Award: OCE-1129227

The element silicon supports the growth of microscopic algae called diatoms.  These organisms are responsible for 20% of the total photosynthesis on our planet. They are unique in that they are the only major group of photosynthetic microbes that need silicon 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 north Atlantic Ocean. 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 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 north Atlantic Ocean was ideal for this work as deep waters of the north Atlantic contain identifiable water masses that have traveled from as far away as Antarctica to others that enter from the Arctic Ocean to still others that form locally in places like the Labrador Sea. This diversity of water masses is ideal for testing that idea that different water masses have distinct isotopic signatures related to their region of origin.

We obtained samples from the surface ocean to samples from depths of over 4,000 m along an ocean section from the Azores off Africa to North America near New England as part of the US GEOTRACES program. The results confirmed a very strong relationship between water mass identity and its isotopic composition.  One of the most striking contrasts was the difference in the isotopic composition of waters from the two poles. Waters that originated in the Southern ocean around Antarctica contained more light isotopes of silicon than did waters originating in the Arctic Ocean. While these findings helped us define how silicon isotopes vary among water masses we also wanted to know how those particular patterns could come about.  For that we turned to computer models of ocean circulation.  By adding diatom productivity and their fractionation of isotopes of silicon to these models we showed that the light isotopic character of deep Southern Ocean waters arises from circulation that essentially traps silicon in the Southern Ocean and pumps light isotopes of silicon into Southern Ocean deep waters.  The opposite is happening in the Arctic where the shallow entrances to the Arctic Ocean exclude the deepest ocean waters from entering the Arctic basin.  The shallower ...