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

该项目将研究元素硅的同位素在太平洋中的分布，以深入了解控制硅在全球海洋中运动的过程。这些研究的动机是，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，它们利用硅制造出被称为“截体”的雕刻精美的外壳。硅藻的独特之处在于，它们是唯一一种需要硅才能生长的海洋光合微生物。硅藻占地球光合作用总量的20%，所以我们每呼吸五分之一的氧气都要感谢硅藻。它们的巨大贡献使得了解是什么控制着它们的分布和数量，对海洋的生态、化学和社会都很重要。它们对硅的需求意味着海水中溶解的硅的数量可以控制硅藻生长的地方和产生的数量。当洋流将富含这些营养物质的深水带到海洋表面时，硅藻获得硅和其他营养肥料。这个项目将研究太平洋中溶解硅的稳定同位素组成是如何变化的。为什么要用同位素呢？事实证明，硅藻在构建它们的晶圆体时优先使用较轻的硅同位素。这在硅藻体和海洋中溶解的硅中产生信号，使得同位素可以用来重建过去硅藻的生产力。不同洋盆中深海溶解硅的同位素组成不同。深海中硅同位素的这些差异似乎是系统性的，并且与深海洋流的运动有关。一旦了解了这些模式，基于同位素的硅藻生产力评估将大大提高。测试硅同位素与构成全球海洋深层环流的水团之间的关系是本研究的主要目标。这项研究将作为2018年从阿拉斯加到塔希提岛的GEOTRACES探险的一部分进行。这次考察队将对几个重要的水团取样。相对于目前对控制Si同位素分布的过程的理解，东北太平洋的深水是最令人困惑的。北太平洋的深水拥有最高浓度的溶解硅和最古老的水域在全球深水循环的“末端”。此外，东北太平洋是特别有趣的，因为它包含可能是全球海洋中最大的硅同位素异常，被称为北太平洋硅酸羽。正如它的名字所暗示的那样，这个羽状物是由一个上升的溶解硅舌组成的，它从北美附近的卡斯卡迪亚盆地一直延伸到日本。到目前为止，有限的可用数据表明，整个羽流的同位素模式与模型预测相反，挑战了我们目前对Si同位素分布控制的理解。在其他地方，计划中的探险将穿越关键的水团，包括地表水，北太平洋中间水，北太平洋深水，以及在该部分的最南端，南极中间水和环极深水，允许测试关于硅同位素如何与太平洋深水块分布相关的假设。检查这些特性需要在GEOTRACES管理团队的后勤支持下对海水进行采样。海水将在塔希提岛和阿拉斯加之间的12个站点从海面到海底附近收集，目标是主要水团和北太平洋羽流。将在三个站点收集整个水柱的硅藻体样本，以探索硅藻体溶解过程中硅的分馏。该项目还将为博士后学者提供部分支持，该博士后学者将参与科学研究，并与首席研究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.