The circumpolar Southern Ocean that surrounds Antarctica represents the main connection between the sunlit surface ocean and the nutrient- and carbon-rich waters of the deep. Deep waters are brought up to the surface here, resulting in outgassing of carbon dioxide (CO2) to the atmosphere and providing nutrients to surface ecosystems at the near-global scale. The Southern Ocean thus has fundamental importance for the carbon cycle, as well as for the cycling of the nutrients required by marine photosynthesisers to grow and multiply -- photosynthetic activity that forms the base of the marine food web, produces half of all atmospheric oxygen, and reduces atmospheric CO2 levels. Understanding how the Southern Ocean controls nutrient cycles, today and in the past, is thus a crucial element in our understanding of the natural system that shapes the chemistry of the Earth’s surface environment, and plays a role in regulating its climate.
The Southern Ocean is particularly important for the marine cycle of the major algal nutrient silicon (Si), since Southern Ocean phytoplankton ecosystems are dominated by the siliceous algae known as diatoms, which form a filigranous cell wall made of opal (amorphous, hydrated silicon dioxide). A principal aim of this project is to understand how the uptake of Si by these siliceous algae interacts with the vigorous, jet-like currents of the Southern Ocean to determine the transport and cycling of Si in the surface Southern Ocean, ultimately governing the supply of Si to surface ecosystems at lower latitudes. During the funding period, the project has also expanded its scope to include the micronutrient metal zinc (Zn), which is vitally required by photosynthesisers, and is taken up especially strongly by diatoms in the Southern Ocean.
The research we have carried out so far has strikingly shown that Southern Ocean diatoms, and their exceptionally strong uptake of both Zn and Si, are the driving factor of the close coupling between the distributions of these two elements in the global ocean, a coupling that is brought about despite the fact that Si is incorporated into the opaline hard parts of diatoms, whilst Zn sits within, and is regenerated with, their more labile organic tissue. By taking them up in large quantities, diatoms rapidly remove both these elements from the Southern Ocean's surface, vastly reducing their northward transport to the low-latitude ocean. These results highlight the disproportionately large influence of the Southern Ocean on marine biogeochemistry, as a result of its unique role in ocean circulation and the unique biochemistry of its phytoplankton ecosystems.
A final aim of the project is to understand how the cycle of Si may have changed over the major glacial climatic cycles of the last 150,000 years, perhaps as the result of changes in the circulation of the ocean, or changes in inputs of the micronutrient metal iron. We are currently investigating these changes by analyzing the isotopic composition of Si within diatom opal preserved in marine sediment recovered from the Southern Ocean.
