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Abstract This paper is the first comprehensive synthesis of what is currently known about the different natural and anthropogenic fluxes of rhenium (Re) on Earth's surface. We highlight the significant role of anthropogenic mobilization of Re, which is an important consideration in utilizing Re in the context of a biogeochemical tracer or proxy. The largest natural flux of Re derives from chemical weathering and riverine transport to the ocean (dissolved = 62 × 10⁶ g yr⁻¹and particulate = 5 × 10⁶ g yr⁻¹). This review reports a new global average [Re] of 16 ± 2 pmol L⁻¹, or 10 ± 1 pmol L⁻¹for the inferred pre‐anthropogenic concentration without human impact, for rivers draining to the ocean. Human activity via mining (including secondary mobilization), coal combustion, and petroleum combustion mobilize approximately 560 × 10⁶ g yr⁻¹Re, which is more than any natural flux of Re. There are several poorly constrained fluxes of Re that merit further research, including: submarine groundwater discharge, precipitation (terrestrial and oceanic), magma degassing, and hydrothermal activity. The mechanisms and the main host phases responsible for releasing (sources) or sequestrating (sinks) these fluxes remain poorly understood. This study also highlights the use of dissolved [Re] concentrations as a tracer of oxidation of petrogenic organic carbon, and stable Re isotopes as proxies for changes in global redox conditions. 

Concentration-discharge (C-Q) relationships of total suspended solids (TSS), total dissolved solids (TDS), particulate organic carbon (POC), and dissolved organic carbon (DOC) were investigated in the tributaries and main-stems of two mountainous river systems with distinct watershed characteristics (Eel and Umpqua rivers) in Northern California and central Oregon (USA). Power-law (C = a × Q b) fits to the data showed strong transport-limited behavior (b > 1) by TSS and POC, moderate transport limitation of DOC (b > 0.3) and chemostatic behavior (b < 0) by TDS in most streams. These contrasts led to significant compositional differences at varying discharge levels, with particle-bound constituents becoming increasingly important (relative abundances of 50% to >90%) at high-flow conditions. Organic carbon contents of TSS displayed marked decreases with discharge whereas they increased in TDS during high-flow conditions. Daily and cumulative material fluxes for different coastal streams were calculated using the C-Q relationships and showed that the delivery of transport-limited constituents, such as TSS and POC (and DOC to a lesser degree), was closely tied to high-discharge events and occurred primarily during the winter season. The coherence between winter fluxes and high wave-southerly wind conditions along the coast highlights how seasonal and inter-annual differences in fluvial discharge patterns affect the fate of land-derived materials delivered to coastal regions. 

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. 

The burial and oxidation of organic carbon (OC) partially regulates global atmospheric CO2 and therefore climate on both modern and geologic timescales. In order to understand fluxes in the carbon cycle, it is imperative to understand the chemical composition of OC, and in turn the fate of different OC sources and sinks. Bulk radiocarbon (14C) techniques are often used to understand environmental OC, but this method only reflects the average 14C age of all contributing C sources in a sample, providing no information on the composition of the OC and obscuring natural heterogeneity in OC ages.