J. Thomson et al., Redox-sensitive element uptake in north-east Atlantic Ocean sediments (Benthic Boundary Layer Experiment sites), EARTH PLAN, 184(2), 2001, pp. 535-547
Regularly increasing radiocarbon age-depth profiles and near-constant sedim
ent composition with depth have demonstrated that sediment accumulation has
been relatively constant during the late Holocene at two north-east Atlant
ic sites on Rockall Bank and Feni Drift (UK Benthic Boundary Layer Experime
nt sites). In such a quasi-steady state situation, the geochemical response
s to early diagenesis of the redox-sensitive elements Cd, Mn, Mo, Re, Se an
d U are readily discerned through changes in element concentrations with de
pth. Collectively, a colour change in the sediments, surficial MnOx-enriche
d layers and Pb-210(excess) profiles allow an estimate of the mean position
of the oxic-post-oxic boundary in the sediments at both sites. This bounda
ry is situated deeper than the Pb-210(excess) surface mixed layer but shall
ower than the C-14 surface mixed layer which represent mixing on 10(2) and
10(3) year time scales, respectively. This implies that the well-defined su
rface MnOx enrichment and its associated Ra-226 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 < 10(3
) years at the Rockall Bank site. Authigenic enrichments of Se, Cd and U ar
e evident immediately below the oxic-post-oxic boundary, although Re enrich
ment 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 assumpt
ions. While diffusion from bottom waters can supply sufficient Se, U and Re
, an additional source (likely C-org) is required for Cd. (C) 2001 Elsevier
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