『Abstract
Chemical weathering of silicate minerals has long been known
as a sink for atmospheric CO2, and feedbacks
between weathering and climate are believed to affect global climate.
While warmer temperatures are believed to increase rates of weathering,
weathering in cool climates can be accelerated by increased mineral
exposure due to mechanical weathering by ice. In this study, chemical
weathering of silicate minerals is investigated in a small temperate
watershed. The Jamieson Creek watershed is covered by mature coniferous
forest and receives high annual precipitation (4000 mm), mostly
in the form of rainfall, and is underlain by quartz diorite bedrock
and glacial till. Analysis of pore water concentration gradients
indicates that weathering in hydraulically unsaturated ablation
till is dominated by dissolution of plagioclase and hornblende.
However, a watershed scale solute mass balance indicates high
relative fluxes of K and Ca, indicating preferential leaching
of these solutes possibly from the relatively unweathered lodgement
till. Weathering rates for plagioclase and hornblende calculated
from a watershed scale solute mass balance are similar in magnitude
to rates determined using pore water concentration gradients.
When compared to the Rio Icacos basin in Puerto Rico, a pristine
tropical watershed with similar annual precipitation and bedrock,
but with dissimilar regolith properties, fluxes of weathering
products in stream discharge from the warmer site are 1.8 to 16.2-fold
higher, respectively, and regolith profile-averaged plagioclase
weathering rates are 3.8 to 9.0-fold higher. This suggests that
the Arrhenius effect, which predicts a 3.5- to 9-fold increase
in the dissolution rate of plagioclase as temperature is increased
from 3.4゜ to 22℃, may explain the greater weathering fluxes and
rates at the Rio Icacos site. However, more modest differences
in K and Ca fluxes between the two sites are attributed to accelerated
leaching of those solutes from glacial till at Jamieson Creek.
Our findings suggest that under conditions of high rainfall and
favorable topography, weathering rates of silicate minerals in
warm tropical systems will tend to be higher than in cool temperate
systems, even if the temperate system is has been perturbed by
an episode of glaciation that deposits regolith high in fresh
mineral surface area.
Keywords: Chemical weathering; Watershed; Diorite; Solute mass
balance; Silicates; Glacial till』
1. Introduction
1.1. Effects of temperature on weathering
1.2. Study area: Jamieson Creek, British Columbia
2. Methods
3. Results
3.1. Rock and regolith properties
3.2. Watershed solute fluxes
3.3. Strontium stable isotope ratios
3.4. Pore and stream water chemistry
4. Discussion
4.1. Availability of mineral matter to weathering
4.2. Weathering on a watershed scale
4.3. Reactive transport on a regolith profile scale
4.4. Apparent enhancement of weathering due to temperature
4.5. Implications for the effects of temperature on weathering
rates
5. Conclusions
Acknowledgements
References