柴达木盆地北缘中侏罗统石门沟组煤中多环芳烃分布特征及其地质意义

Distribution and geological significance of polycyclic aromatic hydrocarbons in coal from the Shimengou Formation of Middle Jurassic in the northern Qaidam basin

多环芳烃(PAHs)是古环境、古野火以及古气候的重要地质记录,本文通过气相色谱质谱法(GC-MS)研究了柴达木盆地北缘中侏罗统石门沟组煤中多环芳烃分布特征,检测出一系列二环至七环芳烃化合物,包括高等植物衍生多环芳烃(卡达烯、6-异丙基-1-异己基-2-甲基萘、惹烯、西蒙内利烯以及二氢惹烯)和燃烧衍生多环芳烃(荧蒽、芘、苯并[a]蒽、[艹屈]、苯并荧蒽、苯并[e]芘、苯并[a]芘、茚并[cd]芘、苯并[ghi]苝和晕苯)等。这些多环芳烃的检出表明石门沟组煤沉积于具有显著陆源高等植物输入的微咸水湖沼环境,成熟度较低(平均随机反射率为0.56%)。石门沟组煤中高等植物衍生多环芳烃以极高的惹烯含量为特征,且存在较高丰度的西蒙内利烯和二氢惹烯,卡达烯丰度极低,反映出成煤期陆地植被类型以松柏类植物的针叶林为主,气候温暖湿润。同时,煤中丰富的燃烧衍生多环芳烃证实了柴达木盆地北缘中侏罗世存在广泛的陆地古野火,较高的大气氧气浓度(25.6%)可能是该时期野火频发的一个重要诱因。本次研究为柴达木盆地中侏罗世古野火事件的研究提供了重要的分子化石证据,也是对中侏罗世古环境和古气候研究的有益补充。

Polycyclic aromatic hydrocarbons(PAHs)are important geological records of paleoenvironment,paleowildfire and paleoclimate.The PAHs extracted from coal of the Shimengou Formation of Middle Jurassic in the northern Qaidam basin(NQB)were investigated by gas chromatography-mass spectrometry in this study.A series of 2-ring to 7-ring aromatic compounds were identified,including higher-plant-derived(cadalene,6-Isopropyl-1-isohexyl-2-methylnaphthalene,retene,simonellite,dihydrogen retene)and combustion-derived PAHs(fluoranthene,pyrene,benz[a]anthracene,chrysene,benzofluoranthenes,benzo[e]pyrene,benzo[a]pyrene,indeno[cd]pyrene,benzo[ghi]perylene and coronene.These aromatic compounds indicate that the Shimengou coal were deposited in a lake swamp with brackish water environment accompanied by significant higher-plant input.These coals were at a relatively low thermal maturity stage(average Vr=0.56%).The higher-plant-derived PAHs are characterized by extremely abundant retene,moderate abundance simonellite,2-methylretene and low abundant cadalene.These characteristics indicate that the vegetation was dominated by conifers(cypresses),and the climate was warm and humid during the during the deposition of coal in the Middle Jurassic in NQB.The abundant combustion-derived PAHs in the coal demonstrate the occurrence of intense wildfires during the Middle Jurassic in NQB,which may have been triggered by the high atmospheric oxygen concentration during this period.This study provides important molecular fossil evidence for the research of Middle Jurassic paleowildfire events in the Qaidam basin,and is also a useful supplement to the study of Middle Jurassic paleoenvironment and paleoclimate.