Geology Reference
In-Depth Information
arsenic are also recycled but in their case it is the negatively charged
anions that are released into solution from an association with solid
phase ferric (hydr)oxides, which dissolve under suciently reducing
conditions. Upward diffusion of reduced species leads ultimately to
surface enrichment via co-precipitation or adsorption of phosphate and
arsenate on iron (hydr)oxides in surface layers. As a result, arsenic
concentrations as high as 675 mg kg
1
can be found near the surface of
Loch Lomond sediments, compared with background values of 15-50
mg kg
1
at depth.
114
Enrichments of phosphorus and arsenic have
similarly been found in association with authigenic iron oxides in
sedimentary Fe/Mn layers in Lake Baikal where, however, enrichments
of molybdenum were found associated with manganese oxides, perhaps
through mixed oxide formation.
115
For lakes which have undergone significant acidification, it has been
suggested that heavy metals could be released from surface sections by
pH-dependent dissolution, resulting in sub-surface maxima in sedimen-
tary heavy metal concentrations. In two Canadian acid lakes, however,
Carignan and Tessier
116
found that downward diffusive fluxes of
dissolved zinc from overlying waters into anoxic pore waters were
responsible for the pronounced sub-surface sediment maxima in solid
phase zinc, presumably as the insoluble sulfide.
Traditionally, heavy metals such as lead, zinc, copper, and cadmium have
been considered diagenetically (i.e. as a consequence of chemically and
biologically induced changes in prevailing sedimentary conditions, e.g. pe,
anionic composition, etc.) immobile and fixed in the sediment after
deposition, partly as a result of the formation of comparatively insoluble
sulfides under reducing conditions at depth. This may well be an over-
simplified view, as interactions and formation of complexes between metals
and dissolved organic matter (e.g. humic substances) may maintain
dissolved metal concentrations at levels greater than those predicted by
simple solubility product calculations (see Sections 3.2.3.5 and 3.2.4.1).
117
A
combination of geochemical modelling, incorporating interactions of me-
tals with mineral phases and naturally occurring organic matter,
34,118
and
the development of increasingly sophisticated analytical technology, such
asDET(diffusiveequ ibrationinthinfilms)
119
and DGT (diffusive
gradients in thin films) for the high-resolution determination on a sub-
millimetre scale of metals in sedimentary pore waters,
120
offers considerable
promise for greater understanding of redox-driven cycling of trace elements
in lakes. Not only have fine structure and sharp-featured maxima been
observed for manganese, iron, sulfide, and some trace metals in pore
waters, but two-dimensional structure in concentration profiles has demon-
strated the potential
importance of sedimentary microniches for metal
