A Holocene history of climate, fire, landscape evolution, and human activity in Northeast Iceland

Abstract

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.

High-resolution Holocene record from Torfdalsvatn, north Iceland, reveals natural and anthropogenic impacts on terrestrial and aquatic environments

“…7C, Alsos et al, 2021), which may partially be due to variable age control between the two sedimentary records. We note, however, that while environmental impacts from settlers have been independently confirmed in lake sediments on the Faroe and Lofoten Islands using fecal biomarkers and/or sheep sedaDNA (D'Anjou et al, 2012;Curtin et al, 2021), these tools have so far proved challenging for use in Icelandic sediments and are needed for confirmation (Ardenghi et al, 2024). In any case, it is unlikely that the small populations that initially settled Iceland (~30,000 people, Landnámabók, i.e.…”

“…These empirical records have analyzed physical properties for glacier history (Larsen et al 2011;Striberger et al, 2012;Harning et al, 2016aHarning et al, , 2016b, pollen, macrofossils, and sedimentary ancient DNA (sedaDNA) for plant community (Rundgren, 1995(Rundgren, , 1998Hallsdóttir and Caseldine, 2005;Gathorne-Hardy et al, 2009;Eddudóttir et al, 2015Eddudóttir et al, , 2016Alsos et al, 2021;Geirsdóttir et al, 2022;Harning et al, 2023), bulk geochemistry for soil erosion and diatom productivity (Geirsdóttir et al, 2009(Geirsdóttir et al, , 2013(Geirsdóttir et al, , 2019(Geirsdóttir et al, , 2020(Geirsdóttir et al, , 2022Larsen et al, 2012;Blair et al, 2015;Harning et al, 2018a;Tinganelli et al, 2018;Bates et al, 2021), lipid biomarkers for fire activity (Ardenghi et al, 2024), and chironomids and lipid biomarkers for quantitative temperature (Caseldine et al, 2003;Axford et al, 2007Axford et al, , 2009Langdon et al, 2010;Holmes et al, 2013;Harning et al, 2020;Richter et al, 2020). The Holocene Thermal Maximum (7900 to 5500 cal a BP, Caseldine et al, 2006;Geirsdóttir et al, 2013) has been a particular focus as it provides a potential analogue for future environmental change -current estimates suggest that summer temperatures were ~3 o C warmer than present (Flowers et al, 2008;Harning et al, 2020).…”