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Peru could be related to the particle fl ux of diatomaceous matter to the sediment
(Böning et al.
2004
). In the suboxic sediment below the OMZ, the observation of
'low-opal sediments' and of a relationship between opal and silver concentrations
was attributed to particle dissolution as they settle in the deep water column and the
subsequent remineralization of opal and silver in equal proportions. 'High-opal
sediments' were observed in anoxic sediments within and in the upper edge of the
OMZ. Consequently, silver distributions there were likely related to an early diage-
netic fi xation with Total Organic Carbon (TOC) or Total Sulfur (TS) during opal
dissolution, refl ecting a redox-control of silver fi xation.
Although organic matter and Ag
2
S species seem to play a role in silver fi xation,
Crusius and Thomson (
2003
) suggested that it may also be sequestered by precipita-
tion of silver selenide species (AgSe or Ag
2
Se) as a result of oxygen exposure of
sediments that were initially anoxic. In a subsequent study conducted off Chile,
Böning et al. (
2005
) found that silver content in sediment increased with water
depth, which they interpreted as an indication that silver enrichment was controlled
by opaline regeneration and/or by a higher availability of silver in seawater. A redox
control of silver content was discarded in that case, based on the contrast with Cd
and Re contents, which decrease with increasing water depth and refl ect decreasing
reducing conditions.
The lack of redox infl uence on silver content was also observed by McKay and
Pedersen (
2008
). They measured concentrations of silver in surface sediments from
the Western Canadian, Mexican, Peruvian, and Chilean continental margins, and
also observed that concentrations increased with water depth. The lack of correla-
tion observed between the concentrations of silver and redox-sensitive trace metals
(Re, Cd, and Mo) led them to conclude that silver accumulation was not controlled
by sedimentary redox conditions. Instead, they hypothesized that silver accumula-
tion results from its scavenging decaying organic particles as they settle through the
water column. More specifi cally, they proposed that silver precipitates as Ag
2
S
within anoxic microenvironments that develop as a result of organic degradation
inside sinking organic particles. In addition, McKay and Pedersen (
2008
) observed
a positive correlation between silver and barium in surface and near surface sedi-
ments, suggesting similar mechanisms of enrichment. Because barium is commonly
used as a tracer of paleoproductivity, these similarities suggest that silver may also
be used as a paleoproxy.
Morford et al. (
2008
) further highlighted the highly dynamic process of silver
diagenesis through the analysis of porewater and sediment profi les in the northeast
Pacifi c. They observed background concentrations of silver in oxic sediments asso-
ciated with high levels in pore waters, indicating a fl ux of silver from the sediments
to the overlying water. In addition, increasing porewater concentrations with water
column depth led them to theorize that silver is delivered to the sediments by scav-
enging—sorbing onto settling particles (a longer path leading to an associated
higher content of silver), and subsequently being released to pore waters. Similarly,
erosion chamber experiments by Kalnejais et al. (
2010
) showed that silver can be
released from sediments to the dissolved phase during erosion events, and over
longer time scales in association with the reaction of suspended particles in the
water column.
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