Iron particles in the ocean are clumping together and sinking, pulling carbon down with them in a process scientists barely understand. This newly identified "colloidal shunt" may act as a hidden gear in the ocean's machinery, linking the cycles of iron and carbon in ways that current climate models do not account for. The research team will collect seawater from the subtropical and tropical North Atlantic, run controlled experiments, and build numerical models to determine exactly how dissolved organic carbon and iron-binding ligands control this aggregation process, and whether the resulting particles dissolve again in deeper, oxygen-poor waters. If the colloidal shunt proves to be a significant pathway for exporting both iron and organic carbon to the deep ocean, it could mean that climate models are underestimating the ocean's capacity to store carbon—especially in vast subtropical and tropical regions where the biological carbon pump is normally weak. This is fundamental science: the immediate payoff is a mechanistic understanding of a previously overlooked process. But that understanding could eventually reshape how scientists predict the ocean's response to climate-driven changes in oxygenation, with consequences for global carbon cycle projections that underpin climate policy.
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The oceanic cycles of iron and carbon are tightly coupled. The supply of dissolved iron regulates ocean biology and biogeochemistry, and organic carbon species impact the solubility and biological availability of iron in seawater. As such, a full mechanistic understanding of the ocean iron cycle and its linkages with the pools and transformations of organic carbon are required to accurately model ocean biogeochemistry and biology, and to explore past, present, and future changes in ocean biogeochemistry and marine primary production. The proposed research aims to characterize these interactions by combining field data from contrasting ocean regimes in the subtropical and tropical North Atlantic with targeted experimental studies and numerical modeling experiments. Specifically, this project will examine the newly identified 'colloidal shunt' mechanism, whereby a portion of the dissolved iron pool in the colloidal size range is not stabilized by complexation with organic ligands but is instead subject to aggregation to form authigenic particulate iron that sinks out of the upper water column - a conceptual model inferred from a prior NSF- NERC collaborative award. In particular, the research will examine the role of dissolved organic carbon and iron-binding organic ligands in mediating the colloidal shunt, the association of organic matter with thus-formed authigenic particulate iron phases, and the dissolution of these phases in the ocean interior including the role of interior oxygen gradients. Potentially transformative implications of this research are that the colloidal shunt might vary in response to climate driven changes in ocean oxygenation, and that this process may provide a conduit for the vertical export of both particulate iron and organic carbon that augments the biological carbon pump in the subtropical and tropical oceans.
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