US GEOTRACES GP17-OCE: Measurements of the trace gases chlorofluorocarbons and sulfur hexafluoride

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（TEIs）的分布，TEIs是同一元素的不同形式。许多tei是浮游植物生长所必需的（如铁），是微量营养素。然而，其他tei是污染物（例如铅）。美国GEOTRACES GP17-OCE考察将在南太平洋和南大洋的太平洋部分进行。了解TEIs如何变化（例如清除和再生）以及估计生化过程速率的关键是确定新被大气气体通风的水团。这些水在过去的几十年里一直与大气接触。该项目的主要目标是测量三种人造海洋通风示踪剂的浓度，这些示踪剂可用于确定海洋表面的水团与大气接触已有多长时间。示踪剂在跟踪水团沿路径老化和估计有效扩散和稀释率方面特别有用。打个比方，在表层海洋中加入一种有色染料，每年增加一点，然后在表层水进入海洋内部时测量这种染料的浓度。将测量的三种示踪剂是氟氯化碳-11 （CFC-11）、氟氯化碳-12 （CFC-12）和六氟化硫（SF6）。收集的微量气体浓度、通风年龄和速率数据将在六个月内提供给其他研究人员，为更广泛的GEOTRACES计划奠定重要基础。项目组将乘坐GP17-OCE考察队在海上测量CFC-11、CFC-12和SF6浓度。除了进行这些测量之外，具体目标是：1)计算示踪剂年龄，并确定tei所在的温跃层，中间和底部水域的相对通风和稀释程度-从末端成员源到下游位置的水年龄，并将GEOTRACES数据置于由于自然和人为气候变化而变化的通风的历史背景中。年龄数据将可用于pi评估tei在下游运输时如何影响清除和再生。2)使用示踪剂年龄，计算生物化学重要过程的速率——例如，表观氧利用率，并有助于区分不同循环制度中物理和生物化学过程的贡献。将根据SF6与CFCs的比率计算示踪剂年龄，对于最古老的水，则计算CFCs彼此的比率。在小于35年的时间尺度上，最可靠的海洋过程示踪剂组合是使用SF6/CFC比率获得的。该奖项反映了美国国家科学基金会的法定使命，并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，被认为值得支持。

The goal of the international GEOTRACES program is to understand the distributions in the ocean of trace chemical elements and their isotopes (TEIs), which are different forms of the same element. Many of the TEIs are essential for phytoplankton growth (e.g., iron) and are micronutrients. Yet, other TEIs are pollutants (e.g., lead). The U.S. GEOTRACES GP17-OCE expedition will occur in the South Pacific and Pacific sector of the Southern Ocean. Essential to understanding how TEIs change, e.g., scavenging and regeneration, and for estimating rates of biochemical processes is to identify water masses that are newly ventilated with atmospheric gases. These are waters that have been in contact with the atmosphere in the past few decades. The main objective of this project will be to measure the concentration of three man-made tracers of ocean ventilation that can be used to determine how long since a water mass was at the ocean surface in contact with the atmosphere. The tracers are particularly useful for following water masses along pathways as they age and estimating rates of effective spreading and dilution. An analogy would be adding a colored dye in the surface ocean, with a bit more dye added each year, and then measuring the concentration of this dye as surface water moves into the interior ocean. The three tracers that will be measured are Chlorofluorocarbon-11 (CFC-11), Chlorofluorocarbon-12 (CFC-12) and Sulfur Hexafluoride (SF6). The trace gas concentrations, ventilation ages, and rate data will all be made available to other investigators within six months of collection, forming an important foundation for the broader GEOTRACES program. The project team will measure CFC-11, CFC-12, and SF6 concentrations at sea aboard the GP17-OCE expedition. In addition to conducting these measurements, the specific objectives are: 1) Calculate tracer ages and identify the relative extent of ventilation and dilution of thermocline, intermediate and bottom waters within which TEIs reside - from the end member sources to downstream locations as waters age, and put the GEOTRACES data in the historical context of changing ventilation due to natural and anthropogenic climate change. The age data will be available for PIs to evaluate how scavenging and regeneration are affected as TEIs are transported downstream. 2) Using tracer ages, calculate rates of biochemically important processes - e.g., apparent oxygen utilization rates, and help to distinguish between contributions from physical and biochemical processes across the different circulation regimes. Tracer ages will be calculated, from the ratio of SF6 to CFCs, and for oldest waters the ratio of CFCs to each other. The most robust tracer combination for ocean processes on time scales of less than 35 years is obtained using the SF6/CFC ratios.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.