The riverine flux of molybdenum and its isotopes to the ocean: Weathering processes and dissolved-particulate partitioning in the Amazon basin

Abstract

Molybdenum (Mo) abundances and isotopes in marine sedimentary rocks have become important tools for understanding the past redox state of the ocean-atmosphere system. Their use depends critically on the size and isotope composition of the dissolved riverine input to the oceans. Previous studies have demonstrated that rivers are isotopically heavier than the upper continental crust, but the reasons why are debated. The debate is important to the question of how the riverine input might change through Earth history, for example in response to tectonic- and climate-driven changes in weathering regime. Here, we present a comprehensive study of Mo and its isotopes in multiple tributaries of the Amazon Basin, across seasons, with the aim of understanding both the controls on riverine transport of Mo and its isotopes and the input to the oceans.For all Amazonian rivers, the dissolved load dominates over the particulate load for Mo, whether the size of the total suspended load is quantified by in-situ filtration or from approaches using cosmogenic data. This finding is common to other highly soluble elements like strontium (Sr), and is very different from published compilations of global rivers, where particulate/dissolved ratios of Mo and Sr have been reported to be an order of magnitude higher than found here for the Amazon. Mo isotope data for the dissolved phase (<0.45 μm) of Amazon rivers (δ98Mo=+0.52 to +1.46, relative to SRM NIST 3134 = +0.25) show substantially less variation than rivers globally (−0.2 to +2.4‰), but Mo concentrations vary over two orders of magnitude (0.06-6.2 nmol kg−1). There is systematic variability between river types, with black and clear water rivers like the Negro and the Tapajós-Trombetas having much lower concentrations and higher Mo isotope ratios than white water rivers. Low water season (November) concentrations are always greater, and Mo isotope compositions lighter, than high water season (June). A small number of analyses shows that the colloidal phase (<0.45 μm, 1 kDa) represents about 20-30% of the total dissolved load, is broadly similar in isotope composition, and invariant in size between seasons. Thus, the greater dissolved concentration and lighter isotope composition in November must predominantly be driven by changes in the “truly-dissolved” fraction.We find little evidence for lithological or mineralogical controls on the Mo isotope composition of Amazonian rivers. Dissolved Mo concentrations are well-correlated with other highly soluble elements found in major minerals, be they silicates, carbonates or sulphides. Rather, molybdenum isotope variations across tributaries and season are best explained by processes related to the weathering regime, including preferential mobilisation of heavy isotopes due to sequestration of the light isotopes to secondary phases in soils. In more detail, soil pH is suggested to play a secondary, but significant, role. The assessment of the global dataset for Mo in rivers in terms of these processes suggests that there could be significant temporal variability in the riverine source of Mo to the oceans, controlled by tectonics and climate and their impact on weathering regime.