Contemporary human activity is altering rates and intensities of wildfire, but the nature of fire activity prior to the modern era is poorly understood. Here we investigate the utility of a novel paleofire proxy: trace pyrogenic organic compounds in stalagmites. Polycyclic aromatic hydrocarbons (PAHs) are linked to burning of biomass and are transported downward through soil and bedrock by infiltrating rainwater and incorporated into stalagmites in underlying caves. We couple PAHs with n-alkanes, which may be used to infer past vegetation conditions, to provide complementary evidence of fire. In this pilot study, we have so far analyzed three fast-growing and precisely-dated aragonite stalagmites from KNI-51, a shallow cave in the fire-prone tropical savanna of Western Australia. The thin layer of fractured bedrock above the stalagmite chamber, coupled with a minimal soil layer allows quick transfer of combustion products to the cave system. To test the stalagmite-paleofire link, we performed several experiments. First, we assessed the possibility of surface contamination on the stalagmites by measuring PAH and n-alkane abundances and distributions in sequential digestions. Second, because KNI-51 often floods and flood sediment is incorporated into the stalagmites, target compounds were measured in soils above the cave, sediments lining the stalagmite chamber floor, and flood-derived sediment incorporated within stalagmites as possible sources of these organic compounds. Third, we considered as additional sources other possible biological and geological influences, such as regional oil and gas fields or microbial activity. And fourth, satellite-mapped fires occurring proximal to the cave were correlated to PAH abundances in a recently active portion of one stalagmite. The results support the contention that PAHs and n-alkanes in KNI-51 stalagmite carbonate likely reflect paleofire activity. While flood sediment contains PAHs, no significant correlation was found between target compounds and the presence of flood layers. We argue that biomass burning and the consequent fire-induced breakdown and particle deposition are the main sources of target organic compounds in these stalagmites. Given that karst is present in many fire-prone environments, and that stalagmites can be precisely dated and grow continuously for millennia, the potential utility of a stalagmite-based paleofire proxy is high.
Stalagmite-based paleofire reconstruction using organic biomarkers: a case study from cave KNI-51, tropical Western Australia
Elena Argiriadis;Giulia Genuzio;Carlo Barbante
2023
Abstract
Contemporary human activity is altering rates and intensities of wildfire, but the nature of fire activity prior to the modern era is poorly understood. Here we investigate the utility of a novel paleofire proxy: trace pyrogenic organic compounds in stalagmites. Polycyclic aromatic hydrocarbons (PAHs) are linked to burning of biomass and are transported downward through soil and bedrock by infiltrating rainwater and incorporated into stalagmites in underlying caves. We couple PAHs with n-alkanes, which may be used to infer past vegetation conditions, to provide complementary evidence of fire. In this pilot study, we have so far analyzed three fast-growing and precisely-dated aragonite stalagmites from KNI-51, a shallow cave in the fire-prone tropical savanna of Western Australia. The thin layer of fractured bedrock above the stalagmite chamber, coupled with a minimal soil layer allows quick transfer of combustion products to the cave system. To test the stalagmite-paleofire link, we performed several experiments. First, we assessed the possibility of surface contamination on the stalagmites by measuring PAH and n-alkane abundances and distributions in sequential digestions. Second, because KNI-51 often floods and flood sediment is incorporated into the stalagmites, target compounds were measured in soils above the cave, sediments lining the stalagmite chamber floor, and flood-derived sediment incorporated within stalagmites as possible sources of these organic compounds. Third, we considered as additional sources other possible biological and geological influences, such as regional oil and gas fields or microbial activity. And fourth, satellite-mapped fires occurring proximal to the cave were correlated to PAH abundances in a recently active portion of one stalagmite. The results support the contention that PAHs and n-alkanes in KNI-51 stalagmite carbonate likely reflect paleofire activity. While flood sediment contains PAHs, no significant correlation was found between target compounds and the presence of flood layers. We argue that biomass burning and the consequent fire-induced breakdown and particle deposition are the main sources of target organic compounds in these stalagmites. Given that karst is present in many fire-prone environments, and that stalagmites can be precisely dated and grow continuously for millennia, the potential utility of a stalagmite-based paleofire proxy is high.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
