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Mehlhorn, Judith ; Gerber, Lukas ; Planer-Friedrich, Britta:
Spatial distribution of metal(loid) depletion and accumulation zones around a natural carbon dioxide degassing site.
In: Chemical Geology.
Bd. 509
(2019)
.
- S. 64-76.
ISSN 0009-2541
DOI: https://doi.org/10.1016/j.chemgeo.2019.01.010
Abstract
Long-term influence of geogenic CO2 affects soil conditions and pedogenesis. Mobilization of metals and metalloids from soil to solution has been reported to occur in natural CO2 degassing sites, so-called mofettes. We determined metal(loid)-specific spatial distribution patterns in soil around a mofette as well as metal(loid) pore water concentrations along a CO2 gradient (0-98% CO2 in soil air). Depletion of Mn, Co, and Ni in soil in the mofette center was caused by leaching due to the long-term soil acidification leading to correspondingly low pore water concentrations and thus indicating no recent influence of CO2 on metal(loid) mobility. Iron and As were also depleted in soil within the mofette center where pedogenic Fe (oxyhydr)oxides could not form due to absence of oxygen. Small-scale variations in redox conditions lead to ongoing Fe cycling and the repeated reduction of Fe (oxyhydr)oxides resulted in increased Fe and As pore water concentrations at high CO2. Precipitation in form of sulfide minerals caused immobilization and accumulation of Cd, Cu, and Zn directly in the degassing center. The highest mobilization risk occurred within 2-4 m distance from the degassing center, where complexation with dissolved organic matter (DOM) increased the mobility of Al, As, Cr, Cu, Fe, and Zn. Our results show that CO2 as soil-forming factor influences the spatial distribution of metal(loid)s. The highest metal(loid) mobilization risk after long-term CO2 influence arises from accumulation of scarcely degraded organic matter, which can easily dissolve and form mobile metal(loid)-DOM complexes.