Sedimentary and environment evolutionary records of the southern Lixiahe area in the Subei Plain during Holocene

The PM4 section, 3.4 m in thickness, occurs in the south of the Lixiahe Plain (32°34′N, 119°56′E). The section lithologically consists of silt, clay and sand. 170 samples every 2cm are taken for grain size analysis in addition to 7 samples for AMS ¹⁴C dating and 86 samples every 4 cm for determination of foraminifera and ostracoda species. 5 stratigraphic units are identified from bottom to top according to the foraminifera and ostracoda assemblages, median grain size and other grain size parameters. The sediments from depth of 1.4~3.4 m or 11900~4850 cal.aBP are dominated by gray-black lacustrine deposits consisting mainly of silt, 83.9% on average. Clay and sand concentrations are 14.9% and 1.2%, respectively on average.Neither foraminifera nor ostracoda has been found. From the depth range of 1.1~1.4 m or 4850~4250 cal.aBP in age, the increase in silt and sand content indicates that water dynamics was intensified comparing to that in previous stage, suggesting an environment of coastal swamps. From 4250~4050 cal.aBP (1.14~0.98 m), sand sharply increased to 11.7%~15.3%, or 13.9% on average, indicating that the depositional environment was a bay behind the north branch of the Huangqiao sand bar. From 4050~1850 cal.aBP or from 0.98~0.68 m in depth, silt increased to 82.9%~88.6%, 86.3% on average, indicating that the hydrodynamics was weakened in that region. Foraminifera was rare, suggesting a lagoonal facies. During the period of 1850~0 cal.aBP (0.68~0 m): sand input was reduced to 1.0%~11.1% or 3.2% on average. No foraminifera is observed excluding small amount of ostracoda, suggesting a lacustrine environment.