Iron speciation in modern sediment from Erhai Lake, southwestern ChinaRedox conditions in an ancient environment

In a core of sub-aquatic sediment from Erhai Lake, southwestern China, 4 Fe species were identified as paramagnetic Fe3+, superparamagnetic Fe3+, hematite Fe3+, and paramagnetic high-spin Fe2+ using Mössbauer spectroscopy. The 120 cm core has a distinct lithological boundary at a depth of about 70 cm. Each Fe species has a distinctive distribution with depth. These results represent the redox conditions within the sediment, and also probably reflect the primary sedimentary environments. With increasing burial depth, hematite (Fe2O3) decreased, especially below depths greater than 25 cm, and finally disappeared at around 95 cm. The summed paramagnetic Fe3+ (superparamagnetic Fe3++paramagnetic Fe3+) did not change as much, only exhibiting a slight decrease at depths greater than 75 cm, about 5 cm beneath the lithological boundary within the core. The intensity of paramagnetic high-spin Fe2+ increased with depth. These vertical variations were in harmony with organic geochemical parameters such as TOC concentration, H-index and O-index, indicating that reducing conditions are strongly intensified in the sediment below 70 cm. The geological, organic geochemical and 14C data combined with the present Mössbauer spectroscopic study give a strong indication that the redox environment of Erhai Lake probably shifted rather rapidly from a deep reducing to a shallow oxic state at about 2 ka ago.