川南地区晚奥陶-早志留世沉积环境与古气候的地球化学特征

Geochemical features of sedimentary environment and paleoclimate during Late Ordovician to Early Silurian in southern Sichuan Basin

奥陶-志留纪之交的地表环境异常活跃,全球冰川事件、海洋缺氧事件以及生物灭绝等事件频发。为了解扬子地区晚奥陶世海洋沉积环境与古气候之间的关系,对四川盆地的双河剖面进行了主元素、微量元素分析。古生产力指标(Ba/Al、Ni/Al)表明晚奥陶世扬子海域初级生产力较高,氧化还原指标(V/Cr、U/Th和U-Mo协变模式)指示凯迪期(五峰组)海水氧化还原条件由早期次氧过渡为厌氧还原条件,赫南特冰期发生氧化,鲁丹期(龙马溪组)则再次缺氧还原。五峰组和龙马溪组的化学风化指数(ICIA)平均值分别为71和72,指示温暖潮湿气候条件,赫南特期观音桥段平均值为61,指示寒冷干旱气候特征。同时,晚奥陶世δ13Corg正偏表明气候变化可能是有机质埋藏和黑色页岩广泛发育的结果。

The Ordovician-Silurian transition is an aberrant interval in geologic history.During this period,global glaciation,oceanic anoxia,and mass extinction occurred.To reveal the relationship between the sedimentary environment and paleoclimate,some geochemical proxies,including the total organic carbon(TOC),carbon isotope,and major and trace elements,from the Shuanghe Section,which is located across the Ordovician–Silurian boundary in the Yangtze Platform,are tested.Productivity proxies Ba/Al and Ni/Al indicate that the surface water of Yangtze Sea experienced high primary productivity during the late Ordovician.Meanwhile,according to the test results of redox parameters V/Cr,U/Th,and U-Mo covariation,the bottom water was mainly suboxic during the early Katian and then transformed to anoxic in the late Katian.During the Hirnantian,the bottom water was briefly oxic and transformed to anoxia again during the early Rhuddanian.The average chemical index of alteration(ICIA)of Wufeng Formation and Longmaxi Formation are 71 and 72,respectively,indicating that warm and humid climate was predominant during the Katian and Rhuddanian.Meanwhile,a relatively cold and arid climate throughout the Hirnantian contributed to low average ICIA.The late Ordovician climate change may have been caused by the development of widespread black shale and increase organic carbon burial according to theδ13Corg results.