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

a r t i c l e i n f o Keywords: molybdenum (Mo) isotopes basaltic weathering riverine inputs estuarine mixing 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 δ 98/95 Mo 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 δ 98/95 Mo 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 (δ 98/95 Mo ice N 1.8‰), basaltic bedrock (δ 98/95 Mo bedrock ∼ 0.0‰) and hydrothermal waters (δ 98/95 Mo hydrothermal ∼−3.4‰). Systematic changes in the dissolved Mo-isotope composition are also observed within river catchments, with δ 98/95 Mo 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 δ 98/95 Mo 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. Molybdenum (Mo) is one of the most abundant transition metals in the oceans and is important for a range of biological and geochemical processes (e. Of particular interest is the highly redox-sensitive behaviour of Mo that is accompanied by significant isotope fractionation, which results in oxic, suboxic, anoxic and euxinic sediments possessing isotopically distinct Mo-isotope compositions (Barling et al. Mo can be utilised as a palaeoredox proxy, and have been used to demonstrate the expansion of reducing marine conditions at a number of times throughout Earth's history (e. A critical assumption in the application of the Mo-isotope redox proxy in this manner is that the isotope composition of Mo entering the oceans has remained more-or-less constant throughout geological history, and that the net …