中二叠世碳同位素漂移事件的天文年代学约束

MIDDLE PERMIAN CARBON ISOTOPE EXCURSION EVENTS AND THEIR ASTRO-CHRONOLOGICAL CONSTRAINTS

中二叠世卡匹敦期发生了两次碳同位素漂移事件,但由于缺乏高精度年代学的约束,其发生机制目前尚存争议。本研究对四川宣汉渡口剖面茅口组共62个碳酸盐岩样品进行无机碳同位素分析,识别出卡匹敦早期的"Kamura"正漂移事件和卡匹敦晚期的负漂移事件,并将其与华南同期事件进行对比。通过剖面已建立的万年级"浮动"天文年代标尺的约束,将发生于Jinogondolella postserrata牙形石带中"Kamura事件"的两个δ13Ccarb尖峰持续时间确定为~28万年,第一个尖峰与地轴斜率高振幅期间导致的海洋高生产力相关,第二个尖峰或与峨眉山火山活动致使的海洋缺氧事件相关。J.xuanhanensis牙形石带中的δ^13Ccarb负漂移事件发生于"Kamura事件"第二个尖峰后的83万年,持续时间较长(~115万年),速率较慢(~0.07‰/万年),推测此次事件可能是由全球海平面缓慢下降导致,或与火山活动导致的二氧化碳和甲烷等气体大量释放无关。

Two carbon isotope excursions during Capitanian(Middle Permian)are well known,but their driving mechanisms are still controversial due to lack of high-precision geochronology.In this study,62 samples from the carbonates in the Maokou Formation at Dukou section in Xuanhan County,Sichuan Province were analyzed for inorganic carbon isotope.The carbon isotope negative and positive excursions have been recognized in the studied section,and compared with the sections from South China.With aid of an existed"floating"astronomical time scale,the duration of twoδ13C positive spikes of the Kamura event in the Jinogondolella postserrata Zone was estimated as^280 kyr.The first spike was derived from the high primary productivity when low obliquity amplitude oscillated,whereas the second spike was associated with the pre-eruptive marine anoxia of the Emeishan volcanism.The negativeδ^13C excursion with a long duration(~1150 kyr)and a low rate(~0.007‰/kyr)in the J.xuanhanensis Zone was initiated^830 kyr later than the second positiveδ13C spike of Kamura event,likely ascribing to the sea-level regression event,but contradicting to volcanism-related 13C-depleted CO2 or methane releases.