摘要
湖泊沉积物是研究古气候、古环境的理想对象。本文选取鹤庆湖泊沉积物开展了样品BeO制备的实验研究。通过对比黄土提取^(10)Be的思路,利用条件实验确定了先沉淀去除常量元素,再进行阳离子交换的新的实验流程。加速器质谱仪的测量结果表明该流程有效可靠,为下一步拓展湖泊^(10)Be环境示踪的新方向提供了实验方法基础。
Background, aim, and scope The cosmogenic radionuclide 10Be is produced primarily by nuclear interactions between secondary cosmic-ray particles with oxygen and nitrogen in the atmosphere. Its production rate is mainly regulated by the geomagnetic field intensity on time scales of thousands to hundreds of thousands years, so that its accumulated concentration in sediments can, in principle, be used to derive high-resolution records of geomagnetic field changes. A lot of researches that trace paleo- geomagnetic intensity variation using l^Be in ice core, lake and loess sediment were reported during recent years. The results have validated the l^Be technique using AMS, as a viable tool to trace past paleo- geomagnetic intensity changes. However, the research about 1~Be in lake sediment hasn＇t been reported in China. Therefore, it is very necessary to study the lacustrine sample, especially the chemical pretreatment method, which will provide the experimental foundation for environment tracing by lacustrine ~~Be. In this paper, we research the procedure of BeO preparation of samples from Heqing lake sediment. Materials and methods The Heqing Basin in Yunnan was the first deep drill core, which obtained about 666 m depth sediment. The sediment was mainly lacustrine. In the paper, we chose a small part of the drill core as our samples. The depth range of the samples is 120--180 m, the interval of which is 8 cm. According the procedure of BeO preparation of loess samples, it is an important step to create the leaching curve. The time scope of Be from the samples in loess and paleosol is the same. But according the result of condition experiment, the time range of Be of different lacustrine samples was different. Obviously, once cation exchange can not separate Be. So we tried twice cation exchange and precipitation-cation exchange respectively, in order to decide which one can separate Be. Results The experiment results show that （1） we collected the leachate between 800--3600 s. The solutions were evaporated and dissolved, and then put in cation exchange resin. The solutions were collected every 400 s. However, the time scope of Be was still different in different samples. Because during 800--3600 s, lots of other element was contained in the solution except Be. And the type and quantity of ion were different in different samples. Therefore, the start time of Be were different. So the twice cation exchange proves infeasible. （2） According the second method, we remove the major element （Mg, Fe, Ca, et al.） by the acid dissolution-alkali precipitation-alkali dissolution, and then purified Be by cation exchange. Finally, we obtained the time range of Be of different samples, which was 2400--6400 s. Be was purified and separate successfully. Discussion However, comparing the former method, the recovery of samples by the second method （precipitation-cation exchange） was lower. Except the blank sample, the recovery of other samples was between 45%--60%. The possible reason was that other precipitation （e.g. Fe（OH）3） caused the precipitation of Be during the experiment. Therefore, the parallel samples were prepared for AMS measurement. The result revealed that the relative error of each parallel samples was less than 3%, which reflected the new procedure is effective and credible. The new procedure can meet the requirement of 10Be measurement although the recovery was lower. Conclusions Because of the different deposition environment, the climatic proxy of the loess and lake sediments, such as carbonate and organic matter are different. For example, the max value of carbonate is 83.35%, and the minimum value is 0.29% in Heqing core. But the carbonate of loess is below 25% in Xifeng from 900 ka. So the procedure of BeO preparation of loess samples was not fit for the Heqing lake sediment. In the paper, we create the new procedure of BeO preparation of samples from Heqing lake sediment. Similarly, the new procedure is fit for other lake sediment samples. Recommendations and perspectives The new procedure of BeO preparation of Heqing lake sediment samples provides an experiment basis for lacustrine 10Be environment tracing. It is significant for tracing paleo-geomagnetic events. Meanwhile, it＇s definitely worth discussing whether the results of lacustrine 10Be and paleo-geomagnetic method are the same or different. It was found that there was offset between the age of the Branhes-Matuyama （B- M） geomagnetic reversal obtained by 10Be and paleomagnetic method in loess. Further more, maybe the research in lacustrine 10Be can provide a new idea for explaining the offset phenomenon.
出处
《地球环境学报》
2016年第5期521-528,共8页
Journal of Earth Environment
基金
国家自然科学基金项目(41230525)
黄土与第四纪地质国家重点实验室开放基金(SKLLQG1305)~~
