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

NSF Award Abstract:
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.