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

Magnetic, biological and chemical studies were performed on lacustrine depo sits from Gucheng Lake, Jiangsu Province, China. Magnetite and haematite ar e identified along the entire profile. It is not clear whether magnetite or /and ferrimagnetic iron sulfide (likely greigite) predominates above a stri king boundary at 12.0 m, expressed by higher susceptibility (chi) and reman ence values. Low NRM intensities above 12.0 m correlate to lower P factors (chi(max)/chi(min)) and shallow inclinations of chi(min), possibly indicati ng some disturbing remanence acquisition processes. Pollen, diatom, pigment and TOC analyses reveal that the early Holocene war ming (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 th e authigenesis of magnetite or ferrimagnetic iron sulphide, resulting in hi gher magnetic susceptibility and remanences values. Progressive freshing wa ter occurred between 6.0 and 4.0 m. NRM changes correlate with these enviro nmental 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. Ow results show that pollen and pigment assembla ges directly reflect palaeoclimatic changes, whereas magnetic properties an d diatoms respond primarily to the changes of hydrology and the limnology o f Gucheng Lake. (C) 1999 Elsevier Science Ltd. All rights reserved.