Deep sea lithogenic weathering a source of iron colloids for the ocean

28 March 2021

Homoky and co-workers (2021, see reference below) determined the isotope composition of dissolved iron (Fe) profiles in shallow surface sediments of the South Atlantic Uruguayan margin, from shelf-top to abyssal floor. They confirmed the presence of lithogenic iron isotope compositions in the oxidising zones of sediment porewaters, and further showed that these signatures are uniquely attributed to the presence of iron colloids (20-200nm). An isotopically constrained porewater mass-balance model is used to show that reductive dissolution and oxidation cannot fully account for the production of iron colloids, whereas non-reductive weathering of lithogenic phases and the production of nano-scale Fe organo-minerals can explain these data. An exchangeable inventory of dissolved iron in porewater is compiled for the ocean depths based on all the sites currently observed and suggests that sedimentary supply to the deep ocean interior will be dominated by organo-mineral iron colloids bearing lithogenic isotope signatures.

**Figure (modified from Homoky et al., PNAS, 2021):** Characterising the exchangeable source of dissolved iron in shallow porewaters beneath the open ocean. (A) Data markers correspond to measured surface (0-1 cmbsf) values compiled from sediments of the western South Atlantic (this study), the eastern South Atlantic, Cape margin¹, the North Pacific, Oregon and California margins and Borderland Basins^2,3, the North Atlantic, Celtic Sea⁴, and the Southern Ocean, Crozet Island abyss². The measured surface inventory of porewater dFe is illustrated by the size of data symbols, and the relative abundance of colloidal iron in porewater is indicated by the colour scale – except for sites with symbols in grey, where dFe speciation was not determined (n.d.). (B) Illustrated summary of key factors attributed to colloidal Fe production, and the nature of its distribution so far observed. Additional data sources used in this figure: [1] Homoky et al. Nature Comms, 4, 2143 (2013); [2] Homoky et al. Geology 37, 751-754 (2009); [3] Severmann et al. Geochimica et Cosmochimica Acta 74, 3984-4004 (2010); [4] Klar et al. Biogeochemistry 135, 49–67 (2017).

You can also read the press releases about this paper: Seafloor nutrient vital in global food chain (University of Leeds) and Deep Sea Sediments Fuel the Oceans (from University of South Florida).

Reference:

W. B. Homoky, T.M. Conway, S.G. John, D. König, F. Deng., A. Tagliabue, and R.A. Mills. (2021) Iron colloids dominate sedimentary supply to the ocean interior. Proc. Natl. Acad. Sci. U.S.A. 118, e2016078118. Access the paper: https://doi.org/10.1073/pnas.2016078118

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