Award: OCE-1434305

This project examined the distribution of isotopes of silicon in the Arctic Ocean. Dissolved silicon in seawater supports the growth of microscopic algae called diatoms that use silicon to build a house of glass called a frustule. For diatoms silicon is an essential nutrient. Without silicon diatoms cannot grow. Understanding what controls diatom growth in the sea is important because these tiny microscopic organisms produce 20% of the oxygen generated through photosynthesis on planet Earth each year. They are also import for food security as diatoms form the base of the food web for most of the world?s largest fisheries. The diatom need for silicon means that the amount of silicon dissolved in seawater can control where diatoms grow and how many grow in a particular place. Diatoms are photosynthetic organisms so they live in the surface ocean where sunlight is abundant. They obtain silicon, and other nutrients, when currents bring water from the deep ocean that is rich in these nutrients to the surface ocean. In this project we investigated how the stable isotopic composition of dissolved silicon varied in the Arctic Ocean. Why bother with isotopes? It turns out that diatoms discriminate against heavy isotopes preferring the lighter ones for building their shells. They do this in a systematic fashion allowing us to use their isotopic composition to assess their cumulative growth and productivity in a given region of the ocean. However, there is a trick. To do this it is necessary to understand the isotopic composition of dissolved silicon that they are using to grow. We are learning that the ocean is far from homogenous when it comes to the isotopic composition of the dissolved silicon in the deep ocean waters that come to the surface. So how will we know what kinds of waters are coming to the surface in a particular place? Developing the understanding that will let us answer this question was a major goal of this study. The Arctic Ocean is somewhat of an enigma for Si isotopes. Prior to our work only six full-depth profiles of the isotopic composition of dissolved silicon in the Arctic Ocean had been published. Those data showed that the isotopic composition of deep waters in the Canada Basin were isotopically heavy (a high ratio of heavy silicon atoms to lighter ones) compared to deep waters from other deep ocean basins. Results from this project confirm the heavy nature of deep waters in the Canada Basin and provide new data that show similar heavy signatures in the deep waters of the Makarov and Amundsen Basins. The broad geographic extent of our data suggests a large-scale common mechanism drives the pattern. That mechanism may be the shallow depth of the ocean gateways that govern the exchange of water between the Arctic Ocean and the adjacent Atlantic and Pacific Oceans. The Bering Strait that is the gateway between the Arctic and the Pacific is generally <100 m deep. The Fram Strait in the Atlantic is deeper, nearly 2000 m, with additional inflows over the Barents Sea which is much shallower at only a few hundred meters. Why does this matter? Well, even 2000 m is relatively shallow in oceanic terms where the global ocean has an average depth of 4000 m! We have learned that the upper half of the Atlantic and Pacific Oceans are isotopically heavy compared to deep waters in these same oceans. Thus the waters that flow through the gateways to fill the deep arctic basins start out with the heavy isotope signature of much shallower waters. Processes in the Arctic such as river inputs, shelf processes and diatom growth modify, but do not erase, this primary signature. We can apply this new understanding to help explain global patterns of Si isotopes in deep waters. The Arctic contributes to the formation of deep waters in the North Atlantic that flow southward causing these waters to be isotopically heavy. The opposite happens in in the Southern Ocean where the deep waters that form and flow northward are isotopically light. The two flows meet and flow past each other sorting out their vertical position in the water column based on water density. These great currents, known as the meridional overturning circulation, redistribute dissolved silicon and its isotopes in systematic ways across the entire global ocean. The measurements of silicon isotopes from across the globe have been successfully simulated in computer models suggesting that our growing understanding of the mechanisms controlling the distribution of Si isotopes is largely correct. Projects like this one provide essential data for testing these models and improving our understanding of how we can use Si isotopes to evaluate diatom productivity in the sea. Last Modified: 04/20/2020 Submitted by: Mark A Brzezinski