It's more than stress - field evidence for climate's influence on rates of rock fracture.

Climate is intimately and complexly linked to all Earth surface processes. Similarly, most surface processes are predicated on the mechanical breakdown of rock, which produces clastic sediment and provides water access via fractures of all scales. Here we explore the climate-dependence of mechanical weathering, not only in the context of climate's influence on the stresses that drive cracking, but also as a function of a hypothesized influence of climate on the bond-breaking processes of rock cracking itself. Numerical modeling and experimental data support this hypothesis, but it has yet to be tested in the field. To do so, we use direct observations of mechanical weathering and weather station data to quantify links among temperature, moisture and mechanical weathering rates. Using acoustic emission sensors, we `listen' to natural rocks cracking over a period of ~4 years. Consistent with published subcritical cracking theory and experimental observations, observed cracking rates increase exponentially as a function of atmospheric water vapor pressure, temperature and relative humidity. Independent of stress, water vapor pressure plays the clearest role in determining cracking rates. By binning observations into limited ranges of stress magnitude, we find rock cracking rates to be consistently and nonlinearly proportional to atmospheric water vapor pressure for all stress levels. Cracking rates increase with increasing temperatures and relative humidity for some stress levels but not all. We infer that, even when stress levels are similar, water vapor pressure strongly impacts cracking rates because of its influence on the bond-breaking that occurs at crack tips when rocks break subcritically. This conclusion strongly diverges from the long-held, underlying assumption of mechanical weathering research on Earth and other planets that mechanical weathering rates hinge solely on factors that impact stresses. Water vapor pressure increases as the atmosphere warms, so these results have important implications for understanding and quantifying the interactions between rock mechanical weathering and changing global climates.