Cr Isotope Oceanography of the Eastern Tropical North Pacific Ocean

表层海水与大气平衡，含氧量高，支持着其中的海洋生物。当海水在冬季冷却时，密度会变得更大，并沉入海洋深处，微生物以从上层海洋落下的下沉的有机物为食，进行呼吸，从而开始失去氧气。在一些地区，如热带北太平洋东部，这一过程导致氧气浓度过低，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，这些地区的面积正在扩大，如果这种趋势继续下去，后果令人担忧。研究古代沉积物的地质学家推测，在过去的某些时期，海洋的缺氧部分要大得多，所以我们知道这种可能性是存在的。我们正在研究一种新的缺氧区示踪剂，海水的铬同位素比率。铬（Cr）是一种遍布海洋的微量元素；在缺氧区，它从高度可溶的6+氧化态（六价铬，一种高浓度的有毒污染物）还原为相对不溶的3+态（三价铬），并附着在下沉的颗粒上从海洋中移除。这个过程有利于较轻的Cr同位素，因此与24质子和29中子的Cr相比，具有24质子和28中子的Cr优先转化。这一转变可用于研究现代海洋和地质古海洋缺氧带的范围和活动。在这些环境中，关于Cr同位素的数据很少，该项目将通过在北太平洋东部热带地区的实地取样和使用等离子体质谱法的实验室测量获得一些第一批数据。该研究有助于加深对沉积地质和地球化学的认识。麻省海洋学院的一名女博士生将参与该项目，这将构成她的大部分论文研究。铬（Cr）在水生环境中以Cr（III）和Cr（VI）两种氧化还原状态存在。Cr（VI）是热力学稳定的，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）具有颗粒反应性，并被清除到下沉的颗粒物质上，而Cr（VI）相对不具有反应性。这两种物质都可以被海洋生物吸收，但硅藻吸收Cr（III）的速度要快20倍。Cr（III）具有生物化学功能（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。当氧气小于2 μ m时，Cr（III）成为热力学稳定的物质，但缓慢的反应动力学使微生物活性或环境还原剂如Fe（II）主导了转化。理论和实验室实验表明，当部分转化发生时，这种氧化还原转化应伴随着稳定的同位素比值变化，Cr（III）富集于较轻的同位素中。在四种Cr稳定同位素中，这个过程是用最丰富的天然同位素53Cr和52Cr的比值来观察的，而用富集的50Cr和54Cr同位素的双尖峰来校正实验室和仪器的质量分馏。这种对环境氧的敏感性促使地质学家使用铬同位素比率作为地球上过去氧合条件的指标。但是，关于海洋水生系统中铬同位素组成的数据很少，无法根据地质推断进行实地验证。我们将通过测量北太平洋东部热带缺氧带样品中Cr氧化还原物质和颗粒物质的Cr浓度水平和垂直分布以及Cr同位素比率来阐明海洋环境的Cr同位素地球化学。Cr同位素比值对含氧海洋中的生物吸收和再生以及缺氧区的氧化还原过程（以及后者随后向邻近的含氧区外混）非常敏感。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的关系进行研究。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。

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.