DEPOSITIONAL FEATURES ANG PALAEOENVIRONMENTAL RECORDS OF THE MUD DEPOSITS IN MIN-ZHE COASTAL MUD AREA,EAST CHINA SEA

During the systemic investigation carried out in the Min-Zhe coastal mud area in 2007-2009 by the "Project on Coastal Investigation and Research", 2949 surface sediment samples and 148 gravity cores were collected. The following results have been achieved based on the detailed study of the samples:(1) As defined by the mean grain size finer than 6.5Φ, the Min-Zhe coastal mud area is distributed between 26°N and 29°N, with a water depth less than 90 m. (2) Gao-Collins grain size trend analysis was carried out to delineate the net sediment transport patterns of the mud area. It indicates that the sediments were mostly transported from northeast towards southwest. The sediments off the Minjiang River mouth were moved in a radiated pattern from the river mouth, upto the area around 27 N as the farthest. (3) The clay minerals in the Min-Zhe coastal mud area could be partitioned into two classes. RegionⅠdistributed from the Yangtze River estuary to the area off the Minjiang River is characterized by high content of illite (64% on average), indicating a provenance from the Yangtze River. Region Ⅱ, located in the Minjiang River estuary, is characterized by high content of kaolinite and chlorite (sum of the two averages 50%), suggesting the source of sediments possibly coming from the Minjiang River. (4) R-factor analysis of macroelements reveals that the mud area is dominated by terrigenous detrital materials instead of biological and marine chemistry materials. (5) The²¹⁰Pb data from seven gravity cores demonstrates that the sedimentation rates change between 0.79 and 3.34 cm/a, with 1.97 cm/a on average. It is lower than the figure from the mud area around the Yangtze River estuary. (6) Based on the environmentally sensitive grain size, clay mineralogy, macroelement assemblages and AMS¹⁴C dating data of the Core MZ01 from the Min-Zhe coastal mud area, 10 extreme values corresponding to contemporaneous cooling events could be recognized since mid-Holocene, and the extreme values are likely to be caused by the enhancement of the East Asia winter monsoon. Furthermore, it is deduced that the East Asia winter monsoon could be divided into four stages, i.e., 8 300~6 300 aBP, strong and unstable; 6300~3800 aBP, strong but stable; 3 800~1 400 aBP, weak and unstable; and after 1 400 aBP, weak but stable.