同位素是钍-230和钚-231，它们已被指定为作为地球物理轨道系统一部分进行测量的关键参数。该项目将侧重于这两种同位素的测量，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调查员、一名博士后研究人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横断面收集海水和悬浮颗粒物的样本，以检查供应和去除微量元素的过程。这些样品将被分析自然产生的放射性核素钍-232、钍-230和钚-231。还将分析沿横断面收集的气溶胶样本和沉积物。与其他参与考察的研究人员合作，该项目将：1)解释Th-232和Th-230的分布，以量化灰尘提供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)确定主要颗粒物相和颗粒微量元素在整个水柱中的下沉通量，以量化它们从海洋中的去除速率；3)比较阿拉斯加至塔希提样带将遇到的不同环境条件下的痕量元素去除速度，以确定关键的物理、化学和生物因素，如沙尘供应和生物生产力，来调节海洋中痕量元素的去除速度。

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.