全新世中期毛乌素沙地炭屑记录与火灾历史

Mid-Holocene Charcoal Records and Fire History in the East Edge of the Mu Us Sandy Land,China

炭屑是探索自然火和人类用火及其驱动机制的重要指标。作者试图通过孢粉提取法获得毛乌素沙地东缘锦界剖面全新世中期(7.5~4.6ka)的炭屑记录,用来揭示该地区炭屑变化特征和火灾历史,并探讨其与气候环境变化之间的关系。结果表明:在7.5~4.6ka期间炭屑浓度变化范围为340~17 420粒·g-1,平均为3 350粒·g-1。剖面炭屑总浓度变化趋势曲线呈"弓形",即在7.2ka以前炭屑浓度较低,平均约为1 840粒·g-1;在7.2~6.6ka浓度最高,平均约为5 280粒·g-1,并在6.8ka前后达到峰值17 400粒·g-1;在6.6ka之后浓度下降,平均为1 720粒·g-1。同时,炭屑粒径指标表明<50μm的炭屑占主导,>100μm的炭屑稀少。炭屑浓度主要反映火灾频率和强度,炭屑粒径则反映火灾距离和范围,即毛乌素沙地东部在全新世中期火灾活动相对频繁并在6.8ka左右达到最大值,同时炭屑物源较远,主要为区域性火灾。锦界剖面的化学风化指数、地球化学元素和粒度变化特征表明该地区在7.5~4.6ka气候条件整体较好,湿度增加,处于全新世适宜期,并在7.2~6.6ka达到最佳值,这表明毛乌素沙地的火灾主要发生在气候最佳时期。结合毛乌素沙地及其邻区的孢粉资料推测,该地区在全新世气候适宜期时植被相对茂盛,为毛乌素沙地的火灾提供了丰富的可燃烧物,可能是火灾增多的主要原因。

Charcoal is an important proxy to explore wildfire and human use of fire and its driving forces.The author tried to collect the mid-Holocene(7.5-4.6 ka)charcoal records by pollen extract method at Jinjie profile to reveal the charcoal concentrations and fire history in the east edge of the Mu Us Sandy Land,then explore its relationships with paleoclimate and paleoenvironment changes.The results showed that during 7.5-4.6 ka,the charcoal concentrations range between340 and 17 420 grains·g-1,with an average of 3 350 grains·g-1.The curve of charcoal concentrations expressed a shape of'bow',characterized by relatively low values before 7.2 ka,with an average of 1 840 grains·g-1;high values during 7.2-6.6 ka,with an average 5 280grains·g-1;the peak occurred at 6.8 ka,attained 17 400 grains·g-1;After 6.6 ka,the concentration decreased,with an average of 1 720 grains·g-1.Meanwhile,the charcoal grain-size showed that the part of<50μm dominated the whole profile.The charcoal concentrations mainly reflect the fire frequencies and intensities,while the charcoal grain-size reflects the fire distances and ranges.So,at the JJ profile,the fire activity was relatively frequent and reached the maximum in 7.2-6.6 ka in the east edge of the Mu Us Sandy Land,and the fire source was from a relatively farther distance.At the same time,the variations of the chemical weathering index,geochemical elements,and grain-size of the sedimentary materials at the JJ profile indicated an optimal climate between 7.2 and 6.6 ka,characterized by the relatively most humid condition.The strongest wildfire in the Mu Us Sandy Land occurred in the relatively better environment.Combining with the pollen data in the Mu Us Sandy Land and the surrounding areas to the southeast(e.g.,Loess Plateau)showed that the relatively exuberant vegetation during the Holocene climatic optimum may have provided enough biomass for the fire.