WEATHERING IN ROCKY MOUNTAIN ALLUVIAL VALLEYS

Silicate weathering is the primary mechanism that removes CO₂ from the atmosphere on geological timescales. Most models of silicate weathering assume that exposure of fresh minerals on mountain hillslopes increases weathering rates. However, in many mountains, high-elevation alluvial valleys—often created by glaciation or faulting—store eroded material. The contribution of these alluvial floodplains to the weathering budget of a mountain range is not well constrained. We use a well-studied alluvial floodplain at the CSU Mountain Campus in the northern Front Range to constrain their role in the weathering budget of a major Colorado Front Range river. The study site is ideal for understanding how an alluvial valley contributes to weathering due to the active connection between the groundwater and surface water, the well-constrained groundwater dynamics, and the silicate lithology of the surrounding catchment. We collected meteoric, surface and ground water to determine if weathering reactions in the alluvial aquifer influence stream [C]. We hypothesize that the overwhelming influence of seasonal discharge from snowmelt combined with the relatively unreactive minerals present in the valley fill will result in only a small effect of alluvial weathering. Preliminary data shows higher alkalinity in the groundwater relative to the stream water and a newly drilled well installed near bedrock in the valley will permit additional study of the relationship between [C] and residence time in this aquifer. Quantifying the contribution of alluvial valley sediment weathering to the mountain’s overall weathering budget is important for better understanding what role these types of mountain environments play in the global carbon cycle.