Wenhao Wang

Chromium (Cr) is a redox sensitive element and Cr isotope composition (δ53Cr) has been widely utilised to reflect the extent of Cr(VI) reduction during Cr pollution mitigation, as well as changes in past atmospheric/oceanic oxygenation. Whilst redox transformations are thought to primarily drive the Cr isotopic variability in modern aquatic environments, other processes, such as ligand promoted dissolution, can occur, potentially overprinting the intrinsic δ53Cr signal.In this study, laboratory-controlled batch and flow-through column experiments on two distinct soil materials were conducted to understand the leaching behaviour of Cr and Cr isotopes, under both oxygenated and O2-free conditions. Significant dissolution of Cr(III), together with Fe and Mn, from the solid phase in the presence of low-molecular-weight organic acids was observed over the time course of all experiments. Initial isotope analyses on Cr(III)-citrate complexes show that δ53Cr values are ~ −0.60 to −0.09‰, reflecting the pristine Cr isotopic signature of the two soil materials. In addition, whilst formation of authigenic Fe particles means that a fraction of dissolved Cr is scavenged, such that solid phase Cr may be associated with Fe, Cr does not seem to be remobilised during reductive dissolution of Fe (and Mn) oxides in these experiments.Results from this study have several implications. Firstly, as Cr is a known carcinogen, increased levels of organic ligands, e.g., in paddy field systems, can cause increased environmental and health risks. Secondly, organic ligands may play an overlooked role in modulating the input and removal processes of dissolved Cr to/from various environments. Finally, ligand-bound Cr(III) likely has a ‘stabilised’ isotopic signature that is distinct from Cr(VI), making it possible to trace this ‘additional’ Cr in aquatic systems; it is difficult to characterise or quantify these Cr-organic complexes using conventional analytical methods.This work is part of the CHROMA project funded by H2020-MSCA-IF (101031974).

Oxidative weathering of organic carbon in sedimentary rocks is a major source of CO₂ to the atmosphere over geological timescales, but the size of this emission pathway in Earth's past has not been directly quantified due to a lack of available proxy approaches. We have measured the rhenium isotope composition of organic‐rich rocks sampled from unweathered drill cores and weathered outcrops in south Texas, whose stratigraphic successions can be tightly correlated. Oxidative weathering of more than 90% of the organic carbon and ∼85% of the rhenium is accompanied by a shift to lower rhenium isotope compositions in the weathered outcrops. The calculated isotope composition of rhenium weathered from the initial bedrock for individual samples varies systematically by ∼0.7‰ with different fractions of rhenium loss. This variation can be empirically modeled with isotope fractionation factors of α = 1.0002–1.0008. Our results indicate that the isotope composition of rhenium delivered to the oceans can be altered by weathering intensity of rock organic matter and that the rhenium isotope composition of seawater is sensitive to past oxidative weathering and associated CO₂ emissions.

Chromium (Cr) is a redox-sensitive element and because Cr isotopes are fractionated by redox and/or biological processes, the Cr isotopic composition of ancient marine sediments may be used to infer changes in past seawater oxygenation or biological productivity. While there appears to be a ‘global correlation’ between the dissolved Cr concentration and Cr isotopic composition of seawater, there is ongoing debate about the relative importance of external sources and internal cycling on shaping the distribution of dissolved Cr that needs to be resolved to validate the efficacy of using Cr isotopes as a paleo proxy. Here, we present full water column depth profiles of total dissolved Cr (Cr(VI)+Cr(III)) and dissolved Cr isotopes (δ⁵³Cr), together with ancillary data, for three stations along a transect (GEOTRACES GApr08) across the sub-tropical North Atlantic. Concentrations of dissolved Cr ranged between 1.84 and 2.63 nmol kg^-1, and δ⁵³Cr values varied from 1.06 to 1.42‰. Although atmospheric dust, hydrothermal vents and seabed sediments have the potential to modify the distribution of Cr in the oceans, based on our observations, there is no clear evidence for substantial input of Cr from these sources in our study region although benthic inputs of Cr may be locally important in the vicinity of hydrothermal vents. Subsurface waters (below the surface mixed layer to 700 m water depth) were very slightly depleted in Cr (by up to ~0.4 nmol kg^-1), and very slightly enriched in heavy Cr isotopes (by up to ~0.14‰), relative to deeper waters and the lowest Cr concentrations and highest δ⁵³Cr values coincided with lowest concentrations of colloidal (0.02 to 0.2 μm size fraction) Fe. We found no direct evidence for biological uptake of dissolved Cr in the oligotrophic euphotic zone or removal of Cr in modestly oxygen depleted waters (O₂ concentrations ~130 μmol kg^-1). Rather, we suggest removal of Cr (probably in the form of Cr(III)) in subsurface waters is associated with the formation of colloid aggregates of Fe-(oxyhydr)oxides. This process is likely enhanced by the high lithogenic particle load in this region, and represents a previously unrecognized export flux of Cr. Regeneration of Cr in deeper waters leads to subtly increased levels of Cr alongside decreased δ⁵³Cr values at individual sites, but this trend is more obvious at the global scale, with δ⁵³Cr values decreasing with increasing radiocarbon age of deep waters, from 1.16 ± 0.10‰ (1SD, n=11) in deep Atlantic waters to 0.77 ± 0.10‰ (1SD, n=25) in deep Pacific waters. Removal of relatively isotopically light Cr from subsurface waters onto particulate material and regeneration of this Cr back into the dissolved phase in deep waters partly accounts for the systematic relationship between δ⁵³Cr and Cr concentrations in seawater discussed by other studies.

Rhenium (Re) is a redox-sensitive element. Recent advances in the precision of measurement of the stable isotopic composition of Re (δ187Re) allow exploration of its potential as a proxy for paleoredox and/or chemical weathering [1]. However, as yet, there have been few studies reporting the geochemical cycling of Re and stable Re isotopes in the modern environments [2] [3], and processes that regulate the Re isotope behavior in hydrothermal systems remain unexplored.Here we present results of the analysis of Re concentration and δ187Re (relative to NIST3143) for water samples collected from hydrothermal and groundwater systems in Iceland. We show that Re in basalt-hosted boiled hydrothermal fluids from Hellisheidi, Nesjavellir, Reykjanes and Svartsengi sites is isotopically heavier (δ187Re = –0.01 to +0.32‰) than Re in Icelandic basalts (δ187Re = ~–0.32‰). The direction of fractionation holds regardless of types of fluid reservoir (meteoric vs. seawater), and is consistent with precipitation of isotopically light sulfides in the hydrothermal system and/or kinetic fractionation of Re during degassing. By contrast, Re in cold (< 10°C) groundwaters collected from the Mývatn area is isotopically indistinguishable from host basalt. Natural hot spring waters exhibit variable δ187Re values (–0.28 to +0.26‰), likely reflecting mixing between hydrothermal and groundwater endmembers. The relatively isotopically heavy δ187Re from hydrothermal sources has the potential to modify the oceanic budget, which has implications for the isotope mass balance of Re.[1] Dellinger et al. (2020) JAAS, 35, 377. [2] Dickson et al. (2020) GCA, 287, 221-228. [3] Dellinger et al. (2021) EPSL, 573, 117131.