Abstract:
Silicon is one of the major nutrients in modern oceans as it is essential for the growth of diatoms, which are responsible for a large proportion of total oceanic primary production. Marine concentration of dissolved silica is, to a large extent, dominated by a balance between riverine input and biological uptake, whereas its distribution around the different ocean basins is controlled by ocean circulation.
However, this picture was significantly different in the geological past. Of significance here, is the evolution of silicifiers, dramatic changes in global climate, position of continents and changes in ocean circulation. The oceanic concentrations of dissolved silica (DSi) were estimated to reach saturation during the Pre-Cambrian due to the absence of silicifiers, decreased during the Paleozoic and Mesozoic due to the appearance of sponges and radiolarians, and further decreased gradually during the Paleogene due to the diversification of diatoms. However, thanks to recent studies using silicon isotopes, we now know that this paradigm is broadly incorrect.
Stable silicon isotope composition of siliceous microfossils is a novel tool to provide information about a variety of aspects of the past silicon cycle. Of crucial importance here is the ability to provide quantitative reconstruction of past oceanic DSi concentrations. Furthermore, when combined with traditional proxy for ocean circulation reconstruction such as Neodymium, Si stable isotopes can provide unprecedented insights into nutrient distribution. Here, we will survey the new findings in past marine DSi concentrations and distribution, as well as discussing further research perspectives.

Bio:
Guillaume Fontorbe is a postdoctoral researcher from Lund University, Sweden. He started his scientific education by a BSc in Chemistry at the University of Orléans before a MSc in Marine Chemistry at the University of Western Brittany. That's where he was first introduced to Silicon biogeochemistry by Prof. Christina De La Rocha, investigating the silicon isotopic composition of the Ganges River. He then started his academic career by joining a PhD program at Lund University under the supervision of Prof. Daniel Conley. Guillaume's PhD project was to investigate the evolution of the marine Silicon cycle during the early Cenozoic. Achievements of his PhD study include the first quantitative reconstruction of dissolved silica concentrations in the early Cenozoic based on sponges and radiolarians silicon isotopes, and evidence of past changes in nutrient levels due to reorganization of ocean circulation. He then continued in the same university as a postdoctoral researcher, exploring further the questions raised from his PhD project.