the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
A Holocene history of climate, fire, landscape evolution, and human activity in Northeast Iceland
Nicolò Ardenghi
David John Harning
Jonathan Henrik Raberg
Brooke René Holman
Thorvaldur Thordarson
Áslaug Geirsdóttir
Gifford H. Miller
Julio Sepúlveda
Abstract. Paleoclimate reconstructions across Iceland provide a template for past changes in climate across the northern North Atlantic, a crucial region due to its position relative to the global northward heat transport system and its vulnerability to climate change. The roles of orbitally driven summer cooling, volcanism, and human impact as triggers of local environmental changes in the Holocene of Iceland, remain debated. While there are indications that human impact may have reduced environmental resilience during Late Holocene summer cooling, it is still difficult to resolve to what extent human and natural factors affected Iceland’s Late Holocene landscape instability. Here, we present a continuous Holocene fire record of northeastern Iceland from proxies archived in Stóra Viðarvatn sediment. We use pyrogenic polycyclic aromatic hydrocarbons (pyroPAHs) to trace shifts in fire regimes, paired with continuous biomarker and bulk geochemical records of soil erosion, lake productivity, and human presence. The molecular composition of pyroPAHs and a wind pattern reconstruction indicate a naturally driven fire signal that is mostly regional. Generally low fire frequency during most of the Holocene significantly increased at 3 ka and again after 1.5 ka BP, before known human settlement in Iceland. We propose that shifts in vegetation type caused by cooling summers over the past 3 kyr, in addition to changes in atmospheric circulation, such as shifts in North Atlantic Oscillation (NAO) regime, led to increased aridity and biomass flammability. Our results show no evidence of faecal biomarkers associated with human activity during or after human colonisation in the 9th century CE. Instead, faecal biomarkers follow the pattern described by erosional proxies, pointing toward a negligible human presence and/or a diluted signal in the lake’s catchment. However, low post-colonisation levels of pyroPAHs, in contrast to an increasing flux of erosional bulk proxies, suggest that farming and animal husbandry may have suppressed fire frequency by reducing the spread and flammability of fire-prone vegetation (e.g., heathlands).
Overall, our results describe a fire frequency heavily influenced by long term changes in climate through the Holocene. They also suggest that human colonisation had contrasting effects on the local environment by lowering its resilience to soil erosion while increasing its resilience to fire.
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Nicolò Ardenghi et al.
Status: open (until 14 Jan 2024)
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RC1: 'Comment on cp-2023-74', Anonymous Referee #1, 24 Nov 2023
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GENERAL COMMENTS
“A Holocene history of climate, fire, landscape evolution, and human activity in Northeast Iceland” by Ardenghi et al. is a complete and well written, well organized research paper that nicely integrates different approaches and proxies for the interpretation of human-environment dynamics during the Holocene. Islands are particularly fit for paleoenvironmental studies since changes are often more visible in lacustrine records and the source area is confined, as is demonstrated by the study of back trajectories reported in this manuscript. This work will certainly be of interest for the readers of Climate of the Past. I thus recommend publication after a minor revision.
SPECIFIC COMMENTS
- Line 109: six extraction cycles seem far too many compared to the usual three. Is there a particular reason for this? If so, please motivate in the text.
- Line 110: I believe ASE350 works at 1500 psi, not 2000. Please check.
- Lines 110-112: why adding the internal standards after the extraction? Although six extraction cycles certainly provide the highest yield for analytes, how do authors account for this? Why using non-labelled standards here while adding deuterated ones as recovery and injection standards?
- Line 129: were alkanes integrated using characteristic m/z ratios extracted from the full scan? Why not using a SIM approach or FID then? What is the purpose of using a triple quadrupole in full scan mode?
- Lines 211-225: are data coincident with the g10ka series tephra omitted due to age uncertainties or was it impossible to determine concentrations of chemical proxies in this layer? Please specify in the text to facilitate non expert readers.
- Lines 358-362: due to small concentrations of n-alkanes in this first part of the record, small changes (as the spike in C19 at about 9 ka BP) may cause significant changes in CPI values. However, does a change from a dominant aquatic source to a dominant terrestrial source of n-alkanes justify the change in the correlation sign between CPI and δ13C? If the terrigenous input increases after 8 ka BP, together with n-alkanes related to terrestrial plants and arboreal pollen, will this explain the change in correlation? Perhaps adding ACL or similar index may help the interpretation.
- Lines 469-470: the discussion of the role of volcanic eruptions as a possible source of PAHs to the lake is limited to one sentence. I believe the manuscript will benefit of a short discussion to support the interpretation of measured PAHs as mainly deriving from fires, e.g. what compounds may be more related to eruptions and which ones are unambiguously related to biomass burning?
- Lines 454-475: observing the record, both peaks in pyroPAHs coincide with drops in Betula pollen, although the one at 2.8 ka BP seems unrelated to human presence since no peak in fecal markers is recorded here, while the second one at 1.5 ka is accompanied by an increase in stanols. In addition, the latter occurs in a cooler (although dry) period. Based on these observations, the first peak could be interpreted as driven by natural causes, as opposed to the second, likely including also some human influence in fire activity (see comments to figure A1), although authors are rightly very cautious in this interpretation. This would also agree with the subsequent reduction in wood availability and fire suppression discussed at the end of the section, in line with several records from the last 2 ka, where initial burning by early settlers is followed by strong changes in fire regimes and/or fire suppression/management, leading to a general decrease in fire tracers prior to the industrial era.
- Figure A1:
- Consider adding smoothing lines to PAH data to aid visual interpretation of the data.
- Is benzo(e)pyrene missing from the plot?
- How do authors explain spikes of benzo(a)pyrene at about 9, 7 and 4 ka BP? Perhaps the axis break should be removed to avoid misinterpreting small fluctuations of the background.
- Retene is believed to be useful as a proxy of coniferous wood and its combustion. This compound is not considered in the discussion but looking at the plot, its trend seems different from others with a peak at the end of phase VII that is synchronous with a peak in LMW PAHs and in sterols/stanols. This peak is prior to the conventional colonization date and authors state at line 422 the stanol one does not have a sufficient s/n ratio to be confidently interpreted as a human signal. However, the synchronicity with peaks in other sites and with retene, considering the dilution effect due to the lake dimensions (resulting in low concentrations and small fluctuations from the background), might indeed link this to early occupation and deforestation of the site. Is any palaeoecological record including softwood species and charcoal available for the area?
TECHNICAL CORRECTIONS
- Please renumber manuscript sections starting from the Introduction (1), not the Abstract.
- Line 136: “MS conditions were as same as” please correct to “the same as”.
- Line 170: please correct “digestive track” to “digestive tract”.
Citation: https://doi.org/10.5194/cp-2023-74-RC1
Nicolò Ardenghi et al.
Nicolò Ardenghi et al.
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