wAbstract
@In the past few decades numerous studies have quantified the
load of dissolved solids in large rivers to determine chemical
weathering rates in orogenic belts and volcanic areas, mainly
motivated by the notion that over timescales greater than `100
kyr, silicate hydrolysis may be the dominant sink for atmospheric
CO2, thus creating a feedback between climate
and weathering. Here, we report the results of a detailed study
during water year 2007 (October 1, 2006 to September 30, 2007)
in the major rivers of the Yellowstone Plateau Volcanic Field
(YPVF) which hosts Earth's largest grestlessh caldera and over
10,000 thermal features. The chemical compositions of rivers that
drain thermal areas in the YPVF differs significantly from the
compositions of rivers that drain non-thermal areas. There are
large seasonal variations in river chemistry and solute flux,
which increases with increasing water discharge. The river chemistry
and discharge data collected periodically over an entire year
allow us to constrain the annual solute fluxes and to distinguish
between low-temperature weathering and hydrothermal flux components.
The TDS flux from Yellowstone Caldera in water year 2007 was 93
t/km2/year. Extensive magma degassing and hydrothermal
interaction with rocks accounts for at least 82 of this TDS flux,
83 of the cation flux and 72 of the HCO3-
flux. The low-temperature chemical weathering rate (17 t/km2/year),
calculated on the assumption that all the Cl- is of
thermal origin, could include a component from low-temperature
hydrolysis reactions induced by CO2 ascending
from depth rather than by atmospheric CO2.
Although this uncertainty remains, the calculated low-temperature
weathering rate of the young rhyolitic rocks in the Yellowstone
Caldera is comparable to the world average of large watersheds
that drain also more soluble carbonates and evaporates but is
slightly lower than calculated rates in other, less-silicic volcanic
regions. Long-term average fluxes at Yellowstone are likely `20
higher than those in the abnormally dry water year 2007, but the
protocol used in this study can be easily adaptable to track future
changes in low-temperature weathering and hydrothermal flux components,
which could provide better monitoring of magmatic unrest.
Keywords: Yellowstone; Chemical weathering; Hydrothermal; Magmatic;
River chemistry; Denudationx
1. Introduction
2. Geologic setting
3. Climate and hydrology of the Yellowstone Plateau
4. Methods
@4.1. Sampling and analytical procedures
@4.2. Solute flux determinations
5. River chemistry and solute flux
@5.1. Chemical composition
@5.2. Solute flux and denudation rates
6. Discussion
@6.1. Atmospheric CO2 vs. acid gas-hydrothermal
weathering
@@6.1.1. Hydrolysis by magma-derived acid gases
@@6.1.2. Mixing of thermal and low-temperature components
@6.2. Comparison with chemical weathering rates in other watersheds
@6.3. Implications for chemical weathering studies in active volcanic
and tectonic areas
7. Conclusions
Acknowledgments
Appendix A. Supplementary data
References