Stable Cu isotope fractionation in soils during oxic weathering and podzolization Moritz Bigalke a,c,⇑ , Stefan Weyer b , Wolfgang Wilcke c a Johannes Gutenberg University Mainz, Earth System Science Research Center, Geographic Institute, Johann-Joachim-Becher-Weg 21, 55128 Mainz, Germany b University of Frankfurt, Institute of Geoscience, Altenho ¨ ferallee 1, 60438 Frankfurt am Main, Germany c Geographic Institute, University of Berne, Hallerstrasse 12, 3012 Berne, Switzerland Received 7 September 2010; accepted in revised form 2 March 2011; available online 6 March 2011 Abstract 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 (d 65 Cu). 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, parti- tioning of Cu in seven fractions of a sequential extraction and d 65 Cu values in bulk soil. Cu concentrations in the studied soils were low (1.4–27.6 lgg 1 ) and Cu was mainly associated with strongly bound Fe oxide- and silicate-associated forms. Bulk d 65 Cu 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 pro- nounced 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 d 65 Cu values with depth was visible in all four profiles. This is the opposite depth distribution of d 65 Cu 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), d 65 Cu 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 lgg 1 ) and a higher d 65 Cu value (0.18&) than oxide-rich sections (1.9 lgg 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 d 65 Cu values in Haplic Podzol 1 indicate a promising potential of d 65 Cu values to improve our knowledge of the fate of Cu during long-term pedogenetic processes. Ó 2011 Elsevier Ltd. All rights reserved. 1. INTRODUCTION Copper in soil deserves scientific attention because it is an essential micronutrient but can also be a potentially toxic pollutant at high concentrations (Alloway, 1990). Copper occurs in unpolluted soils in concentrations of approx. 30 lgg 1 and usually varies in a range of 2–100 lgg 1 (Artiola, 2005). Copper may be present in soil as elemental Cu, Cu + and Cu 2+ but Cu 2+ is the dominating form (Artiola, 2005; Weber et al., 2009). The Cu 2+ ion may adsorb on negatively charged surfaces of clay minerals and organic matter (OM) and be incorporated in the crystal structure of clay minerals. The Cu 2+ ion may also be bound at the surface or incorporated in the crystal structure of oxy(hydr)oxides. A part of the Cu 2+ ions in soil remains 0016-7037/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2011.03.005 ⇑ Corresponding author at: Geographic Institute, University of Berne, Hallerstrasse 12, 3012 Berne, Switzerland. Tel.: +41 (0)316314055. E-mail address: moritz.bigalke@giub.unibe.ch (M. Bigalke). www.elsevier.com/locate/gca Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 75 (2011) 3119–3134