奥陶纪棘皮动物骨骼地球化学特征指示古海水Mg/Ca比的可靠性分析  

Reliability of Ordovician echinoderm skeleton geochemistry as a proxy for seawater Mg/Ca ratio

作　　者：丁娜[1] 刘建波[1] 詹仁斌[2] 

机构地区：[1]北京大学地球与空间科学学院 造山带与地壳演化教育部重点实验室,北京100871 [2]中国科学院南京地质古生物研究所 现代古生物学和地层学国家重点实验室,南京210008

出　　处：《科学通报》2014年第10期924-935,1-4,共12页Chinese Science Bulletin

基　　金：国家自然科学基金(41272035;41221001;41290260);现代古生物学和地层学国家重点实验室资助

摘　　要：显生宙棘皮动物骨骼均由高镁方解石组成，其骨骼的Mg／Ca比受海水Mg／Ca比、古海水温度、成岩作用等因素的影响。研究以华南扬子区下奥陶统棘皮动物海百合化石为主要测试对象，通过能谱仪及波谱仪等微区原位分析方法，获得其骨骼可靠的Mg/Ca比。化石骨骼显微构造和化学成分特征表明，扬子区下奥陶统海百合茎化石骨骼受到成岩作用的影响较弱，依旧保存有与壳体原始矿物有关的Mg含量特征。测试结果显示，根据棘皮动物骨骼的Mg／Ca比与海水Mg／Ca比的线性及指数关系，早奥陶世海水的Mg／Ca比分别在0。16～0。7或0。07～1。17之间变动。经过对其变化趋势的讨论，研究认为扬子区下奥陶统棘皮动物骨骼Mg／Ca比的逐渐增高主要受控于海水Mg／Ca比，而受温度的影响相对较小。研究表明，保存良好的棘皮动物化石可更广泛地应用于古代海洋Mg／Ca比的重建。The Mg/Ca ratios of echinoderm skeletons in the Phanerozoic, composed of high-Magnesium calcite, are controlled by seawater Mg/Ca ratios, seawater temperature, and diagenetic modification. This study analyzed the geochemistry of crinoidal fragments in the Lower Ordovician of the Yangtze Platform with SEM, EDS, WDS and CL to assess reliability of echinoderm skeletal Mg/Ca ratio as a proxy for the composition of palaeoseawaters. Multiple lines of evidences suggest that the well-preserved crinoidal skeletons in the Lower Ordovician of South China remain still preserve their original Mg/Ca ratios even after diagenetic modification. The Early Ordovician seawater Mg/Ca ratios are calculated as 0.16-0.7, and 0.01-1.17, following the linear relationship and power function between echinoderm skeleton and seawater Mg/Ca ratios respectively. Compared with the long-term changes in sea-surface temperature, the Mg/Ca ratios of the Early Ordovician echinoderm skeletons are considered to reflect a long-term fluctuation of seawater Mg/Ca ratios instead of seawater temperature. Thus, well-preserved echinoderm skeletons in the Early Ordovician should be widely used as a proxy taxon for paleocean elemental chemistry reconstruction.

关 键 词：早奥陶世 棘皮动物海百合 古海水Mg Ca比 华南 

分 类 号：Q915[天文地球—古生物学与地层学]