| Details: |
In the face of the climate crisis, the exchange of carbon among Earth's reservoirs has become a critical area of study. Given the global ocean absorbs ca. 30% of anthropogenic CO2, since the beginning of marine biogeochemistry research, scientists have focused on understanding the cycling of essential nutrients (e.g., P, N, Fe, Mn) considering their role in oceanic carbon pumps. Among these, Fe is the limiting micronutrient in the Southern Ocean, and its absence inhibits phytoplankton growth. Despite that, the Southern Ocean absorbs more than 20% of anthropogenic CO2, with seasonal sea ice believed to play a crucial role in the biological carbon pump. However, very few studies were performed on nutrient cycling in the transitional zone between seawater and sea ice, which accounts for ca. 60% of net primary production.
As part of a multi-nation, multidisciplinary study (Southern Ocean Seasonal Experiment) two cruises were undertaken in the winter (sea ice formation) and spring (sea ice retreat) of 2019 along the zero meridian from Cape Town to the Southern Ocean marginal ice zone. We focused on the distribution and speciation of Fe in different reservoirs (e.g., seawater, sea ice, and aerosol). The dissolved Fe (dFe) stock in the surface mixed layer showed an increasing number from winter and spring, which opposed the typical seasonal variability of dFe stock in the Southern Ocean. Such an observation along with the distribution of in situ parameters suggests that the melting of sea ice likely caused an additional supply of dFe in the seawater, which supported the spring phytoplankton bloom observed along the edge of sea ice. Interestingly, our measurements revealed that bioavailable dFe concentrations in sea ice were low (<0.5 nmol/kg) and insufficient to fertilize seawater directly. However, particulate Fe (pFe) concentrations were significantly higher in sea ice (>3 nmol/kg) compared to seawater (<0.6 nmol/kg), indicating that Fe is primarily supplied to seawater as pFe during the melting season.
The results from this study point to a potential Fe reservoir in sea ice that could stimulate phytoplankton bloom and associated C sink in the Fe-deficient Southern Ocean. Despite these findings, our understanding of Fe sourcing and cycling in sea ice remains limited. As climate change continues to reduce sea ice extent around Antarctica, the future impact of sea ice-derived Fe fertilization on the Southern Ocean's carbon sink is uncertain. Further research is essential to explore these dynamics and their implications for the global carbon cycle.
|