SILICATE WEATHERING DURING MOUNTAIN BUILDING LIMITED BY COUPLED INTERACTIONS AMONG CLIMATE, TECTONICS, AND TOPOGRAPHY

Weathering of silicate minerals in tectonically-active mountains is thought to affect atmospheric carbon dioxide concentrations and thus global climate on million-year timescales. Mountain building 1) accelerates the flux of weatherable minerals to the surface via erosion, and 2) generates orographically enhanced rainfall, both of which in isolation should increase silicate weathering rates. However, erosion rates, topography, and orographic rainfall patterns co-evolve during orogenesis, and potential feedbacks within this system may impact silicate weathering rates in complex ways. Here, we present preliminary results of a numerical modeling study exploring the effect of coupling between tectonics, climate, erosion, and topography on silicate weathering in an idealized convergent mountain range. We compare three silicate weathering models, each with varying dependencies on erosion and rainfall, and subject these models to a range of orogenic growth scenarios. We find that threshold behavior separating two topographically mediated climate-tectonic feedback cycles – a relief limiting cycle and a relief enhancing cycle – is a dominant control on modeled silicate weathering fluxes. The relief limiting cycle is characteristic of early stages of orogenesis, where orographically enhanced rainfall is concentrated at high elevations and efficiently limits further topographic growth. These conditions allow sustained, rapid erosion where silicate weathering reactions are kinetically limited despite high rainfall, causing weathering fluxes (per unit area) to decrease with increasing erosion rate. In contrast, the relief enhancing cycle is characteristic of large mountain ranges, with aridity at high elevations and orographically enhanced rainfall at lower elevations. In this scenario, increases in topographic growth enhance aridity at high elevations, promoting further growth and aridity. Increased aridity also limits silicate weathering rates and weathering fluxes quickly become insensitive to further increases in erosion rates. In both feedback cycles, these modeling results challenge the notion of a direct link between orogenesis and increased silicate weathering, suggesting that the climate-weathering feedback is sustained by processes exterior to mountain belts.