『Abstract
During weathering, elements enriched in black shale are dispersed
in the environment by aqueous and mechanical transport. Here a
unique evaluation of the differential release, transport, and
fate of Fe and 15 trace elements during progressive weathering
of the Devonian New Albany Shale in Kentucky is presented. Results
of chemical analyses along a weathering profile (unweathered through
progressively weathered shale to soil) describe the chemically
distinct pathways of the trace elements and the rate that elements
are transferred into the broader, local environment. Trace elements
enriched in the unweathered shale are in massive or framboidal
pyrite, minor sphalerite, CuS and NiS phases, organic matter and
clay minerals. These phases are subject to varying degrees and
rates of alteration along the profile. Cadmium, Co, Mn, Ni, and
Zn are removed from weathered shale during sulfide-mineral oxidation
and transported primarily in aqueous solution. The aqueous fluxes
for these trace elements range from 0.1 g/ha/a (Cd) to 44 g/ha/a
(Mn). When hydrologic and climatic conditions are favorable, solutions
seep to surface exposures, evaporate, and form Fe-sulfate efflorescent
salts rich in these elements. Elements that remain dissolved in
the low pH (＜4) streams ad groundwater draining New Albany Shale
watersheds become fixed by reactions that increase pH. Neutralization
of the weathering solution in local streams results in elements
being adsorbed and precipitated onto sediments surfaces, resulting
in trace element anomalies.
Other elements are strongly adsorbed or structurally bound to
solid phases during weathering. Copper and U initially are concentrated
in weathering solutions, but become fixed to modern plant litter
in soil formed on New Albany shale. Molybdenum, Pb, Sb, and Se
are released from sulfide minerals and organic matter by oxidation
and accumulate in Fe-oxyhydroxide clay coatings that concentrate
in surface soil during illuviation. Chromium, Ti, and V are strongly
correlated with clay abundance and considered to be in the structure
of illitic clay. Illite undergoes minimal alteration during weathering
and is concentrated during illuvial processes. Arsenic concentration
increases across the weathering profile and is associated with
the succession of secondary Fe(III) minerals that form with progressive
weathering. Detrital fluxes of particle-bound trace elements range
from 0.1 g/ha/a (Sb) to 8 g/ha/a (Mo). Although many of the elements
are concentrated in the stream sediments, changes in pH and redox
conditions along the sediment transport path could facilitate
their release for aqueous transport.』
1. Introduction
2. Study site
3. Methods
3.1. Sample collection
3.2. Mineral, chemical, and isotopic analyses
3.3. Efflorescent salt characterization and dissolution/precipitation
experiments
3.4. Geochemical modeling
4. Results
4.1. Bulk sample chemistry
4.2. Stream-sediment samples
4.3. Trace-element content of sulfide minerals
4.4. Efflorescent salts
4.5. Element; Sdisulfide ratios
5. Discussion
5.1. Residence of trace elements in unweathered New Albany
Shale
5.2. Sulfide oxidation and formation of secondary Fe phases
5.3. Modeling changes in element concentrations
5.4. Element mobility
5.5. Impact of trace-element mobility
5.5.1. Element flux rates
5.5.2. Stream sediments
6. Conclusions
Acknowledgements
References