Iron isotopes in the Seine River (France): Natural versus anthropogenic sources

The determination of fluxes and isotope compositions of Fe transported from continents to the ocean is essential for understanding global surface Fe cycle and its effect on oceanic biological productivity. Contrasting to non-polluted rivers, Fe isotope composition in rivers strongly affected by human activities is poorly constrained. In this contribution, we present the first Fe isotope data in suspended particulate matter (SPM) and dissolved load of the human-impacted Seine River (France). Iron concentrations and isotope compositions, together with major and trace element concentrations, were measured for two sample sets: (1) a geographic transect along the river from headwater to estuary, and (2) a temporal series of samples collected in Paris from 2004 to 2007. In the Seine River, Fe is mostly carried by SPM (average 99% of the total Fe) rather than dissolved load. The high Fe enrichment factor (1.40, relative to natural fluvial pre-historical and headwater sediments) and strong correlation between SPM Fe and Zn concentrations (r(2) = 0.70, n = 30) demonstrate a strong anthropogenic Fe input. The Fe isotope compositions in SPM show a very small range (delta Fe-56 from -0.05 parts per thousand to 0.09 parts per thousand) in spite of the large variations of Fe concentrations (from 1.78 to 4.17 wt.%) and are comparable to anthropogenic samples, suggesting that anthropogenic sources have similar Fe isotope composition to that of the natural background. In contrast, larger variations of Fe isotope compositions observed in the dissolved load (from -0.60 parts per thousand to 0.06 parts per thousand) than that of SPM may provide a more promising means for tracing anthropogenic contributions to natural river systems. The delta Fe-56 and delta Zn-66 values of the dissolved loads are positively correlated (r(2) = 0.62, n = 8), indicating a mixing between anthropogenic and natural end-members, enriched in light and heavy Fe isotopes respectively. Correlation between dissolved delta Fe-56 and DOC/Fe ratio (i.e. dissolved organic carbon/dissolved Fe concentrations) suggests that dissolved Fe of natural origin is mainly associated with organic colloids. The Fe compounds with low DOC/Fe ratio and delta Fe-56 values may correspond to anthropogenically-derived Fe-oxyhydroxide or sulfide colloids. Our study clearly demonstrates that polluted rivers transport an anthropogenic surplus flux of Fe that can be traced by coupling Fe and Zn isotopes. This surplus flux will fertilize the ocean and increase the primary productivity of phytoplankton, and thus may ultimately impact the global carbon cycle. (C) 2013 Elsevier Ltd. All rights reserved.