Iron speciation in modern sediment from Erhai Lake, southwestern China - Redox 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+, hema tite Fe3+, and paramagnetic high-spin Fe2+ using Mossbauer spectroscopy. Th e 120 cm core has a distinct lithological boundary at a depth of about 70 c m. Each Fe species has a distinctive distribution with depth. These results represent the redox conditions within the sediment, and also probably refl ect the primary sedimentary environments. With increasing burial depth, hem atite (Fe2O3) decreased, especially below depths greater than 25 cm, and fi nally disappeared at around 95 cm. The summed paramagnetic Fe3+ (superparam agnetic Fe3+ + paramagnetic Fe3+) did not change as much, only exhibiting a slight decrease at depths greater than 75 cm, about 5 cm beneath the litho logical boundary within the core. The intensity of paramagnetic high-spin F e2+ increased with depth. These vertical variations were in harmony with or ganic geochemical parameters such as TOC concentration, I-I-index and O-ind ex, indicating that reducing conditions are strongly intensified in the sed iment below 70 cm. The geological, organic geochemical and C-14 data combin ed with the present Mossbauer spectroscopic study give a strong indication that the redox environment of Erhai Lake probably shifted rather rapidly fr om a deep reducing to a shallow oxic state at about 2 ka ago. (C) 2001 Else vier Science Ltd. All rights reserved.