摘要
黔南宾夕法尼亚亚纪Triticites带内大型Fomitchevella珊瑚礁群落中见附礁生长的腕足动物Striatifera striata和Enteletoides shuichengensis,XRD测试结果显示Striatifera striata壳体次生层为保存较好、成分纯净的低镁方解石,完全未受到成岩作用改造;Enteletoides shuichengensis壳层中则含少量Fe、Ni,可能与成岩作用改造有关。两种腕足动物壳体横切面同一生长带棱柱层中的Ni、Fe、Mn、Na、Sr呈对称性分布,表明这两个种壳体原始信息保存良好,可忽略成岩改造对Enteletoides shuichengensis壳体的影响。测得Striatifera striata和Enteletoides shuichengensis壳体棱柱层中的δ18OPDB分别为-3.249‰和-3.385‰,据此推算出古海水温度为21.60～31.18℃。腕足动物壳体具有较高的δ13CPDB值,分别为4.784‰,4.782‰。利用碳氧同位素分析结果计算得到Z值接近135,表明珊瑚礁区古海水盐度正常。
Phylloid algal reefs are dominant in Pennsylvanian all over the world and well exposed in the south Guizhou, China. While the metazoan framework reefs develop well in this area, especially the presence of a rarely large-scale Fomitehevella coral reef, which is constructed mostly by Big phacelloid Fomitchevella. As a result, South Guizhou becomes a unique case of the Carboniferous reef. Stratigraphieally, South Guizhou reefs belong to the Maping Formation （the Triticites Zone, Gzhelian, uppermost Pennsylvanian）. The depositional environment of the Maping Formation is at shallow marine carbonate platform margin. Furthermore, the faunal association recorded point to a paleogeographical position in a low-latitude close to the equatorial reahn in south China during the Pennsylvanian time. Thus, it can be inferred that the growth environment of Fomitchevella reefs is in a relative warm water, fitting the development of coral reefs. Surely we can use fossil brachiopod shells to attain part of environmental information with respect to the development of Pennsylvanian Fomitchevella eoral reef in South Guizhou. These fossil braehiopod samples were collected from the uppermost Pennsylvanian Fomitchevella coral reef deposits in South Guizhou of China and the preservation state of shells was tested using conventional selection criteria such as eathodoluminescence, microscopy, trace element analy- sis and X-ray diffraction analysis. Despite the selection of well-preserved shells using the conventional evaluation criteria, there are still concerns as to whether the selected braehiopod shells do indeed contain original seawater signals. Analyzed in-situ by laser Ablation-Inductively Coupled Plasma-Mass Spectrometry （LAICPMS）, Fe, Mn, Ni, Sr, Na, Zn, Cr and Cd distribute symmetrically in shell transects of the fossil brachiopods Striatifera striata and Enteletoides shuichengensis. Symmetry of the trace element distribution pattern derived from shell growth bands and is con- sidered an intrinsic and original ontogenetic property of the brachiopod shell chemist~. Ni, Fe, Mn, Na and Sr distribution is symmetrical in the well-preserved shells of the Pennsylvanian brachiopods Striatifera striata and Enteletoides shuiehengensis, indicating that the selected shell by the conventional methods has preserved its original seawater signal for nearly 2.9 Ma. Pennsvlvauian large-scale Fomitchevella coral reef associated with hrachiopods Striatifera striata and Enteletoides shuichengensis in the Triticites Zone, South Guizhou Province. X-ray diffraction analysis indicates that the chemical composition of Striatifera striata shell is pure low-Mg calcite and is free of any signs of diagenesis. Low Fe and Ni contents occurring in EnteletoMes shuichengensis shell are indicative of slightly diagenetie alteration since recrystallization under reducing conditions tends to enrich shell calcite in Fe and Ni. Profiles of Ni, Fe, Mn, Na and Sr contents from hyperplasia symmetrically across the transverse section are nearly dissymmetrical in both brachiopod shells. Therefore, it supports a good preservation of both shells, and the diagenetic effect to fossil Enteletoides shuichengensis shell could be ignored. The carbon and oxygen isotopic composition of well-preserved brachiopod shells were investigated in order to get some information with respect to the depositional environment of large-sized Fomitehevella coral reef. The 8Is OHm values of Striatifera striata and Enteletoides shuichengensisis shells are - 3. 249%c and - 3. 385‰respectively, translated it into sea-surface water temperatures of 21.60℃ to 31. 18℃. These two brachiopod shells are high in δ^13CPDB values of 4. 784‰ and 4. 782‰ respectively. The δ^13CPDB values are slightly higher in comparison to values of Gzhelian brachiopods （ ＋ 4.4 ±1.1%c） from the Russian platform. Z value was 135 point to ＂normal＂ marine values assumed fox the Pennsvlvanian coral reef seawater.
出处
《沉积学报》
CAS
CSCD
北大核心
2011年第3期458-464,共7页
Acta Sedimentologica Sinica
基金
国家自然科学基金项目(批准号:40972004)
中央高校基本科研业务费专项资金(编号:090401011)资助
关键词
腕足壳
碳氧同位素
微量元素
宾夕法尼亚亚纪
黔南
braehiopod shell
oxygen and carbon isotopes
trace element
Pennsylvanian
South Guizhou
