Organic Biomarkers of Fire in Tropical Australian Stalagmites of the Last Millennium

Fire plays a critical role in the ecology of the dry tropics of Western Australia, with ignition generated by human activities and lightning strikes. Attempts to understand temporal changes in rates of biomass burning are hampered by a sparsity of records that are continuous, provide clear evidence of fire at high temporal resolution, and span multiple centuries. Aragonite stalagmites from cave KNI-51, central Australian tropics, have been previously analyzed, with U/Th ratios providing a precise chronology and oxygen isotopic ratios providing a record of past Australian monsoon rainfall variability. We demonstrate that these stalagmites also contain polycyclic aromatic hydrocarbons (PAHs), organic molecules produced by combustion, including biomass burning, and characterized by high persistence in the environment. As such, these stalagmites may serve as a novel proxy for paleofire activity. Cave KNI-51 is shallow, contained within highly permeable rillenkarren limestone, and overlain by extremely thin, carbon-poor soils. PAH sequestration, biodegradation, or mobilization in thick soils and massive bedrock – issues that would complicate transmission of PAHs from the surface to the stalagmite - are minimized. Analysis of stalagmite mud layers, which are common in these samples and are derived from cave flooding events, reveal extremely low PAH concentrations and thus pose minimal or no risk of contamination of the aragonite carbonate. In addition, stalagmite growth rates are high (1-2 mm yr-1), and thus the stalagmites appear capable of recording fire events at ~annual scales. The analytical protocol we have developed for PAHs allows detection in stalagmites at the pg level by guaranteeing the lowest contamination. Samples are drilled from pre-cleaned stalagmite slabs and solvent-extracted in the organic cleanroom after acid digestion at cold temperature. A total of 19 different 2- to 6-ring PAH compounds are analyzed and quantified by GC-MS. Analysis of stalagmite layers deposited over the past millennium are underway, with preliminary data suggesting increased fire activity in the mid-15th century, marked in particular by the presence of fluoranthene, pyrene, benzo(e)pyrene and indeno(1,2,3-c,d)pyrene in the range 2-12 ng g-1.