Levoglucosan and its isomers in terrestrial sediment as a molecular markers provide direct evidence for the low-temperature fire during the mid-Holocene in the northern Shandong Peninsula of China

Black carbon and charcoal's limitation in detecting low-temperature fires could be a major obstacle in observing paleofire. To reconstruct a low-temperature fire pattern and vegetation changes in the Changyi in the northern Shandong Peninsula over the last 5000 years, we used molecular biomarker (levoglucosan and its isomers) and hydrocarbons (n-alkanes), along with charcoal and black carbon in the sediment sites. An increase in the amounts of coniferous flammable fuels could provide the biomass required to maintain low-temperature conditions for smoldering fires during the prehistoric period when wildfires occurred during 5300-5100 yr BP. These fires primarily coincided with cooling climate events that began approximately 5300 yr BP onward. Significant levoglucosan fluxes at approximately 4400-3600 yr BP and 1300-1000 yr BP were thought to be closely linked to intensive anthropogenic burning practices and the climate-controlled sedimentary wet environments, coinciding with the periods of the Longshan (4600-4000 yr B.P), Yueshi Cultures (4000-3500 yr B.P)and Tang Dynasty(1300-1100 yr B.P). During those times, the original forest landscape in the study region was gradually replaced by a fire-prone open forest, where low-temperature smoldering fires could be sustained by the abundance of biomass for producing fuel from open forests. In contrast, the levoglucosan fluxes remained at low values between the intervals of 3600-3200 yr BP and 1000-0 yr BP, whereas the charcoal fluxes showed a significant increase in the Changyi profile. Moreover, the positive correlation between the levoglucosan and char fluxes in sediments suggested that levoglucosan could be an indicator of regional fires or biomass burning at low temperatures. The ratios between levoglucosan and its isomers indicated that levoglucosan are derived from the combustion of softwood at the study site. Futhermore, Levoglucosan in sediments was used in this study to document the changes in vegetation and low-temperature regional biomass burning, as well as the shifts in human land use and the cultural development in response to climate change. Compared to the two biomassburning proxies (black carbon and charcoal), levoglucosan in sediments can additionally provide supplementary information to understand paleofire.