Measurement of Helium Isotopes on the U.S. GEOTRACES Alaska-Tahiti Section (GP15)

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。这项工作的一个重要部分是确定海洋中微量元素的来源。 其中一个重要来源来自海底热液喷发。该项目将支持2018年美国在太平洋进行的GEOTRACES考察中的氦同位素测量，从阿拉斯加向南延伸到塔希提岛。氦同位素提供了关于热液羽流存在和分散的重要信息。在更大的范围内，它们还可用于计算全球尺度的水热来源微量元素和同位素的输入。因此，拟议的工作与与这次考察有关的许多其他项目密切相关。 氦同位素的测量还将使调查上层海洋内的物理混合率成为可能，以帮助解释其他微量元素的命运。在本节的北方端，还将采用这一方法调查海底热液喷口的其他微量元素和同位素输送到生物生产力更高的表层海洋的速度。除了支持GEOTRACES的工作和参与与探险相关的社区规模的外展工作外，调查人员还将与视觉艺术家合作，以新的方式向公众传播他们的科学。美国GEOTRACES太平洋子午线样带（56°N至20°S，沿着152°W）将拦截：强边缘通量，亚北极高营养、低叶绿素沃茨，世界海洋中最古老的深水沃茨，多个热液羽流的远端，氧气最小区，在南纬20°附近的南太平洋，有一些已知的最贫营养的沃茨。这一部分与2013年进行的美国GEOTRACES东太平洋带状样带（EPZT）一起，也将封闭太平洋的大部分区域。因此，这项工作将有助于对区域尺度的水热来源微量元素和同位素流入海洋的情况进行新的、更好的评估。由于2018年的考察将拦截多个热液羽流，该项目还将允许调查美国和国际GEOTRACES计划中出现的三个关键问题：1）是否有一个重要的热液来源的Fe和其他TEI通过上升流进入北太平洋的真光层，正如2013年EPZT结果所证明的那样？ 2)热液羽流中的TEI：He-3比值如何随其喷口源的不同地质背景而变化？ 3)热液羽流的TEI：He-3比率如何随水柱化学性质的变化而沿着温盐输送带变化？ 在整个部分中，研究人员还将结合联合收割机上层水柱氦-3测量与水柱氚浓度（从过去探险收集的数据推断），以估计水柱通风时间尺度，该尺度可用于量化要拦截的广泛地球化学体系（热带、亚热带、亚极地）中TEI转化率。 在以上升流为特征的地区，氦同位素的精确测量可以与典型的气体交换率估计相结合，以确定一些关键TEIs的上升流率。该奖项反映了NSF的法定使命，并被认为值得通过使用基金会的知识价值和更广泛的影响审查标准进行评估来支持。

The goal of the international GEOTRACES program is to understand the distributions of trace chemical elements and their isotopes in the oceans. An essential part of this effort is determining the sources of trace elements to the oceans. One important such source comes from seafloor hydrothermal venting. This project will support the measurement of helium isotopes on the 2018 U.S. GEOTRACES expedition in the Pacific Ocean, running south from Alaska to Tahiti. Helium isotopes provide important information on the presence and dispersion of hydrothermal plumes. On a larger scale, they can also be used to calculate global-scale inputs of hydrothermally-sourced trace elements and isotopes. Thus, the proposed work is closely tied to many other projects associated with this expedition. The measurement of helium isotopes will also allow enable the investigation of physical mixing rates within the upper ocean to help interpret the fate of other trace elements. At the northern end of the section, this approach will also be used to investigate the rate at which other trace elements and isotopes from seafloor hydrothermal venting are delivered to the more biologically productive surface ocean. In addition to supporting the GEOTRACES effort and participating in community-scale outreach efforts associated with the expedition, the investigators will also work with a visual artist to communicate their science to the public in new ways.The U.S. GEOTRACES Pacific Meridional Transect (56°N to 20°S, along 152°W) in late 2018 will intercept: strong margin fluxes, sub-Arctic high-nutrient, low-chlorophyll waters, the oldest deep waters in the world's oceans, the distal ends of multiple hydrothermal plumes, oxygen minimum zones, subpolar and equatorial upwelling, and, in the South Pacific near 20°S, some of the most oligotrophic waters known. This section, together with the US GEOTRACES East Pacific Zonal Transect (EPZT) conducted in 2013, will also close off a large volume of the Pacific Ocean. Consequently, this work will facilitate new, improved evaluations of regional-scale fluxes of hydrothermally-sourced trace elements and isotopes (TEIs) to the oceans. Because the 2018 expedition will intercept multiple hydrothermal plumes, this project will also allow the investigation of three key questions emerging from the larger U.S. and international GEOTRACES programs: 1) Is there a significant source of hydrothermally-sourced Fe and other TEIs to the euphotic zone in the North Pacific, through upwelling, as was demonstrated for the Southern Ocean from the 2013 EPZT results? 2) How do TEI:He-3 ratios in hydrothermal plumes vary with different geologic settings of their vent-sources? 3) How do the TEI:He-3 ratios imparted to hydrothermal plumes vary along the thermohaline conveyor as a function of varying water column chemistry? Throughout the section the investigators will also combine upper-water column helium-3 measurements with water column tritium concentrations (extrapolated from data collected on past expeditions) to estimate water column ventilation time scales that can be used to quantify rates of TEI transformation across the wide range of biogeochemical regimes to be intercepted (tropical, subtropical, subpolar). In regions characterized by upwelling, precise measurements of helium isotopes can be combined with canonical gas-exchange rate estimates to make flux-gauge determinations of upwelling rates for some key TEIs.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.