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Iron isotope compositions of sediment with depth-dependent iron reduction in a temperate eutrophic freshwater lake
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DOI：10.1016/j.gca.2026.07.017.png)
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Abstract
En 中文
Pathway-dependent iron isotope fractionation is a powerful tool for evaluating iron biogeochemical cycle. Lake sediments host complex iron redox transformations, yet very few iron isotope investigations have been performed on lake sediments. Here, we combined high depth-resolution multi-biogeochemical analyses with stable iron isotope measurements to investigate a sediment core (21 cm) from Baiyangdian, a temperate eutrophic freshwater lake in North China. The excess 210Pb analysis indicated an undisturbed sedimentary record in the lake. In the top 4 cm, sulfide accumulated in the porewaters and the reactive iron was mainly present as iron sulfides (51–86%), indicating that Fe(III) (oxyhydr)oxides were primarily reduced by sulfide. Accordingly, the δ56Fe values of sorbed Fe(II) plus tightly-bound Fe(II) (mostly FeS with little FeCO3) gradually decreased from −0.21 ± 0.06‰ to −0.95 ± 0.10‰ with depth. Meanwhile, the isotope fractionation between Fe(III) (oxyhydr)oxides and sorbed Fe(II) plus tightly-bound Fe(II) ranged from 1.02 ± 0.10‰ to 1.60 ± 0.10‰. Due to the great extent of pyritization, pyrite exhibited relatively heavy δ56Fe values (−0.32 ± 0.04‰ to 0 ± 0.09‰) within the 1–4 cm layer. Below 4 cm depth, microbial iron reduction became dominant in the absence of sulfide. Fe2+ concentrations in the porewaters gradually increased with depth, and non-sulfur-bound iron species dominated the reactive iron pool (67–94%). The sorbed Fe(II) and tightly-bound Fe(II) (mostly FeCO3 with little FeS) together constituted 26–54% of the reactive iron. The δ56Fe values (−1.97 ± 0.08‰ to −1.49 ± 0.09‰) of sorbed Fe(II) plus tightly-bound Fe(II) were lighter than those at 1–4 cm depth. The isotope fractionation between Fe(III) (oxyhydr)oxides and sorbed Fe(II) plus tightly-bound Fe(II) ranged from 1.56 ± 0.11‰ to 2.31 ± 0.11‰ below 4 cm depth, which was larger than that observed at 1–4 cm depth. Our results show that the iron isotopes in the sediment exhibit distinct fractionation depending on the iron reduction pathways, and offer the first comprehensive iron isotope dataset for abiotic sulfide-driven iron redox cycling acquired directly from temperate eutrophic freshwater lake sediments. The findings provide a framework for comprehending how iron undergoes redox cycling and associated isotope compositions in analogous lacustrine sediments.
Keywords:
Iron
Reduction
Iron isotope fractionation
Freshwater lake
Sediment
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