MINERALOGICAL CHARACTERIZATION OF AN ARGILLITE WEATHERING PROFILE, EEL RIVER CRITICAL ZONE OBSERVATORY, NORTHERN CALIFORNIA

The mineralogical transformations that occur as a result of rock weathering influence how water and solutes are routed from hillslopes to stream channels. Therefore, studies of hillslope weathering profile evolution may elucidate linkages between climate, lithology, and topography. In this study, we examine samples of soil, weathered bedrock, and fresh bedrock from a heavily instrumented experimental hillslope within the Eel River Critical Zone Observatory (ERCZO) in northern California to understand the relationship between weathering processes, mineralogy, and landscape evolution. The Rivendell site of the ERCZO is situated on steeply dipping turbidite sequences of the Franciscan Formation. Monitoring of climate, groundwater, soil moisture, water chemistry, and vegetation within the steep, 4000 m² unchanneled hillslope has highlighted the role that the weathered bedrock plays in biological and hydrological processes. In this study, we document mineralogical changes within the weathering profile using XRD, SEM, EMP and light microscopy. Soil XRD analyses documenting the abundance of vermiculite indicate that the argillite produces a clay rich soil, which is consistent with field observations. Within the weathered bedrock zone, parent rock mineralogy is dominated by quartz, albite, vermiculite, anorthoclase, apatite and illite. SEM microscopy revealed a complex manganese-phosphorous coating on many grains, and a titanium rich phase with six-point-star symmetry on some basal planes. In fresh bedrock, light and electron microscopy reveals the presence of framboidal pyrite below the chronically saturated zone. As the weathering front progresses and these minerals become exposed to the water table fluctuations leading to the oxidation. Additionally, the dissolution of detrital grains of calcite are likely influenced by the groundwater dynamics within the weathered, fractured bedrock zone. Both pyrite oxidation and calcite dissolution are mechanisms of porosity generation and are therefore important processes in determining the spatial extent of the bedrock weathering within hillslopes. This study documents where these mineralogical changes occur in the weathering profile in an effort to understand feedbacks between weathering and hydrologic processes at the hillslope scale.