现代沉积年分辨的137Cs计年──以云南洱海和贵州红枫湖为例

1964,1975及1986年三个¹³⁷CS时标计算出红枫湖和洱海沉积物平均堆积速率完全一致,说明1975年和1986年次级蓄积峰作为计年时标的可靠性。洱海沉积物平均堆积速率为0．047±0．002g／(cm²·a),与²¹⁰Pb方法所获得的结果一致；红枫湖为0．17±0．01g／(cm²·a),与其建湖以来沉积物实际堆积的观察结果吻合。洱海1964年沉积物中¹³⁷CS的比活度(校正到沉积年代)仅46．4Bq／kg,而红枫湖达145Bq／kg。由于红枫湖沉积物堆积速率约为洱海的3．7倍,所以其1964年层节沉积物中¹³⁷CS蓄积量比洱海大11.5倍。红枫湖沉积物中¹³⁷CS累计值的99．4％分配于1985年以前,而洱海的19．4％分配于1986年以后,显示出¹³⁷CS在洱海沉积物中的较大扩散能力。

137Cs DATING BY ANNUAL DISTINGUISH FOR RECENT SEDIMENTATION:SAMPLES FROM ERHAI LAKE AND HONGFENG LAKE

Accurate dating of lake sediments is the basis of establishing precise geochemical cycle model and revealing regional pollution history and global environmental changes. The fallout radionuclide ¹³⁷Cs is an efficient tracer for lake sedimentation and watershed erosion. ¹³⁷CS vertical profiles have been successfully used for comparing sedimentation rates. However, it is very difficult to use the ¹³⁷CS dating technique for determining low sedimentation rate (< 1cm / a) because of sampling problem, so it is necessary to improve on the differentiation level of sampling and determining. In the NOrth Hemisphere, the 1954 and 1964 bine markers of ¹³⁷CS can be used for dating sediments, but the ¹³⁷CS-activity peak of sediment horizon in the 1954 is too low to be distinginshed.The sampling equipment, which don' t disturb the sedimentS with the precision of sample separation as good as 0.1cm, were used to collect samples in Erhai Lake and Hongfeng Lake in 1997 and 1988. ¹³⁷CS vertical profiles in sedimentS show three Peaks; the highest peak corresponds to the fallout accumulation in sediment horizon of the year 1964, the other tWo correspond to the global fallout delivery of 1975 and the leakage of the Chernobyl reactor of 1986. The three Peaks indicate the order of the ¹³⁷CS accumulation in lake sedimentS, so they can be used as new bine markers. The discrepancy betWeen the ¹³⁷CS vertical distribution characteristics and the deposihonal fluxes is probably caused by the ¹³⁷CS diffusion in eariydiagenesis process, hilt the ¹³⁷CS diffusion can' t change the location of ¹³⁷CS peaks. Sediment accumulation rates, which are calculded according to the 1964, 1975, and 1986 bine markers respechvely, show good consistency. TIs reveals the possibility and reliability of the 1975 and 1986bine markers. The average sedimentation rate of Erhai Lake is 0.047 ± 0.0029 / (cm².a) which agrees with the result of the ²¹⁰pb method, and that of Hongfeng Lake is0.17 t 0.01g / (cm². a) which is consistent with the observahon result since theestablishment of HOngfeng Lake. The ¹³⁷CS achvity of the sediment horizon in 1964for Erhai Lake is 46.4Bq / kg and that of Hongfeng Lake is 145Bq / kg. Because thesedimentahon accumulahon rate of Hongfeng Lake is hoe bines of that of Erhai Lake,the '3'Cs ashvity of the sediment horizon in 1964 for Hongfeng Lake is 10 bines ofthat for Erhai Lake. 99.4% of ¹³⁷CS inventory is distributed in sedimentS older than1985 for Hongfeng Lake and 19.4% of that is distributed in sedimentS younger than1986 for Erhai Lake, which indicates larger ¹³⁷CS diffusion capacity in sedimentS ofErhai Lake than in Hongfeng Lake.