海洋沉积物^(32)Si测年方法研究

Study on dating method of ^(32)Si in marine sediments

天然^(32)Si是宇宙射线成因的,随着降水进入湖泊或海洋,被硅质生物摄取后最终存在于生物硅中。^(32)Si来源单一,生产速率相对恒定,半衰期为150 a,可测年的时间尺度为100～1 000 a,是该时间尺度最合适的测年核素,填补了百年到千年时间尺度测年方法的空白。本文建立了海洋沉积物^(32)Si的测量方法,主要步骤为:(1)样品前处理;(2)生物硅的分离与纯化;(3)生物硅样品中磷的分离与纯化;(4)^(32)P的制样与β计数测量。全程通过硅钼蓝和磷钼蓝分光光度法监测实验过程硅和磷的损失情况,对南沙海域采集的沉积物岩心进行研究,得到南沙海域沉积物岩心^(32)Si的平均活度为16.60 mBq/kg,范围值为8.39～33.34 mBq/kg;^(32)Si在SiO_2中的平均比活度为0.356 Bq/kg,^(32)Si的核素丰度平均值为1.29×10^(-16 )(^(32)Si/SiO_2);根据岩心^(32)Si活度估算得深水区(水深1 335～1 537 m)和浅水区(121～141 m)岩心的沉积速率分别为0.106 cm/a、0.191 cm/a;根据^(32)Si活度计算^(32)Si的平均沉降通量为2.14×10^(-6 )Bq/(cm^2·a),与参考文献的结果较为吻合。

32Si generates from the spallation reaction of cosmic rays and argon in atmosphere,which is ingested by siliceous organisms such as diatoms after falling into the lakes and sea from atmosphere.The cosmogenic radioactive isotope 32Si,with a half-life of approximately 150 a,represents an excellent dating tool for the age range of approximately 100-1000 a.32Si,with a single source and a constant production rate,is of importance to the ancient environments and paleoclimatology.The method of determining 32Si activity in the marine sediments was established.The main steps were as follows:(1)sample pretreatment;(2)separating and purifying biogenic silica;(3)separating and purifying phosphorus from biogenic silica;(4)preparation of phosphorus and counting 32P by β counter.The experimental process was monitored by acidic silicomolybdenum blue spectrophotometry and phosphomolybdenum blue spectrophotometry to control loss of silicon and phosphorus during the process.The specific activity of 32Si in sediments from Nansha sea area varies from 8.39 mBq/kg to 33.34 mBq/kg,with 16.60 mBq/kg as the mean value.The average specific activity of 32Si in SiO2 is 0.356 Bq/kg.The average specific abundance of 32Si in biogenic silica is 1.29×10^-16(32Si/ SiO2).Sedimentary rates of sediment cores from deep water(1335-1537 m)and shallow water(121-141 m)are 0.106 cm/a and 0.191 cm/a respectively.The average setting flux of 32Si is 2.14×10^-6 Bq/(cm^2·a),within the range of references.