Collaborative Research: US GEOTRACES PMT: Pb and Cr isotopes

海洋中的大部分铅是由人类活动排放到那里的。这些活动包括冶炼、燃煤和焚烧等高温工业过程，以及20世纪中叶的含铅汽油消费。来自这些来源的铅以细小颗粒的形式通过大气在世界各地移动-S风并最终沉积在海洋表面，在那里它溶解。幸运的是，根据铅的同位素组成(同位素是同一元素的原子，但原子核中的中子数目不同)，可以确定这些铅源的不同来源(例如，美国、欧洲和亚洲的输入)。铅被证明具有不同的同位素比率，因为铅是从不同地质时期的矿床中提取出来的。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了北太平洋深处。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海底的海水样本，并分析这些样本的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，这些变化是在东热带太平洋极低氧区选择性地将较轻的铬同位素从海洋中移走时产生的。这一工具可以帮助理解海洋中氧气减少的持续演变，以及根据沉积物等地质档案建立的海洋中氧气过去的变化。1981年，Schaule和Patterson首次对海洋中的人为铅进行了测量，得到了北太平洋的剖面图，此后几个实验室陆续测量了其他剖面，表明太平洋中的铅对铅通量的区域变化做出了响应-从逐步淘汰含铅汽油(主要在日本、加拿大、墨西哥和美国)到增加燃煤(主要在中国)。从1981年到现在，夏威夷附近的铅浓度下降了两倍，206Pb/207Pb同位素组成从~1.2(主要是美国的铅)降到了1.165(主要是中国的煤铅)。美国的GeoTraces太平洋子午线断面巡航将给我们一个前所未有的机会，让我们详细了解人为铅进入太平洋深处的情况，这可以用来确定溶解和颗粒铅进入深海的路径。该项目还将确定铬的稳定同位素组成，铬是一种元素，其氧化还原状态由海洋中的氧气浓度决定。在[O2]2微克分子/公斤的缺氧区(ODZ)中，铬从铬酸盐CrO4-2(六价铬)还原为Cr3离子(三价铬)。同位素较轻的铬原子优先被还原，还原的Cr3离子是粒子反应的(附着在下沉的粒子上)，并从水柱中移走。这使得残留的铬同位素更重，所以我们可以通过测量~(53)Cr/~(52)Cr来检测这个过程。这一过程与氮素还原(反硝化)同时发生，可用于追踪ODZ水域混入含氧海洋的后果。

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.