【摘要】 晚奥陶世赫南特冰期是地球历史上最重要的冰川事件之一,宜昌王家湾剖面作为奥陶系赫南特阶GSSP完整地记录了奥陶纪末期生物绝灭和环境变化事件.对王家湾小河边剖面(GSSP以南180m小河边)奥陶系-志留系界线地层及其碳氧稳定同位素组成进行了高分辨率研究.其中观音桥层主体为一套厚度小于30cm的泥质灰岩,保存有丰富的赫南特贝动物群化石记录,下伏上奥陶统五峰组笔石页岩,上覆下志留统(兰多维列统)龙马溪组笔石页岩.高分辨率测试分析了其中厚约25cm的观音桥层碳、氧稳定同位素组成,发现δ13Ccarb呈现出一次明显的负偏移趋势,在其上部达到最低值-12.8‰,是目前全球该层段无机碳同位素报道的最低值.认为这一明显的δ13Ccarb负偏事件指示了观音桥层沉积环境的突然变化,反映在冰盛期海平面下降时扬子海溶解无机碳库(DIC库)中加入了大量额外轻碳(12C)组份,其来源可能与海底甲烷有氧及厌氧氧化(AOM)或有机质有氧及厌氧氧化过程有关.更多还原

【Abstract】 The Late Ordovician glaciation(Hirnantian)is one of the most critical glaciation events in Earth history.The endOrdovician mass extinction and associated environmental changes are well recorded at the GSSP section(Wangjiawan)in Yichang,South China.In this study,we investigated and sampled across Ordovician-Silurian boundary from Wangjiawan riverside section(～180msouth to the GSSP section),attempted to reconstruct the carbon and oxygen isotope variation in high-resolution.Guanyinqiao Formation is underlain by Upper Ordovician Wufeng graptolite shale and overlain by Lower Silurian Longmaxi graptolite shale.It is characterized by a set of argillaceous limestone with less than 30cm in thickness and preserves abundant Hirnantia fauna fossils.High-resolution measurement of stable carbon and oxygen isotope composition of 25cm-thick Guanyinqiao interval suggests thatδ13 Ccarbvariation appears an obvious negative excursion with the nadir of-12.8‰as the most depletedδ13 Ccarbvalue reported at this interval so far globally.We interpret that the distinct negative excursion indicates abrupt changes in environment during Guanyinqiao period,which reveals the injection of massive extra light carbon(12 C)component into the dissolved inorganic carbon reservoir(DIC)with sea-level drop during glaciations.It is inferred to be associated with the aerobic and anaerobic oxidation of methane(AOM)and organic matter.更多还原