High resolution palaeofire signals from Christina Lake, Alberta: a comparison of the charcoal signals extracted by two different methods

Two high resolution palaeofire signals were extracted from the annually-laminated sediment of Christina Lake in the boreal forest of Alberta, Canada. A charcoal curve focusing on large charcoal fragments (> 125 mum) was extracted by the sieving method described by Whitlock and Millspaugh. A carbon curve which includes all size fractions of charcoal was extracted by a method modified from Winkler's chemical assay method. Modifications included increasing the duration of nitric acid treatment to 2 h and using a total carbon analyser (TCA) rather than standard loss on ignition. These adjustments resulted in a carbon curve which is unaffected by clay water losses and subtle changes in sedimentology which were problematic in the original gravimetric technique. The sensitivity of the TCA allowed subtle shifts in carbon (i.e. charcoal) abundance to be detected in lake sediments with low levels of charcoal. When compared with the known fire history near the lake since 1900, the sieved charcoal curve was most responsive to fires known to burn near the shore of the lake and less responsive to fires burning elsewhere in the watershed. The TCA carbon curve was somewhat more sensitive than the sieved charcoal curve for detecting fires in the drainage basin which did not burn to the lake edge. The TCA carbon curve was strongly correlated with sediment influx, suggesting stream inputs of charcoal are the dominant influence on this signal. The peak in response of the TCA curve to known fire events was sometimes lagged by several years, suggesting that it can take several seasons for the bulk of stream-transported charcoal to make its way from the watershed to its final deposition site in the lake. Very large fires burning just outside the drainage basin of the lake were not detected by either method, suggesting that both of the methods used are sensitive only to fires burning in the watershed and that long-distance aerially dispersed charcoal is insignificant at this site. (C) 2000 Elsevier Science B.V. All rights reserved.