Abstract:

Fire plays an important role in studies of paleoclimate change and human activities.Microcharcoal in sediments can indicate the extent,frequency and variation of wildfires throughout geological history.Studies of microcharcoal fires based on pollen methodologies have received extensive attention from researchers,however,statistical approaches to lycopodium spores related to the needs of sporopollen statistics are uncertain,and this may result in the instability of results of microcharcoal.The Lingtai (LT) profile (33°04′ N,107°39′) chosen for this study is located in Lingtai County,Gansu Province,China.16 samples from different layers were selected throughout the 13 m profile (S0~L2),nine of which were in the loess layers and seven in the paleosol layers.All samples were extracted by a standard pollen methodology (adding a known number of lycopodium spores to each sample in order to calculate microcharcoal concentrations).To obtain the number of lycopodium spores that should be counted in microcharcoal research,20 groups were counted for each sample and 50 lycopodium  spores for each group,giving a total of 1 000 lycopodium spores.Two distinct shapes of microcharcoal were identified: sub-round (length/width＜2.5,R) and sublong (length/width＞2.5,L),which respectively represent woody and herb sources.The data were then divided into four groups of grainsize groups: ~10-30 μm,~30-50 μm,~50-100 μm,and ＞100 μm.Results showed that the percentages and concentrations of microcharcoal in the 20 groups studied were similar but exhibited some fluctuation (standard deviation: 0.06% ~ 8.7%).We compared the data from the first accumulating group (i.e., one group) to the twentieth accumulating group (i.e.,a total of twenty groups),finding that when the total number of lycopodium spores is greater than 500,the accumulated microcharcoal concentration in each sample no longer shows significant change,and the percentages of each grainsize group tend to be stable.When the total number of lycopodium spores is 300 or greater,L/R ratios tend to be stable (standard deviation: ~0.01-0.24; r = 0.87).The results of Spearman correlation analysis of the data are consistent with the above results.Therefore,we suggest that the number of lycopodium  spores to be counted in microcharcoal research should be 500±50.Additionally,the microcharcoal concentration range between 2.33 × 10³ -5.30 × 10⁴ grains·g^-1 has an average of 18 424.34 grains·g^-1.The “L/R” ratios range between 0.26 and 2.96,averaging 1.53.Our data show that,since 165 ka,fire events have been more frequent in the Lingtai area,and herbaceous vegetation has been more dominant than woody vegetation.These findings correspond well with the elemental carbon content of the loess-paleosol,indicating that the occurrence of paleo-fires in the area is closely related to environmental change.This study explores the reasonable number of lycopodium  spores that should be counted to ensure the quality of data in work based on the standard pollen methodology.The results provide references for statisticians and statistical standards for subsequent studies of microcharcoal in loess-paleosols.

Key words: Loess Plateau, microcharcoal statistics, paleo-fire; environmental change