Adams和Couch法古盐度恢复结果的可靠性检验——以青海湖布哈河口区沉积物为例

Testing the Reliability of Adams Formula and Couch Formula for Estimating Paleosalinity: Case study of the Buha River Estuary,Qinghai Lake

Adams公式和Couch公式是利用沉积物中硼元素浓度对沉积水体古盐度进行定量恢复的常用方法,但由于两种方法的恢复结果经常存在较大差异,古盐度的计算结果常难以让人信服。为了验证Adams公式和Couch公式的可靠性,本次研究采集了青海湖布哈河口区不同沉积环境的表层沉积物和底层水体样品,处理后分别分析了沉积物中硼元素浓度、总有机碳(TOC)含量及矿物组成,同时对水体盐度及其硼浓度进行了测量,结果表明:湖泊水体中硼浓度与盐度存在极好的线性正相关性,但沉积物中硼元素的浓度与沉积水体中硼的浓度并无直接联系。根据沉积物中硼的浓度及黏土矿物含量和组成,分别采用Adams公式和Couch公式对水体盐度进行计算,盐度计算值均远高于水体盐度实测值,其中Adams公式的计算结果与实测值差别最大,盐度计算值与沉积物中硼含量呈正相关关系。沉积物中的硼元素主要由继承自母岩的硼、黏土矿物吸附硼和有机质中富集的硼三部分组成,只有黏土矿物中的吸附硼能够反映水体盐度。青海湖布哈河口区沉积物富含有机质,有机质对硼强烈的富集作用是导致Adams公式和Couch公式盐度恢复结果失效的重要原因,同时不同类型黏土矿物对硼吸附能力的差异也对恢复结果有重要影响:黏土矿物在盐水中的浸泡试验表明蒙脱石对硼的吸附能力最强,次为伊利石,高岭石吸附能力最差,推翻了Couch关于伊利石对硼吸附能力最强的认识。原Adams公式和Couch公式仅没有考虑有机质对硼的影响,对不同类型黏土矿物吸附能力的认识有误,不适合陆相沉积物沉积水体古盐度的恢复。沉积物黏土矿物中吸附硼浓度仍是反映沉积水体盐度的可靠指标,古盐度的恢复首先必须消除沉积物中有机质对硼的影响,然后根据一系列的浸泡试验确定不同类型黏土矿物对硼的吸附系数,并建立新的计算公式。

The Adams formula and Couch formula are commonly used for quantitative estimation of the paleosalinity of sedimentary water by measuring the concentration of boron in the sediment.However,the paleosalinity values are open to doubt due to the great difference between the values calculated by the two methods.To test the reliability of the two formulas,samples were collected from surface sediments and underlying water in different sedimentary envi ronments in the Buha River estuary of Qinghai Lake.The concentration of boron,total organic carbon(TOC)and min eral composition of the deposits were derived,and the salinity and boron concentration of the water body were mea sured.The results for the lake water showed an excellent linear positive correlation between the concentration of boron and salinity,but no direct correlation between the concentration of boron in sediments and in sedimentary water.Based on the concentration of boron in sediments and the content and composition of clay minerals,the water salinity was calculated by both the Adams formula and the Couch formula.The calculated water salinity was higher than the measured salinity;although the value calculated by the Adams formula differed most from the measured values,it was positively correlated with the boron content in the sediments.The boron in the sediments mainly comprises boron in herited from the mother rock,boron adsorbed by clay minerals and boron enriched organic matter.Of these,only the adsorbed boron reflects the salinity of water.The sediments in the Buha River estuary of Qinghai Lake are rich in or ganic matter.The strong enrichment effect of organic matter on boron content is an important reason for the failure of the salinity estimation by both formulas.Differences in the boron adsorption capacity of different types of clay miner als also significantly influence the results:the immersion tests of clay minerals in salt water showed that montmorillon ite has the strongest affinity for boron adsorption,followed by illite and kaolinite.This contradicts Couch’s under standing that illite has the strongest boron adsorptive capacity.Neither the original Adams formula nor the Couch for mula consider the effect of organic matter on the boron content,and the misunderstanding of the adsorptive capacity of different types of clay minerals renders both formulas unsuitable for estimating paleosalinity in terrestrial sediments.Nevertheless,the concentration of adsorbed boron in clay minerals in the sediment remains a reliable index of the sa linity of sedimentary water.The recovery of paleosalinity must first eliminate the influence of organic matter on the bo ron content,then determine the boron adsorption coefficient of the different clay minerals by carrying out a series of immersion tests and establishing a new formula.