WARMER AND WETTER CLIMATES ACCELERATE MECHANICAL WEATHERING (Invited Presentation)

One of the few negative feedbacks proposed for Earth’s warming climate is centered on the relationship between silicate weathering and CO₂ drawdown. Prior studies, including those examining how low-latitude mountain-building influences this system (e.g. Macdonald et al., 2019), have focused almost solely on chemical weathering. Yet, chemical weathering rates are highly dependent on fresh mineral exposure, which mechanical weathering facilitates. Here, we present a field-test of the hypothesis that mechanical weathering rates are also strongly dependent on temperature and moisture – beyond the influence that these variables have on stress-loading via processes like thermal cycling or freezing. Eppes and Keanini (2017) so proposed, noting that the bond-breaking mechanisms of mechanical weathering are subcritical in nature. Using physically based numerical models, they demonstrate that very low external stress-loading – by gravity, tectonics or environment - leads to molecular-scale, chemo-physical reactions that serve to propagate cracks at rates strongly dependent on both moisture and temperature. Here we seek to document to what extent these results can be observed in the field.

We instrumented two granite boulders with Acoustic Emission sensors to record real-time cracking for one year (North Carolina) and three years (New Mexico), while simultaneously recording adjacent ambient air temperature and relative humidity. We find that cracking rates increase exponentially as a function of vapor pressure, relative humidity and temperature – in agreement with fracture mechanics theory, experiments and numerical modeling results presented in Eppes and Keanini (2017). Further analyses of these observations show that vapor pressure, a measure of the gross water content of the air that increases with both temperature and relative humidity, appears to be the primary driver of the correlations. Given that globally, vapor pressure for Earth’s land surface will rise by about 6% per +1°C, our results suggest that – for a given stress - mechanical weathering rates will also increase. These results have strong implications for the parameterization of the weathering-CO₂-temperature system in climate models, and for understanding the contributions of mountain building to the carbon cycle and climate.