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

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 Basins2,3, 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).


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:

Latest highlights

Science Highlights

Quantifying the weathered fluxes to the ocean is far from over: the overlooked role of rock coast erosion

This study reveals that cliff derived sediment supply is only three times less than the solid discharge of rivers for Europe.


Science Highlights

Contrasting distributions of dissolved manganese, nickel, cadmium and zinc in the Mediterranean Sea

Extensive trace metal clean sampling during the Dutch GEOTRACES cruise in the Mediterranean Sea allowed Middag and his colleagues to establish the basin scale distribution of these trace metals.


Science Highlights

Specific features characterize the dissolved iron distribution in the North Western Indian Ocean

Venkatesh Chinni and Sunil Kumar Singh propose dissolved iron profiles along two meridional transects realized during spring and fall seasons between the Arabian Sea and the sub-tropical western Indian Ocean…


Science Highlights

Anthropogenic aerosol has become a dominant source of zinc in the deep water of the Northern South China Sea

Liao and colleagues determined zinc concentrations and isotope compositions in sinking particles collected in the Northern South China Sea…