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Open questions remain around the Holocene variability of climate in Iceland, including the relative impacts of natural and anthropogenic factors on Late Holocene vegetation change and soil erosion. The lacustrine sediment record from Torfdalsvatn, north Iceland, is the longest known in Iceland (≤12000 cal a BP) and along with its high sedimentation rate, provides an opportunity to develop high-resolution quantitative records that address these challenges. In this study, we use two sediment cores from Torfdalsvatn to construct a high-resolution age model derived from marker tephra layers, paleomagnetic secular variation, and radiocarbon. We then apply this robust age constraint to support a complete tephrochronology (>2200 grains analyzed in 33 tephra horizons) and sub-centennial geochemical (MS, TOC, C/N, δ13C, and BSi) and algal pigment records. Along with previously published proxy records from the same lake, these records demonstrate generally stable terrestrial and aquatic conditions during the Early and Middle Holocene, except for punctuated disturbances linked to major tephra fall events. During the Late Holocene, there is strong evidence for naturally driven algal productivity decline beginning around 1800 cal a BP. These changes closely follow regional Late Holocene cooling driven by decreases in Northern Hemisphere summer insolation and the expansion of sea-ice laden Polar Water around Iceland. Then at 880 cal a BP, ~200 years after the presumed time of human settlement, a second shift in the record begins and is characterized by a strong uptick in landscape instability and possibly soil erosion. Collectively, the Torfdalsvatn record highlights the resilience of low-elevation, low-relief catchments to the pre-settlement soil erosion in Iceland, despite a steadily cooling background climate. The precisely dated, high-resolution tephra and paleoenvironmental record from this site can serve as a regional template for north Iceland. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

As global warming progresses, changes in high-latitude precipitation are expected to impart long-lasting impacts on earth systems, including glacier mass balance and ecosystem structure. Reconstructing past changes in high-latitude precipitation and hydroclimate from networks of continuous lake records offers one way to improve forecasts of precipitation and precipitation-evaporation balances, but these reconstructions are currently hindered by the incomplete understanding of controls on lake and soil water isotopes. Here, we study the distribution of modern water isotopes in Icelandic lakes, streams, and surface soils collected in 2002, 2003, 2004, 2014, 2019 and 2020 to understand the geographic, geomorphic, and environmental controls on their regional and interannual variability. We find that lake water isotopes in open-basin (throughflowing) lakes reflect local precipitation with biases toward the cold season, particularly in lakes with sub-annual residence times. Closed-basin lakes have water isotope and deuterium excess values consistent with evaporative enrichment. Interannual and seasonal variability of lake-water isotopes at repeatedly sampled sites are consistent with instrumental records of winter snowfall, summer relative humidity, and atmospheric circulation patterns, such as the North Atlantic Oscillation. In contrast to the cold-season bias in Icelandic lakes, summer surface soil water isotopes reflect summer precipitation overprinted by evaporative enrichment that can occur throughout the year, although the soils sampled were shallower than rooting depths for many plant types. This dataset provides new insight into the functionality of water isotopes in Icelandic environments and offers renewed possibilities for optimized site selection and proxy interpretation in future paleohydrological studies on this North Atlantic outpost. [ABSTRACT FROM AUTHOR]

Most extant ice caps mantling low-relief Arctic Canada landscapes remained cold based throughout the late Holocene, preserving in situ bryophytes killed as ice expanded across vegetated landscapes. After reaching peak late Holocene dimensions ∼1900 CE, ice caps receded as Arctic summers warmed, exposing entombed vegetation. The calibrated radiocarbon ages of entombed moss collected near ice cap margins (kill dates) define when ice advanced across the site, killing the moss, and remained over the site until the year of their collection. In an earlier study, we reported 94 last millennium radiocarbon dates on in situ dead moss collected at ice cap margins across Baffin Island, Arctic Canada. Tight clustering of those ages indicated an abrupt onset of the Little Ice Age at ∼1240 CE and further expansion at ∼1480 CE coincident with episodes of major explosive volcanism. Here we test the confidence in kill dates as reliable predictors of expanding ice caps by resampling two previously densely sampled ice complexes ∼15 years later after ∼250 m of ice recession. The probability density functions (PDFs) of the more recent series of ages match PDFs of the earlier series but with a larger fraction of early Common Era ages. Post 2005 CE ice recession has exposed relict ice caps that grew during earlier Common Era advances and were preserved beneath later ice cap growth. We compare the 106 kill dates from the two ice complexes with 80 kill dates from 62 other ice caps within 250 km of the two densely sampled ice complexes. The PDFs of kill dates from the 62 other ice caps cluster in the same time windows as those from the two ice complexes alone, with the PDF of all 186 kill dates documenting episodes of widespread ice expansion restricted almost exclusively to 250–450 CE, 850–1000 CE, and a dense early Little Ice Age cluster with peaks at ∼1240 and ∼1480 CE. Ice continued to expand after 1480 CE, reaching maximum dimensions at ∼1880 CE that are still visible as zones of sparse vegetation cover in remotely sensed imagery. Intervals of widespread ice cap expansion coincide with persistent decreases in mean summer surface air temperature for the region in a Community Earth System Model (CESM) fully coupled Common Era simulation, suggesting the primary forcings of the observed snowline lowering were both modest declines in summer insolation and cooling resulting from explosive volcanism, most likely intensified by positive feedbacks from increased snow cover and sea ice and reduced northward heat transport by the oceans. The clusters of ice cap expansion defined by moss kill dates are mirrored in an annually resolved Common Era record of ice cap dimensions in Iceland, suggesting this is a circum-North-Atlantic–Arctic climate signal for the Common Era. During the coldest century of the Common Era, 1780–1880 CE, ice caps mantled >11000 km 2 of north-central Baffin Island, whereas <100 km 2 is glaciated at present. The peak Little Ice Age state approached conditions expected during the inception phase of an ice age and was only reversed after 1880 CE by anthropogenic alterations of the planetary energy balance. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Most extant small ice caps mantling low-relief Arctic Canada landscapes remained cold-based throughout the late Holocene, preserving in situ bryophytes killed as ice expanded across vegetated landscapes. As Arctic summers warmed after 1900 CE, ice caps receded, exposing entombed vegetation. The calibrated radiocarbon ages of dead moss collected near ice-cap margins (kill-dates) define when ice advanced across the site, killing the moss, and remained over the site until the year of their collection. In an earlier study we reported 94 Last Millennium radiocarbon dates on in situ dead moss collected at the margins of two upland ice complexes on northern Baffin Island, Arctic Canada. Tight clustering of those ages indicated an abrupt onset of the Little Ice Age ~1240 CE, and further expansion ~1480 CE, coincident with episodes of major explosive volcanism. Here we test the confidence in kill dates as reliable predictors of expanding ice caps by re-sampling those previously sampled ice complexes 14 years later, after ~250 m of ice recession. The probability density functions (PDF) of the more recent series of ages matches PDFs of the earlier series, but with a larger fraction of early CE ages; post 2005 CE ice recession has exposed relict ice caps that grew during earlier Common Era advances, and were preserved beneath later ice-cap growth. We compare 107 kill dates from the two ice complexes with 79 kill dates from 62 other ice caps within 250 km of the two densely sampled ice complexes. The PDF of kill dates from the 62 other ice caps cluster in the same time windows as those from the two ice complexes alone, with the PDF of all 186 kill dates documenting episodes of widespread ice expansion restricted almost exclusively to 250–450 CE, 850–1000 CE and a dense early Little Ice Age cluster with peaks at ~1240 and ~1480 CE. Ice continued to expand after 1480 CE, reaching maximum dimensions ~1880 CE, still visible as zones of limited vegetation cover in remotely sensed imagery. Intervals of widespread ice-cap expansion coincide with persistent decreases in mean summer surface air temperature for the region in a Community Earth System Modeling (CESM) fully coupled Common Era simulation, suggesting primary forcing of the observed snowline lowerings were both modest declines in summer insolation, and cooling resulting from explosive volcanism, most likely intensified by positive feedbacks from sea-ice expansion and reduced northward heat transport by the oceans. The clusters of ice cap expansion defined by moss kill-dates are mirrored in an annually resolved Common Era record of ice-cap dimensions in Iceland, suggesting this is a circum-North-Atlantic-Arctic climate signal for the Common Era. During the coldest century of the Common Era, 1780–1880 CE, ice caps mantled > 11,000 km2 of north-central Baffin Island, whereas < 100 km2 is glaciated at present. That state approached conditions expected during the inception phase of an ice age, and was only reversed after 1880 CE by anthropogenic alterations of the planetary energy balance. [ABSTRACT FROM AUTHOR]

The Vedde Ash, first described in Norway and dated to ~12000 cal a BP, has been taken to represent tephra derived from a large eruption of the Katla volcano in Iceland and dispersed across the North Atlantic and Europe. However, evidence for tephra layers with similar composition to the Vedde Ash, but of different ages, questions the utility of isolated Vedde-like tephra layers as reliable and independent age control. Here, we report three stratigraphically separated Early Holocene Katla tephra layers from the lake Torfdalsvatn, in north Iceland, that have bimodal chemical composition similar to the Vedde Ash. By using previously published conventional 14C ages and revised calibration curves, we provide new ages for these tephra layers of ~11375, 11360, and 11200 cal a BP – all substantially younger than the Vedde Ash. Torfdalsvatn's record stands as an important reminder that repeated explosive eruptions of Iceland's major volcanos during the deglacial cycle have produced multiple tephra plumes with similar geochemistry that may span 1000s of years. As a result, we urge caution when using isolated Icelandic tephra layers in distant regions as precise geochronometers without supporting age control. [ABSTRACT FROM AUTHOR]

Lake sediment records give valuable insight into the dynamic events that characterized the last deglaciation in Iceland. Here, we focus on the well‐dated sediment record from Hestvatn, a low‐elevation lake in south Iceland, that features six graded bedding events deposited by outburst floods from glacial lakes dammed by the decaying Iceland Ice Sheet (IIS) in the time period of the Vedde Ash and the G10ka Series tephra. Using climate proxies preserved in the sediment cores, in conjunction with regional glacial geomorphology, we reconstruct the retreat of the IIS in south Iceland, from a marine‐based glacier during the Younger Dryas to a land‐based glacier during the Preboreal. As the ice sheet margin withdrew to the central highlands, ice‐dammed lakes formed along glacier margins. The ice‐dams were occasionally breached, generating large‐scale jökulhlaups (catastrophic outburst floods) that deposited thick turbidite sequences preserved in the sediment record of Hestvatn. The high concentration of volcanic material incorporated within deglacial sediments indicates that along with IIS retreat, subglacial volcanic activity may have helped initiate some of the jökulhlaups. Onset of more stable Holocene conditions was reached after the final turbidite at ~10 ka bp, when the IIS had withdrawn from most of the highlands of Iceland. [ABSTRACT FROM AUTHOR]