Cr isotopes fractionate during oxidative weathering of the continents; the oxidation of Cr (III) bearing minerals produces soluble Cr (VI) which is enriched in the heavy isotope, Cr (VI) is lost to local rivers resulting in a Cr depleted, isotopically light residual soil [1] [2]. To date, research in this area has focused on laterites, a unique soil type produced by intense tropical weathering. To evaluate if this phenomenon is unique to tropical regions we have measured the Cr isotope composition (d53/52Cr ‰) of soils and river water from the Antrim Plateau, Northern Ireland, a marine temperate climate.
Cr is enriched in the soil horizon relative to basaltic bedrock. Cr isotope fractionation is minimal with d53/52Cr values (-0.27 + 0.02 ‰ to -0.15 + 0.03‰) near to bedrock values (-0.25 + 0.04 ‰) indicating a lack of oxidative weathering. However, local river waters are enriched in isotopically heavy Cr (VI) (+0.8 + 0.1 ‰) which is a consequence of oxidative weathering processes. The apparent non-fractionation in soils is a function of 1) the accumulation weather resistant Cr-spinels and 2) characteristics unique to soils in this area such as low pH and high organic content which tend to retain Cr as immobile Cr (III). The Cr (VI) lost from the system is insufficient to effect the isotopic composition of the soils. This study together with Cr data from other basaltic river catchments has found that the d53/52Cr of river water is catchment specific [1] and is controlled by local soil forming factors such as protolith, climate and biota. Our continuing research into modern analogues help link the d53/52Cr value of river water and the d53/52Cr value of seawater and contribute to our ultimate aim of using Cr isotopes as a palaeoclimate indicator.