Incursion of sea water into Gucheng Lake detected by magnetic, biologic and chemical data

Abstract

Magnetic, biological and chemical studies were performed on lacustrine deposits from Gucheng Lake, Jiangsu Province, China. Magnetite and haematite are identified along the entire profile. It is not clear whether magnetite or/and ferrimagnetic iron sulfide (likely greigite) predominates above a striking boundary at 12.0 m, expressed by higher susceptibility (χ) and remanence values. Low NRM intensities above 12.0 m correlate to lower P factors (χmaxχmin) and shallow inclinations of χmin, possibly indicating some disturbing remanence acquisition processes.Pollen, diatom, pigment and TOC analyses reveal that the early Holocene warming (started at the 13.7 m) led to a pulsative sea water incursion at 12.0 m. This incursion resulted in a period of brackish water, which favours the authigenesis of magnetite or ferrimagnetic iron sulphide, resulting in higher magnetic susceptibility and remanences values. Progressive freshing water occurred between 6.0 and 4.0 m. NRM changes correlate with these environmental trends. Magnetic changes above 2.5 m seems to be a result of human-driven deforestation and cultivation, as evidenced from pollen studies and lithological observations. Our results show that pollen and pigment assemblages directly reflect palaeoclimatic changes, whereas magnetic properties and diatoms respond primarily to the changes of hydrology and the limnology of Gucheng Lake.