An organic proxy approach to the high-resolution reconstruction of fire history from stalagmites

The history of human management of bushfire in Australia dates to at least 50 kyr BP but at the end of the 18th century, the arrival of European settlers disrupted indigenous management, leading to changes in patterns of fire regimes, which likely contributed to the decline of land mammals and fire-sensitive plants. Lake sediments and tree scars record these shifts across many parts of the continent. However, the fire-prone tropical savannas of north­western Australia lack such long-term, high-resolution paleofire archives. As a result, our understanding of fire prior to the arrival of European pastoralists in this area in the late 19th century are sparse, heterogeneous, and often poorly resolved. The region does contain karst areas, and stalagmites are being investigated as paleofire proxies. Paleofire reconstruction from stalagmites is in its infancy, but several recent studies investigated Holocene fire activity through various geochemi­cal proxies. Here, we present the first high-resolution paleofire record obtained from polycyclic aromatic hydrocarbons (PAHs) in stalagmites, encompassing most of the last millennium (CE ~1100-2009). Using this method, we can define the timing and intensity of fires proximal to the cave. Cave KNI-51, located in the Kimberley region of north-central Australia, is the ideal site for this pilot study. Previous U-Th dating of KNI-51 stalagmites has yielded high-precision age models and revealed fast growth (1-2 mm/yr), that together offer the possibility of near annual resolution. The cave is shallow, formed in permeable and fractured limestone and overlain by thin and poorly developed soils. From our data we infer short residence times for PAHs in soil over the cave and long time preservation in stalagmites. A strong positive correlation exists between PAH abundance and satellite data of burn scars in the CE 2000-2009 period using a 3 km buffer around KNI-51. Our results suggest an increase in the frequency of low intensity fire beginning around CE 1880, coincident with the arrival of European pastoralists to the Kimberley. In contrast, our data suggest few high intensity fires since CE 1550 relative to the first half of the millennium. Comparison with oxygen isotope data from the same stalagmites also allows an examination of (multi)decadal shifts in rainfall and fire activity.