Trace Metal Effluxes From Peruvian Shelf Sediments Constrained in Parallel by Benthic Lander Mounted Pumps and Pelagic Rosette Sampling

Abstract

Shelf sediments receive trace metals (TM) from sinking particles and return TMs to the water column via dissolved effluxes and resuspension events. For dissolved redox sensitive elements such as iron (Fe), shelf sediments underneath oxygen minimum zones are typically a major source to the water column. However, quantifying the associated dissolved fluxes is methodologically challenging. During an oceanographic campaign to the Peruvian shelf, we deployed benthic landers fitted with in situ lander pumps and a GEOTRACES compliant sampling rosette system. By overlapping the deployment of these instruments, we aimed to quantitatively assess the known deficiencies in assessment of TM concentrations and fluxes close to the seafloor. Across 7 locations over the Peruvian shelf, we observed consistent spatial trends between the two approaches with <20% difference for measured dissolved copper, nickel, and silicic acid concentrations. Dissolved Fe gradients were however notably stronger near the seafloor. Samples from in situ lander pumps typically suggested much higher concentrations at 0.5–5 m from the seafloor than samples from the rosette system deployed within 2 m of the seafloor. Similarly, estimated diffusive dissolved Fe (dFe) fluxes were a factor of 30–570× higher when derived using the concentration gradient from the in situ lander pumps over the inner/midshelf. The calculated residence time of dFe thus varies markedly with a range from 5 days to 7 years depending on how the benthic flux is constrained. These differences largely reflect the rapid attenuation of dFe concentrations from benthic effluxes by both scavenging and dilution on spatial scales of ∼0.1–10 m.