近300年来福建长乐海岸沙丘记录的风沙环境演变

RECENT 300-YEAR COASTAL ENVIRONMENT EVOLUTION BASED ON AEOLIAN SAND IN CHANGLE, FUJIAN, CHINA

对福建长乐东海(DH)海岸沙丘剖面加密采样,通过光释光(OSL)测年分析确定了DH海岸沙丘的发育年代。在统一的时间标尺上,以粒度参数作为主要代用指标,综合对比东亚和北半球其他地区小冰期(LIA)风沙沉积记录,以及东亚冬季风变化序列、北半球温度变化序列、北大西洋涛动指数(NAO)、福建海岸线历史变迁等记录,探讨长乐海岸沙丘发育过程及其环境意义。结果表明:(1)DH沙丘砂以中砂和细砂为主,总体分选极好,负偏至近对称和中等峰态,基本符合现代典型海岸风成砂的特征;(2)OSL年代学分析表明,DH沙丘是近300年来的小冰期中后期发育的,这一时期总体气候特征是冬季风偏弱条件下的干冷多风,叠加频繁的人类耕作活动,海岸沙丘广泛发育;(3)LIA晚期以来,DH沙丘平均粒径代表的风沙活动强度与东亚冬季风和北半球温度变化序列对应较好,但是与欧亚大陆西海岸的NAO位相变化晚期序列的相关性不明显。区域台风、风暴潮可能在短时间内在海岸带产生快速的风沙侵蚀和堆积,造成粒度记录信号的局部缺失或突变;(4)以DH沙丘为代表的海岸沙丘演变过程与区域海平面变动密切相关,可能记录了晚全新世海平面总体下降背景下的次级波动信息。自汉代以来,长乐海岸线逐渐后退,至今向东推进约5—10km,海退过程有效的促进了海岸沙丘向海进积发育。

Coastal dunes are formed under the interaction of air, land, and sea, in which wind and sea level change play important roles in the development of the dunes. Using optical stimulated luminescence（OSL） technique, we studied six samples of coastal dunes along Donghai（DH） section（25°52′29.9″N, 119°34′44.5″E; 4.42 km from coastline） in Changle, Fujian, East China. Samples were collected with stainless pipe（30cm long, diameter 6cm）. Grain size characteristics were determined and their fluctuation was compared against those of the East Asian winter monsoon sequence, the Northern Hemisphere temperature series, and the North Atlantic Oscillation Index（NAO）. These samples were mainly medium-fine-grained sand, well sorted, in negative to nearly symmetrical skewness, and medium kurtosis, indicative of typical coastal aeolian sands. OSL dating show that they were formed in the past 300 years since the late Little Ice Age（LIA）. The climate conditions were weaker East Asia winter monsoon, dry and windy, and frequent human activities promoted coastal dunes. Since late LIA, the intensity of aeolian activity as represented by the mean grain size, the East Asian winter monsoon and Northern Hemisphere temperature series are well related each other, but not with the North Atlantic Oscillation. Regional typhoon and storm surge might have produced rapid sand accumulation and erosion, resulting in information loss or alteration in grain size. We found that the development of the dunes was closely related to sea level change as the dunes recorded secondary fluctuations of late Holocene sea level. Since the Han Dynasty（202—220BC） in China, the Changle coast retreated gradually, and had progressed eastward for 5—10km, which promoted efficiently the seaward growth of the coastal dunes.