| dc.description.abstract | A combination of mineralogical (SEM–EDS, EMPA) and geochemical (redox dynamics, selective extractions)
approaches was applied to mining- and smelting-impacted sediments from Lake Uru Uru and from
the Cala Cala Lagoon, a non-impacted reference site, in the Bolivian Altiplano. The purpose was to assess
the factors controlling the post-depositional redistribution (mobilization/sequestration) of trace metals
and metalloids. As expected, trace metals and metalloids are less reactive at the reference site than in
Lake Uru Uru. In the latter, trace metals and metalloids are principally hosted by Fe- and Mn-oxyhydroxides,
authigenic sulfides and silicate particulates. Post-depositional redistribution is ascribed to early
diagenetic processes driven by organic matter (OM) mineralization, including dissolution of trace
metal-bearing phases and precipitation of authigenic sulfide and carbonate phases. Seasonal climate variability
exerts a strong influence on these processes. Evaporation of surface water during the dry season
in the northern part of Lake Uru Uru promotes large redox front oscillations in the sediments and, therefore,
transient redox conditions, likely leading to the weakening of anoxia in near-surface sediments and
to a ‘compressed’ redox zonation compared to the southern part of the lake. Seasonal disappearance of
the water column in the Northern Lake Uru Uru entails an alternation of: (i) low trace element mobility
in the dry season due to elemental precipitation; and (ii) an increase of trace element mobility via diffusive
transport during the wet season due to release from OM, Fe- and Mn-oxyhydroxides and carbonates
upon mineralization, reductive dissolution and destabilization processes, respectively. Reoxidation of
authigenic Fe-sulfides likely following the re-instalment of the water column above the sediment at
the end of the dry season and prior to the return of anoxia probably favors transport of trace elements
to the water column. As a consequence of this intricate web of redox- and climate-related processes, both
northern and southern sediments of Lake Uru Uru are a source of dissolved trace elements, particularly As
and Cd, for the overlying water column via diffusive transport. However, much of the diffused As and Cd
is likely to be removed from the dissolved phase by the redox loop through Fe-oxyhydroxide precipitation.
Finally, it is suggested that long term sequestration of trace metals and metalloids in the sediments
could be controlled by inter-annual climate variability such as ENSO events. | |
