This study examines dissolved rhenium (Re) concentrations as a function of water runoff using river samples
from two contrasting mountainous 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 15-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 ancient/fossil organic matter because of its close association with petrogenic
carbon (OCpetro) in rocks. In both the Eel and Umpqua Rivers, our measurements show that Re behaves
similarly to major weathering derived-solutes corrected for atmospheric input, such as Ca2+*, Mg2+*,
and Na+* with modest dilution across all tributaries with increasing runoff. Rhenium behaves dissimilarly
from other trace elements, such as Mo and U, and is also dissimilar to biologically-cycled nutrients, such
as NO3 – , PO4 3 , 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 (SO4 2 *) have distinct CR
relationships, in which SO4 2 * undergoes greater dilution with increasing runoff. This implies that Re
is not dominantly sourced from sulfide weathering, which leaves primary bedrock minerals and OCpetro
hosted in bedrock of these watersheds as the likely dominant sources 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). Furthermore, comparison of two tributary watersheds with
similar bedrock but marked differences in erosion rates show higher Re concentrations in Bull Creek (erosion
rate of 0.5 mm yr 1) relative to Elder Creek (erosion rate of 0.2 mm yr 1). The results of this study
suggest that dissolved Re in the Eel and Umpqua River basins is likely derived from primary mineral dissolution
or OCpetro oxidation, and Re fluxes are higher in areas with higher erosion rates, suggesting that
tectonic setting is one factor that controls Re release and therefore OCpetro oxidation.
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Assessing molybdenum isotope fractionation during continental weathering as recorded by weathering profiles in saprolites and bauxites
- Award ID(s):
- 1757313
- NSF-PAR ID:
- 10215415
- Date Published:
- Journal Name:
- Chemical Geology
- Volume:
- 566
- Issue:
- C
- ISSN:
- 0009-2541
- Page Range / eLocation ID:
- 120103
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation