Molybdenum isotope behaviour accompanying weathering and riverine transport in a basaltic terrain

Abstract

The application of the molybdenum (Mo) isotope system as a proxy for determining changes in the redox state of the oceans is predicated on the assumption that the composition of continental input can be characterised from crustal rock types, and remains constant. However, it has recently been shown that the δ98/95Mo composition of global rivers varies between 0.15‰ and 2.4‰ and is therefore systematically heavier than the average composition of the continental crust (∼0‰). In order to understand the processes that control Mo-isotope fractionation during weathering this study presents δ98/95Mo and Mo abundance data for rivers (and estuarine samples) from Iceland that drain predominantly basaltic terrains. Resolvable differences are observed in the isotopic composition of the riverine Mo sources; ice (δ98/95Moice >1.8‰), basaltic bedrock (δ98/95Mobedrock ∼0.0‰) and hydrothermal waters (δ98/95Mohydrothermal ∼−3.4‰). Systematic changes in the dissolved Mo-isotope composition are also observed within river catchments, with δ98/95Mo values increasing from ∼0‰ in glacial rivers (close to the source) to ∼1‰ downstream, consistent with Mo-isotopes being fractionated during weathering. Analysis of other riverine phases (bedload, colloids and iron-precipitates) demonstrates that these phases preferentially incorporate light Mo-isotopes, and remain coupled to the dissolved load during riverine transportation. A δ98/95Mo profile through the Borgarfjörður estuary exhibits a predominantly conservative mixing behaviour, but suggests that the release of isotopically light Mo from the particulate and/or colloidal phases may occur in the low salinity part of the estuarine mixing zone.