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| Titel |
Balance of pCO2 in the carbonate-bearing unsaturated zone controlling mineral precipitation: Implications for geo-environmental analyses in caves |
| VerfasserIn |
S. Sanchez-Moral, A. Fernandez-Cortes, S. Cuezva, E. Garcia-Anton, D. Benavente |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250059027
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| Zusammenfassung |
| The chemical composition of shallow groundwater reflects quickly and effectively any
changes occurring in atmospheric and soil-substrate conditions in catchment area.
Hydrobiogeochemical processes are highly dynamic at the seasonal and shorter temporal
scales. Weathering processes in carbonate substrates involve a series of chemical reactions
that implies short-term contributions to the net ecosystem carbon balance via consumption or
emission of CO2. After rainwater infiltrates into the soil, CO2-enriched water reaches the
underlying unsaturated porous rock (vadose zone). The imbalance of CO2-partial pressure
(pCO2) between the infiltration soil-derived water and the cave air favor the processes of
mineral precipitation. Precipitation of carbonate mineral in subsurface environments implies
long-term net carbon sequestration, but also short-term CO2 release from the substrate to
the atmosphere. Moreover, the variations of cave air pCO2, on daily to seasonal
timescales, potentially affect the timing of mineral deposition and, consequently, the
Ca-mineral proxy climate records derived from stalagmites collected at the same
sites.
In this study an exhaustive monitoring program of the geochemical characteristics of
seepage water and mineral precipitation rates was carried out in several karstic caves from
Spain. These caves represent a wide range of possible karst scenarios with different
geological and environmental features as well as several management models. The changes
on the oversaturation/precipitation states of carbonated mineral are controlled by the
CO2-recharge or depletion of the cave atmospheres, which is under the direct influence of the
external conditions. For each cave environment an inherent stationary range of pCO2 has
always been identified, within which two coeval processes are detected: 1) slight and gradual
mineral oversaturation of the seepage water with low precipitation rates of calcite and
aragonite, and 2) decrease of the subsaturation degree of the Mg2+-carbonate phases. These
thermodynamically equilibrium ranges for pCO2 determine the type and rate of mineral
precipitation in each cave. Overcoming this theoretical pCO2 range could entail
an inhibition the mineral deposition or, even provoke the beginning of corrosion
process. Conversely, the intense depletion of pCO2 level below this range would alter
the type of calcium-carbonated phases that contributes in the current speleothems
growth.
Study results demonstrate the subsurface water is an indicator to understand the
geochemical processes in the local carbon cycle on short timescales, especially
those related to terrestrial ecosystems located in karst terrains. The environmental
conditions, related to the suitable range of CO2 defined for each cave environment,
should be considered in order to avoid any alteration of the deposition rates and
mineral typology of speleothems, both to establish correct climate-proxy relationships
in speleothems and channeling the management efforts into cave conservation. |
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