Stable Cu isotope fractionation in soils during oxic weathering and podzolization

• Published in: Geochimica et cosmochimica acta Vol. 75; no. 11; pp. 3119 - 3134
• Main Authors: Bigalke, Moritz, Weyer, Stefan, Wilcke, Wolfgang
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.06.2011
• Subjects: A horizons; aluminum; B horizons; Cambisols; Copper; E horizons; Fractionation; Horizon; hydromorphic soils; Iron; isotope fractionation; Isotope ratios; Isotopes; organic horizons; organic matter; Podzols; soil formation; Soils; stable isotopes; Weathering
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2011.03.005

Copper stable isotope ratios are fractionated during various biogeochemical processes and may trace the fate of Cu during long-term pedogenetic processes. We assessed the effects of oxic weathering (formation of Cambisols) and podzolization on Cu isotope ratios (δ 65Cu). Two Cambisols (oxic weathered soils without strong vertical translocations of soil constituents) and two Podzols (soils showing vertical translocation of organic matter, Fe and Al) were analyzed for Cu concentrations, partitioning of Cu in seven fractions of a sequential extraction and δ 65Cu values in bulk soil. Cu concentrations in the studied soils were low (1.4–27.6 μg g −1) and Cu was mainly associated with strongly bound Fe oxide- and silicate-associated forms. Bulk δ 65Cu values varied between −0.57‰ and 0.44‰ in all studied horizons. The O horizons had on average significantly lighter Cu isotope compositions (−0.21‰) than the A horizons (0.13‰) which can either be explained by Cu isotope fractionation during cycling through the plants or deposition of isotopically light Cu from the atmosphere. Oxic weathering without pronounced podzolization in both Cambisols and a weakly developed Podzol (Haplic Podzol 2) caused no significant isotope fractionation in the single profiles, while a slight tendency to lower δ 65Cu values with depth was visible in all four profiles. This is the opposite depth distribution of δ 65Cu values to that we observed in hydromorphic soils (soils which show indication of redox changes because of the influence of water saturation) in a previous study. In a more pronounced Podzol (Haplic Podzol 1), δ 65Cu values and Cu concentrations decreased from Ah to E horizons and increased again deeper in the soil. Humus-rich sections of the Bhs horizon had higher Cu concentrations (2.8 μg g −1) and a higher δ 65Cu value (−0.18‰) than oxide-rich sections (1.9 μg g −1, −0.35‰) suggesting Cu translocation between E and B horizons as organo-Cu complexes. The different depth distributions in oxic weathered and hydromorphic soils and the pronounced vertical differences in δ 65Cu values in Haplic Podzol 1 indicate a promising potential of δ 65Cu values to improve our knowledge of the fate of Cu during long-term pedogenetic processes.