The behaviour of Cu and Zn isotopes during soil development: Controls on the dissolved load of rivers

Abstract

Highlights • Light Cu, and unfractionated Zn, isotopes retained in oxic soils. • Development of anaerobic conditions can lead to loss of this signature. • Mineral aerosol addition and biological cycling overprint weathering processes. • Oxic soils contain Cu-Zn isotopes complementary to riverine dissolved pool.

Cu and Zn isotope fractionation during extreme chemical weathering

“…In contrast to Zn, Cu in the riverine dissolved pool is isotopically heavy relative to rocks (Vance et al, 2008); this behaviour is similar to observations for Mo (Archer and Vance, 2008;Neubert et al, 2011), Li (Huh et al, 1998), Ni (Cameron and Vance, 2014), and Cr isotopes (Frei et al, 2014). Accordingly, there are two proposed explanations for the heavy Cu isotope composition in rivers: (1) An equilibrium isotope fractionation in rivers (Vance et al, 2008) and/or in soils (Vance et al, 2016) between an isotopically heavy, organic ligand bound, dissolved pool and an isotopically light pool sorbed to particulates; (2) Redox-driven release of isotopically heavy Cu during oxidative weathering of sulphides in black shales or supergene systems (Mathur et al, 2005;Mathur et al, 2012;Mathur and Fantle, 2015;Lv et al, 2016).…”

“…Thus, mass balance constraints dictate that organically complexed Cu in solution will closely reflect the isotopic composition of the Cu that is mobilized during weathering, while the free Cu species will be fractionated by as much as -1.1‰. Consequently, in a scenario in which Zn and Cu are organically complexed in pore waters, secondary mineral phases may simply scavenge the isotopically light uncomplexed (or weakly-complexed) pool of Zn and Cu (as proposed by Vance et al, 2016).…”

“…Surface organic-rich layers of soils are often enriched in light Zn and Cu isotopes, which has been related to the uptake and recycling of plants (e.g., Bigalke et al, 2011Bigalke et al, , 2010aLiu et al, 2014;Vance et al, 2016;Weiss et al, 2007). Therefore, the presence of isotopically light solid organic matter in the uppermost sample of the SQ weathering profile is plausible, given that the surface of the lateritic mesa into which the quarry has been excavated is characterized by well-developed sub-tropical scrub vegetation, predominantly acacia.…”

“…A study of the dissolved phase in rivers found a discharge-weighted Zn isotope composition unfractionated from continental rocks (Little et al, 2014b). The absence of systematic Zn isotope fractionation in rivers implies little or no Zn isotope fractionation during weathering, a hypothesis partially supported by studies of Zn isotopes in soils (Bigalke et al, 2010a;Vance et al, 2016;Opfergelt et al, 2017;Suhr et al, 2018). However, studies carried out in extreme, (sub-) tropical weathering regimes have observed preferential release of heavy Zn isotopes (Viers et al, 2007;Lv et al, 2016;Guinoiseau et al, 2017;Suhr et al, 2018).…”

Response of copper concentrations and stable isotope ratios to artificial drainage in a French Retisol

“…The lack of a relationship of the δ 65 Cu values of dissolved Cu with the redox potential might be explained by the fact that the Cu isotope ratio of dissolved Cu in soils and rivers is more strongly controlled by complexation with strong dissolved ligands (Vance et al, 2008;Vance et al, 2016) than by redox changes. The responsible ligands were identified by cathode-stripping voltammetry and are subdivided in the ligand classes L1 and L2 (Muller et al, 2001).…”