Extreme iron isotope fractionation between colloids and particles of boreal and temperate organic-rich waters

Significant and systematic variations of iron isotopic composition in surface water sample fractions obtained by frontal cascade filtration and ultrafiltration have been recorded in (1) subarctic organic-rich boreal river and stream, mire, lake and soil solutions in northern taiga zone (Karelia, NW Russia) and (2) temperate river and lake waters of the southern boreal zone (Central Russia). Water samples were filtered in the field employing progressively decreasing pore size from 100 μm to 1 kDa followed by iron isotope analysis. In all river samples, there was a gradual increase of δ57Fe relative to IRMM-14 with decreasing pore size, from +0.4 ± 0.1‰ at 100 μm up to +4.2 ± 0.1‰ at 10 kDa fraction in the subarctic zone and from −0.024 ± 0.2‰ at 100 μm up to +1.2 ± 0.2‰ at 10 kDa in the temperate zone. In the series of filtrates/ultrafiltrates of subarctic and temperate streams and rivers, the δ57Fe value decreases with increasing molar Fe/Corg ratio. Therefore, small-size, Fe-poor, C-rich colloids (1–10 kDa) and Low Molecular Weight (LMW) fractions of oxygenated water exhibit strong enrichment in heavy isotope whereas High Molecular Weight Fe-rich colloids (100 kDa–0.22 μm) and particles (1–100 μm) are isotopically lighter and closer to the continental crust Fe isotope composition.The relative enrichment of 1–10 kDa ultrafiltrates in heavy isotopes suggests that low molecular weight ligands bind Fe more strongly (Fe–O–C bonds) than Fe(III)oxy(hydr)oxides (Fe–O–Fe bonds), in accord with quantum mechanics principles. Highly positive δ57Fe of the LMW fraction of labile and potentially bioavailable Fe in small subarctic rivers may turn out to be a very important source of isotopically heavy Fe in the Arctic Ocean. The mechanisms involved in the production of this isotopically heavy Fe may lead this tracer to become a new indicator of environmental changes occurring in the boreal zone.