US GEOTRACES Pacific Meridional Transect (GP-15): Resolving Silicon Isotope Anomalies in the Northeast Pacific

该项目将研究元素硅在太平洋中的同位素分布，以深入了解控制硅在全球海洋中移动的过程。这类研究的动机是，溶解在海水中的硅支持硅藻的生长。硅藻是一种微小的藻类，它们使用硅来产生造型华丽的贝壳，称为锥体。硅藻的独特之处在于，它们是海洋光合作用微生物中唯一需要硅才能生长的主要群体。硅藻占地球上总光合作用的20%，所以我们每呼吸五分之一的氧气就得感谢一个硅藻。它们的巨大贡献使得了解是什么控制了它们的分布，对海洋的生态和化学以及社会更加重要。它们对硅的需求意味着，海水中溶解的硅量可以控制硅藻生长的位置和数量。当洋流将富含这些营养物质的深水带到表层海洋时，硅藻就会获得硅和其他营养肥料。该项目将研究太平洋中溶解硅的稳定同位素组成如何变化。为什么要纠结于同位素呢？事实证明，硅藻在建造其锥体时，优先使用较轻的硅同位素。这在硅藻锥体和海洋中溶解的硅中产生了信号，使得同位素可以用来重建过去的硅藻生产力。不同的大洋盆地，深海水中溶解硅的同位素组成不同。深水中硅同位素的这些差异似乎是系统性的，并与深海中的洋流运动有关。一旦了解了这些模式，基于同位素的硅藻生产力评估将会有很大改进。测试硅的同位素与构成全球海洋深层循环的水团之间的关系是这项研究的主要目标。这项研究将作为2018年从阿拉斯加到塔希提岛的地球轨道探险队的一部分进行。这次探险将对几个重要的水团进行采样。与目前对控制硅同位素分布的过程的理解相比，东北太平洋的深水是最令人费解的。北太平洋的深水是溶解硅密度最高的水域，也是全球深水循环“末端”最古老的水域。此外，东北太平洋尤其令人感兴趣，因为它可能包含全球海洋中最大的硅同位素异常，即北太平洋硅酸羽流。顾名思义，这股烟柱是由从北美近海的卡斯卡迪亚盆地延伸到日本的一条隆起的溶解硅组成的。到目前为止可获得的有限数据表明，羽流中的同位素模式与模型预测相反，挑战了我们目前对硅同位素分布控制的理解。在其他地方，计划中的探险将与关键水团相交，包括地表水、北太平洋中水、北太平洋深水，以及在该区段的南端，南极中水和环极深水，从而可以测试硅同位素如何与太平洋深水团的分布有关的假设。检查这些功能将涉及使用GEOTRACES管理团队的后勤支持对海水进行采样。海水将在塔希提岛和阿拉斯加之间的12个站点从表层到近海底收集，目标是关键水团和北太平洋羽流。将在三个站点采集整个水柱中的硅藻小柱样本，以探索硅藻小柱溶解过程中硅的分馏作用。该项目还将为一名博士后学者提供部分支持，该学者将参与科学研究，并与主要研究人员合作，向公众传播发现。

This project will examine the distribution of isotopes of the element silicon in the Pacific Ocean to gain insights into processes that control the movement of silicon through the global ocean. Such studies are motivated by the fact that the silicon that is dissolved in seawater supports the growth of diatoms. Diatoms are microscopic algae that use silicon to produce ornately sculpted shells called frustules. Diatoms are unique in that they are the only major group of marine photosynthetic microbes that need silicon in order to grow. Diatoms are responsible for 20% of the total photosynthesis on Earth so we can each thank a diatom for every fifth breath of oxygen that we breathe. The sheer scale of their contribution makes understanding what controls their distribution and abundance important for the ecology and chemistry of the oceans and for society. Their need for silicon means that the amount of silicon dissolved in seawater can control where diatoms grow and how many are produced. Diatoms obtain silicon, and other nutrient fertilizer, when currents bring deep waters that are rich in these nutrients to the surface ocean. This project will investigate how the stable isotopic composition of dissolved silicon varies in the Pacific Ocean. Why bother with isotopes? It turns out that diatoms preferentially use lighter isotopes of silicon when building their frustules. This produces signals in diatom frustules and in the dissolved silicon in the sea that allows isotopes to be used to reconstruct diatom productivity in the past. The isotopic composition of the dissolved silicon in deep ocean waters is different in different ocean basins. These differences in isotopes of silicon in deep waters appear to be systematic and are tied to the movement of currents in the deep sea. Once these patterns are understood evaluations of diatom productivity based on isotopes will improve enormously. Testing the relationship between isotopes of silicon and the water masses that comprise the deep circulation of the global ocean is a major goal of this study.This study will take place as part of the 2018 GEOTRACES expedition from Alaska to Tahiti. This expedition will sample several important water masses. The deep waters of the northeast Pacific are among the most puzzling relative to current understanding of the processes controlling Si isotope distributions. Deep waters of the north Pacific possess the highest concentration of dissolved silicon and oldest waters at the "end" of the global deep water circulation. Moreover, the northeast Pacific is of particular interest as it contains what could be the largest silicon isotope anomaly in the global ocean, known as the North Pacific Silicic Acid Plume. The plume, as its name implies, consists of a tongue of elevate dissolved silicon that extends from the Cascadia Basin off North America nearly to Japan. The limited data available so far implies that isotope patterns across the plume are the opposite of model predictions challenging our current understanding of controls on Si isotope distributions. Elsewhere, the planned expedition will intersect key water masses including surface waters, North Pacific Intermediate Water, North Pacific Deep Water, and at the southern extreme of the section, Antarctic Intermediate Water and Circumpolar Deep Water allowing tests of hypotheses on how silicon isotope relate to the distribution of deep water masses in the Pacific. Examining these features will involve sampling seawater using the logistical support from the GEOTRACES management team. Seawater will be collected from the surface to near ocean bottom at twelve stations between Tahiti and Alaska targeting key water masses and the North Pacific Plume. Samples of diatom frustules from throughout the water column will be collected at three stations to explore fractionation of silicon during the dissolution of diatoms frustules. This project will also provide partial support for a postdoctoral scholar who will both participate in the science and also collaborate with the principal investigator on disseminating the discoveries to the public.