Incorporation mechanisms of iron and organic matter into newly-formed sea ice

Sea ice plays a critical role in the global ocean, including polar biogeochemical cycles and ecosystems. Importantly, sea ice serves as temporal reservoir for key nutrient iron (Fe), which is known to limit primary productivity in large parts of the Southern Ocean. Iron released from melting sea ice contributes to the formation of important phytoplankton blooms, and the carbon drawdown in the marginal ice zone. Although the importance of sea ice as a source of Fe for the Southern Ocean has been clearly established, the processes leading to the high levels of Fe in sea ice have yet to be identified and quantified. In this study, the mechanisms responsible for Fe enrichment in sea ice are explored through observations, including in situ ice-growth experiments, laboratory ice-growth experiments, and one-dimensional numerical modelling. A combination of natural young ice sampling and in situ ice-growth experiments conducted under trace metal clean conditions during a winter voyage in the Weddell Sea shows that enrichment of Fe and organic matter starts at the very first stages of sea ice formation. Results show that physical processes occurring during sea ice growth are responsible for early enrichment of both Fe and organic matter into newly formed sea ice. The particle size plays a key role, with bigger particles exhibiting higher enrichment indices (EI). Dissolved compounds behaved conservatively with salt, except dissolved Fe (DFe) and ammonium, which were enriched in the ice compared to concentrations in the underlying seawater. Differential behavior of DFe and particulate Fe (PFe) is observed for the first time during sea ice formation, suggesting a decoupling throughout the whole year. The role of organic matter (particulate organic carbon and extracellular polymeric substances (EPS)) as a carrier for Fe is then investigated using laboratory ice growth experiments. The form of organic matter determines its entrapment rate in forming sea ice. The quality of organic matter also influences its association with Fe, and therefore the EI of Fe. Biogenic PFe is preferentially enriched compared to lithogenic PFe. Higher EI for biogenic PFe are explained by a combination of factors, such as size and association to EPS. Organic ligands are thought to drive the enrichment of DFe into forming sea ice. Finally, a Fe-biogeochemical component was added to the Louvain-la-Neuve sea Ice Model (LIM-1D) to explore the initial entrapment of Fe in young sea ice, and to identify the biological and physical mechanisms driving the Fe dynamic in older sea ice. LIM-1D is a one-dimensional (vertical) biogeochemical sea ice model configured to reproduce the typical thermodynamic regimes of first-year Antarctic sea ice. By implementing the model with an entrapment factor for PFe and an adsorption rate for DFe it is possible to reproduce the high DFe and PFe concentrations observed in young sea ice. The activation of biological processes is then needed in order to reproduce Fe pattern observed in older sea ice. Specifically, it is crucial to better constrain the parameters regulating the fluxes between pools of Fe. This represents a first step toward an improved representation of sea ice biogeochemical processes in order to introduce them in Earth System Models used for climate simulations.