Past fire dynamics inferred from polycyclic aromatic hydrocarbons and monosaccharide anhydrides in a stalagmite from the archaeological site of Mayapan, Mexico

Abstract

Abstract. Speleothems (cave stalagmites) contain inorganic and organic substances that can be used to infer past changes in local and regional paleoenvironmental conditions. Specific biomarkers can be employed to elucidate the history of past fires, caused by interactions among climate, regional hydrology, vegetation, humans, and fire activity. We conducted a simple solid–liquid extraction on pulverised carbonate samples to prepare them for analysis of 16 polycyclic aromatic hydrocarbons (PAHs) and three monosaccharide anhydrides (MAs). The preparation method requires only small samples (0.5–1.0 g); PAHs and MAs were measured by GC–MS and LC–HILIC–MS, respectively. Detection limits range from 0.05–2.1 ng for PAHs and 0.01–0.1 ng for MAs. We applied the method to 10 samples from a ∼ 400-year-old stalagmite from Cenote Ch'en Mul, at Mayapan (Mexico), the largest Postclassic Maya capital of the Yucatán Peninsula. We found a strong correlation (r = 0.75, p < 0.05) between the major MA (levoglucosan) and non-alkylated PAHs (Σ15). We investigated multiple diagnostic PAH and MA ratios and found that although not all were applicable as paleo-fire proxies, ratios that combine PAHs with MAs are promising tools for identifying different fire regimes and inferring the type of fuel burned. In the 1950s and 1960s, levoglucosan and Σ15 concentrations roughly doubled compared to other times in the last 400 years, suggesting greater fire activity at Mayapan during these two decades. The higher concentrations of fire markers may have been a consequence of land clearance at the site and exploration of the cave by Carnegie Institution archaeologists.

Combustion Completeness and Sample Location Determine Wildfire Ash Leachate Chemistry

“…Homann et al. (2023) found that high molecular weight PAHs were often <LOD in a Mexican speleothem. They attributed this to filtering of high molecular weight PAHs by overlying soils and epikarsts, as earlier suggested by Perrette et al.…”

“…Additionally, that low molecular weight PAHs are generally more abundant than high molecular weight PAHs, and that total PAH concentrations are higher in black and gray ashes than in white ashes both suggest that PAH-derived records of past fires may be biased toward less-severe burns. Homann et al (2023) found that high molecular weight PAHs were often <LOD in a Mexican speleothem. They attributed this to filtering of high molecular weight PAHs by overlying soils and epikarsts, as earlier suggested by Perrette et al (2013).…”

“…The legacies of fire impacts on ash and soil are also evident in organic compounds resulting from the combustion of plant material. Pyrogenic biomarkers such as levoglucosan and polycyclic aromatic hydrocarbons (PAHs) are used to investigate past fire activity in environmental archives such as ice and sediment cores, and speleothems (Argiriadis et al, 2019(Argiriadis et al, , 2023Denis et al, 2012;Homann et al, 2022Homann et al, , 2023Rubino et al, 2016;Vachula et al, 2019). Levoglucosan (1,) is a water-soluble anhydrosugar which is formed through the thermal breakdown of cellulose and hemicellulose (Bhattarai et al, 2019;Elias et al, 2001;Li et al, 2021;Simoneit et al, 1999).…”

“…These proxy archives have allowed past fire activity to be reconstructed at local (e.g., Rehn et al., 2021), continental (e.g., Zennaro et al., 2015), and global scales (e.g., Marlon et al., 2008). In recent years, speleothems (primarily stalagmites) have been developed as archives of past fire activity, with both inorganic proxies (predominantly trace metals) and pyrogenic biomarkers used to investigate past fire activity (Argiriadis et al., 2019, 2023; M. Campbell et al., 2023; Homann et al., 2022, 2023; McDonough et al., 2022). The inorganic fire proxy signal in speleothems is thought to be sourced from ashes deposited over the cave, which are subsequently leached by rainfall, transported via karst flowpaths, and deposited with speleothem calcite.…”

“…The inorganic fire proxy signal in speleothems is thought to be sourced from ashes deposited over the cave, which are subsequently leached by rainfall, transported via karst flowpaths, and deposited with speleothem calcite. It is less clear whether the pyrogenic biomarker signal is transported via the leaching of ashes or via aerosol inputs (Homann et al., 2023), although Homann et al. (2022) sampled cave dripwaters and demonstrated in that cave that levoglucosan was transported with infiltrating waters through the karst.…”