锶同位素地层学新方法探究

Exploration of New Methods in Marine Strontium Isotope Stratigraphy

锶同位素地层学(Strontium Isotope Stratigraphy,简称SIS)是建立海相沉积岩层年代模型的一种重要方法。该方法在油气勘探、古气候重建等领域有着广泛的应用。应用SIS的前提是:(1)全球海水锶同位素成分相对均一且随时间推移逐渐变化;(2)样品的锶同位素代表样品形成时原始海水的成分。但是,后期成岩蚀变以及样品表面吸附的杂质会改变样品的锶同位素组成,从而造成无意义的SIS年龄。因此,样品选择与前处理至关重要。文章全面总结了已有的海相碳酸盐岩锶同位素地层学的测试方法,包括样品前处理和锶同位素测试。近年来,为了精准地提取古海洋成分信息,多项应用碳酸盐钕、铅和锂同位素的研究对样品前处理流程进行了深入的探讨。结合这些领域的新发展,作者全面地研究了一个具有独立年龄限制的采集于埃及苏黎世海湾浅海中新世沉积地层中的牡蛎化石样品,根据淋洗溶液中的元素含量变化,推荐一套新的碳酸盐样品前处理流程。文章同时也总结和比较了使用热电离质谱仪(TIMS)和多接收电感耦合等离子质谱(MC-ICPMS)测试的锶同位素的准确度和精度。尽管对于锶含量较高的样品,新一代MC-ICP-MS能够达到与老一代TIMS相似的精度,但是对于含量较低或者对精度要求较高的样品,TIMS仍然是SIS研究的首选。最后,作者对使用MC-ICP-MS开展锶同位素地层学提出一些建议。

Strontium Isotope Stratigraphy(SIS)is an important tool for establishing age models for oceanic sedimentary deposits.It has wide applications in many fields including oil and gas exploration and paleoclimate reconstructions.SIS is based on the premise that global seawater is homogeneous relative to Sr isotopes but with variations through time.A key assumption of SIS is that the samples retain the Sr isotope composition of the original seawater where they form.However,diagenesis and loosely attached impurities can alter the Sr isotope composition of the fossil samples,thereby undermining the meaningfulness of the SIS age.Therefore,sample selection and preparation are crucial for obtaining meaningful SIS ages.This article evaluates existing methods for SIS,including sample preparation and Sr isotope analysis.In recent years,to precisely extract the composition of ancient seawater,studies using Nd,Pb,and Li isotopes in carbonates have thoroughly investigated different methods of sample preparation.Based on these technical advances,this paper thoroughly investigated an oyster sample with independent age constraints from the Miocene shallow-marine deposits of the Gulf of Suez,Egypt.By comparing the elemental composition of sequential leachates of the oyster with those of present-day organisms,this paper proposes a new protocol for SIS sample preparation.This paper also compiles and compares the accuracy and precision of Sr isotope data measured using Thermo-ionizing mass spectrometry(TIMS)vs.Multi-collector inductively coupled plasma mass spectrometer(MC-ICP-MS).While for samples with abundant Sr,the new generation of MC-ICP-MS can reach similar precision and accuracy as the old generation TIMS,for smaller samples and samples that require higher precision,the new generation TIMS is still the best choice for SIS research.Finally,this paper makes some suggestions for Sr analysis on the MC-ICP-MS.