| Summary: | High nutrient, low chlorophyll (HNLC) regions, where the availability of iron (Fe) limits primary production, comprise approximately 40% of the global ocean. Variability in Fe supply to these regions has the potential to impact Earth's climate by affecting the efficiency of the biological carbon pump, and thereby carbon dioxide uptake by the oceans. Characterizing Fe sources to HNLC regions is thus crucial for a better understanding of the connections and feedbacks between the ocean and climate change. This work addresses the question of Fe supply to two HNLC regions: the Southern Ocean and the subarctic northeast (NE) Pacific Ocean. In both regions, dissolved Fe (dFe) and the reduced form of iron, Fe(II), were measured in the water column. In the Southern Ocean, measurements were undertaken under the seasonal pack ice in the East Antarctic south of Australia. The results indicate that the sea ice represents a significant dFe source for the under-ice water column in spring, and that the Fe delivered from brine drainage and sea ice-melt likely contributes to the formation of the spring bloom at the ice edge. Shelf sediments were also found to supply dFe to the water column. Their effect was most pronounced near the shelf break and at depth, but offshore transport of Fe-enriched waters was also implicated. Fe(II) concentrations in spring were very low, most likely due to a lack of electron donors in the water column and limited solar radiation underneath the sea ice. Repeat measurements along a transect in the subarctic NE Pacific indicate that shelf sediments supply dFe and Fe(II) at depth, but their influence does not appear to extend offshore beyond several hundred kilometres. Episodic events such as the passage of sub-mesoscale eddies may transport subsurface waters a limited distance from the shelf break, supplying Fe(II) in a depth range where upwelling and deep mixing could bring it to the surface. Offshore, dFe shows little variability except in June 2012, where an aerosol deposition event is suspected to ...
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