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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−1 and 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−1 for 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−1 Re, 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. Plain Language Summary: This study examines how the rare element rhenium (Re) moves throughout the Earth's surface. The largest natural source of Re is rock weathering, but human activities such as mining, coal combustion, and petroleum combustion have accelerated the natural Re cycle. Understanding the behavior of Re in a variety of Earth surface environments helps us to evaluate fundamental biogeochemical questions about Earth's carbon and oxygen cycles. Key Points: We compile existing rhenium (Re) concentration data and flux estimates in Earth's surface reservoirsAlthough rhenium is one of the rarest elements in the Earth's crust, human activity has enhanced Re mobilization by around 3‐ to 4‐foldRhenium may help track the oxidation of rock‐bound organic carbon, track anthropogenic pollution, and reconstruct paleoredox conditions [ABSTRACT FROM AUTHOR]

Redox state fluctuations are a primary mechanism controlling the mobilization of trace metals in soils. However, underlying lithology may modulate the effect that redox fluctuations have on trace metal mobility by influencing soil particle size and mineral composition. To investigate the relationships among trace metal behavior, lithology, and redox state, we subjected surface soils from two intensely weathered soil profiles formed on contrasting lithologies to consecutive, 8-day redox cycles. A suite of metals (Al, Mn, Fe, Ti, Rb, Zr, Nb, Mo, REEs, Pb, Th, U) were quantified in the aqueous phase (<10 nm) and solution (<415 nm, including colloids) from soil slurries. In soil formed on volcaniclastic bedrock with high clay content and a high abundance of short-range-ordered Fe-(oxyhydr)oxides phases (e.g. nano-goethite; quantified by Mössbauer spectroscopy), reducing events and colloidal dynamics drove trace metal mobilization. In contrast, in soil formed on granite bedrock with lower clay content and a low abundance of short-range-ordered Fe-(oxyhydr)oxides phases (nano-goethite and lepidocrocite), overall trace metal mobilization was lower, and mobilization was not predictable from redox state. Molybdenum isotopes were also measured through redox cycles but did not exhibit redox-dependent behavior. This study provides direct evidence that lithology remains an overarching factor governing the characteristics of metal mobility in soils, even after extended and intense chemical weathering and soil development processes., (Copyright © 2019 Elsevier B.V. All rights reserved.)