浙江湘湖早-中全新世的硅藻记录及沉积环境演变

EARLY TO MID-HOLOCENE SEDIMENTARY ENVIRONMENT EVOLUTION IN XIANGHU LAKE AREA, ZHEJIANG PROVINCE, BASED ON A DIATOM RECORD

通过对浙江省萧山湘湖地区XH-2孔上部24 m（全新世以来）沉积物的粒度及硅藻组合进行分析，揭示了研究区早-中全新世期间的沉积环境及相对海平面变化。结果显示，约9300～8850 cal.a B.P.期间，淡水硅藻占绝对优势，半咸水种含量极少，表明湘湖地区当时基本未受海水影响，主要为淡水环境；8850～8550 cal.a B.P.期间，半咸水硅藻含量上升，但整个硅藻组合中仍以淡水硅藻为主，表明湘湖地区虽仍以淡水环境为主，但开始受到间歇性潮汐或风暴潮的影响，对应潮上带的沉积环境；8550～8150 cal.a B.P.期间，淡水种硅藻显著减少，半咸水种、沿岸种及海水种含量增加，表明湘湖地区当时受海水影响显著，对应潮下带的沉积环境；8150～7700 cal.a B.P.期间，淡水种含量开始逐渐回升，海水种含量减少，表明湘湖地区受海水影响开始减弱；7700 cal.a B.P.以后几乎没有发现硅藻，研究区当时可能已经为陆地环境，古人类开始居住，对应于跨湖桥文化（8000～7000 cal.a B.P.）的出现与发展。综合XH-2孔上部24 m沉积物中的粒度变化及硅藻组合结果，发现该地区在8550～8150 cal.a B.P.期间可能存在一个海平面上升事件，该事件在长江三角洲、东南亚甚至世界其他地区均有记录，其成因很可能与全新世早-中期劳伦泰德冰盖融水事件以及Agassiz和Ojibway两个冰川湖溢水事件导致的全球海平面快速上升有关。

Xianghu Lake is located in the Ningshao Plain and very close to the Kuahuqiao archaeological site. A 60-m-long core named XH-2（30°07＇17″N, 120°11＇38″E） was collected in the Xianghu Lake area, and we analyzed the diatom assemblages and grain size of 24-m-long in upper core XH-2 to study the sedimentary environment evolution during the Early to mid-Holocene in this area. The diatom records show that freshwater species were dominant during 9300～8850 cal. a B.P.（2400～1900 cm） whereas brackish water species were rare, indicating a freshwater environment in the Xianghu Lake area, with little influence of seawater. A remarkable increase in brackish water species occurred at the interval 8850～8550 cal. a B.P.（1900～1550 cm）, but freshwater species are still dominated this period, which indicates that tidal water especially storm tide had an intermittent impact on the Xianghu Lake area, corresponding to a supratidal sedimentary environment. Abrupt increase in the abundance of marine and costal water species with the remarkable decline in the freshwater species characterized the period of 8550～8150 cal. a B.P.（1550～1175 cm）, which indicated strong influence of seawater on the Xianghu Lake area, inferring an obviously subtidal sedimentary environment. At the interval 8150～7700 cal. a B.P.（1175～950cm）, there was a gradually increase in freshwater species and decrease in marine species. This implies the weakened impact of seawater on the Xianghu Lake area. Diatoms disappeared after 7700 cal. a B.P., possibly resulted from the land exposure due to the decelerated sea level rise, when the Kuahuqiao culture developed during 8000～7000 cal. a B.P. The rapid sea level rise based on the diatom and grain size record in Xiang Lake area during 8550～8150 cal. a B.P. corresponded well with records from the Yangtze delta, Southeast Asia and other places in the world. And the rapid deglaciation of the Laurentide Ice Sheet and Agassiz-Ojibway outbrust flood may cause rapid sea-level rise in Xianghu Lake area during the early to mid-Holocene.