Sources, Fluxes, and Biogeochemical Cycling of Silver in the Oceans - Reviews of Environmental Contamination and Toxicology - page 38

further suggested the predominance of aeolian fl uxes of silver from Asian industrial
emissions to the North Pacifi c—again with the qualifi cation that the biogeochemical
processes governing the residence times of silver and selenium in oceanic surface
waters are also quite dissimilar. Because natural emissions measurably contribute to
the aeolian fl uxes of selenium—but are presumably negligible for silver—in the
North Pacifi c, variations in the [Ag]:[Se] ratios may be used to distinguish between
natural and industrial fl uxes of selenium to those oceanic waters.
However, Zhang et al. ( 2004 ) proposed that different, natural processes accounted
for the distribution of silver in the northwest Pacifi c. They observed that silver con-
centrations increased with latitude in the surface waters of the Bering Sea, the cen-
tral North and South Pacifi c and the Southern Ocean. They attributed the increased
silver in those waters to an upwelling and vertical mixing similar to that of silica.
They also attributed elevated Ag/Si ratios in the surface vs. deep waters to differ-
ences in biological uptake. Then, they used a scavenging/regeneration model to
characterize the Ag-Si relationship.
Finally, Kramer et al. ( 2011 ) recently proposed another natural process to account
for the distribution of silver in the North Pacifi c. Their account was based on correla-
tions between low oxygen and silver concentrations in the broad subsurface oxygen
minimum zone (OMZ) of the North Pacifi c, which could produce the spatial vari-
ability observed in profi les in the central eastern Pacifi c. Specifi cally, they noted
that Ag:Si profi les and oxygen profi les present relatively similar shapes, and that
the intensity of the subsurface Ag/Si minima follows the same trend as that of the
OMZ. In Fig. 8 , we show Ag:Si and O 2 profi les in the Pacifi c Ocean that are available
in the literature, as well as a plot of Ag:Si vs. O 2 . These observations were interpreted
by Kramer et al. ( 2011 ) as an indication that dissolved oxygen content could apply a
secondary control on the dissolved silver concentration.
Kramer et al. ( 2011 ) also hypothesized that silver may be removed from oxygen-
depleted waters by scavenging and/or precipitation of AgS species and subsequent
sequestration in the underlying reducing sediments. Such mechanisms could be the
result of an exchange of dissolved silver with thermodynamically less stable metal-
sulphide nanoclusters (e.g., Cu, Cd, Zn) present in the oxic water column or its
complexation with nanomolar concentrations of free sulphide, as described by
Rozan and Luther ( 2009 ). In addition, silver may be scavenged in the water column
and precipitated as Ag 2 S in anoxic microenvironments within settling decaying
organic matter, as proposed by McKay and Pedersen ( 2008 ).

3.2 Sediments

Similarly to waters, few researchers have investigated the distribution and biogeo-
chemical cycling of silver in marine sediments. The relative paucity of information
is illustrated by the fact that we found only ten peer-reviewed references on silver
in marine sediments, which are listed in Table 2 . This absence of information is
surprising, because there is potential for silver (as a biogeochemical analogue of
silicate) to be used as a proxy of past diatom fl uxes to the sea fl oor. But the