Concentration-discharge relationships and the dissolved chemistry of contrasting small mountainous rivers in the Pacific Northwest, USA

Abstract:

This study investigates the effect of erosion rate on the dissolved geochemistry and chemical weathering of two contrasting watersheds, the Eel River and the Umpqua River in the Pacific Northwest, USA. These two rivers share many key characteristics in terms of size, discharge, climate, and vegetation, but they have a 10-fold difference in sediment yield due to their tectonic setting and uplift and erosion rate. We evaluate this question by using concentration-discharge (C-Q) relationships for major cations, anions, dissolved organic carbon, and other geochemical tracers, such as redox sensitive elements. We also examined riverine dissolved rhenium (Re), which is released and delivered conservatively to rivers during the weathering of bedrock. Recent research outlines the potential of Re to serve as a tracer for the oxidation of petrogenic carbon because of its close association with petrogenic carbon (OC_petro) in rocks. In both the Eel and Umpqua Rivers, most major cation and anion concentrations generally behave somewhat chemostatically across all tributaries. The C-Q patterns of Re in the dissolved load of rivers behave similarly to those of major weathering derived solutes such as sodium, calcium, and magnesium. Average Re concentrations in the faster eroding Eel river are more than two times greater than Re concentrations in the Umpqua River, 3.60 pmol/L and 1.48 pmol/L, respectively, despite similar Re concentrations in bedrock. The results of this study suggest that dissolved Re fluxes increase with increased erosion rates, suggesting that tectonic setting is one factor that controls OC_petro oxidation. The data from this study further supports the observations that Re could serve as a potential tracer for oxidation of (OC_petro).