Silicon (Si), the second most abundant element in Earth’s crust, is an essential nutrient for marine organisms (phytoplankton, sponges, radiolarians) and plays crucial roles in marine carbon cycling and ecosystems and, atmospheric CO₂ inventory. The ocean receives Si from different sources, including continental erosion (riverine and groundwater discharge, glacial meltwaters…), sedimentary recycling (reverse weathering, diagenesis…), hydrothermal input, and atmospheric deposition. However, anthropogenic activities significantly alter Si supply to the ocean: damming restricts the riverine input of Si, while climate-induced ocean stratification limits vertical mixing, curtailing Si supply to surface waters. Contrarily, increasing nitrogen (N) and phosphorus (P) inputs to the coastal areas are disrupting the Redfieldian Si:N:P stoichiometry, causing a regional shift from siliceous to non-siliceous species, impacting carbon export and trophic dynamics. Understanding how climate-driven changes—such as enhanced weathering, glacial melt, dust input, and deep-sea geochemical processes—reshape the marine Si cycle is critical for predicting shifts in elemental cycling, phytoplankton community structure, and carbon storage. This interdisciplinary session explores evolving Si fluxes across ocean’s interfaces and their biogeochemical impacts, encouraging studies using isotopic tracers, omics, satellite products, and coupled Earth system models, to identify hidden fluxes, feedbacks, and vulnerabilities in the Si cycle under changing oceanic state.

• Ocean Biology and Biogeochemistry -