苏北平原沉积孢粉组合记录的全新世气候突变

通过中国东部苏北平原厚3.4 m的PM4剖面孢粉分析结果,重建了该地区全新世以来古植被更替过程,查明了5.20~3.00 cal.ka B.P.中国东部沿海平原百年尺度的海平面变化历史,并探讨了沿海平原植被对海平面变化的响应。孢粉结果表明,全新世以来该地区的植被与气候经历了以下5个阶段:1)11.90~8.80 cal.ka B.P.,亚热带常绿、落叶阔叶混交林繁盛,气候温暖湿润;2)8.80~7.40 cal.ka B.P.,发育了常绿、落叶阔叶混交林-草地,气候相对冷干;3)7.40~5.20 cal.ka B.P.,植被为常绿、落叶阔叶混交林,温度降低,降水增加;4)5.20~1.08 cal.ka B.P.,植被退化为盐生草甸;5)1.08 cal.ka B.P.以来植被中湿生草本群落繁盛。根据藜科、蒿属、禾本科和莎草科花粉的含量变化及粒度、有孔虫特征重建了相对海平面的变化历史,5.20~4.55 cal.ka B.P.、4.25~4.05 cal.ka B.P.与4.55~4.25 cal.ka B.P.、4.05~3.00 cal.ka B.P.分别为海平面上升和下降的阶段。研究区4.25~4.05 cal.ka B.P.海平面上升可与大范围尺度的"4.2 ka B.P.气候事件"相对应,可能受到了当时东亚季风减弱、苏北平原强降水与大规模洪水事件的共同驱动。

Abrupt climate changes recorded by palynological evidence in Subei plain during the Holocene

The studied section PM4 is located in the north of Taizhou City(32°34'N, 119°56'E), southwestern Subei plain, the basal age of the section is 11.9 cal.ka B.P. The lithology of the section are composed of silt, silty clay and clay, and abundant organic residues were also found in the sediment. In total, 82 samples were collected for palynological analysis at an interval of 4 cm in the 3.4 m-thick section. Based on the results of palynological analysis, we reconstructed 5 stages of vegetation change:(1) 11.90~8.80 cal.ka B. P., subtropical evergreen and deciduous broadleaved forests developed as the zonal vegetation in the study area. They were dominated by Fagaceae trees(e.g. Cyclobalanopsis, Castanea, Quercus, Castanopsis). The climate was warm and humid.(2) 8.80~7.40 cal.ka B. P., herb taxa showed an expansion into the evergreen and deciduous broadleaved forests, as indicated by the increase of Chenopodaceae and Artemisia. The inferred climate condition turned to be relatively cold and dry.(3) 7.40~5.20 cal.ka B. P., the pollen contents of Betula, Castanea, Pinus, Curpressaceae, Poaceae and Cyperaceae increased, while the Cyclobalanopsis, Quercus and Castanopsis, Chenopodiaceae and Asteraceae decreased. It is suggested that the precipitation would increase and the temperature decrease during this period.(4) 5.20~1.08 cal ka B. P., herb taxa expanded with a dramatic increase of Chenopodiaceae. The vegetation is salt marsh in the study area.(5) 1.08~0 cal.ka B. P, the contents of Chenopodiaceae and arboreal pollen decreased, due to the eastward movement of the coastline and possible human activity. The content variations of Chenopodiaceae, Artemisia, Poaceae and Cyperaceae can reflect the sea-level fluctuations by their implications for the vegetation response under the changing sea level.(1) During 5.20~4.55 cal.ka B. P, the contents of Chenopodiaceae(as a typical taxon in coastal saline vegetation) and Cyperaceae pollen increased, while those of Artemisia decreased, indicating that the sea level rose up.(2) During 4.55~4.25 cal.ka B. P., Artemisia increased, while Chenopodiaceae and Cyperaceae sharply decreased, suggesting a relative fall of sea level.(3) From 4.25 cal.ka B.P. to 4.05 cal.ka B. P., Chenopodiaceae and Cyperaceae sharply decreased, indicating that the sea level rose up again and more area was submerged.(4) During 4.05~3.00 cal.ka B. P., local vegetation was mainly composed of Chenopodiaceae plants with a small amount of Artemisia, Poaceae and Cyperaceae, indicating that the sea level decreased.(5) During 3.00~1.85 cal.ka B. P., Chenopodiaceae reduced along with the decrease of soil salinity, while Artemisia and Poaceae plants gradually increased, reflecting the further withdraw of sea level. The sea-level rising in the Subei plain during 4.25~4.05 cal.ka B. P. can be coincided with the worldwide "4.2 ka B. P." climatic event. The weakening of the East Asian monsoon, related strengthening of precipitation and large-scaled flood events in Eastern China could be responsible for the rise of the sea level in the Subei plain.