Controls on granitic weathering fronts in contrasting climates

Granitic weathering profiles display highly diverse morphologies, reflecting the complex relationships between climate and weathering rates. Some profiles exhibit abrupt transitions from fresh bedrock to highly weathered material over short ( < 1 m) distances, while others exhibit only limited weathering extending 10s of meters into the bedrock. Although granitic weathering processes have been well studied, the controls on profile morphology and weathering rates within granitic, and many other lithologies remain poorly understood; these are likely influenced by a range of both intrinsic and extrinsic factors, which in turn will have crucial implications for understanding, for example, climate-weathering feedbacks. In this study we present mull-scale elemental and mineralogical analyses of a > 30 m granitic weathering profile from the cool, temperate, Lysina catchment in the NW Czech Republic, from which we calculated mass transfer, weathering indices, and mineral specific weathering rates. The Lysina profile exhibits limited weathering extending > 30 m into fractured bedrock, dominated by albite weathering at a rate of 9.3 x 10(-17 )mol m(2) s(-1). To identify environmental and geological controls on weathering front morphology and chemical weathering rates, Lysina was compared to previously published granitic weathering profiles from around the world. Weathering front morphology and weathering rates were calculated for the additional sites from published data and were correlated to mean annual precipitation (MAP), mean annual temperature (MAT), and erosion rates, with MAP having the strongest relationship. Higher MAP likely promotes lower saturation indices in pore waters, allowing weathering reactions to occur further from equilibrium. Comparison of erosion rates amongst the granitic catchments revealed an inconsistent effect on chemical weathering rates, but high erosion rates may promote weathering by reducing the thickness of the regolith and exposing the bedrock to reactive fluids. Mean annual temperatures appear to only have significant impacts on weathering fronts in environments with high precipitation and high erosion rates. Fractured bedrock profiles (Lysina and Rio Icacos) have higher weathering intensities, than the other sites studied here. High connected porosity in fractured rocks enhances water movement allowing more efficient removal of weathering products, thus reducing thermodynamic saturation, increasing weathering rates, and producing sharper weathering gradients. These findings indicate that CO2 drawdown on geological timescales is also likely to be governed by precipitation rates, as well as temperature, and that much of the climate-significant weathering may occur within very narrow zones of the Earth's surface.