全新世中期相对稳定的长江水位造成约3000年的湖泊沉积缺失

长江中下游地区浅水湖泊密布,全新世该区湖泊沉积的模式还不清晰。本研究在长江中下游的南漪湖、升金湖和菜子湖这3个湖泊开展了多钻孔AMS^14C测年工作,测年结果显示这些湖泊沉积地层中广泛出现长时间尺度的沉积物缺失。南漪湖湖泊钻孔的沉积物14C年龄介于5668~7828cal.aB.P.,菜子湖湖泊钻孔的沉积物^14C年龄介于6221~7929cal.aB.P.,升金湖围垦区钻孔14C年龄介于6302~7049cal.aB.P.。结合该地区以往湖泊钻孔研究资料,发现全新世长江中下游两岸洼地湖泊存在较广泛的6~3ka的沉积间断。结合长江水位重建资料,笔者提出关于全新世湖泊沉积存有长期间断的新认识:即6~3ka,长江水位相对平稳,湖泊沉积物虽有堆积,但易于被侵蚀搬运造成沉积间断;与此对应的是,在约8~7ka,海面上升造成长江水位较快上升,由于顶托作用,湖泊沉积物持续堆积;在约3ka以来,由于人类活动的影响,以及长江水位的进一步上升,湖泊沉积物也易于堆积,但在一些湖区沉积物也会被侵蚀。在6~3ka之间湖泊沉积物易于被侵蚀的一个可能原因是该时段长江上游来沙来水减少,自然堤易被破坏,对两岸湖泊洼地的封堵作用减少,使得湖泊泥沙易被侵蚀入江。

ABOUT 3000 YEARS OF LAKE SEDIMENT HIATUS DUE TO RELATIVELY STABLE WATER LEVEL OF THE YANGTZE RIVER IN THE MIDDLE HOLOCENE

The length of the Yangtze River ranks third in the world and first in Asia. Shallow lakes are densely distributed in the middle and lower reaches of the Yangtze River. Since the late 1940s, the reclaimed area of lakes in this area was over 13000 km^2. At present, there are 651 lakes over 1 km^2 in the middle and lower reaches of the Yangtze River, with an area of 16476 km2. The largest is Poyang Lake with an area of 2993 km^2. Sever sediment accumulation occurred in many lakes in the region in the past decades. However, the pattern of lake sediment accumulation in Holocene is not clear. We carried out multi AMS^14C dating in multi cores at Nanyi Lake, Shengjin Lake and Caizi Lake in the middle and lower reaches at the Yangtze River Basin. The AMS^14C dating results of plant residues in the three lakes cores showed that long time sediment hiatus were ubiquitous in these lakes. For example, the14C ages of sediments from cores in Nanyi Lake ranged from 5668 cal.a B. P. to 7828 cal.a B. P., and those from Caizi Lake ranged from 6221 cal.a B. P. to 7929 cal.a B. P., while for Shengjin Lake14C ages ranged between 6302 cal.a B. P. and 7049 cal.a B. P. Combining with the reliable dating results of plant residues from Poyang Lake, Taihu Lake, Taibai Lake, Chaohu Lake, Chenghu Lake in this region, we found that there were widespread hiatus of sediment during 6~3 ka in lakes from both sides of the middle and lower reaches of the Yangtze River in Holocene. Based on the reconstructed water level data of the Yangtze River and sea level, we propose a hypothesis for the long-term hiatus of Holocene lake sediments. During 6~3 ka, water level of the Yangtze River was relatively stable. Although sediments were accumulated in lakes, they were easy to be eroded and transported due to relatively high elevation of water-sediment and the impact of Yangtze River, resulting in discontinuity of deposition. While at about 8~7 ka, water level of the Yangtze River rose rapidly due to the rising sea level, and lake sediments continued to accumulate due to the supporting effect. Since about 3 ka, lake sediments were also easy to accumulate due to human activities and the further rise of water level in the Yangtze River, but sediments in some areas of some lake were also eroded. One of the possible reasons for lake sediments hiatus during 6 ka and 3 ka is that the inflow of sediment in the upper reaches of the Yangtze River decreases and natural levees are easily destroyed, resulting the deduction of blocking effect on lakes from both sides of the Yangtze River and making lake sediments easily to be eroded into the river.