赣北黄茅潭近代湖泊137Cs蓄积特点、SCP计数和事件性沉积及其对210Pb计年的矫正

摘要

Profiles of 210Pb and 137Cs of two cores drilled from Huangmaotan Lake were rebuilt,and were compared and adjusted against those of SCP (spheroidal carbonaceous particle) counting,and grain size records.The lake is situated in North Jiangxi,China along the middle reach of the Changjiang (Yangtze) River,5.1 km2 in full-water area,and 7.7km2 in catchment area.The basements of the catchment area are composed of Triassic-Cretaceous-Tertiary muddy sandstones and conglomerates mostly in continental facies.The two cores,HMT-1 and HMT-3,were taken in center of the lake,100m apart,with a Dutch-made Beeker corer,6cm cross,and 80cm long.Several sedimentary events in 1953-1954,1963-1964,1974-1975,1998-1999,respectively,were determined based on correlation of above-mentioned records to the annual rainfall and the annual days of heavy rain during 1952-2012.The age control for Core HMT-1 was reconstructed,which is largely different from sedimentary chronology of 210Pb.We believe that this discrepancy was mainly resulted from 210Pb dating.As the sedimentary environment in the middle reaches of Changjiang River is complicated as revealed in the Huangmaotan Lake,an event horizon settled from large flood could cause errors in 210Pb dating.In addition,we found that year 1986 is the turning point of 137Cs environmental activity in sediment of the Changjiang River catchment.Before 1986,correlation between rainfall and 137Cs concentrations was overall positive,which is mainly due to atmospheric deposition during 1952-1986;and after 1986,it became overall negative during 1986-2010 as the hosting sediments were derived mostly from the river catchment areas.A typical peak of 137Cs activity in 1963-1964 was found related to high atmospheric fallout and to low sedimentation due to small rainfall and less flooding.Therefore,the 137Cs peak of 1963-1964 can be used as a reliable time maker horizon for recent sediments in the river catchment areas,so be another peak in 1986.Although 137Cs dating needs to be validated in the future,it remains as a tool to set time maker horizons for the recent sediments.However,it is necessary to compare 137Cs activity profile against those of grain size,SCP counting,and other records to search for characteristic time maker layers,and adjust the result of 210pb dating.%对赣北黄茅潭近代湖泊沉积岩芯进行了137Cs、210Pb测试和SCP(球状碳粒)计数分析,阐述了137Cs蓄积特征,结合SCP计数、粒度指标及降水记录厘定了一些事件性沉积层位.研究表明,1986年前后是沉积环境中137Cs行为的转折点;在这之前,137Cs以大气散落为主,其蓄积行为大致与降水存在正相关关系,在这之后,137Cs以流域侵蚀为主,其蓄积行为与降水呈负相关关系.1953-1954年、1974-1975年、1998-1999年,流域降水丰沛,相应沉积层位137Cs比活度低,这与流域强烈侵蚀稀释了进入湖泊的137Cs有关.1963-1964年沉积层位137Cs蓄积峰稳定而显著,与高通量的大气散落有关,也与当时降水量低,大雨、暴雨次数少,流域侵蚀强度低造成较低的沉积速率等密切相关,是可靠的定年时标.1986年存在同样的气候环境特点,其蓄积峰可能也是存在的,但需要进一步确认.基于210Pb方法,利用多种计年模式计算了沉积岩芯的年代,发现与这些事件性沉积层位具有较大差异.研究认为,在长江中游这种降水高、流域侵蚀强度高的较为复杂的沉积环境中,210Pb计年存在较大误差.复杂沉积环境中近代沉积的定年,有必要深度挖掘137Cs环境行为,在全面阐述其蓄积特点的基础上,辅以SCP计数、粒度指标及降水等识别事件性沉积层位,矫正210Pb计年,是精确建立近代沉积时标的必要方法.