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

• Published in: Geochimica et Cosmochimica Acta
• Main Authors: Hirst, Catherine, S. Andersson, Per, Mörth, Carl-Magnus, J. Murphy, Melissa, Schmitt, Melanie, Kooijman, Ellen, Kutscher, Liselott, Petrov, Roman, Maximov, Trofim, Porcelli, Don
• Other Authors: UCL - SST/ELI/ELIE - Environmental Sciences
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier BV 2023
• Subjects: Geochemistry and Petrology; Arctic; Arctic Ocean; Ice; lena river; permafrost; Siberia
• Online Access: http://hdl.handle.net/2078.1/278480; https://doi.org/10.1016/j.gca.2023.09.016

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 (δ56Fe) for particulate (> 0.22 µm) and colloidal (< 0.22 µm – 1kDa) 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., δ56FePart = -0.37 ± 0.16 ‰), and crustal-like values during river ice break-up and spring flood (e.g., δ56FePart = 0.07 ± 0.08 ‰), 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 um) have δ56Fe values that are comparable to or lower than crustal values during winter (e.g., δ56FeCol = -0.08 ± 0.05 ‰) but similar to or higher than crustal values during spring flood (e.g., δ56FeCol = +0.24 ± 0.11 ‰). Low δ56Fe ratios for colloidal Fe during winter are consistent with precipitation from isotopically light Fe(II)aq transported in sub-permafrost groundwaters. Higher colloidal δ56Fe 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 ...