Global synchroneity of a positive carbon isotope excursion at the Cenomanian/Turonian boundary: Validation by calcareous microfossil biostratigraphy of the Yezo Group, Hokkaido, Japan
Global synchroneity of a positive carbon isotope excursion at the Cenomanian/Turonian boundary: Validation by calcareous microfossil biostratigraphy of the Yezo Group, Hokkaido, Japan
复制标题
塞诺曼阶/土伦阶边界正碳同位素偏移的全球同步性：通过日本北海道虾夷群钙质微化石生物地层学验证
DOI：
10.1111/j.1440-1738.1993.tb00085.x
复制
发表时间：
1993
期刊：
影响因子：
1.5
通讯作者：
Tsunemasa Saitō
中科院分区：
文献类型：
--
作者：
T. Hasegawa;Tsunemasa Saitō
对日本北海道白垩纪中期层序的有机碳同位素分析表明，在被生物地层学定义的森诺曼阶/土伦阶（C/T）界线处，δ 13 C值出现了2‰的正偏移。在北海道中部的大夕张地区，根据常见的浮游有孔虫的分布，确认了世界上其他地方已知的Cenomanian/Turonian边界的浮游有孔虫Whiteinella archaeocretacea带。在北海道西北部的塔布地区，诊断性的浮游有孔虫很少，但钙质超微浮游生物却很常见。archaeocretacea区也可以通过识别Corollithion kennedyi和Axopodorhabdus albianus（两种钙质微型浮游生物物种）的联合最后出现水平来建立。碳同位素剖面表现出相似的模式，具有可比的波峰和波谷出现在层序中的相同地层位置。δ 13 C值在W中部出现显著的2‰的正位移，这里称之为“δ 13 C峰”。在Oyubari地区的archaeocretacea区和塔普地区的上述两个微型浮游生物分类群的最后出现的连体上方。塞诺曼阶/土伦阶的界线可以在两个剖面的尖峰位置的正上方画出。岩石评价分析和有机碳生物标志物分析表明，有机碳同位素分析表明，有机碳是陆源。因此，观测到的2%的偏移应该反映了大气CO2同位素组成的变化。一个独特的层主要由沙粒大小的spumellarian放射虫是目前直接在Oyubari和Tappu地区的δ 13 C尖峰上方，这表明在表面沃茨的营养物质和二氧化硅的可用性增加。
Isotopic analyses of organic carbon from the mid-Cretaceous sequence in Hokkaido, Japan, revealed a 2‰ positive excursion of δ13C values at the biostratigraphically defined Cenomanian/Turonian (C/T) boundary recognized in the Yezo Group. The planktonic foraminiferal Whiteinella archaeocretacea Zone, which is known to bracket the Cenomanian/Turonian boundary elsewhere in the world, was recognized in the Oyubari area of central Hokkaido based on the distribution of commonly occurring planktonic foraminifera. In the Tappu area of northwestern Hokkaido, where diagnostic planktonic foraminifera are rare but calcareous nannoplankton occur commonly, the interval coeval with the W. archaeocretacea Zone can also be established by recognizing the conjoined last appearance levels of Corollithion kennedyi and Axopodorhabdus albianus, both calcareous nannoplankton species. Carbon isotope profiles exhibit a similar pattern with comparable peaks and troughs occurring in the same stratigraphic position in the sequences. A prominent, positive 2‰ shift of δ13C values, here called ‘δ13C spike’ occurs in the middle of the W. archaeocretacea Zone in the Oyubari area and just above the conjoined last appearances of the two above-mentioned nannoplankton taxa in the Tappu area. The Cenomanian/Turonian boundary can be drawn just above the peak position of the spike in both sections. The Rock Eval analyses and biomarker analyses of organic carbon indicate that organic carbon subjected to our isotope analyses is of terrestrial origin. Therefore, the observed 2%o shift should reflect changes in the isotopic composition of the atmospheric CO2. A unique layer composed predominantly of sand-grain sized spumellarian Radiolaria is present immediately above the δ13C spike both in the Oyubari and Tappu areas, suggesting an increasing availability of both nutrients and silica in surface waters.