Iron colloids dominate sedimentary supply to the ocean interior

Abstract

Dissolution of marine sediment is a key source of dissolved iron (Fe) that regulates the ocean carbon cycle. Currently, our prevailing understanding, encapsulated in ocean models, focuses on low-oxygen reductive supply mechanisms and neglects the emerging evidence from iron isotopes in seawater and sediment porewaters for additional nonreductive dissolution processes. Here, we combine measurements of Fe colloids and dissolved δ56Fe in shallow porewaters spanning the full depth of the South Atlantic Ocean to demonstrate that it is lithogenic colloid production that fuels sedimentary iron supply away from low-oxygen systems. Iron colloids are ubiquitous in these oxic ocean sediment porewaters and account for the lithogenic isotope signature of dissolved Fe (δ56Fe = +0.07 ± 0.07‰) within and between ocean basins. Isotope model experiments demonstrate that only lithogenic weathering in both oxic and nitrogenous zones, rather than precipitation or ligand complexation of reduced Fe species, can account for the production of these porewater Fe colloids. The broader covariance between colloidal Fe and organic carbon (OC) abundance suggests that sorption of OC may control the nanoscale stability of Fe minerals by inhibiting the loss of Fe(oxyhydr)oxides to more crystalline minerals in the sediment. Oxic ocean sediments can therefore generate a large exchangeable reservoir of organo-mineral Fe colloids at the sediment water interface (a “rusty source”) that dominates the benthic supply of dissolved Fe to the ocean interior, alongside reductive supply pathways from shallower continental margins.

Carboxyl-richness controls organic carbon preservation during coprecipitation with iron (oxyhydr)oxides in the natural environment

“…Recent work indicates that marine sediments might be sources of CRAM to both sediment porewaters and the overlying water column 74 , but the mechanism(s) that generate the CRAM signal are unknown 73 . Given the chemical fractionation of NOM with iron (oxyhydr)oxides towards carboxyl-rich components 14,66 and new work showing that OC-Fe colloidal organominerals diffuse out of marine sediments 75 , we suggest that nanoscale OC-Fe colloids, macromolecules and nanoparticles carry a CRAM signal from the sediments into the overlying water column that might be measured as part of the dissolved NOM seawater pool.…”

Bioactive Trace Metals and Their Isotopes as Paleoproductivity Proxies: An Assessment Using GEOTRACES‐Era Data

“…Bulk marine sediments typically possess a composition similar to the crustal composition of +0.1‰ (Beard et al., 2003; Poitrasson, 2006). However, dissolved Fe(II) derived from bacterially mediated reductive dissolution in sediments has been characterized by δ 56 Fe between −1‰ and −3‰ (Bergquist & Boyle, 2006, 2010; Henkel et al., 2018; Homoky et al., 2009, 2013; Klar, Homoky et al., 2017; Severmann et al., 2006), whereas Fe derived from non‐reductive dissolution processes is thought to be considerably heavier, between +0.1‰ and +0.3‰, and also likely to be present in the colloidal phase (Homoky et al., 2009, 2013; 2021; Radic et al., 2011). Further modification of reductive endmember compositions is possible upon contact with oxidizing seawater, potentially masking true source signatures.…”

Slow Particle Remineralization, Rather Than Suppressed Disaggregation, Drives Efficient Flux Transfer Through the Eastern Tropical North Pacific Oxygen Deficient Zone

“…Such a community would likely be comprised primarily of small organisms which the EK60 is not able to measure at this depth. One possible source of horizontal transport is colloids in a deep iron plume (Homoky et al., 2021; Lam et al., 2020).…”