Past fire dynamics inferred from polycyclic aromatic hydrocarbons and monosaccharide anhydrides in a stalagmite from the archaeological site of Mayapan, Mexico
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, 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 be due to land clearance at the site and explorations of the cave by the Carnegie Institution archaeologists.
Julia Homann et al.
Status: open (extended)
- RC1: 'Comment on bg-2023-63', Anonymous Referee #1, 10 May 2023 reply
Julia Homann et al.
Julia Homann et al.
Viewed (geographical distribution)
The paper "Past fire dynamics inferred from polycyclic aromatic hydrocarbons and monosaccharide anhydrides in a stalagmite from the archaeological site of Mayapan, Mexico" presents an interesting comparison between different biomarkers of past fire activity as recorde in a speleothem. Such comparisons are necessary in order to better understand the strengths and weaknesses of biomarkers, and especially such differences as recorded in a matrix in which they are not often analyzed. However, the concentrations of these biomarkers are often below the limit of detection (LOD), thereby making the comparisons less useful. The values below the LOD also influence their ability to be used in ratios that help determine the type of past burned vegetation. It is unclear if these low values are due to the matrix, the loction, or the sampling procedure. This paper still deserves to be published, but the limited values should be mentioned in the introduction. Please also address the following points:
Line 57: Please explain what you mean by “cave-internal sources”.
Lines 165-170: What are the benefits of running in the full scan mode rather than targeting the known m/z? If the retention times are known, then why not just use the targeted MS2 mode? Is anything gained from using the full scan mode?
Section 3.1: At what point in the study was the recovery rates tested? As the existing method resulted is relatively low recovery rates, why was the method not adapted accordingly to minimize volatilization and therefore to improve recovery? Were other solvents tested that would minimize volatilization? Other solvents may also influence other aspects of the recovery, but it is surprising that these solvents were not systematically tested and represented in this study as this work is a “proof-of-concept” work.
Line 214: Although Perette et al., 2013 discuss the possible effects of filtering by the overlying soil, this concept needs details within the submitted paper in order to be clear to the reader. The reader can hunt down Perette et al., 2013 and read the work, but ideally mentions of other literature should include sufficient detail that the readers understand why the work was cited. In this case, does the soil preferentially remove the high molecular weight PAHs? If so, what is the mechanism for this fractionation between the high molecular weight PAHs and the lower molecular weight PAHS? As noted in Lines 218-222 this filtration effect influences examining any ratios that incorporate low molecular weight PAHs.
Line 234-244: Is Retene influenced by the method (i.e. the potential loss during volatilization as mentioned in Section 3.1)? Could the method cause the fact that only two samples have quantifiable concentrations of Retene above the LOD? Could the filtration through the soil (Lines 218-222) also affect the Retene concentrations? With only two samples above the LOD, this paragraph is speculative, although the authors do acknowledge the limits of trying to interpret data with only two points.
Figure 4b: Why is the fire intensity data plotted with a reverse y-axis? The viewer does not gain any additional information from this flipped axis.
Lines 40-41: The following existing sentence should be used to start a new paragraph: “The presence of specific biomarkers is indicative of the fuel source”.
Line 48: Remove the word “for”.
Table 2: “Common name” is a more accepted form than “trivial name”.
Line 189: Only one “Figure 2” reference is necessary.