Our story starts, like so many do, in a land far, far away. A parcel of water persistently marches north past the east coast of North America. Like a content traveler returning from a tropical holiday with bronzed or blistered red skin, the water parcel shows signs of where it has been. It is warm and moderately salty from time spent at low latitudes.
Along its northward journey, evaporation causes the water parcel to cool and become saltier. (Remember: 1. it takes a lot of energy to transform water from liquid to its gas form, and heat is a measure of the energy in the water, and 2. evaporation only removes water and leaves behind salt, so the remaining water has a higher concentration of salt than it did before). The parcel of water becomes colder and saltier until it reaches the seas near Greenland. Since cold, salty water is denser, by this time it has it become so dense that it begins to sink.
Down, down it flows to the bottom of the ocean. Then pushed along by the sinking water behind it and pulled forward by the moving water before it, the parcel slowly makes a long journey south, hugging the seafloor, to join the deep-water currents that encircle Antarctica. The journey south changes the chemistry of the water parcel. Biological activity in the overlying surface waters rains a constant supply of detrital material to the deep-water. Dissolution and microbes disassemble the raining material back into the fundamental nutrients that fuel primary production. By the time the water parcel reaches the Southern Ocean, it has some of the highest concentrations in the world of nitrate, phosphate, and dissolved silica (silicic acid). The water parcel travels around Antarctica until the wind forces the overlying water away from the continent, creating space for the parcel to upwell to the surface.
The diatoms are ready. The water parcel reaches the surface and then flows north. Diatoms capitalize on ideal growth conditions; they outcompete other phytoplankton, using carbon dioxide and the readily available nutrients to create biomass (the amount of living matter). By the time the water parcel travels north into the other ocean basins, diatoms have used up nearly all the dissolved silica and a large portion of the nitrate and phosphate. The burst of diatom biomass is passed up the food web. What isn’t eaten falls through the water column, becoming part of the vast diatom ooze that blankets the sea floor below the Southern Ocean.
There is evidence that the Southern Ocean has gone through periods of higher and lower diatom productivity. During times of high production, diatoms are able to use more of the available nutrients and thus more carbon dioxide, the loss of which from the atmosphere creates a colder climate.
The silicon and nitrogen isotopes in the sediments below the Southern Ocean have recorded the history of how diatoms used nutrients during primary production. So here we are, in the Southern Ocean, getting a better understanding of what happens to the silicon and nitrogen isotopes as they go from water, to diatom, to depth, to sediment, so that we can reconstruct the history of Southern Ocean diatom production.
We’re studying silicon and nitrogen isotopes because changes in diatom productivity here have global consequences. The waters of the Southern Ocean feed the other ocean basins, and the amount of production that occurs here affects the global carbon cycle.