Abstract:

Spring wildfires frequently occur in the alpine swamp wetlands of the Qinghai-Tibet Plateau,yet their spontaneous ignition mechanism remains unclear.This study reveal the combustion properties of red sediments at the bottom of wetland pits and their potential as a spontaneous ignition source from the perspective of the chemical characteristics of iron oxides.Red sediment samples(at a depth from 0 to 0.2 cm)were collected from the pit bottom in the fire-prone area of Azigou,Muli Town,Tianjun County,Qinghai Province.Their micromorphology,elemental composition,thermal stability,and iron chemical states were analyzed using field emission scanning electron microscopy coupled with energy-dispersive spectroscopy(FE-SEM-EDS),simultaneous thermal analysis(STA),and X-ray photoelectron spectroscopy(XPS).The results showed that:1)The sediments are mainly composed of iron oxyhydroxide(FeOOH),and organic matter(OM),exhibiting a porous honeycomb structure;2)The laboratory apparent ignition point is approximately 213 ℃,the combustion of organic matter is the main exothermic process;3)The fine structure of energy-dispersive X-ray spectroscopy confirms that iron mainly exists in the form of FeOOH;4)FeOOH is non-combustible however,owing to its high specific surface area and surface hydroxyl properties,it can act as a “photothermal conversion medium” to adsorb and enrich organic matter.The red sediments in the Qinghai alpine swamp wetlands have the potential to convert light energy into heat under specific conditions(e.g.,sunlight focused by ice lenses)and trigger the smoldering of organic matter.The ignition mechanism results from the coupling of seasonal freeze-thaws,micro-topography,solar radiation,and the special material attributes.This study provides a new perspective for understanding the causes of natural fires in alpine wetlands from a material basis standpoint.

Key words: natural wildfire, red sediment, FeOOH, photothermal conversion, ignition mechanism