中国煤山剖面事件界线层粘土中石膏化生物化石碎屑初探

Recognition of Gypseous Broken-fossils in the White Clay near Permian/Triassic Boundary of the Meishan Section in South China

中国煤山剖面第 2 5层亦即俗称的“白粘土”层代表着曾发生在二叠纪与三叠纪 (P/T)地史转折期 (约 2 .5亿年前 )的灾变事件地层记录。继发现“白粘土”中存在自形晶石膏之后 ,这里新发现其中保存有石膏化生物化石碎屑。中子活化分析显示 ,白粘土中的石膏化生物化石碎屑与自形晶石膏乃至与白粘土本身具有基本相同的稀土分布模式 ,指示了三者在相同或极其相似环境中同期或准同期形成。联系“白粘土”呈天然强酸性的事实 ,提出了酸性硫酸盐化学置换碳酸盐导致生物化石碎屑石膏化的成因机制。石膏化生物化石碎屑的发现 ,为P/T转折期的海洋曾出现严重酸性化事件提供了新证据 ;

It is known that marine biodetritus deposits were often carbonated to form carbonate fossils with carbonate compositions such as CaCO 3. However, an exceptional fact is provided. The marine Permian/Triassic (P/T) boundary clay, the famous 'white clay' in the Meishan section, located in Changxing county, Zhejiang province of China, contributes rich pieces of gypseous broken-fossils. They appear with macro and micro morphological features under optical microscope and scanning electron microscope (SEM), with mineralogical phase of gypsum determined by powder X-ray diffraction analysis (XRD) and with chemical form of CaSO 4·2H 2O supported by measurements using energy-diffractive spectroscopy (EDS). In contrast, no carbonate fossils or biodetritus could be found from the same boundary clay at the same location. Undoubtedly, the gypseous broken-fossils were recasts and they were high likely derived from primary carbonate fossils or carbonate detritus as precursors. But, due to carbonate (such as calcite) much stable than sulfate (such as gypsum) in general, dependent on K sp(CaSO 4·2H 2O)K sp(CaCO 3) representatively, geochemical replacement of sulfate to carbonate is not permit, unless in acidic condition stronger than pH4. Following study through neutron activation analysis (NAA) confirms that there are identical mode of rare-earth elements (REE) distributions between the gypseous broken-fossils and the pure boundary clay (without any gypsum by separation), indicating both originating at least in similar marine environments and at close times. Therefore, associated with the REE mode, the finding of the gypseous broken fossils supports the proposal that a marine acidification event by hydrate sulfuric acid would occur near the time of the P/T boundary in the Earth history.