Redox-sensitive element uptake in north-east Atlantic Ocean sediments (Benthic Boundary Layer Experiment sites)

Regularly increasing radiocarbon age–depth profiles and near-constant sediment composition with depth have demonstrated that sediment accumulation has been relatively constant during the late Holocene at two north-east Atlantic sites on Rockall Bank and Feni Drift (UK Benthic Boundary Layer Experiment sites). In such a quasi-steady state situation, the geochemical responses to early diagenesis of the redox-sensitive elements Cd, Mn, Mo, Re, Se and U are readily discerned through changes in element concentrations with depth. Collectively, a colour change in the sediments, surficial MnOx-enriched layers and 210Pbexcess profiles allow an estimate of the mean position of the oxic–post-oxic boundary in the sediments at both sites. This boundary is situated deeper than the 210Pbexcess surface mixed layer but shallower than the 14C surface mixed layer which represent mixing on 102 and 103 year time scales, respectively. This implies that the well-defined surface MnOx enrichment and its associated 226Ra and Mo fractions must have been efficiently recycled first by downwards mixing into anoxic conditions, and then by subsequent reduction and diffusive migration back up into oxic conditions. This occurs in <40 years at the Feni Drift site and in <103 years at the Rockall Bank site. Authigenic enrichments of Se, Cd and U are evident immediately below the oxic–post-oxic boundary, although Re enrichment does not occur until a few centimetres deeper. The long-term fluxes of authigenic Se, Cd, U and Re uptake are evaluated with steady-state assumptions. While diffusion from bottom waters can supply sufficient Se, U and Re, an additional source (likely Corg) is required for Cd.