Iridium profile for 10 million years across the Cretaceous-Tertiary boundary at Gubbio (Italy).
Iridium profile for 10 million years across the Cretaceous-Tertiary boundary at Gubbio (Italy).
复制标题
古比奥（意大利）白垩纪-第三纪边界一千万年的铱剖面。
作者：
W. Álvarez;F. Asaro;A. Montanari
在对12个样品进行中子活化分析的基础上，在古比奥的远洋石灰岩序列中发现了白垩纪-第三系(KT)界线上的Ir异常，并被解释为KT大灭绝时有一个大型地外物体的影响。关于古比奥KT边界红外线剖面形状及其解释的持续争议要求进行更详细的后续研究。利用改进的NAA技术对57m厚、距今1000万年的古比奥层序部分进行了分析，发现在KT边界只有一个Ir异常，但这种异常显示出复杂的精细结构，其成因尚不能完全解释。Kt ir异常的峰值出现在1厘米厚的粘土层中，那里的平均Ir浓度为万亿分之3000(Ppt)；这个峰的两侧是Ir浓度在20到80ppt的尾巴，高于12ppt到13ppt的背景。尾巴的精细结构可能部分是由于侧向改造、扩散、洞穴，或许还有米兰科维奇旋回。
The iridium anomaly at the Cretaceous-Tertiary (KT) boundary was discovered in the pelagic limestone sequence at Gubbio on the basis of 12 samples analyzed by neutron activation analysis (NAA) and was interpreted as indicating impact of a large extraterrestrial object at exactly the time of the KT mass extinction. Continuing controversy over the shape of the Ir profile at the Gubbio KT boundary and its interpretation called for a more detailed follow-up study. Analysis of a 57-meter-thick, 10-million-year-old part of the Gubbio sequence using improved NAA techniques revealed that there is only one Ir anomaly at the KT boundary, but this anomaly shows an intricate fine structure, the origin of which cannot yet be entirely explained. The KT Ir anomaly peaks in a 1-centimeter-thick clay layer, where average Ir concentration is 3000 parts per trillion (ppt); this peak is flanked by tails with Ir concentrations of 20 to 80 ppt that rise above a background of 12 to 13 ppt. The fine structure of the tails is probably due in part to lateral reworking, diffusion, burrowing, and perhaps Milankovitch cyclicity.