chemical weathering

Erosional and climatic effects on long-term chemical weathering rates in granitic landscapes spanning diverse climate regimes

C. Riebe, J.W. Kirchner, R. Finkel, Erosional and climatic
effects on long-term chemical weathering rates in granitic C. Riebe, J.W. Kirchner, R. Finkel, Erosional and climatic
effects on long-term chemical weathering rates in granitic

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 jC, average precipitation ranging from 22 to 420 cmyear 1, and denudation rates ranging from 23 to 755 tkm 2year 1. Long-term chemical weathering rates range from 0 to 173 tkm 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 freshmaterial 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.

Influence of landscape position and vegetation on long-term weathering rates at the Hubbard Brook Experimental Forest, New Hampshire, USA

Nezat, C. A., J. D. Blum, A. Klaue, C. E. Johnson, and T. G. Siccama
(2004), Influence of landscape positions and vegetation on long-term
weathering rates at the Hubbard Brook Experimental Forest, New
Hampshire, USA, Geochem. Cosmochim. Acta, 68(14), 3065– 3078.

Abstract: 
The spatial variability of long-term chemical weathering in a small watershed was examined to determine the effect of landscape position and vegetation. We sampled soils from forty-five soil pits within an 11.8-hectare watershed at the Hubbard Brook Experimental Forest, New Hampshire. The soil parent material is a relatively homogeneous glacial till deposited 14,000 years ago and is derived predominantly from granodiorite and pelitic schist. Conifers are abundant in the upper third of the watershed while the remaining portion is dominated by hardwoods. The average long-term chemical weathering rate in the watershed, calculated by the loss of base cations integrated over the soil profile, is 35 meq m2 yr1—similar to rates in other 10 to 15 ka old soils developed on granitic till in temperate climates. The present-day loss of base cations from the watershed, calculated by watershed mass balance, exceeds the long-term weathering rate, suggesting that the pool of exchangeable base cations in the soil is being diminished. Despite the homogeneity of the soil parent material in the watershed, long-term weathering rates decrease by a factor of two over a 260 m decrease in elevation. Estimated weathering rates of plagioclase, potassium feldspar and apatite are greater in the upper part of the watershed where conifers are abundant and glacial till is thin. The intra-watershed variability across this small area demonstrates the need for extensive sampling to obtain accurate watershed-wide estimates of long-term weathering rates.

Extreme storm events, landscape denudation, and carbon sequestration: Typhoon Mindulle, Choshui River, Taiwan

Goldsmith, S. T.; Carey, A. E.; Lyons, W. B.; Kao, S. J.; Lee, T. Y.;
Chen, J. Extreme storm events, landscape denudation, and
carbon sequestration: Typhoon Mindulle, Choshui River, Taiwan.
Geology 2008, 36 (6), 483–486.

Abstract: 
We have performed the fi rst known semicontinuous monitoring of particulate organic carbon (POC) fl uxes and dissolved Si concentrations delivered to the ocean during a typhoon. Sampling of the Choshui River in Taiwan during Typhoon Mindulle in 2004 revealed a POC fl ux of 5.00 × 105 t associated with a sediment fl ux of 61 Mt during a 96 h period. The linkage of high amounts of POC with sediment concentrations capable of generating a hyperpycnal plume upon reaching the ocean provides the fi rst known evidence for the rapid delivery and burial of POC from the terrestrial system. These fl uxes, when combined with storm-derived CO2 consumption of 1.65 × 108 mol from silicate weathering, elucidate the important role of these tropical cyclone events on small mountainous rivers as a global sink of CO2.
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