『Abstract
Grasslands of north-central Kansas are underlain by carbonate
aquifers and shale aquitards. Chemical weathering rates in carbonates
are poorly known, and, because large areas are underlain by these
rocks, solute fluxes are important to estimating global weathering
rates. Grasslands exist where the amount of precipitation is extremely
variable, both within and between years, so studies in grasslands
must account for changes in weathering that accompany changes
in precipitation. This study: (1) identifies phases that dominate
chemical fluxes at Konza Prairie Biological Station (KPBS) and
Long-Term Ecological Research Site, and (2) addresses the impact
of variable precipitation on mineral weathering. The study site
is a remnant tallgrass prairie in the central USA, representing
baseline weathering in a mid-temperate climate grassland.
Groundwater chemistry and hydrology in the 1.2 km2
watershed used for this study suggest close connections between
groundwater and surface water. Water levels fluctuate seasonally.
High water levels coincide with periods of precipitation plus
low evapotranspiration rather than during precipitation peaks
during the growing season. Precipitation is concentrated before
recharging aquifers, suggesting an as yet unquantified residence
time in the thin soils.
Groundwater and surface water are oversaturated with respect
to calcite within limitations of available data. Water is more
dilute in more permeable aquifers, and water from one aquifer
(Morrill) is indistinguishable from surface water. Cations other
than Ca co-vary with each other, especially Sr and Mg. Potassium
and Si co-vary in all aquifers and surface water, and increases
in concentrations of these elements are the best indicators of
silicate weathering at this study site. Silicate-weathering indices
correlate inversely to aquifer hydraulic conductivity.
87Sr/86Sr in water ranges from 0.70838
to 0.70901, and it decreases with increasing Sr concentration
and with increasing silicate-weathering index. Carbonate extracted
from aquifer materials, shales, soil, and tufa has Sr ranging
from about 240 (soil) to 880 ppm(Paleozoic limestone). 87Sr/86Sr
ranges from 0.70834±0.00006 (limestone) to 0.70904±0.00019 (soil).
In all cases, 87Sr/86Sr of aquifer limestone
is lower than 87Sr/86Sr of groundwater,
indicating a phase in addition to aquifer carbonate is contributing
solutes to water.
Aquifer recharge controls weathering: during periods of reduced
recharge, increased residence time increases the total amount
of all phases dissolved. Mixing analysis using 87Sr/86Sr
shows that two end members are sufficient to explain sources of
dissolved Sr. It is proposed that the less radiogenic end member
is a solution derived from dissolving aquifer material; longer
residence time increases its contribution. The more radiogenic
end member solution probably results from reaction with soil carbonate
or eolian dust. This solution dominates solute flux in all but
the least permeable aquifer and demonstrates the importance that
land-surface and soil-zone reactions have on groundwater chemistry
in a carbonate terrain.』
1. Introduction
2. Setting
2.1. Geology
2.2. Climate and precipitation
2.3. Hydrology and physical hydrogeology
3. Methodology
4. Results
4.1. Aqueous geochemistry
4.2. Isotope geochemistry of solutes
4.3. Isotope geochemistry of solids
5. Discussion
5.1. Fatmosphere: atmospheric input
5.2. Fweathering: Sr isotope ratios as discriminators
of weathering phase
5.3. Proportions of phases weathering at KPBS
5.4. Weathering and recharge at KPBS
6. Summary
Acknowledgements
References