Search

Anthropogenic stressors to freshwater environments have perpetuated water quality and quantity challenges for communities across Arctic Canada, making drinking water resources a primary concern for northern peoples. To understand the ecological trajectory of lakes used as freshwater supply, we conducted a paleolimnological assessment on two supplemental sources in Igloolik, Nunavut, Arctic Canada. A stratigraphic examination of biological indicators (Insecta: Diptera: Chironomidae) allowed for paleotemperature reconstructions with decadal and centennial resolution over the past 2000 years. Between 200 and 1900 CE, the sub-fossil chironomid community was comprised of cold-water taxa, such as Abiskomyia, Micropsectra radialis-type, and Paracladius. Reconstructed temperatures were consistent with known climate anomalies during this period. A rapid shift in the composition of the chironomid assemblages to those with higher temperature optima ( Chironomus anthracinus-type, Dicrotendipes, and Tanytarsus lugens-type) in the late 20th century was observed in both systems. Our results demonstrate that these ecosystems are undergoing marked transformations to warmer, more nutrient-rich environments, and suggest that water sustainability pressures will likely continue in tandem with ongoing climate change. To contextualize the influence of recent warming and elucidate the status of freshwater resources over the longer term, paleolimnological methods can be usefully applied as components of vulnerability assessments.

This research has been supported by the Leverhulme Trust (grant no. RC-2018-023), the European Research Council (grant no. 694481), the German Research Foundation (grant no. FE-1096/6-1), the Swiss Government Excellence Postdoctoral Scholarships (grant no. FIRECO 2016.0310), the National Science Centre of Poland (grant no. 2015/17/B/ST10/01656), the SCIEX Scholarship Fund (grant no. PSPB-013/2010), and the Estonian Research Council (grant no. MOBJD313)., Sedimentary charcoal records are widely used to reconstruct regional changes in fire regimes through time in the geological past. Existing global compilations are not geographically comprehensive and do not provide consistent metadata for all sites. Furthermore, the age models provided for these records are not harmonised and many are based on older calibrations of the radiocarbon ages. These issues limit the use of existing compilations for research into past fire regimes. Here, we present an expanded database of charcoal records, accompanied by new age models based on recalibration of radiocarbon ages using IntCal20 and Bayesian age-modelling software. We document the structure and contents of the database, the construction of the age models, and the quality control measures applied. We also record the expansion of geographical coverage relative to previous charcoal compilations and the expansion of metadata that can be used to inform analyses. This first version of the Reading Palaeofire Database contains 1676 records (entities) from 1480 sites worldwide. The database (RPDv1b – Harrison et al., 2021) is available at https://doi.org/10.17864/1947.000345., Leverhulme Trust RC-2018-023, European Research Council (ERC) European Commission 694481, German Research Foundation (DFG) FE-1096/6-1, Swiss Government Excellence Postdoctoral Scholarships FIRECO 2016.0310, SCIEX Scholarship Fund PSPB-013/2010, Estonian Research Council MOBJD313

Age-depth models provide essential temporal frameworks in paleoenvironmental science. We use a sample of 80 recently-published age-depth models to comment on current practices in building and reporting radiocarbon-based age-depth models. We address options for model building, sampling strategies, dating densities, and best practices for reporting age-depth models and associated data. Our review reveals incomplete reporting of14C ages, model-building methods, age-depth models and associated meta-data in many recent studies. All information needed to evaluate, reproduce and update an age-depth model should accompany every published model. We also present a case study of building age-depth models for a lake sediment core that has both14C ages and an independent varve chronology. The case study illustrates that choosing the ‘best model’ is not a simple task, and that model accuracy is ultimately controlled by differences between14C ages and true age that likely occur in many late Quaternary records.

Sedimentary charcoal records are widely used to reconstruct regional changes in fire regimes through time in the geological past. Existing global compilations are not geographically comprehensive and do not provide consistent metadata for all sites. Furthermore, the age models provided for these records are not harmonised and many are based on older calibrations of the radiocarbon ages. These issues limit the use of existing compilations for research into past fire regimes. Here, we present an expanded database of charcoal records, accompanied by new age models based on recalibration of radiocarbon ages using INTCAL2020 and Bayesian age-modelling software. We document the structure and contents of the database, the construction of the age models, and the quality control measures applied. We also record the expansion of geographical coverage relative to previous charcoal compilations and the expansion of metadata that can be used to inform analyses. This first version of the Reading Palaeofire Database contains 1681 records (entities) from 1477 sites worldwide. The database (DOI: 10.17864/1947.319) is available from https://researchdata.reading.ac.uk/id/eprint/319.

The failures of the popular Black-Scholes-Merton (BSM) model led to an interest in new, robust models which could more accurately model the behavior of historical prices. We consider one such model, the regime switching time-changed Levy process, which builds upon the BSM model by incorporating jumps through a random clock, as well as randomly varying parameters according to a continuous-time Markov chain. We develop the characteristic function as well as two methods for pricing European call options. Finally, we estimate the parameters of the model by incorporating historic energy data and option quotes using a variety of methods.

ESS, the European Spallation Source, will adopt a single family of double spoke cavities for accelerating the beam from the normal conducting section to the first family of the elliptical superconducting cavities. It will be the first double spoke cavities in the world to be commissioned for a high power proton accelerator. The first double spoke cavity for the ESS project was tested with high power in the HNOSS cryostat at Uppsala University. A pulse-mode test stand based on a self-excited loop was used in this test. The qualification of the cavity package involves a double-spoke superconducting cavity, a fixed fundamental power coupler, tuner, a low-level radiofrequency (LLRF) system and a high-power radiofrequency (RF) station. The test represents an important verification milestone before the module assembly. This cavity had unfortunately a high dynamic loss of 12 W @ 9 MV/m, where potential causes for such a high value have been studied and corresponding suggestions are listed. This paper presents the test configuration, RF conditioning history, first high power performance and experience of this cavity package.

Latitudinal treeline in northwestern Quebec, Canada, is a broad band of forest-tundra between 56 and 58°N. Three new pollen records from the forest-tundra zone were combined with ten previously-published records to form a transect of paleoecological data across this treeline. The area was deglaciated ∼6–7 ka, with only a brief period of shrub tundra vegetation in the oldest sediments of all sites. Noticeable values of Populus, Juniperus and Larix pollen percentages were found during this time in the southernmost sites, suggesting they comprised part of the initial plant migration. Maximum Picea pollen percentages were found between 2 and 4 ka across the boreal forest and forest-tundra, with values remaining constant in the boreal forest and decreasing in the forest-tundra in the past 3 ka. Across the entire area, pollen influx decreased in the past 4 ka, and non-arboreal pollen increased in the forest-tundra, suggesting an opening of the forest and decrease in terrestrial production. Charcoal influx also declined, indicating a reduction in fire activity in the past 2–4 ka. Although the treeline has remained in the same average location in the region over the past 6 ka, the strength of the temperature gradient across the frontal zone changed over the course of the Holocene.

A pollen record from a lake sediment core from southeastern Prince of Wales Island, Nunavut, Canada (SW08; 72.3177, −97.2678, 104 m a.s.l) provides the first high-resolution July temperature reconstruction for the last 1,000 years for the central Canadian Arctic Archipelago. The vegetation underwent marked transitions during the Little Ice Age (LIA; 1500–1800 CE) and Medieval Climate Anomaly (MCA; 1090–1250 CE), which was primarily observed in the proportion of Cyperaceae, Poaceae, and Salix pollen. Cyperaceae pollen was highest in the samples corresponding to the MCA, whereas Poaceae increased during the LIA. In the last 30 years, Salix and Betula pollen increased. The mean July temperature reconstruction showed a long-term cooling from 1080–1915 CE with a sustained cold period from 1800–1915 CE prior to twentieth-century warming. A synthesis of paleoclimate records from across the Arctic demonstrates that pollen-based reconstructions record both high and low frequency climate variability, when sampling resolution is sufficient, and can improve regional climate reconstructions.

Diatom assemblages in lake sediments of Lac Noir, southwestern Québec, were studied at a resolution of 10 yr to determine principal drivers of primary producers for the past ~1000 yr. Generalized additive modeling revealed strong links between broadscale climate intervals of the late Holocene, forest composition, and diatom flora. During the Medieval Warm Period (~AD 1200) and onset of the Little Ice Age (~AD 1450), increases in Tabellaria flocculosa str. IIIp at the expense of Discostella stelligera reflected low lake productivity. At AD 1630, an abrupt shift to cooler temperatures and dry conditions triggered a decline in hemlock (Tsuga), replaced by disturbance and cool-adapted taxa. Greater nutrient availability and soil erosion in the catchment led to a corresponding and rapid increase in diatoms with higher optima for nitrogen, such as Asterionella formosa and Fragilaria crotonensis. After AD 1870, an increase in pollen of taxa associated with disturbances signaled the arrival of Euro-Canadians, and associated nutrient inputs to the lake triggered increases in Stephanodiscus minutulus and Achnanthidium minutissimum. Overall results of the study indicate that climate played an important underlying role in lake-ecosystem dynamics; however, disturbances affecting forest composition had more direct influences on the diatom communities of Lac Noir.

The FREIA Laboratory at Uppsala University focuses on superconducting technology and accelerator development. It actively supports the development of the European Spallation Source, CERN, and MAX IV, among others. FREIA has developed test facilities for superconducting accelerator technology such as a double-cavity horizontal test cryostat, a vertical cryostat with a novel magnetic field compensation scheme, and a test stand for short cryomodules. Accelerating cavities have been tested in the horizontal cryostat, crab-cavities in the vertical cryostat, and cryomodules for ESS on the cryomodule test stand. High power radio-frequency amplifier prototypes based on vacuum tube technology were developed for driving spoke cavities. Solid-state amplifiers and power combiners are under development for future projects. We present the status of the FREIA Laboratory complemented with results of recent projects and future prospects., Comment: 30 pages, 18 figures

A comprehensive database of paleoclimate records is needed to place recent warming into the longer-term context of natural climate variability. We present a global compilation of quality-controlled, published, temperature-sensitive proxy records extending back 12,000 years through the Holocene. Data were compiled from 679 sites where time series cover at least 4000 years, are resolved at sub-millennial scale (median spacing of 400 years or finer) and have at least one age control point every 3000 years, with cut-off values slackened in data-sparse regions. The data derive from lake sediment (51%), marine sediment (31%), peat (11%), glacier ice (3%), and other natural archives. The database contains 1319 records, including 157 from the Southern Hemisphere. The multi-proxy database comprises paleotemperature time series based on ecological assemblages, as well as biophysical and geochemical indicators that reflect mean annual or seasonal temperatures, as encoded in the database. This database can be used to reconstruct the spatiotemporal evolution of Holocene temperature at global to regional scales, and is publicly available in Linked Paleo Data (LiPD) format. Fil: Kaufman, Darrell. Northern Arizona University.; Estados Unidos Fil: McKay, Nicholas. Northern Arizona University.; Estados Unidos Fil: Routson, Cody. Northern Arizona University.; Estados Unidos Fil: Erb, Michael. Northern Arizona University.; Estados Unidos Fil: Davis, Basil. University Of Lausanne; Suiza Fil: Heiri, Oliver. University Of Basel; Suiza Fil: Jaccard, Samuel. University Of Bern; Suiza Fil: Tierney, Jessica. University of Arizona; Estados Unidos Fil: Dätwyler, Christoph. University Of Bern; Suiza Fil: Axford, Yarrow. Northwestern University; Estados Unidos Fil: Brussel, Thomas. University of Utah; Estados Unidos Fil: Cartapanis, Olivier. University Of Bern; Suiza Fil: Chase, Brian. Universite de Montpellier; Francia Fil: Dawson, Andria. Mount Royal University; Canadá Fil: de Vernal, Anne. Université du Québec a Montreal; Canadá Fil: Engels, Stefan. University of London; Reino Unido Fil: Jonkers, Lukas. University Of Bremen; Alemania Fil: Marsicek, Jeremiah. University of Wisconsin-Madison; Estados Unidos Fil: Moffa Sánchez, Paola. University of Durham; Reino Unido Fil: Morrill, Carrie. University of Colorado; Estados Unidos Fil: Orsi, Anais. Université Paris-Saclay; Francia Fil: Rehfeld, Kira. Heidelberg University; Alemania Fil: Saunders, Krystyna. Australian Nuclear Science And Technology Organisation; Australia Fil: Sommer, Philipp. University Of Lausanne; Suiza Fil: Thomas, Elizabeth. University At Buffalo; Estados Unidos Fil: Tonello, Marcela Sandra. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones Marinas y Costeras. Universidad Nacional de Mar del Plata. Facultad de Ciencias Exactas y Naturales. Instituto de Investigaciones Marinas y Costeras; Argentina Fil: Tóth, Mónika. Balaton Limnological Institute; Hungría Fil: Vachula, Richard. Brown University; Estados Unidos Fil: Andreev, Andrei. Alfred Wegener Institut Helmholtz Centre for Polar and Marine Research; Alemania Fil: Bertrand, Sebastien. Ghent University; Bélgica Fil: Massaferro, Julieta. Administración de Parques Nacionales. Parque Nacional "Nahuel Huapi"; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina

A comprehensive database of paleoclimate records is needed to place recent warming into the longer-term context of natural climate variability. We present a global compilation of quality-controlled, published, temperature-sensitive proxy records extending back 12,000 years through the Holocene. Data were compiled from 679 sites where time series cover at least 4000 years, are resolved at sub-millennial scale (median spacing of 400 years or finer) and have at least one age control point every 3000 years, with cut-off values slackened in data-sparse regions. The data derive from lake sediment (51%), marine sediment (31%), peat (11%), glacier ice (3%), and other natural archives. The database contains 1319 records, including 157 from the Southern Hemisphere. The multi-proxy database comprises paleotemperature time series based on ecological assemblages, as well as biophysical and geochemical indicators that reflect mean annual or seasonal temperatures, as encoded in the database. This database can be used to reconstruct the spatiotemporal evolution of Holocene temperature at global to regional scales, and is publicly available in Linked Paleo Data (LiPD) format., Measurement(s)climateTechnology Type(s)digital curationFactor Type(s)temporal interval • geographic location • proxy typeSample Characteristic - Environmentclimate systemSample Characteristic - LocationEarth (planet) Machine-accessible metadata file describing the reported data: https://www.ncdc.noaa.gov/paleo/study/27330

Fossil pollen records are well-established indicators of past vegetation changes. The prevalence of pollen across environmental settings including lakes, wetlands, and marine sediments, has made palynology one of the most ubiquitous and valuable tools for studying past environmental and climatic change globally for decades. A complementary research focus has been the development of statistical techniques to derive quantitative estimates of climatic conditions from pollen assemblages. This paper reviews the most commonly used statistical techniques and their rationale and seeks to provide a resource to facilitate their inclusion in more palaeoclimatic research. To this end, we first address the fundamental aspects of fossil pollen data that should be considered when undertaking pollen-based climate reconstructions. We then introduce the range of techniques currently available, the history of their development, and the situations in which they can be best employed. We reviewthe literature on how to define robust calibration datasets, produce high-quality reconstructions, and evaluate climate reconstructions, and suggest methods and products that could be developed to facilitate accessibility and global usability. To continue to foster the development and inclusion of pollen climate reconstruction methods, we promote the development of reporting standards. When established, such standards should 1) enable broader application of climate reconstruction techniques, especially in regions where such methods are currently underused, and 2) enable the evaluation and reproduction of individual reconstructions, structuring them for the evolving open-science era, and optimising the use of fossil pollen data as a vital means for the study of past environmental and climatic variability. We also strongly encourage developers and users of palaeoclimate reconstruction methodologies to make associated programming code publicly available, which will further help disseminate these techniques to interested communities.

The Regional Estimates of VEgetation Abundance from Large Sites (REVEALS) model was used to quantify Holocene changes in vegetation cover in the deciduous forest of southeastern Quebec, Canada. The Extended R-Value (ERV) model was used to obtain relative pollen productivity estimates (PPEs) for eight tree taxa and to determine the relevant source area of pollen (RSAP) for lakes in this ecosystem. Modern vegetation was estimated using pollen data from 16 small (2) lakes and a species-level vegetation survey of southern Quebec. The RSAP was estimated to be within 1600 m of the lakes. Tsuga, Fagus, and Quercus were the most productive taxa, and Populus and Acer were the lowest. Reconstructed changes in absolute vegetation cover show a high abundance of Picea followed by Populus in the early Holocene. The reconstructed values for Populus suggest that it was widely distributed across the landscape. Abies and Acer were dominant on the landscape during the late to mid-Holocene, and an increase in Picea during the Neoglacial is more significant than in percentage diagrams. The REVEALS results provide estimates of land-cover change that are more realistic and informative than the use of pollen percentages alone.

We present a 2000-year pollen-based reconstruction of total annual precipitation (ANNP) for North America. Application of Local Indices of Spatial Association (LISA) permitted us to distinguish reg...

This study presents a diatom-based analysis of the post-glacial Holocene environmental history at Lake RS29 on Somerset Island in the Canadian High Arctic. Earliest post-glacial diatom assemblages (10 200–10 000 cal yr BP) consisted mainly of small, benthic fragilarioid taxa. Poor diatom preservation in the early Holocene (~10 000–6200 cal yr BP) is associated with warm conditions, as determined by pollen data from the same core and other paleoclimate estimates from the region. Analysis of this and other sites from across the Canadian Arctic suggest that zones of poor diatom preservation or diatom absence in lake sediment records may be associated with warm conditions. After 6200 cal yr BP, acidophilic assemblages consisting of Aulacoseira spp. and a suite of periphytic taxa indicate acidification since the mid-Holocene. During this time period, cooling causing changes in lake ice phenology was likely a major driver of the reconstructed mid-Holocene pH decline. Watershed processes, including reduced fluxes of base cations as the rate of sediment accumulation slowed, may also be contributors to long-term shifts in lake water pH and associated changes in diatom assemblages. The uppermost sediments in the Lake RS29 record were characterized by abrupt declines in Aulacoseira alpigena and increases in benthic diatom taxa Cyclotella sensu lato, suggesting an increase in lake water pH and longer ice-free seasons.

This study employed a 6.48 m long dated sediment core to characterize the amount of variation in background trace metal concentrations, and develop an understanding of the role of climatic influences on sediment inorganic and organic geochemistry in a High Arctic lake over ∼7000 years of the Holocene. Element geochemistry reflected varying detrital contributions from two geological sources within the catchment: carbonates (dolomites and limestones) which provided the dominant major element by weight (i.e., Ca), and carbonaceous mudrocks (primarily shales) which may have contributed most of the other trace and minor elements. The presence of eroded shale-hosted organic matter (OM) in sediments was confirmed by reflectance measurements on reworked vitrinitic macerals which had similar values to Palaeozoic carbonaceous rocks in the watershed. Bimodal distribution of the reflectance data suggested that two sources of vitrinitic macerals were present, from different formations. Variable dilution of shale-sourced elements by carbonates, related to climate (temperature)-influenced rates of dissolution and erosion, was the dominant process controlling inorganic geochemical composition. RockEval pyrolysis of bulk sediment OM revealed the unusual finding of two distinct sources of sediment kerogen which alternated in importance during the Holocene: a “baseline” state of eroded shale-hosted OM which was probably always present but which dominated sediment OM when autochthonous (algal) production was minimal, and an “enhanced algal state” which dominated when limnological conditions favored higher autochthonous productivity. Periods with more frequent examples of the enhanced algal state occurred during the mid- and late-Holocene, coincident with periods of relatively high summer air temperatures in the region. This study provides evidence that climate, particularly air temperature, influenced sediment inorganic and organic geochemical compositions in this lake through its effect on catchment geology erosion rates and aquatic primary productivity. It shows the value of studying very long periods of sediment accumulation as a background context for recent sediment metal concentrations.

Las investigaciones geomorfológicas han estudiado la relación entre los aspectos culturales del paisaje, como los usos del suelo, con las geoformas en diferentes contextos geográficos. De manera más específica, estas líneas de investigación se han enfocado a la manera como el terreno condiciona las actividades humanas. El estudio de estas relaciones, que generalmente son complejas, ha tendido a simplificarse a partir de meras sobreposiciones cartográficas en los sistemas de información geográfica (SIG). El presente artículo argumenta que los resultados de la sobreposición de datos geográficos pueden beneficiarse, para comprender los resultados, de un análisis de estadística multivariada. Estos análisis permiten interpretar la naturaleza de las relaciones espaciales entre los diferentes elementos que conforman el paisaje. Por lo tanto, se exploró el grado de correlación cuantitativa entre geoformas y usos del suelo en una región tropical seca de la costa Pacífico mexicana. Se eligió al valle bajo de Nexpa, en la región Sierra-Costa del estado de Michoacán, como caso de estudio por su grado de complejidad morfogenética y por su composición litológica y estructural heterogénea. La colecta de datos se realizó en dos fases: una fase preliminar de interpretación de imágenes satelitales (Quickbird 2008) y una fase de verificación en campo y levantamiento del terreno. La clasificación del relieve se hizo utilizando pares estereoscópicos impresos a una escala 1:10 000. Los datos de uso del suelo, por otra parte, se interpretaron utilizando los datos de cubierta del terreno como insumo. Posteriormente, se clasificaron estos datos a partir de técnicas de levantamiento participativo y etnografía de campo. Las capas de información geográfica fueron digitalizadas, vectorizadas e incorporadas a un SIG. De esta forma, se pudo obtener una matriz con los datos de sobreposición espacial de los polígonos. Posteriormente, se aplicó un Análisis de Correspondencia (AC) que fue utilizado para identificar el grado de correlación entre usos y geoformas. Los resultados muestran una correspondencia significativa entre diferentes entidades. Se aplicó un primer AC con todas las clases del terreno. Los resultados revelaron que la “planicie costera” y el “lecho mayor” se asocian, mayormente, a usos como la pesca artesanal, el nado y las actividades turísticas. Una segunda etapa fue aplicada excluyendo las clases de “planicie costera” y “lecho mayor”, que capturaron demasiada varianza en el primer análisis. La prueba posterior develó mayor detalle en asociaciones espaciales más finas y menos evidentes en el paisaje. Las geoformas de origen fluvial se asociaron con patrones más simples de actividad humana. Como contraparte, en las geoformas de denudación, el análisis reveló patrones más complejos y diversos con los usos del suelo. Las actividades agrícolas y ganaderas se presentaron, de diferentes maneras, tanto en geoformas fluviales como en las de denudación. Los usos forestales no maderables, como la caza y la extracción de leña, se asociaron con áreas de mayor pendiente y podrían haber favorecido ciertos procesos de conservación del bosque tropical seco en el área. Esta última interpretación requiere de un análisis a mayor profundidad. Los resultados se discutieron en término de su relevancia y utilidad para la planeación territorial. Este enfoque podría fortalecer los procedimientos espaciales para la toma de decisiones, particularmente en áreas en donde los datos de geografía aplicada se requieren para el sondeo de usos y actividades a escala local.

The development of landscapes of the central part of the Middle-Volga region in the last 2500 years was a discontinuous process of the explosive growth of population and land utilization alternating with stages of depopulation and desolation. The periods of depopulation and transitions of cultures occurred at similar times to climate changes. Some cultures were associated with distinct climatic episodes, such as the association of the Dark Ages Cold Period with Hun, post Hun, Heraldic, and Khasarian times, and the Medival Warm Period with the time of Volga Bulgaria. A combination of archaeological and paleoecological analyses allowed us to reconstruct a sequence of landscape and land use changes in relation to the historical development of the region. The first millennium CE was a time of major changes in population, agricultural technologies, social structure, and settlement patterns in the forest-steppe zone. The MiddleVolga region underwent a transition from a non-populated, mainly-forested landscape of first centuries CE to a highly deforested agricultural landscape of the Volga Bulgarian state by the 11th century CE. Within several centuries, the landscape was transformed by shifting cultivation, wood and ore extraction, and the formation and expansion of pastures and road networks. The process of deforestation in the region was facilitated by the relatively warm climates of the Medieval Warm Period.

This study investigates the spatio-temporal postglacial development of the vegetation in the northwestern part of the forest-tundra of northern Quebec, Canada. Six lake sediment cores and 59 modern sediment samples were collected in an area of 16 by 7 km and analyzed for pollen and charcoal. Following a brief herb and shrub tundra period and a period with abundant Alnus, Picea mariana grew in the region for at least the past 4000 years, but there was a gradient across the area, with Picea growing to the west but not toward the east. The period of maximum Picea abundance varied between sites, but was generally between 1500 and 2500 years ago. Neoglacial cooling resulted in a decrease in Picea on the landscape, but the timing of the decline at any site depended on removal of Picea by fire. There was a gradient in Picea abundance across this study area, which resulted in a reconstructed spatial July temperature gradient of ∼1 °C across this 16 km region. In spite of the importance of fire in affecting local Picea populations around any lake, the paleoclimate reconstructions showed a long-term neoglacial cooling of ∼1 °C over the past 3800 cal yr BP, with superimposed century-scale fluctuations of ∼0.2 °C, corresponding to the Little Ice Age, the Medieval Warm Period, the Dark Ages Cold Period and the Roman Warm Period. High-resolution pollen records can therefore be used to reconstruct not only regional vegetation and climate history, but also local differences within small areas.

A high-resolution sediment record from Lac Brule, southwestern Quebec, was studied to determine the effects of long-term climate change and anthropogenic impacts on cladoceran assemblages of the late Holocene. Temporal shifts in cladoceran communities were closely associated with known century-scale climatic episodes, namely the Medieval Warm Period (750–1250 CE), the Little Ice Age (1450–1850 CE) and the twentieth-century warming. Forward selection indicated the importance of catchment variables and aquatic primary production in governing cladoceran communities. This suggests that higher trophic levels in Lac Brule were responsive to changes in the physical and chemical properties of the lake, perhaps as an indirect consequence of climatic change. The exploitation of the Wallingford-Back Mine in the immediate watershed of Lac Brule from 1924 to 1972 CE also had notable impacts on cladoceran assemblages. To strengthen interpretations of the Lac Brule record, relationships between cladoceran assemblages and limnological variables were studied in surface samples from 31 lakes from the surrounding region. This spatial analysis identified nutrients (phosphorus and nitrogen) and pH as the most influential variables driving changes in present-day cladoceran communities. However, the performance of cladocera-based inference models was insufficient to quantitatively reconstruct variables in the down-core analysis of Lac Brule.

•A synthesis of existing literature on Holocene climate change in Arctic Canada and Greenland.•Warmest-to-coldest millennium temperature change in the Holocene in Arctic Canada and Greenland is 3.0 ± 1.0 °C.•Thermal maximum conditions occurred earlier in high latitudes and at sites less influenced by North Atlantic circulation.

Reconstructing climates of the past relies on a variety of evidence from a large number of sites to capture the varied features of climate and the spatial heterogeneity of climate change. This review summarizes available information from diverse Holocene paleoenvironmental records across eastern Beringia (Alaska, westernmost Canada and adjacent seas), and it quantifies the primary trends of temperature- and moisture-sensitive records based in part on midges, pollen, and biogeochemical indicators (compiled in the recently published Arctic Holocene database, and updated here to v2.1). The composite time series from these proxy records are compared with new summaries of mountain-glacier and lake-level fluctuations, terrestrial water-isotope records, sea-ice and sea-surface-temperature analyses, and peatland and thaw-lake initiation frequencies to clarify multi-centennial- to millennial-scale trends in Holocene climate change. To focus the synthesis, the paleo data are used to frame specific questions that can be addressed with simulations by Earth system models to investigate the causes and dynamics of past and future climate change. This systematic review shows that, during the early Holocene (11.7–8.2 ka; 1 ka = 1000 cal yr BP), rather than a prominent thermal maximum as suggested previously, temperatures were highly variable, at times both higher and lower than present (approximate mid-20th-century average), with no clear spatial pattern. Composited pollen, midge and other proxy records average out the variability and show the overall lowest summer and mean-annual temperatures across the study region during the earliest Holocene, followed by warming over the early Holocene. The sparse data available on early Holocene glaciation show that glaciers in southern Alaska were as extensive then as they were during the late Holocene. Early Holocene lake levels were low in interior Alaska, but moisture indicators show pronounced differences across the region. The highest frequency of both peatland and thaw-lake initiation ages also occurred during the early Holocene. During the middle Holocene (8.2–4.2 ka), glaciers retreated as the regional average temperature increased to a maximum between 7 and 5 ka, as reflected in most proxy types. Following the middle Holocene thermal maximum, temperatures decreased starting between 4 and 3 ka, signaling the onset of Neoglacial cooling. Glaciers in the Brooks and Alaska Ranges advanced to their maximum Holocene extent as lakes generally rose to modern levels. Temperature differences for averaged 500-year time steps typically ranged by 1–2 °C for individual records in the Arctic Holocene database, with a transition to a cooler late Holocene that was neither abrupt nor spatially coherent. The longest and highest-resolution terrestrial water isotope records previously interpreted to represent changes in the Aleutian low-pressure system around this time are here shown to be largely contradictory. Furthermore, there are too few records with sufficient resolution to identify sub-centennial-scale climate anomalies, such as the 8.2 ka event. The review concludes by suggesting some priorities for future paleoclimate research in the region.

Archaeological radiocarbon (14C) dates are a fundamental source of information documenting patterns in paleodemographic change from prehistoric to modern times. Several open access databases (Canadian Archaeological Radiocarbon Database, Radiocarbon Palaeolithic Europe Database, CONTEXT, RAdiocarbon Dates ONline, and AustArch) and publications which include lists of dates provide easy access to archaeological 14C data, presently totalling over 70,000 dates worldwide. Some parts of the world are more extensively sampled than others including North America, Australia and China, whereas in others the databases have not yet been prepared. A comparison of frequency distributions of 14C dates from North America and Australia to modeled estimates of historical population growth for these continents from the HYDE 3.1 database shows similarities, providing confidence in long-term estimates of population growth using both methods. Our capacity to study global demographic change is currently limited by the spatiotemporal completeness of regional 14C databases. These results suggest the systematic collection and entry of dates into an openly-accessible, global 14C database will allow for significant advances to be made in archaeology, anthropology and Quaternary paleogeography.

A study of chironomid remains in the sediments of Lake JR01 on the Boothia Peninsula in the Central Canadian Arctic provides a high-resolution record of mean July air temperatures for the last 6.9 ka. Diatom and pollen studies have previously been published from this core. Peak Holocene temperatures occurred prior to 5.0 ka, a time when overall aquatic and terrestrial biological production was high. Chironomid-inferred summer air temperatures reached up to 7.5°C during this period. The region of Lake JR01 cooled over the mid- to late-Holocene, with high biological production between 6.1 and 5.4 ka. Biological production decreased again at ∼2 ka and the rate of cooling increased in the past 2 ka, with coolest temperatures occurring between 0.46 and 0.36 ka, coinciding with the Little Ice Age. Although biological production increased in the last 150 yr, the reconstructed temperatures do not indicate a warming during this time. During transitions, either warming or cooling, chironomid production increases, suggesting an ecosystem-level response to climate variability, seen at a number of lakes across the Arctic.

Human paleodemographic changes were related to environmental fluctuations for the North American Arctic and boreal region, including Greenland. Using the frequency of archaeological radiocarbon dates as a proxy for population size, past changes in population were estimated and quantitatively examined in relation to reconstructions of temperature and sea ice conditions. This analysis was conducted across three spatial scales in order to better identify potential climate impacts on population size: the entire area, four major cultural-environmental regions and sixteen sub-regions. The timing of initial settlement differed by region, and population size fluctuated through time, but there was nevertheless an overall, accelerating increase in most areas. There was a high correspondence between millennial and centennial-scale climate variability and paleodemographic changes across the region, with population size generally increasing during warmer periods and decreasing during cooling episodes. Late Holocene cooling (neoglaciation) triggered a nearly-synchronous population decline across the entire region at 3.9 ka, a time when significant societal disruptions have been identified around the world, and are here shown in the Arctic. Decreasing temperatures and increased sea ice coverage also influenced large-scale migration patterns of Paleo-Inuit peoples as well as their cultural evolution. The growth of Paleo-Inuit populations after 3 ka during continued climate cooling is consistent with archaeological evidence suggesting Paleo-Inuit technological change enabled adaptations to increased sea ice.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Thermokarst lakes cover nearly one fourth of ice-rich permafrost lowlands in the Arctic. Sediments from an athalassic subsaline thermokarst lake on Herschel Island (69°36′N; 139°04′W, Canadian Arctic) were used to understand regional changes in climate and in sediment transport, hydrology, nutrient availability and permafrost disturbance. The sediment record spans the last ~ 11,700 years and the basal date is in good agreement with the Holocene onset of thermokarst initiation in the region. Electrical conductivity in pore water continuously decreases, thus indicating desalinization and continuous increase of lake size and water level. The inc/coh ratio of XRF scans provides a high-resolution organic-carbon proxy which correlates with TOC measurements. XRF-derived Mn/Fe ratios indicate aerobic versus anaerobic conditions which moderate the preservation potential of organic matter in lake sediments. The coexistence of marine, brackish and freshwater ostracods and foraminifera is explained by (1) oligohaline to mesohaline water chemistry of the past lake and (2) redeposition of Pleistocene specimens found within upthrusted marine sediments around the lake. Episodes of catchment disturbance are identified when calcareous fossils and allochthonous material were transported into the lake by thermokarst processes such as active-layer detachments, slumping and erosion of ice-rich shores. The pollen record does not show major variations and the pollen-based climate record does not match well with other summer air temperature reconstructions from this region. Local vegetation patterns in small catchments are strongly linked to morphology and sub-surface permafrost conditions rather than to climate. Multidisciplinary studies can identify the onset and life cycle of thermokarst lakes as they play a crucial role in Arctic freshwater ecosystems and in the global carbon cycle of the past, present and future.

This paper summarizes current knowledge about the postglacial history of the vegetation of the Canadian Arctic Archipelago (CAA) and Greenland. Available pollen data were used to understand the initial migration of taxa across the Arctic, how the plant biodiversity responded to Holocene climate variability, and how past climate variability affected primary production of the vegetation. Current evidence suggests that most of the flora arrived in the area during the Holocene from Europe or refugia south or west of the region immediately after local deglaciation, indicating rapid dispersal of propagules to the region from distant sources. There is some evidence of shrub species arriving later in Greenland, but it is not clear if this is dispersal limited or a response to past climates. Subsequent climate variability had little effect on biodiversity across the CAA, with some evidence of local extinctions in areas of Greenland in the late Holocene. The most significant impact of climate changes is on vegetation density and/or plant production.

We examine the influence of multiple environmental factors on quantitative reconstructions of past climate that are based on conventional transfer function approaches using subfossil midge remains preserved in lake sediments. Chironomid assemblages from the uppermost sediments of 366 lakes spanning northern North America were compared to environmental parameters using direct gradient analysis. While temperature expectedly explained the largest amount of variation in the chironomid assemblages, dissolved organic carbon (DOC) and total Kjeldahl nitrogen (TKN) accounted for significant amounts of the remaining variance. By constraining chironomid assemblages to mean-July air temperature (JulyT), we found a cluster of 70 lakes that were orthogonal to the primary temperature gradient in ordination. These lakes tended to be cold with higher DOC and TKN concentrations, yet had chironomid assemblages that were similar to the assemblages found in less productive, warmer lakes. In order to examine how secondary gradients affected paleotemperature reconstructions, three chironomid-based transfer function models were generated: a full dataset model (all 366 lakes), a reduced temperature-constrained model (296 lakes), and a reduced secondary-constrained model (70 lakes). Application of these models to four previously published Holocene paleotemperature reconstructions were used to distinguish the influence of temperature versus secondary-gradient effects. While unproductive Arctic lake paleotemperature reconstructions were found to be robust, reconstructions for two lakes in the boreal-tundra ecotone suggest secondary gradients influenced parts of their records. We recommend that passive core trajectory analysis upon our expansive surface-sediment calibration set provides a means to assess the veracity of paleotemperature reconstructions and potential influence of secondary gradients.

Pollen profiles from two lakes, Tawny Pond (44°48′59″N, 77°10′54″W, 276 m) and Stoll Lake (44°58′16″N, 77°17′22″W,303 m) in Addington Highlands, eastern Ontario, Canada were analyzed to determine the effects of late-Holocene climate change and European settlement on eastern Ontario's forests. Both lakes were analyzed at high temporal resolution and record vegetation dynamics over the last 1000 years. Throughout the past 1000 years, Pinus , Tsuga , Betula , Quercus , Acer and Fagus were the dominant taxa in the pollen record. From 950–1550 AD the forest was dominated by Tsuga , Fagus and Acer . Between 1550 and 1730 AD Pinus and boreal tree species became more abundant and/or had increased relative pollen production. Low pollen influx to the sediment, high values of Pinus and relatively low values of Tsuga and hardwoods between 1730 and 1870 AD suggest colder temperatures. Since 1870 AD, herbaceous plants (weeds) increased in abundance, whereas softwoods decreased and hardwoods increased, due to landscape changes associated with European settlement. A comparison of the temperature reconstruction based on the pollen assemblages and the Toronto and Ottawa climate records shows a close correspondence, suggesting pollen records can record decadal-scale climate fluctuations.

Chironomid assemblages from the uppermost sediments of 435 lakes spanning northern North America were compared to environmental parameters using direct gradient analysis. This large calibration set was merged from several previously developed regional datasets, and increases the number of modern analogues that are available for use for paleoenvironmental interpretations in this region. Air temperature explained the largest amount of variation in the chironomid assemblages with several other environmental factors accounting for statistical significant amounts of the remaining variance. A robust inference model for deriving past mean July air temperatures from subfossil chironomid assemblages was developed and applied to previously published paleoclimate reconstructions from the High-Arctic, Middle-Arctic, Boreal treeline, and Alpine regions of northern North America. The patterns of the temperature reconstructions from the combined dataset were generally similar to the original reconstructions, but with colder inferred temperatures reflecting the incorporation of a larger number of modern sites from colder climates in the combined dataset. This analysis demonstrated that the larger temperature gradient available in the new training set, when compared to the temperature gradients in the original training sets, provides a better estimation of chironomid-environment relationships. In particular, the optima and tolerances estimated using the larger, combined dataset should be more accurate, and therefore, improve midge-based paleoclimate reconstructions for northern North America. Despite the much larger spatial scale and greater associated environmental heterogeneity now incorporated in the combined dataset, this study suggests that in most cases the overarching constraint governing chironomid distributions in northern North America is temperature.

A high-resolution diatom analysis of a varved sediment sequence from Lac Brule, southwestern Quebec, was used to study temporal patterns of environmental change in the late Holocene. Key periods of interest in the record included the Medieval Warm Period (~800–1300 CE), the Little Ice Age (~1450–1850 CE), and post-European settlement (~1850–present). Subfossil diatom assemblages were compared to previously published pollen, cladocera, and sediment records from Lac Brule, revealing complex dynamics between terrestrial vegetation succession, nutrient fluxes, and trophic interactions. Generalized additive models showed a response to long-term climate variability in the diatom record, although it was not the most influential driver of community changes at Lac Brule. Catchment-mediated processes instead played the largest role in governing the structure of diatom assemblages in the lake. For example, nutrient loading following a local fire in the watershed at 1345 CE led to an abrupt and significant increase in Fragilaria spp. Human activity associated with deforestation and the Wallingford-Back Mine (1924–1972 CE) also had strong impacts on the landscape, which triggered further responses in the aquatic communities of Lac Brule.

Along with Arctic amplification, changes in Arctic hydroclimate have become increasingly apparent. Reanalysis data show increasing trends in Arctic temperature and precipitation over the 20th century, but changes are not homogenous across seasons or space. The observed hydroclimate changes are expected to continue, and possibly accelerate, in the coming century, not only affecting pan-Arctic natural ecosystems and human activities, but also lower latitudes through changes in atmospheric and oceanic circulation. However, a lack of spatiotemporal observational data makes reliable quantification of Arctic hydroclimate change difficult, especially in a long-term context. To understand hydroclimate variability and the mechanisms driving observed changes, beyond the instrumental record, climate proxies are needed. Here we bring together the current understanding of Arctic hydroclimate during the past 2000 years, as inferred from natural archives and proxies and palaeoclimate model simulations. Inadequate proxy data coverage is apparent, with distinct data gaps in most of Eurasia and parts of North America, which makes robust assessments for the whole Arctic currently impossible. Hydroclimate proxies and climate models indicate that the Medieval Climate Anomaly (MCA) was anomalously wet, while conditions were in general drier during the Little Ice Age (LIA), relative to the last 2000 years. However, it is clear that there are large regional differences, which are especially evident during the LIA. Due to the spatiotemporal differences in Arctic hydroclimate, we recommend detailed regional studies, e.g. including field reconstructions, to disentangle spatial patterns and potential forcing factors. At present, it is only possible to carry out regional syntheses for a few areas of the Arctic, e.g. Fennoscandia, Greenland and western North America. To fully assess pan-Arctic hydroclimate variability for the last two millennia additional proxy records are required.

High-elevation ecosystems, such as those on Mount Kenya are undergoing significant changes, with accelerated glacial ice losses over the twentieth century creating new space for alpine plants to establish. These ecosystems respond rapidly to climatic variability and within decades of glacial retreat, Afroalpine pioneering taxa stabilize barren land and facilitate soil development, promoting complex patches of alpine vegetation. Periglacial lake sediment records can be used to examine centennial and millennial scale variations in alpine and montane vegetation compositions. Here we present a 5300-year composite pollen record from an alpine tarn (4370 m asl) in the Hausberg Valley of Mount Kenya. Overall, the record shows little apparent variation in the pollen assemblage through time with abundant montane forest taxa derived and transported from mid elevations, notably high abundances of aerophilous Podocarpus pollen. Afroalpine taxa included Alchemilla, Helichrysum and Dendrosenecio-type, reflecting local vegetation cover. Pollen from the ericaceous zone was present throughout the record and Poaceae percentages were high, similar to other high elevation pollen records from eastern Africa. The Oblong Tarn record pollen assemblage composition and abundances of Podocarpus and Poaceae since the late Holocene (similar to 4000 cal yr BP-present) are similar to pollen records from mid-to-high elevation sites of nearby high mountains such as Mount Elgon and Kilimanjaro. These results suggest a significant amount of uphill pollen transport with only minor apparent variation in local taxa. Slight decreasing trends in alpine and ericaceous taxonomic groups show a long-term response to global late Holocene cooling and a step decrease in rate of change estimated from the pollen assemblages at 3100 cal yr BP in response to regional hydroclimatic variability. Changes in the principal component axis scores of the pollen assemblage were coherent with an independent mid-elevation temperature reconstruction, which supported the strong influence of uphill pollen transport from montane forest vegetation and association between temperatures and montane vegetation dynamics. Pollen accumulation rates showed some variability related to minerogenic sediment input to the lake. The Oblong Tarn pollen record provides an indication of long term vegetation change atop Mount Kenya showing some decreases in local alpine and ericaceous taxa from 5300-3100 cal yr BP and minor centennial-scale variability of montane taxa from mid elevation forests. The record highlights potentials, challenges and opportunities for the use of proglacial lacustrine sediment to examine vegetation change on prominent mountain massifs.

Reproducible climate reconstructions of the Common Era (1 CE to present) are key to placing industrial-era warming into the context of natural climatic variability. Here we present a community-sourced database of temperature-sensitive proxy records from the PAGES2k initiative. The database gathers 692 records from 648 locations, including all continental regions and major ocean basins. The records are from trees, ice, sediment, corals, speleothems, documentary evidence, and other archives. They range in length from 50 to 2000 years, with a median of 547 years, while temporal resolution ranges from biweekly to centennial. Nearly half of the proxy time series are significantly correlated with HadCRUT4.2 surface temperature over the period 1850-2014. Global temperature composites show a remarkable degree of coherence between high- and low-resolution archives, with broadly similar patterns across archive types, terrestrial versus marine locations, and screening criteria. The database is suited to investigations of global and regional temperature variability over the Common Era, and is shared in the Linked Paleo Data (LiPD) format, including serializations in Matlab, R and Python., PAGES, a core project of Future Earth, is supported by the U.S. and Swiss National Science Foundations. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Some of this work was conducted as part of the North America 2k Working Group supported by the John Wesley Powell Center for Analysis and Synthesis, funded by the U.S. Geological Survey. B. Bauer, W. Gross, and E. Gille (NOAA National Centers for Environmental Information) are gratefully acknowledged for helping assemble the data citations and creating the NCEI versions of the PAGES 2k data records. We thank all the investigators whose commitment to data sharing enables the open science ethos embodied by this project.