Graduate Thesis Or Dissertation
 

Concentration-Runoff Relationships of Contrasting Small Mountainous Rivers in the Pacific Northwest, USA: Insights into the Weathering of Rhenium

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  • This study examines dissolved rhenium (Re) as a function of water runoff using river samples from two contrasting watersheds, the Eel and Umpqua Rivers in the Pacific Northwest, USA. These watersheds share many key characteristics in terms of size, discharge, climate, and vegetation, but they have a 10-fold difference in sediment yield due to differences in their tectonic setting and uplift and erosion rates. We evaluate concentration-runoff (C-R) relationships and ratios of coefficients of variation (CVC/CVR) for major cations, anions, dissolved inorganic carbon, selected trace elements including Re, and 87Sr/86Sr ratios. 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 (OCpetro) in rocks. In both the Eel and Umpqua Rivers, the major cation and anion concentrations generally show modest dilution across all tributaries with increasing runoff. We observe that Re behaves similarly to major weathering derived-solutes corrected for atmospheric input, such as Ca2+*, Mg2+*, and Na+* with modest dilution behavior. Rhenium behaves dissimilarly from other trace elements, such as Mo and U, and is also dissimilar to biologically-cycled nutrients, such as NO3-, PO43-, and K+*, suggesting differences in sources, solute generation mechanisms, and flowpaths. Rhenium behavior is also distinct from that of colloids, which have increasing concentrations with increasing runoff. We find that Re and sulfate corrected for atmospheric input (SO42-*) have distinct C-R relationships, in which SO42-* undergoes greater dilution with increasing runoff. This implies that Re is not dominantly sourced from sulfide weathering, which leaves oxidation of OCpetro as the likely dominant source of dissolved Re release. At mean discharge, Re concentration in the Eel river (3.5 pmol L-1) is more than two times greater than Re concentrations in the Umpqua River (1.5 pmol L-1). Similarly, two tributary watersheds within the Eel basin, Bull Creek and Elder Creek, which have the same bedrock also show higher Re concentrations where erosion rates are higher. The results of this study suggest that dissolved Re in the Eel and Umpqua River basins is likely derived from OCpetro oxidation, and Re fluxes are higher in areas with higher erosion rates, suggesting that tectonic setting is one factor that controls OCpetro oxidation.
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