Early Cretaceous Charcoal Fossils And Their Palaeoenvironmental Significance From Jiuxi Basin

The Cretaceous is considered to be a period of high temperature,high oxygen concentration,high carbon dioxide concentration,during this time,early angiosperms experienced an explosive radiation.At the same time,the Cretaceous period is also one of the most frequent periods of wildfire in the entire geological history.The study of Cretaceous wildfire events helps us better understand the environmental effects of wildfire in greenhouse climate.Many macroscopic charcoal fossils were colleccted from the continuous deposites of the Lower Cretaceous Hanxia section in Jiuxi Basin,Gansu Province.We used the SEM to observe the micro-structure of charcoal and identify its related type,and measured the reflectivity of charcoal to calculate the wildfire temperature,and explored the wildfire types.A total of 5 genera of charcoal fossils belongs to conifers were identified,including Taxodioxylon,Protaxodioxylon,Protophyllocladoxylon,Protocupressinoxylon and Brachyoxylon.The reflectivity values of charcoal fossil were between 1.32% and 4.32%,revealed that the wildfire temperature was between 328°C and 692°C during the Aptian-Albian in Jiuxi Basin.And the Early Cretaceous wildfire events can be divided into three stages.Fire evolved from ground fires or surface fires to surface fires and crown fires,and finally developed to ground fires.Combined with the climatic environment evolution and vegetation succession during the late Early Cretaceous in the study area,it is speculated that in the first stage of wildfire,transitory arid climate led to the hindrance of fire development.In the second stage,warm and humid climate as well as coniferous vegetation provide abundant sources for wildfires,leading to high temperature surface fires and crown fires.The aridification during the third stage and the vegetation evolution caused the wildfire developed to ground fires.Wildfire records were all over the world during the AptianAlbian period,but mostly concentrated in Boreal Tropical zone.It is speculated that it may be related to hot and relatively humidity climatic conditions.Moreover,frequent wildfires under the "greenhouse climate" conditions during the late Early Cretaceous promoted the radiation evolution of early herbaceous angiosperms in the study area.