Iron isotopes reveal seasonal variations in the mechanisms for iron-bearing particle and colloid formation in the Lena River catchment, NE Siberia

Large Arctic rivers are an important source of iron (Fe) to the Arctic Ocean, though seasonal variations in the terrestrial source and supply of Fe to the ocean are unknown. To constrain the seasonal variability, we present Fe concentrations and isotopic compositions (delta Fe-56) for particulate (>0.22 mu m) and colloidal (<0.22 m-1 kDa) Fe from the Lena River, NE Russia. Samples were collected every month during winter baseflow (September 2012-March 2013) and every 2-3 days before, during and after river ice break-up (May 2015).Iron in particles have isotope ratios lower than crustal values during winter (e.g., delta Fe-56(Part) = -0.37 +/- 0.16 parts per thousand), and crustal-like values during river ice break-up and spring flood (e.g., delta Fe-56(Part) = 0.07 +/- 0.08 parts per thousand), indicating a change in the source of particulate Fe between winter and spring flood. Low isotope values are indicative of mineral dissolution, transport of reduced Fe in sub-oxic, ice-covered sub-permafrost groundwaters and near-quantitative precipitation of Fe as particles. Crustal-like isotopic compositions result from the increased supply of detrital particles from riverbank and soil erosion during river ice break-up and flooding. Iron colloids (<0.22 mu m) have delta Fe-56 values that are comparable to or lower than crustal values during winter (e.g., delta Fe-56(Col) = -0.08 +/- 0.05 parts per thousand) but similar to or higher than crustal values during spring flood (e.g., delta Fe-56(Col) = +0.24 +/- 0.11 parts per thousand). Low delta Fe-56 ratios for colloidal Fe during winter are consistent with precipitation from isotopically light Fe(II)(aq) transported in sub-permafrost groundwaters. Higher colloidal delta Fe-56 ratios during the spring flood indicate that these colloids are supplied from surface soils, where Fe is fractionated via oxidation or organic carbon complexation, similar to during summer. Approximately half of the annual colloidal Fe flux occurs during spring flood while most of the remaining colloidal Fe is supplied during summer months. The total amount of colloidal Fe transported during winter was relatively low. The seasonal variation in colloidal Fe isotope values may be a useful tool to trace the source of colloidal Fe to the Arctic Ocean and monitor future changes in the sources and supply of Fe from the permafrost landscape to the Lena River basin.