『Abstract
We used cosmogenic nuclide and geochemical mass balance methods
to measure long-term rates of chemical weathering and total denudation
in granitic landscapes in diverse climatic regimes. Our 42 study
sites encompass widely varying climatic and erosional regimes,
with mean annual temperatures ranging from 2 to 25℃, average precipitation
ranging from 22 to 420 cm・year-1, and denudation rates
ranging from 23 to 755 t・km-2・year-1. Long-term
chemical weathering rates range from 0 to 173 t・km-2・year-1,
in several cases exceeding the highest granitic weathering rates
on record from previous work. Chemical weathering rates are highest
at the sites with rapid denudation rates, consistent with strong
coupling between rates of chemical weathering and mineral supply
from breakdown of rock. A simple empirical relationship based
on temperature, precipitation and long-term denudation rates explains
89-95％ of the variation in long-term weathering rates across our
network of sites. Our analysis shows that, for a given precipitation
and temperature, chemical weathering rates increase proportionally
with fresh-material supply rates. We refer to this as “supply-limited”
weathering, in which fresh material is chemically depleted to
roughly the same degree, regardless of its rate of supply from
breakdown of rock. The temperature sensitivity of chemical weathering
rates is two to four times smaller than what one would expect
from laboratory measurements of activation energies for feldspar
weathering and previous inter-comparisons of catchment mass-balance
data from the field. Our results suggest that climate change feedbacks
between temperature and silicate weathering rates may be weaker
than previously thought, at least in actively eroding, unglaciated
terrain similar to our study sites. To the extent that chemical
weathering rates are supply-limited in mountainous landscapes,
factors that regulate rates of mineral supply from erosion, such
as tectonic uplift, may lead to significant fluctuations in global
climate over the long term.
Keywords: chemical weathering; granitic landscape; precipitation;
physical erosion; temperature; climate』
1. Introduction
2. Methods, field sites and sampling
2.1. Chemical weathering rates from immobile element enrichment
in eroding landscapes
2.2. Quantifying denudation rates with cosmogenic nuclides
2.3. Field sites: general information
2.4. Sampling and analysis
3. Results and discussion
3.1. Coupling of chemical weathering and total denudation
3.2. Quantifying erosional and climatic effects on chemical weathering
3.3. Predicted chemical depletion fractions
3.4. Supply-limited versus weathering-limited behavior
3.5. Temperature-dependence of chemical weathering rates
4. Conclusions and implications
Acknowledgements
References