the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 07 Jul 2026
Rain, wind or fire: which led the PAH fallout in Northern Madagascar across the Holocene?
Abstract. The interactions between wildfire, human and climate variability are not well understood. To have better predictions, past studies are useful for observing their behaviours on a long-time scale. To infer past fire activity, studies used charcoal and molecular biomarkers derived from biomass burning, such as Polycyclic Aromatic Hydrocarbons (PAHs). In the north of Madagascar, charcoals are of local origin and have shown increased abundance since 1000 cal a BP. In contrast, PAH are present throughout the Holocene. Therefore, PAHs likely provide a more regional information. Due to the seasonal wind regime, they are thought to originate from East Africa. However, the PAH signal pattern differs from the fire activity record of East Africa. Instead, it shows a similar pattern to precipitation variability in northern Madagascar. Higher PAH concentrations are observed during wetter period, whereas lower concentrations occur during drier periods. This suggest that PAH preservation is enhanced, which may facilitate leaching processes prior to their deposition in sediments. These results challenge the direct interpretation of PAH as straightforward indicators of fire events and dry periods.
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Status: open (until 01 Sep 2026)
- RC1: 'Comment on egusphere-2026-3726', Anonymous Referee #1, 05 Aug 2026 reply
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- 1
General comments
This manuscript presents the results of a scientific research into the use of Polycyclic Aromatic Hydrocarbons (PAHs) as biomarkers of biomass burning, measured in a mountain lake in northern Madagascar throughout the Holocene, and their relationship with regional climatic drivers such as wind and rainfall patterns. This work is relevant to the research field, presenting an interesting approach based on the different chemical characteristics of PAH compounds to improve the estimation of their origin and location, and to determine whether they can be used as a proxy for wildfire signals preserved in environmental archives. Text editing and additional information, where demanded, would be beneficial.
Specific Comments
This manuscript addresses relevant scientific questions within the scope of CP by presenting novel data that supports the use of the concept of physicochemical properties to trace the geographic origin of chemical compounds and the main mechanisms by which they are transported over time.
The scientific hypothesis is clearly outlined, as is the methodology used to test it. However, the analytical methodology for determining PAHs requires further clarification to ensure it is fully understandable and reproducible without consulting the references.
While some sentences and figures require clarification, the results of this study are clearly presented and sufficient to test the stated hypothesis, providing adequate support for the interpretations, particularly given the approach based on the physicochemical properties of PAHs. However, the originality of this work should be better highlighted. Some insights have been presented and a comprehensive understanding based on this data has been achieved. However, the conclusions section needs improvement as it is too condensed.
The references used are adequate for the discussion of this topic. The title is OK. Although it contains all the mandatory elements, the abstract and the rest of the text should be improved in terms of language editing to make them clearer and more fluent. Some terms/words are highlighted in the technical corrections section.
Technical Corrections
I would prefer the terms 'Cal BP' or 'Cal yr BP', as these are more common, but this is just a suggestion.
Lines 57-59: It is missing a reference for this information.
Line 60: Why? Neither the above paragraph nor this text makes it clear why charcoal is limited as a regional wildfire proxy. Please could you improve this explanation?
Line 63: “current observations” - is this related to the 2017 reference? Is there a more recent study on the seasonality of PAH concentrations in the environment?
Line 81: “no CHARCOAL was found”.
Line 120 (Figure 1 Legend): Please complete the legend of Figure A with the winter, summer and annual mean precipitation information shown.
Line 133: Correct to “Fig. 1D”.
Line 134: I suggest the following text correction: “…we USED the 11-metre-LONG sediment cores LM1A and LM1B, taken in 2017 using a Russian PEAT corer (Teixeira et al., 2021).”
Please also include any other information from the sampling methodology described by Teixeira et al. (2021) that is relevant to PAH analysis and interferences, such as sample preservation. Also, please specify which analytical controls were employed to address potential contamination issues arising from the sampling procedures.
Lines 145-147: “Simultaneously, charcoal counts exhibit that fires started to occur, probably not in Montagne d’Ambre, but more likely occurring in the low elevation areas surrounding the mountains.”
Could you please rewrite this sentence to make it clearer?
Line 162: Were the samples spiked with internal standards prior to extraction? If so, which internal standards were added?
Lines 165-170: Please standardise the chemical notation for solutions according to the IUPAC recommendations. For example: 1:1 (v/v); heptane/dichlorometane (98:2, v/v)…
Do you really mean 98/2 v/v? Mostly heptane (4.9 ml), with just a few drops of DCM (100 µl)? Or was a different proportion used for this fraction's eluent? Which fraction is this, F2? Which proportion was used for the 2mL HEP/DCM solution for F2?
It is informed that “If pigment remained in fraction 1, a second separation by SiO2 column chromatography was performed.” How exactly? Is it just for F1, and are the same eluents used?
Please review the methodology information regarding the eluents and their proportions used to recover each fraction of compounds and use standard terms throughout the text.
Lines 171-172: Could you please review this sentence, as it is not clear enough? Could you also provide more information on whether fractions 1 and 2 were evaporated together or separately after being combined? Or were the two extracts analysed for PAHs separately? Was the DMSO removed from the extract prior to injection?
More detailed information on these steps is needed to make them reproducible.
Line 180: Instead of merely citing the reference, I strongly recommend that you provide a brief description of the applied method, highlighting any possible adaptations.
Line 196: Please correct “at at 25ºC”
Line 197: I would be careful using this concept, given that PAHs are not naturally hydrophilic. Instead, I would use the concept of increasing hydrophobicity as described by Garagnon et al. (2024).
Line 197: “solubility coeficient > 10-2 mg.L-1 (N, Ace, Flu, Phe, Ant, Fla and Pyr)”. Please provide the reference for this information, which was used to support this grouping.
Lines 205-206: This is an incomplete sentence. Please review it and provide the full information, including the equation, not just the reference.
Line 235 (Legend of Table 1): I suggest rewriting this sentence, e.g.: "Bold and italic lines represent inverse age, which were not taken into account in the model.”
Line 239 (Legend of Figure 2): I suggest rewriting these sentences, e.g.: "Linear age-depth model of Lake Maudit created using the CLAM package. Red points represent inverse age, which were not considered in the model.”
Line 258: “since 2800 cal a BP” is repeated. Please review this sentence.
Line 271 (Legend of Figure 3): I suggest using the lettering scale (A–D) for these four graphs and including this information in the legend.
Line 282: Please format this reference “(Ana Lúcia C. Lima et al., 2005)” along the text in accordance with the journal's rules.
Line 298: “(Pyr, Flu, Fla, Fig. SM 3)” - Figure 3 shows the distribution of N, Phe, Flu and Fla. It does not show Per or Pyr. Please adapt the figure to show the comparisons mentioned in the text.
Line 307: Please correct to correct to “… few CHARCOAL particles”
Lines 309-310: Where can it be seen/checked? In which figure? It cannot be seen clearly in Fig. 4.
“Particles >160um”? Which results show this? Do you mean the results of Teixeira et al. (2021)? If so, please clarify the size fractionation used.
Line 312: What do you mean by “presently occur”? When?
Lines 312-314: These two sentences are not easy to understand. Could you please rewrite this text to explain it further?
Lines 320-321: “Therefore, the northern region of Madagascar cannot be considered as a source of PAHs”
Which PAHs does this refer to? The pyrogenic ones? Could you please explain this relationship further, given that you pointed out a contrast between charcoal and PAHs above?
Line 322-323: “but this would reflect more remote sources supplying PAHs.”
Supplying all PAHs, or the "hydrophilic" ones?
Line 327: Please correct to “originated”
Line 329: “Where came from PAH during the Early and Mid-Holocene?”
Please review the grammar of this sentence.
Lines 341-342: “Therefore, this implies that although distant, the fire source is not excessively far.”
What does this mean in terms of distance? Is there any study of the atmospheric transportation distances of PAHs that can be cited here?
Line 367: Please correct to “In a different site” and clarify which one is it.
Line 371: Please correct to “Lake Masoko recorded high value of charcoal accumulation…”
Lines 375-376: “PAHs likely originate from regional biomass burning, but the variation was not due to fire activity variation.”
Please clarify this information.
Lines 378-383 (Legend of Figure 4):
Is there any data within the blue scale in Figure 4A? Could the scale be adapted to display the values more clearly, as was done in Fig. 4-C?
In the axis titles of Figures 4-D and 4-E, please indicate the locations and references of the charcoal data.
The legend is incoherent with Figure 4D. Please check it.
Line 443-452: I recommend rewriting the conclusion to make it clearer and address/clarify the following points:
“which are molecules produced by incomplete combustion” - that CAN be produced.” As mentioned in this manuscript, some PAHs can also have a biogenic origin.
“However, the signal of PAH differs from the fire activity signal” – fire activity of where?
“These results challenge the direct interpretation of PAHs in terms of fire events and dry periods”. Could you please clarify here the challenges you mention in terms of dry periods?
Lines 501-502: Please complete this reference
Lines 503-504: Please inform this reference’s DOI
Lines 596-599: Please inform this reference’s DOI
Lines 611-612: Please complete this reference
Line 643: DOI not found. Please check it.