Cr Isotope Oceanography of the Eastern Tropical North Pacific Ocean

东部热带北太平洋 Cr 同位素海洋学

• 批准号: 1924050
• 负责人: Edward Boyle
• 金额: $ 43.62万
• 依托单位: Massachusetts Institute of Technology
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2019
• 资助国家: 美国
• 起止时间: 2019-08-01 至 2023-01-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1924050&HistoricalAwards=false
• 关键词: Cr; Isotope; Oceanography; Eastern; Tropical

Surface seawater equilibrates with the atmosphere and has high oxygen levels, supporting the sea life within it. When this water is cooled in the winter, it becomes denser and sinks into the depths of the ocean, and begins to lose its oxygen as microbes respire while feeding on sinking organic matter falling from the upper ocean. In some regions, such as the eastern tropical North Pacific Ocean, this process leads to such low oxygen concentrations that oxygen-respiring organisms such as fish cannot survive. There is evidence that these regions are growing larger in recent decades raising concerns over the consequences should these trends continue. Geologists looking at ancient sediments have surmised that some periods in the past have had much larger oxygen-depleted portions of the ocean, so we know the possibility exists. We are working on a new tracer of these oxygen deficient zones, the chromium isotope ratio of seawater. Chromium (Cr) is a trace element found throughout the ocean; in oxygen depleted zones it is reduced from the highly soluble 6+ oxidation state (hexavalent Cr, a toxic pollutant at high concentrations) to the relatively insoluble 3+ state (trivalent Cr) which is removed from the ocean attached to sinking particles. This process favors lighter Cr isotopes hence Cr with 24 protons and 28 neutrons is preferentially transformed compared to Cr with 24 protons and 29 neutrons. This transformation can be used to study the extent and activity of oxygen deficient zones in the modern ocean and in geological ancient oceans. There is very little data for Cr isotopes in these environments, and this project will obtain some of the first data with fieldwork sample collection in the eastern tropical North Pacific Ocean and laboratory measurements using plasma mass spectrometry. This research can contribute to sedimentary geological and geochemical understanding. A female Ph.D. student at Massachusetts Institute of Oceanography will participate in the project that will comprise most of her thesis research. Chromium (Cr) in the aquatic environment exists in two redox states, Cr(III) and Cr(VI). Cr(VI) is thermodynamically stable and the dominant form observed in the oxic ocean, although a few percent of unstable Cr(III) is also observed. Cr(III) is particle-reactive and is scavenged onto sinking particulate matter, whereas Cr(VI) is relatively unreactive. Both species can be taken up by marine biota, although diatoms take up Cr(III) ~20 times faster. Cr(III) has biochemical functionality (e.g. lipid and protein metabolism), whereas Cr(VI) is a toxin that damages cellular biochemistry. When oxygen is less than about 2 uM, Cr(III) becomes the thermodynamically stable species, but slow reaction kinetics lets microbial activity or environmental reductants such as Fe(II) dominate the conversion. Theory and lab experiments indicate that this redox transformation should be accompanied by a stable isotope ratio shift when partial conversion occurs, with Cr(III) enriched in the lighter isotopes. Of the four Cr stable isotopes, this process is observed using the ratio of the most abundant natural isotopes 53Cr and 52Cr while using an enriched double spike of the scarce 50Cr and 54Cr isotopes to correct for laboratory and instrument mass fractionation. This sensitivity to environmental oxygen has inspired geologists to use Cr isotope ratios as an indicator of past oxygenation conditions on earth. But there is little data on the Cr isotope composition in marine aquatic systems with which to ground-truth the geological inferences. We will illuminate the Cr isotope geochemistry of the marine environment with measurements on the horizontal and vertical distribution of Cr concentrations and Cr isotope ratios for the Cr redox species and particulate matter for samples from the eastern tropical North Pacific oxygen deficient zone. Cr isotope ratios are sensitive to biological uptake and regeneration in the oxic ocean and redox processes in oxygen deficient zones (and the latter's consequent outmixing into adjacent oxic zones). This data will be examined in relationship to standard hydrographic properties and nutrients and to the oceanic redox systems of nitrogen, iron, and manganese.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.

表层海水与大气平衡，含氧量高，支持海洋生物在其中生存。当这些水在冬季冷却时，密度变大，沉入海洋深处，并开始失去氧气，因为微生物在以海洋上层下沉的有机物为食的同时进行呼吸。在某些地区，如热带北太平洋东部，这一过程导致氧气浓度如此之低，以至于鱼类等呼吸氧气的生物无法生存。有证据表明，近几十年来，这些区域的面积越来越大，这引起了人们对这些趋势继续下去的后果的关切。地质学家通过研究古代沉积物推测，在过去的某些时期，海洋中有更大的缺氧区域，因此我们知道这种可能性是存在的。我们正在研究这些缺氧区的一种新的示踪剂，即海水的铬同位素比。铬（Cr）是一种在整个海洋中发现的微量元素;在缺氧区，它从高度可溶的6+氧化态（六价铬，高浓度的有毒污染物）还原为相对不溶性的3+状态（三价铬），它附着在下沉颗粒上从海洋中去除。该过程有利于较轻的Cr同位素，因此与具有24个质子和29个中子的Cr相比，具有24个质子和28个中子的Cr优先转化。 这种转换可以用来研究现代海洋和地质古海洋中缺氧带的范围和活动性。这些环境中的铬同位素数据很少，本项目将通过在热带北太平洋东部进行实地采样和使用等离子体质谱法进行实验室测量来获得首批数据。该研究有助于沉积地质和地球化学的认识。 女博士马萨诸塞州海洋学研究所的一名学生将参与该项目，该项目将包括她的大部分论文研究。铬（Cr）在水环境中以两种氧化还原状态存在，Cr（III）和Cr（VI）。Cr（VI）是化学稳定的，是在含氧海洋中观察到的主要形式，尽管也观察到少数不稳定的Cr（III）。Cr（III）是颗粒反应性的，并被清除到沉降的颗粒物质上，而Cr（VI）相对不反应。这两种物种都可以被海洋生物所吸收，尽管硅藻吸收Cr（III）的速度要快20倍。Cr（III）具有生物化学功能（例如脂质和蛋白质代谢），而Cr（VI）是一种破坏细胞生物化学的毒素。当氧小于约2 μ M时，Cr（III）变成化学稳定的物质，但缓慢的反应动力学使微生物活性或环境还原剂如Fe（II）主导转化。理论和实验室实验表明，这种氧化还原转化应该伴随着稳定的同位素比移动时，发生部分转换，与铬（III）富集在较轻的同位素。在四种Cr稳定同位素中，使用最丰富的天然同位素53 Cr和52 Cr的比率观察到该过程，同时使用稀有的50 Cr和54 Cr同位素的富集双尖峰来校正实验室和仪器质量分馏。这种对环境氧的敏感性激发了地质学家使用铬同位素比率作为地球上过去氧化条件的指标。 但是，很少有数据的铬同位素组成的海洋水生系统，以地面真理的地质推断。我们将阐明海洋环境的铬同位素地球化学与测量的水平和垂直分布的铬浓度和铬同位素比值的氧化还原物种和颗粒物的样品从热带北太平洋缺氧区东部。 铬同位素比值是敏感的生物吸收和再生的氧海洋和氧化还原过程中的缺氧区（和后者随之而来的outmixing到相邻的氧区）。这些数据将与标准水文特性和营养物质以及氮、铁和锰的海洋氧化还原系统进行相关研究。该奖项反映了NSF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。

项目成果

Investigating the cycling of chromium in the oxygen deficient waters of the Eastern Tropical North Pacific Ocean and the Santa Barbara Basin using stable isotopes

• DOI: 10.1016/j.marchem.2020.103756
• 发表时间: 2020-04
• 期刊: Marine Chemistry
• 影响因子: 3
• 作者: S. B. Moos;E. Boyle;M. Altabet;A. Bourbonnais

Trivalent chromium isotopes in the eastern tropical North Pacific oxygen-deficient zone

• DOI: 10.1073/pnas.1918605118
• 发表时间: 2021-02
• 期刊: Proceedings of the National Academy of Sciences
• 作者: Tianyi Huang;S. B. Moos;E. Boyle