Multiple Isotopes as Tracers of Human Impacts on Geochemistry of Elements in River Systems

English Abstract:

Zinc, Iron and Copper are anthropophile elements and are essential for biogeochemical and physiological functioning of terrestrial and oceanic organisms. Changes in the supply of these metals in continental rivers may affect their global bio-geochemical cycles and even the related metamorphic activities of aquatic organisms. It is thus essential to precisely determine the sources of these metals in river systems and the variations of fluxes of these metals input by human activities and to the ocean in modern environment. The recently developed isotope methods may be used for tracing the sources of these metals and characterizing their geochemical behaviors and fluxes in rivers. Multi isotopes may provide more complete picture of anthropogenic impact on geochemistry and the fluxes of these elements in rivers.

We report multiple isotopic compositions in both dissolved and suspended loads of the anthropogenically-impacted Seine River (France). The Seine basin displays two important characteristics: a relatively simple carbonated-dominated lithology and an increasing anthropogenic impact from the headland towards the estuary. In this study, the concentrations and isotope compositions of these metals, together with major and trace element concentrations, were measured for a wide sample sets including a geographic transect from headwater to estuary, a temporal series of samples collected in Paris and an anthropogenic sample series. Our data show that the concentrations of these metals in SPM clearly increase downstream, while their isotope compositions show a decrease. Calculation of enrichment factor relative to natural background points to an important anthropogenic input of these metals. This input could be traced by combining multiple isotopic data with the geochemistry of other major and trace elements. Taken as a whole, though Zn, Fe and Cu concentrations correlate very well, Cu displays different isotopic trends compared to those of Zn and Fe. Our study demonstrated the importance of multi-isotopes in studying the anthropogenic contribution and characterizing the geochemical behaviors of heavy metals in river systems.