Estimating benthic Fe and reactive solute fluxes

The flux of dissolved Fe (Fe_d) from sediments is a critical component of the biogeochemical Fe cycle. Common approaches to estimating these fluxes are based on modeling of Fe_d porewater profiles or incubating sediment and overlying water using benthic chambers or retrieved cores. Inadequate resolution of pore water Fe profiles in the uppermost mm of sediments and rapid Fe²⁺oxidation can greatly compromise these approaches. These limitations may be partially responsible for large variations in reported benthic Fe_d fluxes at given values of independent controlling variables such as sedimentary C_org remineralization rates. A series of laboratory experiments were conducted to compare methods to estimate Fe fluxes and confirmed that Fe_d²⁺ is oxidized to Fe³⁺ within a few minutes in oxygenated chamber water. The resulting Fe³⁺ colloids do not pass through 0.2 μm pore size filters and are also rapidly lost from suspension. Loss from suspension is partially reversible. Most benthic chamber studies do not correct for such losses and likely significantly underestimate Fe_d fluxes. Additional reactive components such as P and trace metals that associate with precipitated Fe-oxides must be subject to similar major losses during flux measurements. We modified the standard incubation methods by circulating overlying water through Fe extractors and accumulating Fe_d released into chambers over a known period at constant O₂ and pH. We found using a range of experimental conditions that in cylindrical chambers ∼70–93% of introduced Fe²⁺ is typically recovered on accumulators at saturated O₂ (∼270 μM) and seawater pH (∼8; NBS). We recommend that flux incubations that target reactive solutes (e.g., Fe, P) be configured with flow-through extractors, and pre-calibrated for losses specific to chamber designs, chamber water residence times, and deployment conditions (e.g., O₂, pH).