Riverine Re and Mo Inputs to Seawater

We present Re, Mo, and major ion concentration data for a variety of natural fluids as an update to previous geochemical studies (Bertine, 1970; Colodner et al., 1993). The majority of samples are rivers, though data on rain and hydrothermal fluids are also presented. If the data represent natural fluxes, previous assessments of the world average river Re concentration are only ~¼ of the true value, necessitating a shorter marine residence time for this element. Alternatively the data may indicate that the surface Re cycle is anthropogenically perturbed on a global scale. Average Re concentration in world rivers has been previously assessed at 2.1 pM (Colodner et al., 1993) based on a subsample of 4 rivers (Amazon, Orinoco, Ganges, and Brahmaputra) which together account for about 23% of the global river water flux. We believe this estimate to be too low due to the disproportionately large influence of the Re-poor Amazon, which accounts for 75% of the flux considered, though it is only 17% of the global flux. Our larger sample set of more than 50 rivers, accounting for >30% of the global river water flux, supports this as does consideration of the correlative Re vs. SO22-, and Re vs. Σcations relationships first noted by Dalai et al. (2002) for Himalayan rivers. On this basis we believe the average global river water Re concentration to be about 4x higher than the value given by Colodner et al. (1993). This would shorten the estimated marine residence time of Re from 750 ka to about 175 ka. Alternatively, the higher estimate of world river average Re concentration may reflect an anthropogenic perturbation in the global Re cycle. Initial assessment of Re in rivers was heavily biased by the Amazon and Orinoco which accounted for 88% of total water flow considered (Colodner et al.,1993); these two rivers are considered to be relatively pristine. In contrast our evaluation encompasses rivers such as the Mississippi, and Hudson, which are more anthropogenically influenced and have much higher Re levels, often greatly in excess of seawater concentrations of 40 pM. Mississippi Re concentrations of more than 90 pM (Walker et al., 2006) and Hudson concentrations exceeding 200 pM are observed. At more local scales in areas of known human influence, such as in the heavily irrigated South Platte River valley or the acid mine pit in Butte MT, dissolved Re concentrations are higher still, 1,200 pM and 11,000 pM respectively. Previous studies of marine Re have observed an imbalance, with sinks exceeding sources by up to two-fold (Morford and Emerson, 1999). If our data better represent the natural Re flux, this imbalance persists, but now requires additional sinks; shallow sub-oxic margin sediments are a possibility that has been not quantitatively considered to this point. The shorter residence time of 175 ka resulting from this assessment implies that Re should respond more rapidly to changes in sources and sinks than previously thought.