晚泥盆世弗拉期-法门期(F-F)之交海洋氧化:来自华南垌村剖面碳酸盐岩Ce异常证据

MARINE OXYGENATION PULSES DURING LATE DEVONIAN FRASNIAN-FAMENNIAN TRANSITION:EVIDENCE FROM CE ANOMALY RECORD FROM THE DONGCUN SECTION,SOUTH CHINA

晚泥盆世弗拉期-法门期(Frasnian/Famennian,以下简称F-F)之交的生物灭绝事件是显生宙以来五大生物灭绝事件之一,这次灭绝一般认为与全球海洋缺氧有关。华南广西桂林是我国F-F地层最为发育、研究程度最高的地区之一,本研究利用碳酸盐岩Ce异常指标,对桂林地区垌村剖面F-F期间的古氧化还原条件进行了重建。结果表明,F-F期间碳同位素的正漂移对应着表层海洋的氧化而非先前广泛认为的缺氧。将这一发现与先前同一地区的地球化学和古生物数据相结合并进行综合分析,我们认为气候快速波动在F-F期间生物与环境巨变中扮演了最为关键的角色。气候变冷使得海水氧溶解量增加和赤道—极地之间海水温度梯度变大,海洋表层环流和垂直翻转变得更加活跃,导致深水营养物质向海洋表层的输送量增多,有效促进表层海洋初级生产力和有机碳埋藏通量升高,进而引起全球海水碳同位素值的提高。因此,本研究支持全球气候快速变冷而非海洋缺氧导致了F-F大灭绝事件。

The Frasnian/Famennian(F-F)extinction occurred in the Late Devonian is one of the five major extinction events in the Phanerozoic,the event is generally linked to global marine deoxygenation.In this study,a new redox proxy(sensitive rare element Ce anomaly)was used to reconstruct the paleo-redox conditions during the F-F transition in the Dongcun Section,South China.Results show that ocean oxygenation rather than anoxia as previously thought coincide with two positive carbon isotope excursions during the F-F interval.This observation combined with previous data from South China suggests that climate cooling may play a more important role in driving environmental fluctuations during the F-F transition.The cooler climate could increase the dissolved oxygen concentration in seawater,resulting in the enhancement of seawater temperature gradient between the equator and the polar regions.This process could accelerate ocean circulation and overturning enhancing regional upwelling,later promoting the transport of nutrients from deep-water to the ocean surface,stimulating the marine productivity in surface water and organic carbon burial fluxes,and consequently resulting in s13Ccarb positive excursions.This study supported the idea that global cooling rather than ocean deoxygenation was the major cause for the F-F mass extinction event.