用^(87)Sr/^(86)Sr研究海平面变化与全球对比问题

VARIATIONS OF RATIO OF ^(87)Sr/^(86)Sr IN SEAWATER WITH TIME:IMPLICATIONS FOR SEA LEVEL CHANGES AND GLOBAL CORRELATION

综述了近年来国际上在 Sr同位素研究应用方面的新成果 ,着重讨论了如何应用海相沉积岩 (物 ) 87Sr/ 86Sr比值来研究海平面变化和全球对比问题。综合分析表明 ,海水 Sr同位素组成受 Sr的来源控制 ,海水87Sr/ 86Sr比值随时间变化与海平面变化有内在的联系 ,高频旋回 (时限约 2 .0Ma)的87Sr/ 86Sr变化与三级海平面变化相对应。研究认为牙形石是分析 Sr同位素组成最理想的样品 ,用之建立的 87Sr/ 86Sr变化曲线最具全球对比意义。与国外相比 ,我国在这方面研究成果较少 ,但从地质条件分析 ,我国独有的一些地质记录 ,在古海平面变化和全球对比研究中占据着不可缺少或不可代替的地位 ,其中扬子地块西南缘二叠—三叠纪界限附近的 87Sr/ 86Sr变化曲线 ,可以弥补国外一些学者建立的自显生宙以来全球海水87Sr/ 86Sr变化曲线在该时段存在的“断层”现象。

Strontium(Sr) is one of the most sensitive indicators to seawater. It has been widely exploited both in terms of its concentration and particularly in its isotope, which has been used to investigating ancient sea level fluctuations. Appreciable progresses have been achieved in recent years, but problems still remained with the seawater 87 Sr/ 86 Sr curve construction for the purpose of global correlation. It is shown that the variations of 87 Sr/ 86 Sr throughout the Phanerozoic are related to the sea level changes. High frequence(2.0 Ma) fluctuations in seawater 87 Sr/ 86 Sr consistent with third order sequence. These changes are controlled by Sr sources of the seawater. There are two main Sr sources, one is “the continental Sr” with a high 87 Sr / 86 Sr, which is input to seawater by flux, the other one is “the oceanic Sr” with a low 87 Sr / 86 Sr, which is got into seawater directly by hydrothermal circulation through seafloor. Conodont is the idea sample for the studies because of its precise stratigraphic age and high Sr concentrations as well as relatively resistant to digenetic alternation. The 87 Sr/ 86 Sr curve of seawater constructed by using conodont is the best standard for global correlation. China possesses some unique geological records, such as the continuous sedimentation at the Permian/Triassic boundary, which is very helpful to supplement the gap in the 87 Sr/ 86 Sr curve constructed for seawater from Permian to Jurassic by R. B. Koepnick et al(1998) on the basis of Burke et al(1982).