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Southern Ocean Silicon Cycling: combining views of the past and present using silicon isotopes

Periodic Reporting for period 1 - SOSiC (Southern Ocean Silicon Cycling: combining views of the past and present using silicon isotopes)

Reporting period: 2016-04-01 to 2018-03-31

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.
In the course of the research carried out during this project, we developed a simple biogeochemical model that simulates the cycling of Si, the macronutrient phosphate and the micronutrient Zn in the ocean. This model was coupled to a physical model that simulates the currents of the ocean’s circulation, in order to identify the main processes that control the distributions of Si and Zn in the ocean. These simulations showed that it is the peculiar physiology of unicellular siliceous algae that grow in the remote polar Southern Ocean that dominantly controls the similarity in the large-scale marine distributions of Zn and Si, through a series of interactions that take place between biological uptake and the ocean’s physical circulation. As can be seen in the image, by simulating the particular biochemistry of these algae, our model (upper panels) is able to reproduce the strikingly close similarity between Zn and Si distributions in the ocean seen in observations (lower panels). Our simulations have thus shown that the cellular-level biochemistry of minute microalgae plays an important role in the marine cycle of Zn at the scale of the global ocean. These results have been presented to researchers at scientific conferences, in scientific publications and to the scientifically-interested lay public in public lectures and press releases.
Furthermore, we are still in the process of studying changes in the Southern Ocean cycle of Si over the last 150,000 years, through the analysis of a final total of ~150 samples from a marine sediment core retrieved from the sea-floor of the Southern Ocean. When completed, these results will give us an unprecedented well-resolved view of how Si cycling changed during the major climate changes of the last glacial cycle.
The results of our modelling work, as well as further work investigating the transport of the micronutrient iron by eddies shed from meanders in the Gulf Stream, have highlighted the importance of the ocean’s physical circulation in determining the distribution of macro- and micronutrients in the ocean, and their supply to the surface-ocean ecosystems of photosynthesisers that form the base of the marine food web, contribute half of all atmospheric oxygen that we breathe, and help to modulate the atmospheric levels of the greenhouse gas CO2. The impact of the circulation will need to be considered more carefully in future research on marine biogeochemistry. Furthermore, we have also developed the first three-dimensional biogeochemical model of the marine cycle of zinc, and have demonstrated the power of combining observational and modelling approaches for studying the biogeochemical cycles of macro- and micronutrients.
More broadly, our research provides an incremental step towards improving our understanding of the natural system that plays a role in regulating Earth’s climate, helping to tackle the Societal Challenge “Climate Action, Environment, Resource Efficiency and Raw Materials” identified in Horizon 2020.
Comparison of biogeochemical model results with observations of marine Si and Zn distributions