Collaborative Research: NSFGEO-NERC: The Colloidal Shunt as a Critical Nexus of the Ocean Iron and Carbon Cycles

The turnover of organic carbon in the ocean plays an important role in regulating the ocean carbon cycle. The oceanic cycles of iron and carbon are tightly coupled. The supply of dissolved iron regulates ocean biology while organic carbon impacts the solubility and biological availability of iron in seawater. We strive to better understand the mechanisms and linkages between pools of iron and organic carbon in the oceans and to predict their sensitivity to future environmental and climatic changes. In this collaborative project, jointly funded with the U.K. Natural Environment Research Council, scientists from the U.S. and U.K. will combine field data from the Bermuda Atlantic Time-series Study (BATS) region and from the Eastern North Atlantic with targeted experimental studies and a state-of-the-art ocean biogeochemical model to better characterize organic carbon - iron linkages and their roles in past, present, and future changes in ocean biology and chemistry. The project will support the education and training of undergraduate, graduate, and postdoctoral researchers, and will connect rural K-12 and undergraduate students with the research through outreach activities. Field observations from the BATS and Cape Verde Ocean Observatory regions will be integrated with experimental studies targeting iron-organic carbon interactions across seasonal and spatial gradients. An ocean biogeochemical model will be used to constrain the processes that modulate interactions of iron with dissolved and particulate organic matter. Specifically, this project will examine the a ‘colloidal shunt’ mechanism, whereby a portion of the dissolved iron pool in the colloidal size range is not stabilized by complexation with organic ligands. This iron instead forms aggregates with organic carbon to form particulate matter that sinks out of the upper water column. The research will focus on 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 as a function of oxygen. 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. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.