Award: OCE-1233028

The small amount of the element silicon that is dissolved in seawater supports the growth of microscopic algae called diatoms. Diatoms alter both the concentration and isotopic composition of dissolved silicon in the sea. This project examined how diatom activity redistributes isotopes of Si in the ocean with the ultimate goal of reconstructing diatom growth and activity. Why is this important? Diatoms live in the surface ocean and they are an abundant group of organisms that use photosynthesis to transform carbon dioxide from the air into their cell biomass. A famous oceanographer Henry Bigelow once said "All fish is diatoms" reflecting the importance of diatoms as the base of the food chain that supports some of the world?s largest fisheries. Despite their small size diatom produce 20% of the oxygen generated through photosynthesis on planet Earth each year. That?s more than all of the tropical rainforests on land. So for every fifth breath of oxygen that you take, you can thank a diatom. Diatoms are unique in that they are the only major group of photosynthetic microbes that need silicon to grow which they use to form ornately sculpted cell walls. 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 nutrients, when currents bring deep waters that are rich in these nutrients to the surface ocean. Scientists have been investigating the role that dissolved silicon availability plays in diatom ecology and how it affects their contribution to the Earth?s carbon cycle. In this project we investigated how the stable isotopic composition of dissolved silicon varied across the eastern Pacific Ocean between Peru and Tahiti. Why bother with isotopes? It turns out that diatoms discriminate against heavy isotopes preferring the lighter ones. They do this in a systematic fashion such that their isotopic composition reflects their cumulative growth and productivity in a given region of the ocean. This means that we can use the isotopic composition of diatoms to quantify diatom productivity in the modern ocean by examining the composition of diatoms growing in the surface ocean and in the geologic past if we examine diatoms from dated sediment cores. To use isotopes of silicon for such studies it is necessary to understand the isotopic composition of dissolved silicon that they are using to grow. The isotopic value of the dissolved silicon sets the stage for the whole isotope fractionation process that allows isotopes to be used to reconstruct diatom productivity. We are learning that the ocean is far from homogenous when it comes to the isotopic composition of the dissolved silicon in deep ocean waters. So how will we know what kinds of waters are coming to the surface in a particular place? Fortunately, the variation in Si isotopes in deep waters appears systematic and tied to the great deep currents in the sea. Testing this idea was the main goal of this study. The deep basins of the eastern Pacific contain identifiable water masses that have traveled north from the Southern Ocean and south from the North Pacific. The area sits near the end of the global deep circulation and theory predicts that deep waters should have high dissolved silicon concentrations of relatively uniform isotopic composition. We obtained samples from the surface ocean to depths of over 5,000 m along an ocean section from Peru to Tahiti. The results confirmed a very strong relationship between water mass identity and its isotopic composition with deep waters having the uniform isotopic composition predicted by theory. One of the most striking contrasts was the difference in the isotopic composition of intermediate and deep waters flowing into the region from the Southern Ocean around Antarctica. A relatively shallow water mass known as Antarctic Intermediate Water was low in dissolved Si and had an isotopically heavy Si isotope signature. The opposite was true of the deeper waters from the south, Circumpolar Deep Water, where dissolved Si concentrations are high and isotope values light. These patterns have been predicted in computer simulations and we are beginning to verify those predictions in nature. This project was the first to produce a major ocean section of silicon isotopes in the Pacific Ocean. The observed patterns are intriguing and fit emerging ideas of how we can use isotopes of silicon to learn about diatoms and carbon cycling on our planet. The patterns observed in the Pacific differ strongly from previous measurements in the Atlantic, but those contrasts largely fit predictions of models that reconstruct the distribution of Si isotopes across the global ocean. Additional data collected by other US GEOTRACES studies will further test our understanding of the controls on Si isotope distributions allowing us to better apply the proxy for reconstructing diatom activity in the oceans. Last Modified: 01/31/2018 Submitted by: Mark A Brzezinski