Collaborative Research: US GEOTRACES PMT: Pb and Cr isotopes

合作研究：US GEOTRACES PMT：Pb 和 Cr 同位素

• 批准号: 1737152
• 负责人: Robert Rember
• 金额: $ 30.18万
• 依托单位: University of Alaska Fairbanks Campus
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2017
• 资助国家: 美国
• 起止时间: 2017-08-01 至 2022-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1737152&HistoricalAwards=false
• 关键词: Collaborative; Research; US; GEOTRACES; PMT

Most of the lead (Pb) in the ocean has been put there by human activities. These activities include high temperature industrial processes such as smelting, coal combustion, and incineration, as well as leaded gasoline consumption during the middle portion of the 20th century. Lead from these sources moves as fine particles around the world by the atmosphere?s winds and eventually deposits on the surface ocean where it dissolves. Fortunately, it is possible to determine the different origins of these Pb sources (e.g. U.S., European, and Asian inputs) from its isotopic composition (isotopes are atoms of the same element with different numbers of neutrons in the nucleus). Lead has been shown to have different isotope ratios because the Pb has been extracted from mineral deposits from different geological periods. This project aims to determine how much of this Pb has moved into the deep North Pacific by ocean circulation or by attaching to sinking particles. This will be done by collecting seawater samples from the surface to the bottom of the ocean at about 30 stations between Tahiti and Alaska and analyzing these for their Pb concentration and Pb isotope ratios. This project will also determine variations in the chromium (Cr) isotope composition of Pacific seawater that are created when the lighter isotopes of Cr are selectively removed from the ocean in extremely low oxygen zones in the eastern tropical Pacific Ocean. This tool can help understand the ongoing evolution of decreasing oxygen in the ocean and past changes in the oxygen in the ocean established from geological archives such as sediments. The first measurements of anthropogenic Pb in the ocean resulted in a profile in the North Pacific Ocean by Schaule and Patterson in 1981, and since then several labs have sparsely and erratically measured other profiles showing that the Pb in the Pacific Ocean is responding to regional changes in Pb fluxes - from the phasing out of leaded gasoline (mainly in Japan, Canada, Mexico, and the United States) to increasing amounts of coal combustion (mainly in China). The concentration of Pb decreased near Hawaii by a factor of two between 1981 and the present, and the 206Pb/207Pb isotopic composition of Pb decreased from ~1.20 (mainly U.S. Pb) to 1.165 (mainly Chinese coal Pb). The U.S. GEOTRACES Pacific Meridional Transect cruise will give us an unprecedented opportunity to obtain a detailed view of the penetration of anthropogenic Pb into the deep Pacific Ocean which can be used to determine the pathways that dissolved and particulate Pb take in arriving in the deep sea. This project will also determine the stable isotope composition of Cr, an element whose redox state is determined by the oxygen (O2) concentration in the ocean. In oxygen deficient zones (ODZs) where [O2] 2 µmol/kg, Cr is reduced from chromate CrO4-2 (hexavalent chromium) to Cr3+ ion (trivalent chromium). The isotopically lighter atoms of Cr are preferentially reduced, and the reduced Cr3+ ion is "particle-reactive" (attaches to sinking particles) and removed from the water column. That leaves the residual Cr isotopically heavier, so we can detect this process by measurements of 53Cr/52Cr. This process occurs in conjunction with nitrogen reduction (denitrification) and can be used to trace the consequences as ODZ waters mix out into the oxic ocean.

海洋中的大部分铅（Pb）是人类活动造成的。这些活动包括高温工业过程，如冶炼、煤燃烧和焚烧，以及世纪中期的含铅汽油消费。 来自这些来源的铅通过大气层以细颗粒的形式在世界各地移动？最后沉积在海洋表面，在那里溶解。幸运的是，可以确定这些Pb源的不同来源（例如，美国，欧洲和亚洲的输入）从它的同位素组成（同位素是同一元素的原子，原子核中的中子数不同）。 由于铅是从不同地质时期的矿床中提取出来的，因此铅具有不同的同位素比。 该项目旨在确定有多少铅通过海洋环流或附着在下沉颗粒上进入北太平洋深处。这将通过在塔希提岛和阿拉斯加之间的大约30个站点收集从海洋表面到底部的海水样本并分析其铅浓度和铅同位素比率来完成。该项目还将确定太平洋海水中铬（Cr）同位素组成的变化，这些变化是在东太平洋热带极低氧区有选择地从海洋中去除较轻的铬同位素时产生的。该工具可以帮助了解海洋中氧气减少的持续演变以及从沉积物等地质档案中建立的海洋中氧气的过去变化。1981年，Schaule和Patterson首次测量了海洋中的人为铅，并在北太平洋绘制了一个剖面图，此后，几个实验室对其他剖面图进行了稀疏和不规则的测量，显示太平洋中的铅对铅通量的区域变化作出了反应--铅汽油的逐步淘汰（主要在日本、加拿大、墨西哥和美国）到增加燃煤量（主要在中国）。从1981年到现在，夏威夷附近的Pb浓度下降了2倍，Pb的206 Pb/207 Pb同位素组成从~1.20（主要是美国Pb）下降到1.165（主要是中国煤Pb）。美国GEOTRACES太平洋经向样带巡航将为我们提供一个前所未有的机会，以获得人为Pb渗透到太平洋深处的详细情况，可用于确定溶解和颗粒Pb到达深海的途径。该项目还将确定铬的稳定同位素组成，这种元素的氧化还原状态取决于海洋中的氧（O2）浓度。在[O2] 2 µ mol/kg的缺氧区（ODZ）中，Cr从铬酸盐CrO4 - 2（六价铬）还原为Cr3+离子（三价铬）。Cr的同位素较轻的原子优先被还原，还原的Cr3+离子是“颗粒反应性的”（附着在下沉的颗粒上），并从水柱中去除。这使得残留的Cr同位素更重，因此我们可以通过测量53Cr/52Cr来检测这个过程。这一过程与氮还原（反硝化）一起发生，可用于追踪ODZ沃茨混入含氧海洋的后果。