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Upper Ocean Cycling of Iron South of the Polar Front: Biological Patterns and Processes
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DOI：10.1029/2025GB008803.png)
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Abstract
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Samples from the upper 300 m along the 52°S band, south of the Antarctic Polar Front in the Atlantic sector of the Southern Ocean, including a 3-week monitoring of a persistent bloom, were analyzed to advance our understanding of iron cycling and supply to recurrent phytoplankton blooms. We measured dissolved Fe (dFe, <0.2 μm) and labile particulate iron using a mild acid leach (pLFe48h) targeting primarily detrital and fecal material. Particulate iron was partitioned into size classes to distinguish small, “slow” sinking particles (<53 μm) from large, “fast” sinking particles (>53 μm), while intermediate fractions were analyzed to investigate aggregation and export processes. Across all stations, dFe exhibited a consistent vertical structure, including a previously undescribed ubiquitous minimum below mixed and euphotic layers. The absence of pLFe48h in the 0.2–3 μm fraction indicates reduced aggregation and is consistent with minimal authigenic Fe formation. Instead, pLFe48h was concentrated in “slow” sinking particles, indicating surface entrainment. Biomass was negatively correlated with dFe but positively correlated with pLFe48h, linking bloom development to particulate iron availability rather than dFe. Overall, rapid surface recycling of “slow” sinking pLFe48h appeared to meet most of the daily iron demand during a prolonged diatom bloom. Near South Georgia Island, wind-driven mobilization of iron-rich material from the island snowpack and deposition into leeward waters likely supplied an additional iron source. By applying new protocols to characterize labile particulate iron, we reveal the elusive magnitude of the recycling dynamics of particulate iron in fecal and detrital materials sustaining Southern Ocean blooms.
Keywords:
chemical oceanography
trace elements
iron
chemical partitioning
biochemical cycling
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