An official website of the United States government
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
This content will become publicly available on December 16, 2025
Testing sources and pathways of pyrogenic chemical markers to caves in tropical Australia for speleothem paleorecord calibration
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.
more »
« less
- Award ID(s):
- 2147186
- PAR ID:
- 10614579
- Publisher / Repository:
- American Geophysical Union
- Date Published:
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
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
-
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
-
Australia has long been recognized as one of the world’s fire hotspots, but the Black Summer of 2019-2020, when 97,000 km2 were scorched across southeastern Australia, and the larger fires of northern Australia’s savanna and desert in 2023, may indicate a shift toward a higher level of fire activity. Placing these events in context requires developing precisely-dated, high resolution records of bushfire through periods with different climate and land use mean states. We reconstructed bushfire activity for the period 1110-2009 CE using polycyclic aromatic hydrocarbons (PAH) in three precisely-dated, fast-growing, and partially overlapping aragonite stalagmites from cave KNI-51, located in the central Australian tropical savanna. PAH molecular weights are tied to combustion temperature (i.e., low molecular weights form in lower temperature fires), and thus our record preserves evidence of both the timing and intensity of bushfire over the majority of the last millennium. Comparisons of burn scar satellite imagery with temporal changes in PAH abundances in recently deposited stalagmite suggest that airfall (smoke and ash) from fires within a 5 km radius is primarily responsible for transmitting PAHs to the land surface over the cave, a finding supported by our recent controlled burn and irrigation experiment. The rapid growth rate of KNI-51 stalagmites (1-2 mm yr-1), coupled with the extremely thin soils above the cave, appear to allow for transmission and preservation of multi-annual paleofire signals. To investigate the effects of external forcing on bushfire activity over the last millennium, we applied linear mixed-effect regression to the PAH data, and also included monsoon rainfall (using oxygen isotope ratios from the same stalagmites), annual surface air temperature (using output from the CESM-Last Millennium Ensemble), antecedent fire (using the same stalagmite PAH record), and timing with respect to the arrival of European pastoralists (EP) and their cattle in the 1880s. The model reveals significant differences prior to and following the arrival of EP. Most notably, prior to the arrival of EP, rainfall was significantly correlated with low and medium intensity fires, but not high intensity ones. After the arrival of EP, the correlation between rainfall and fire activity decreased markedly, and showed no statistically significant correlation to any fire intensity. Similarly, prior to the arrival of EP, antecedent fire activity (determined as the sum of PAH within the previous 5 years) was correlated with all levels of fire intensity, but after EP arrival, only high intensity fires are correlated with such antecedent burning. Our findings thus suggest that fire activity following the arrival of EP in the eastern Kimberley has been distinct from any other extended period of the last nine centuries.more » « less
-
Because traditional paleofire archives (e.g., burn scars on trees, charcoal in lake sediments) are not available in all settings, new ways of reconstructing past fire activity are needed. We focus here on polycyclic aromatic hydrocarbons (PAHs) in stalagmites. PAHs are organic molecules composed of two or more fused aromatic rings formed through incomplete combustion of organic matter, and vary in molecular weight depending on combustion conditions. Because the use of PAHs in stalagmites as a paleofire indicator is still in its infancy and because the production, deposition, and transport of PAHs into a cave is a complex and multi-faceted system, we tested the reproducibility of PAHs in two coeval and precisely-dated aragonite stalagmites – KNI-51-F and KNI-51-G - from KNI-51 (15.3°S, 128.6°E), a shallow cave located in the Kimberley region of tropical Western Australia. KNI-51-F and KNI-51-G span 1110-1620 CE and 1310-1630 CE, respectively. Each was hand-milled for analysis in continuous sections spanning approx. 2 mm-tall intervals at Ca’ Foscari University. Owing to differences in growth rate, temporal resolutions for KNI-51-F and KNI-51-G were 3±2 and 1±0.4 yr/sample, respectively. Chemical preparations and analysis methods follow those of Argiriadis et al. (2019) Analytical Chemistry, volume 91. In order to assess replication between the two stalagmites, we compared total abundances of low molecular weight (LMW: Napthalene, Acenaphthylene, Acenaphthene, Fluorene), medium molecular weight (MMW: Phenanthrene, Anthracene, Fluoranthene, Pyrene, Benzo(a)Anthracene, Chrysene, Retene), and high molecular weight (HHM: Benzo(b)Fluoranthene, Benzo(k)Fluoranthene, Benzo(e)Pyrene, Benzo(a)Pyrene, Perylene, Benzo(ghi)Perylene, Indeno(1,2,3-c,d)Pyrene, Dibenzo(A,H)Anthracene) PAHs. Total abundances of LMW, MMW, and HMW PAHs are similar (<10 ng/g) except for HMW PAHs in KNI-51-G, which are generally <1 ng/g. Total LMW and MMW abundance time series replicate well, with multiple synchronous multidecadal periods characterized by consistently low PAH abundances, suggestive of reduced bushfire activity, punctuated by intervals of high PAH abundances, likely reflecting frequent bushfire. Less coherence exists between HMW PAHs.more » « less
