Stable Cu isotope fractionation in soils during oxic weathering and podzolization

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 (delta Cu-65). 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 delta Cu-65 values in bulk soil. Cu concentrations in the studied soils were low (1.4-27.6 mu g g(-1)) and Cu was mainly associated with strongly bound Fe oxide-and silicate-associated forms. Bulk delta Cu-65 values varied between -0.57 parts per thousand and 0.44 parts per thousand in all studied horizons. The O horizons had on average significantly lighter Cu isotope compositions -0.21 parts per thousand than the A horizons (0.13 parts per thousand) 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 delta Cu-65 values with depth was visible in all four profiles. This is the opposite depth distribution of delta Cu-65 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), delta Cu-65 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 mu g g(-1)) and a higher delta Cu-65 value (-0.18 parts per thousand) than oxide-rich sections (1.9 mu g g(-1), -0.35 parts per thousand) 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 delta Cu-65 values in Haplic Podzol 1 indicate a promising potential of delta Cu-65 values to improve our knowledge of the fate of Cu during long-term pedogenetic processes. (C) 2011 Elsevier Ltd. All rights reserved.