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Pyrogenic compounds, such as polycyclic aromatic hydrocarbons (PAHs), can track the type and intensity of fires and are preserved in many environmental matrices including speleothems. We recently reported on a stalagmite record of PAH abundance distributions from cave KNI-51, located among the eucalypt savanna in the Ningbing range of tropical Western Australia. In order to better understand the manner by which PAHs from local bushfires are deposited on the land surface and transported into caves, we performed a controlled burn and irrigation experiment at cave KNI-140, located near to and in the same bedrock as cave KNI-51. Samples of soil, vegetation, ash, and air were collected prior to and immediately succeeding the prescribed burn. The fire, which burned predominantly grasses, was ignited by matches (no accelerants were used) and covered approximately 30,000 square meters upwind from the cave. The land surface above the cave was irrigated prior to and immediately succeeding the burn with resulting dripwater collected for analysis. Next, ash samples were deposited directly above the cave and then similarly irrigated, with the drip water also collected. The PAHs present in these samples were measured via gas chromatography-mass spectrometry at Ca’ Foscari University, Venice. Our results reveal that low molecular weight PAHs were the most abundant species of PAH in the drip water and heavier PAHs were substantially less abundant. This result is likely due to the low combustion temperature of the burn, with abundances increasing through each of the three stages of sample collection, demonstrating that deposition from smoke and cinders produces identifiable signals in dripwater (and thus stalagmite) PAHs, supporting the contention that KNI-51 stalagmites record fire activity occurring not just above the cave but within km of the cave. On-going analyses of soil, vegetation, and ash samples will further clarify the role of fire on production and transmission of PAHs at this site, and thus how these organic compounds preserved in speleothems can help delineate the fire history in the region.
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Controls on PAH production in modern East African soils using satellite data
Polycyclic aromatic hydrocarbons (PAHs) are a suite of molecules produced by incomplete combustion processes (i.e., pyrogenic) as well as by thermal maturation of organic matter (i.e., petrogenic processes). PAHs are increasingly being used to reconstruct fire activity in ancient environments amid efforts to determine best practices for PAH-biomass normalization and efforts to differentiate between petrogenic and pyrogenic sources. Long chain alkanes (LCAs) are commonly used account for changes in biomass production (or biomarker preservation) relative to PAH production, but this normalization hasn’t been tested with modern observations. Other indices for PAH sources (e.g., APDI), fuel types (e.g., DMPx and DMPy) and transport pathways (e.g., LMW ratios) have been developed based on data from burn experiments and environmental chemistry literature, but there are limited studies linking PAH distributions to natural fire characteristics. We measured PAHs in a collection of modern soil samples spanning a range of environments in East Africa. Previous work on this sample set includes soil carbonate and plant wax carbon isotope measurements, two common paleoenvironmental proxies for vegetation. We used satellite data (MODIS and GFED burn products) to estimate the burned area, fire return frequency, and fire intensity experienced by each of the sites in the decade preceding soil sample collection. We compared our measured soil PAH data to satellite-inferred fire characteristics and existing plant wax carbon isotope data to assess both the fidelity of PAH-LCA normalization and the utility of PAH proxies such as DMPx, DMPy, and APDI in the East African paleo-record.
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- Award ID(s):
- 2018924
- PAR ID:
- 10438496
- Date Published:
- Journal Name:
- Eos
- ISSN:
- 2533-0306
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Human activity and climate change are altering natural rates and intensities of wildfire, but the scale and extent of burning prior to the modern era are poorly understood. Prehistoric fire activity can be reconstructed using a variety of records including charcoal deposited along with sediment at the bottom of lakes and burn scars on tree rings, but these are not available in all environmental settings. We are developing a new paleofire proxy: polycyclic aromatic hydrocarbons (PAHs) in stalagmites. PAHs are produced by the burning of vegetation, with molecular weights reflecting combustion conditions. After being formed in a fire, PAHs are transported downward by infiltrating rainwater and in cave areas can become incorporated into stalagmites as they crystallize from drip water in underlying caves (Perrette et al., 2008; Denniston et al., 2018). Thus, the potential exists for PAHs in stalagmites to preserve evidence of the presence and intensity of fire through time. Because this is a new method, several important tests need to be performed to evaluate its veracity. I assessed how well PAH abundances, ratios, and trends replicate between two coeval stalagmites from cave KNI-51 located in the tropics of Western Australia. Stalagmite KNI-51-F was previously analyzed and I analyzed the overlapping portion of stalagmite KNl-51-G: overlap in age from CE 1310-1630. This work was done at Ca' Foscari University, Venice, in the fall of 2022, under the direction of Dr. Elena Argiriadis. The results show similarities between the area of overlap in the G and F stalagmites. The commonalities of the concentrations of PAH in the stalagmites indicate confidence in the developed method in assessing pyrogenic compounds in coeval stalagmites.more » « less
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Recent developments in speleothem science are showing their potential for paleofire reconstruction through a variety of inorganic and organic proxies including trace metals (1) and the pyrogenic organic compound levoglucosan (2). Previous work by Argiriadis et al. (2019) presented a method for the analysis of trace polycyclic aromatic hydrocarbons (PAHs) and n -alkanes in stalagmites (3). These compounds reflect biogeochemical processes occurring at the land surface, in the soil, and in the cave. PAHs are primarily related to combustion of biomass while n-alkanes, with their potential for vegetation reconstruction (4), provide information on fuel availability and composition, as well as fire activity. These organic molecules are carried downward by infiltrating water and incorporated into speleothems (5), thereby creating the potential to serve as novel paleofire archives. Using this approach, we developed a high-resolution stalagmite record of paleofire activity from cave KNI-51 in tropical northwestern Australia. This site is well suited for high resolution paleofire reconstruction as bushfire activity in this tropical savanna is some of the highest on the continent, the cave is shallow and overlain by extremely thin soils, and the stalagmites are fast-growing (1-2 mm yr-1) and precisely dated. We analyzed three stalagmites which grew continuously in different time intervals through the last millennium - KNI-51-F (CE ~1100-1620), KNI-51-G (CE ~1320-1640), and KNI-51-11 (CE ~1750-2009). Samples were drilled continuously at 1-3 mm resolution from stalagmite slabs, processed in a stainless-steel cleanroom to prevent contamination. Despite a difference in resolution between stalagmites KNI-51-F and -G, peaks in the target compounds show good replication in the overlapping time interval of the two stalagmites, and PAH abundances in a portion of stalagmite KNI-51-11 that grew from CE 2000-2009 are well correlated with satellite-mapped fires occurring proximally to the cave. Our results suggest an increase in the frequency of low intensity fire in the 20th century relative to much of the previous millennium. The timing of this shift is broadly coincident with the arrival of European pastoralists in the late 19th century and the subsequent displacement of Aboriginal peoples from the land. Aboriginal peoples had previously utilized “fire stick farming”, a method of prescribed, low intensity burning, that was an important influence of ecology, biomass, and fire. Prior to the late 1800s, the period with the most frequent low intensity fire activity was the 13th century, the wettest interval of the entire record. Peak high intensity fire activity occurred during the 12th century. Controlled burn and irrigation experiments capable of examining the transmission of pyrogenic compounds from the land surface to cave dripwater represent the next step in this analysis. 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. (1) L.K. McDonough et al., Geochim. Cosmochim. Acta. 325, 258–277 (2022). (2) J. Homann et al., Nat. Commun., 13:7175 (2022). (3) E. Argiriadis et al., Anal. Chem. 91, 7007–7011 (2019). (4) R.T. Bush, F. A. McInerney, Geochim. Cosmochim. Acta. 117, 161–179 (2013). (5) Y. Sun et al., Chemosphere. 230, 616–627 (2019).more » « less
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Polycyclic aromatic hydrocarbons (PAHs) are produced by the burning of biomass, with molecular weights reflecting combustion conditions. After being formed, PAHs are transported downward through soil and bedrock by infiltrating rainwater (Perrette et al., 2013), and in karst areas can become incorporated into stalagmites as they crystallize from dripwater in underlying caves (Perrette et al., 2008; Denniston et al., 2018). Thus, when stalagmite growth is high, infiltration times short, and fluid mixing minimized, there exists the potential for PAHs in stalagmites to preserve evidence of the presence and intensity of fire through time. We have previously reported a high-resolution analysis of PAH distributions in two non-overlapping aragonite stalagmites from cave KNI-51, tropical Western Australia, that together span the majority of the last 900 years. The geologic conditions of this site make it well suited for the transmission of discrete pulses of fire-derived compounds from the land surface to the stalagmite. Soils are thin to absent above the stalagmite chamber and the cave is shallow. As a result, homogenization of infiltrated water (and thus PAHs) is expected to be small on interannual time scales. In addition, intense summer monsoon rains flush fire debris from the hillsides over the cave. These characteristics, coupled with the fast growth rates (1-2 mm/yr) and precise radiometric dates (±1-30 years 2 s.d. over the last millennium) of KNI-51 stalagmites suggest that they hold the potential for extremely high resolution paleofire reconstruction. Here we provide the first test of replication of PAH abundances, ratios, and trends in coeval stalagmites. Samples were analyzed at Ca’ Foscari University using methods of Argiriadis et al. (2019) and the results validated by comparing them with fire activity detected through satellite images. Stalagmites KNI-51-F and -G overlap in age from CE 1310-1630, allowing an examination of the consistency of the PAH signal along different infiltration pathways. References Argiriadis, E. et al. (2019) European Geosciences Union Annual Meeting, Vienna, Australia. Denniston, R.F. et al. (2018) American Geophysical Union Annual Meeting, Washington, D.C. Perrette, Y. et al. (2008) Chemical Geology, 251, 67-76. Perrette, Y. et al. (2013) Organic Geochemistry, 65, 37-45.more » « less
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