EVALUATING THE RELATIONSHIP BETWEEN OROGENESIS, CLIMATE, AND SILICATE WEATHERING WITH A COUPLED LANDSCAPE EVOLUTION MODEL

Conventional wisdom suggests that enhanced silicate weathering during orogenesis is an important part of Earth’s inorganic carbon cycle and a regulator of global climate on million-year timescales. In part, this is based on a simple conceptual framework where associated increases in erosion rate increase the supply of fresh rock to the surface and where building of topographic relief concentrates rainfall on windward flanks of orogens, both of which increase silicate weathering rates. However, erosion and climate evolve in a complex manner, both in space and time, in response to external forcings and internal feedback cycles as orogens grow, and changes in these factors have the potential to complicate the relationship between orogenesis and silicate weathering. Here, we present a coupled landscape evolution model developed to explore the effect of changing landscape conditions during orogenesis on silicate weathering. We simulate steady-state solutions for a range of landscape evolution and orogenic growth scenarios and use these scenarios to parameterize three different widely-used silicate weathering models. Key differences among weathering models are varying dependencies of silicate weathering rate on climate and soil/weathering zone thickness. We find that weathering models where soil thickness is sensitive to erosion rate tend to exhibit complex, non-monotonic changes in silicate weathering rate as orogens grow larger. We interpret that this results from a transition to kinetically-limited weathering, and this transition typically occurs under conditions expected for small- to moderately-sized, or relatively young orogens. Importantly, this result also implies that further orogenic growth may result in negligible changes or even decreases in silicate weathering flux, which runs contrary to the conventional view of enhanced silicate weathering with orogenic growth. Meanwhile, a weathering model where weathering zone thickness is insensitive to erosion consistently predicts a simple, monotonic increase in silicate weathering flux with increasing orogen size, highlighting contrasting predictions among current weathering models. Our modeling framework also provides a means to establish predictions for how key variables in a given setting may influence silicate weathering, making it a potentially useful tool to test competing models.