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Holocene climate change in Arctic Canada and Greenland

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... Ice sheet reconstructions based on indicators of past ice extent and relative sea level data indicate that the GrIS was substantially larger during the onset of the last deglaciation (18-16 kiloyear before present, hereafter referred to as ka) than present [3][4][5][6]. In contrast, the GrIS reached a minimum extent at around 6-4 ka after a period of warm climate called the Holocene Thermal Maximum (8 ka) [5,[7][8][9][10][11][12][13][14][15]. Besides these direct geologically based reconstructions, several modelling efforts on the paleo GrIS have been conducted by using forcing constrained by paleo proxies of temperature, ice extent and relative sea level [16][17][18][19]. ...
... By constraining the ice sheet model using field observations of relative sea level and ice extent, previous studies [5,16,17] propose that the GrIS reached its maximum size around 18-16 ka. Paleo proxies of temperature, ice extent and ice elevation [8,11,[81][82][83]87] combined with ice sheet model simulations [16][17][18] reveal that the GrIS retreated to a minimum size most likely between 7-4 ka. ...
... Paleoclimate proxies [7,11,62,80,82,[95][96][97] indicate that the Northern Hemisphere summer temperature was warmer during the Holocene than the Pre-Industrial era, primarily due to the fact that the perihelion occurred during the boreal summer. This would logically drive the retreat of the GrIS. ...
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Using transient climate forcing based on simulations from the Alfred Wegener Institute Earth System Model (AWI-ESM), we simulate the evolution of the Greenland Ice Sheet (GrIS) from the last interglacial (125 ka, kiloyear before present) to 2100 AD with the Parallel Ice Sheet Model (PISM). The impact of paleoclimate, especially Holocene climate, on the present and future evolution of the GrIS is explored. Our simulations of the past show close agreement with reconstructions with respect to the recent timing of the peaks in ice volume and the climate of Greenland. The maximum and minimum ice volume at around 18-17 ka and 6-5 ka lag the respective extremes in climate by several thousand years, implying that the ice volume response of the GrIS strongly lags climatic changes. Given that Greenland's climate was getting colder from the Holocene Thermal Maximum (i.e., 8 ka) to the Pre-Industrial era, our simulation implies that the GrIS experienced growth from the mid-Holo-cene to the industrial era. Due to this background trend, the GrIS still gains mass until the second half of the 20th century, even though anthropogenic warming begins around 1850 AD. This is also in agreement with observational evidence showing mass loss of the GrIS does not begin earlier than the late 20th century. Our results highlight that the present evolution of the GrIS is not only controlled by the recent climate changes, but is also affected by paleoclimate, especially the relatively warm Holocene climate. We propose that the GrIS was not in equilibrium throughout the entire Holocene and that the slow response to Holo-cene climate needs to be represented in ice sheet simulations in order to predict ice mass loss, and therefore sea level rise, accurately.
... Both climate and Greenland's ice-sheet geometry have varied substantially since the Last Glacial Maximum (Lecavalier and others, 2014). In the Kangerlussuaq sector, the GrIS was advanced ∼100 km west of its current boundary, reflecting a colder historical climate (Lecavalier and others, 2014;Briner and others, 2016;Young and others, 2020). While the ice formed at the divide over the Holocene period has yet to flow across the profile from divide-to-margin (Dahl-Jensen and others, 1998), the ice thickness and surface slope of the ice sheet have changed commensurately with the 25% reduction in extent that has taken place (Lecavalier and others, 2014). ...
... Because the outer flanks of western GrIS contain numerous deep and steep-sided subglacial troughs, we focus our investigation on a single flowline transect so that we can employ high time/space resolution in our model. Our transient model includes higher order stresses, a well-informed paleoclimate (Buizert and others, 2018;Downs and others, 2020), thermally active bedrock, and honors previously published records of terminus retreat (Briner and others, 2016;Lesnek and others, 2020;Young and others, 2020). We track the constitutive components of the basal heat budget during the transient simulation to isolate the energy sources and sinks associated with the migration of frozen/melted bed conditions. ...
... Detailed chronologies of Holocene ice margin retreat have been established for this sector of the ice sheet based on glacial geologic evidence with 10 Be and 14 C age dating (Briner and others, 2016;Lesnek and others, 2020;Young and others, 2020). The ice margin retreated most rapidly during the first half of the last 11 ka. ...
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The geometry and thermal structure of western Greenland ice sheet are known to have undergone relatively substantial change over the Holocene. Evolution of the frozen and melted fractions of the bed associated with the ice-sheet retreat over this time frame remains unclear. We address this question using a thermo-mechanically coupled flowline model to simulate a 11 ka period of ice-sheet retreat in west central Greenland. Results indicate an episode of ~100 km of terminus retreat corresponded to ~16 km of upstream frozen/melted basal boundary migration. The majority of migration of the frozen area is associated with the enhancement of the frictional and strain heating fields, which are accentuated toward the retreating ice margin. The thermally active bedrock layer acts as a heat sink, tending to slow contraction of frozen-bed conditions. Since the bedrock heat flux in our region is relatively low compared to other regions of the ice sheet, the frozen region is relatively greater and therefore more susceptible to marginward changes in the frictional and strain heating fields. Migration of melted regions thus depends on both geometric changes and the antecedent thermal state of the bedrock and ice, both of which vary considerably around the ice sheet.
... Joynt and Wolfe (2001) developed a diatom-based quantitative inference model from sedimentary assemblages collected in lakes located throughout eastern Baffin Island, and were able to reconstruct summer water temperatures covering the past 5000 cal a BP from sediment records preserved in Fog lake, on Baffin's eastern coast. Their results generally agreed with other independent paleoclimatic reconstructions for this region (Briner et al., 2016). ...
... The two diatom-based proxies used here to infer summer water temperatures, mean annual air temperatures (diatom assemblages) and hydroclimatic trends (δ 18 O diatom ) from ca. 5000 to 500 a BP, showed an overall cooling trend through the Holocene, in line with previous paleoclimatic studies from Baffin Island lakes (Briner et al., 2016). ...
Article
Sedimentary diatoms have been used to quantitatively reconstruct climate‐related variables, such as temperature at different timescales. Even though temperature is often less of a key driver of diatom ecology than other environmental parameters (water chemistry), diatom inference models have been shown to be reliable in deducing past temperature trends. In addition, the oxygen isotope composition (δ18Odiatom) preserved in buried diatom frustules has demonstrated its potential to reflect climatic and hydrological conditions at the time of frustule formation. This study combines results from both diatom‐based climate proxies to reconstruct summer water and mean annual air temperatures, and hydrological trends in Nettilling Lake, Baffin Island, from ca. 5000 to 500 cal a bp. Diatom‐inferred temperatures revealed an overall ca. 2 °C cooling throughout the Late‐Holocene. The δ18Odiatom values showed an increasing trend up to ca. 1900 cal a bp, where they reached their highest values (+24.8‰ at 15 cm) and thereafter decreased to their lowest values (+21.4‰ at 4 cm). These trends were linked to meltwater inflows associated with Penny Ice Cap thaw rate that was in turn controlled by regional climatic conditions which went from intensified cooling during the Neoglacial period to slight warming thereafter. Our results suggest that diatom‐ and diatom‐isotope‐based temperature and hydrological reconstructions can identify trends related to the natural climate system variability. The diatom oxygen isotopes are useful for paleoenvironmental studies of terrestrial aquatic ecosystems, but not for all hydrological systems are the ideal temperature proxy. Hence, the combination of proxies helps to disentangle temperature and hydrological effects for paleoclimatic reconstructions and may support future studies of postglacial environmental change in northern lakes.
... BP, when lacustrine conditions similar to those of today were established. While Narancic et al. (2016) documented its basin evolution during the regional mid to late Holocene warming trend, the present study provides additional insights into the lake's freshwater phase during late Holocene cooling and compares its results to other sedimentary records from southern Baffin Island (e.g., Briner et al. 2016). We generated the late Holocene pH reconstruction for Nettilling Lake using a diatom-based quantitative inference model developed by Joynt and Wolfe (2001) from sedimentary assemblages collected in 61 lakes distributed throughout eastern and southern Baffin Island. ...
... With the onset of intensified Neoglacial cooling ca. 2 000 yrs. cal. BP (Briner et al. 2016), Ti concentrations decreased together with PIC melt rates corresponding to slightly depleted isotopic trends. The Ti concentrations then culminated in the Little Ice Age, ca. ...
Article
The Arctic has warmed significantly over the past decades. However, the evolution of Arctic climate during the Holocene remains to be clarified in more detail, and regional factors controlling aquatic ecosystem evolution need to be better defined to grasp the sensitivity of lakes to rapid environmental change. Nettilling Lake was studied for changes in sedimentary diatom assemblages over the last 5 000 years. Lake water pH was reconstructed by applying a diatom-based lake water pH inference model. We hypothesized that the changes in diatom assemblages were driven by variations in lake water transparency and attendant water turbidity associated with the input of fine suspended solids from glacial meltwaters. Reduced underwater light resulted in greater abundance of planktonic over benthic taxa from ca. 5 000 to 3 000 yrs. cal. BP, followed by less turbid conditions and proliferation of benthic taxa during regional cooling. The lake water was slightly alkaline throughout the Holocene, ranging between pH 7.1 and 7.7. Our results support the notion that hydrological processes, dependent on climate variations, have a first-order influence on the regulation of the lake water pH through glacial meltwater inputs, which will likely continue to control the lake's long-term chemical and biological evolution. RÉSUMÉ
... This interpretation is in line with other records of perturbations of the North Atlantic circulation during the late Deglacial as an effect of the sustained meltwater input (Hoffman et al., 2012;Hoogakker et al., 2011;Olafsd ottir et al., 2010;Thornalley et al., 2013). In this case, especially the synchronous deglaciation of the Laurentide Ice Sheet (LIS) Carlson et al., 2008) and SW-GrIS (Briner et al., 2016;Larsen et al., 2014;Weidick et al., 2012;Young and Briner, 2015) in the earliest Holocene functioned as potential meltwater sources. ...
... During the mid/late Holocene transition, the significant fining of the SS, accompanied by a contemporaneous fining of the coarse mode and a slight decrease of EM2-AR between 3.3 and 2 ka clearly suggests a further slowdown of the WGC off SW Greenland. As this slowdown coincides with the onset of the Neoglaciation, i.e. the period defined by the cooling following the Holocene Thermal Maximum (Briner et al., 2016;Jennings et al., 2002;Reusche et al., 2014;Weidick et al., 2012), this phenomenon is now termed the Neoglacial Slowdown of the WGC. ...
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The hydrodynamics of the Labrador Sea, controlled by the complex interplay of oceanographic, atmospheric and ice-sheet processes, play a crucial role for the Atlantic Meridional Overturning Circulation (AMOC). An improved understanding of the hydrodynamics and its forcing in the past could therefore hold a key to understanding its future behaviour. At present, there is a remarkable temporal mismatch, in that the largely microfossil-based reconstructions of Holocene Atlantic-water inflow/influence in the Labrador Sea and Baffin Bay appear to lag grain size-based current strength reconstructions from the adjacent North Atlantic by > 2ka. Here, we present the first current strength record from the West Greenland shelf off Nuuk to reconstruct Atlantic Water (AW)-inflow to the Labrador Sea via the West Greenland Current. Our data show that the Holocene AW-inflow into Labrador Sea is well aligned with the Holocene Speed Maximum documented in the North Atlantic (McCave and Andrews, 2019; Quat. Sci. Rev. 223), suggesting a close coupling with the AMOC. The observed lag between the microfossil-based records and the Holocene Speed Maximum can be explained when considering the presence of an extended meltwater lens that prevented the shoaling of the inflowing Atlantic waters. Once the melt-water discharge waned after the cessation of large-scale melting of the surrounding ice sheets, the AW could influence the surface waters, independently of the strength of its inflow. Only then was an effective ocean-atmosphere heat transfer enabled, triggering the comparably late onset of the regional Holocene Thermal Maximum. Furthermore, sediment geochemical analyses show that short term cooling events, such as the 8.2 ka event related to the final drainage of glacial Lake Agassiz, lead to glacier advances of the Greenland Ice Sheet. Since the grain size data show that these events had no influence on the AW-inflow to the north eastern Labrador Sea, these advances must have been caused by atmospheric cooling. Consequently, we argue that (i) in this region, surface water-based proxies register AW influence rather than inflow (ii) the AW inflow into the Labrador Sea is controlled by the AMOC, but (iii) its impact on an effective ocean-atmosphere heat transfer was hindered by a prevailing meltwater lens in the early Ho-locene, i.e. until the cessation of large-scale melting of the surrounding ice sheets.
... In contrast, POC from Zackenberg mainstem exhibited more invariant Δ 14 C values, ranging between −514 and −318‰. These 14 C ages (5,700-3,000 years) likely reflect the composite of OC inputs from different sources and ages, yet the overall interval corresponds to the Holocene thermal maximum in eastern Greenland (Briner et al., 2016), an interval when the Greenland icecap and the A.P. Olsen glacier are thought to have retreated, allowing for plant and soil development in locations currently covered by glaciers (Bhatia et al., 2013). We therefore suggest that Zackenberg river, fed mainly by subglacial flow from the A.P. Olsen glacier, currently exports POC comprising a mixture of aged soil organic matter, in-glacier necromass from present and past production (i.e., bacteria, ice algae) and deposited organic aerosols (Bhatia et al., 2013). ...
Article
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On‐going shrinkage of Greenland's icecap, permafrost thaw, and changes in precipitation are exposing its landscapes to erosion and remobilization of ancient organic carbon (OC) held in soils and sedimentary rocks. The fate of this OC and potential feedbacks to climate are still unclear. Here, we show that the glacial Zackenberg river (Northeastern Greenland) exports aged particulate OC (POC, uncalibrated radiocarbon ages of ∼4,000 years). Many of the smaller periglacial streams affected by abrupt permafrost thaw transport substantially older POC (up to 32,000 years), especially with enhanced discharge following intense precipitation. Mineralogical analysis, and density and size fractionation of soils and both glacial and nonglacial river sediments reveal that OC is largely associated with phyllosilicate minerals, suggesting stabilization against microbial processing. Enhanced export of ancient, mineral‐associated OC as a consequence of summer rainfall may accelerate translocation of OC from terrestrial to marine environments, but could have limited consequences for climate.
... Greenland (Kjaer et al., 2022) and Arctic Canada (Briner et al., 2016). ...
Article
Quantifying past changes in erosion rate is essential for deciphering Earth surface processes and their driving mechanisms. Coupled in situ cosmogenic 14C‐10Be has been shown to be a useful chronometer in detecting changes in erosion rate, but its detection ability is limited to rapidly eroding fluvial landscapes and changes greater than a factor of two; and in any case, it cannot uniquely quantify both the magnitude and the timing of change in erosion rate. Here, we establish a theoretical framework to show that this limitation can be overcome by combining the recently developed optically stimulated luminescence (OSL) rock surface exposure dating with the 14C‐10Be chronometer. We demonstrate that the resulting OSL‐14C‐10Be composite geochronometer can determine both the magnitude and the timing of an abrupt change in bedrock erosion rate by a factor of < 0.8 or > 1.2 over 100‐104 a timescales, where erosion rates are < 10‐2 cm.a‐1.
... Was the mid-Holocene expansion of ice caps driven predominantly by cooling or precipitation increases? The timing of regional thermal maxima is difficult to establish, not only due to the paucity of records from NW Greenland, but also local paleoclimate proxy bias near the ice margin (Axford et al., 2021;Briner et al., 2016). ...
Article
Relative Sea Levels (RSLs) derived primarily from marine bivalves near Petermann Glacier, NW Greenland, constrain past regional ice-mass changes through glacial isostatic adjustment (GIA) modeling. Oxygen isotopes measured on bivalves corrected for shell-depth habitat and document changing meltwater input. Rapid RSL fall of up to 62 m/kyr indicates ice loss at or prior to ∼9 ka. Transition to an RSL stillstand starting at ∼6 ka reflects renewed ice-mass loading followed by further mass loss over the past few millennia. GIA simulations of rapid early RSL fall suggest a low regional upper-mantle viscosity. Early loss of grounded ice tracks atmospheric warming and pre-dates the eventual collapse of Petermann Glacier's floating ice tongue near ∼7 ka, suggesting grounding zone stabilization during early phases of deglaciation. We hypothesize mid-Holocene regrowth of regional ice caps in response to cooling and increased precipitation, following loss of the floating shelf ice. Remnants of these ice caps remain present but are now melting.
... Reconstructions of their past fluctuations have provided valuable insights into climate conditions prior to instrumental records (Oerlemans, 2005;Joerin et al., 2008). Available evidence from glacier fluctuations and other paleoclimate proxies in Greenland generally demonstrates large spatial and temporal heterogeneity in climatic shifts throughout the Holocene (Dahl-Jensen et al., 1998;Kaufman et al., 2004;Kelly and Lowell, 2009;Kobashi et al., 2011Kobashi et al., , 2017Briner et al., 2016;McKay et al., 2018;Axford et al., 2021;Larocca and Axford, 2022), highlighting a complex response to changes in Northern Hemisphere summer insolation and to other regional and/or sub-millennial-scale forcings. Studies comparing temperature inferred from alpine glacier variations and independent instrumental or proxy temperature records show close agreement Axford et al., 2021). ...
Article
Alpine glaciers are sensitive responders to climate fluctuations, especially to changes in summer temperature, and thus their past extents are invaluable indicators of past climate. Here, we leverage geomorphological evidence of past glacier extent to reconstruct the three-dimensional surfaces of 42 land-terminating paleoglaciers in southernmost Greenland and calculate their equilibrium-line altitudes (ELAs) during the presumed coldest interval of the Little Ice Age (LIA). We compare LIA paleoglacier ELAs with observed modern glacier ELAs (i.e. snowlines) to estimate the LIA summer temperature depression relative to present. On average, we find that ELAs rose 122 ± 64 m since the LIA, which corresponds to a summer temperature change of 1.1± 0.6 ° C, assuming no change in precipitation. Because there are minimal constraints on LIA precipitation anomalies in southernmost Greenland, we also report summer temperature changes for precipitation values ranging from a substantially drier to substantially wetter than modern LIA. When compared with independent temperature evidence, our results suggest LIA precipitation was similar to or slightly less than today. The alpine glaciers studied lost (on average) 56± 16% of their area and 44± 13% of their centerline length between the LIA culmination and 2019 CE. In general, smaller and lower elevation glaciers lost larger fractions of their area. We find no clear geographic patterns in the magnitudes of ELA, area, or length changes across the 9000 km² study area despite its heterogeneous climate. This study demonstrates a workflow for regional-scale paleoglacier reconstruction and provides the first quantitative LIA summer paleotemperature estimate for southernmost Greenland via region-wide glacial evidence. Our work also suggests the need for reconstruction of a large number of glaciers to infer past regional climatic information, as individual glaciers may respond heterogeneously to climate perturbations leading to variable changes in their ELA and length.
... In a review of the early Holocene Thermal Maximum (HTM, also called the Holocene Climatic Optimum, ~ 9 to 5 ka), in Greenland and adjacent regions, Axford et al. (2021) concluded that insolation forcing led to HTM atmospheric temperatures that were 3-5 • C warmer than pre-industrial temperatures, with the largest warming in the northern part of Greenland. In the Northern Hemisphere, boreal summer insolation peaked about 11 ka during the last phase of the final deglaciation (Laskar et al., 2004;Briner et al., 2016). However, the response of Greenland and adjacent regions to insolation is complex such that the timing of the atmospheric, oceanic, and ice sheet response varied due to regional factors, albedo feedbacks, and the oceanic impact of fresh water from ice melt (Vasskog et al., 2015). ...
Article
We reconstructed Holocene paleoceanography of the Sherard Osborn Fjord (SOF), N Greenland, and Lincoln Sea in the eastern Arctic Ocean using sediment properties and micropaleontology from cores obtained during the Ryder 2019 Expedition. Our aims were to better understand faunal indicators of water mass influence on Ryder Glacier and the Lincoln Sea at water depths >500 m. Benthic microfaunal reflect glacio-marine interval during late deglaciation ~10.5 to 8.5 ka (kiloannum) during the Holocene Thermal Maximum (HTM) with dominant benthic foraminiferal species Cassidulina neoteretis, Cassidulina reniforme, and the ostracode Rabilimis mirabilis. Casssidulina neoteretis is considered an indicator of Atlantic Water (AW) throughout the Arctic Ocean and Nordic Seas; C. reniforme reflects glacio-marine conditions from the retreating Ryder Glacier. Deglaciation was followed by a period of elevated productivity and diverse ostracode faunal assemblages that suggest AW influence from 8.5 to 6 ka in the Lincoln Sea and inside SOF. The Holocene occurrence of the ostracode species Acetabulastoma arcticum, that appears in low numbers in the Lincoln Sea and briefly (~ 4–3 ka) in SOF, reflects the presence of variable sea ice in this region. Based on the similarities of the Lincoln Sea and fjord ostracodes to modern and glacial-deglacial faunas from the central Arctic Ocean, the AW influence likely originates from recirculation of AW water from the central Arctic Basin. In general, our results suggest a strong but temporally varying influence of AW during the entire 10.5 kyr record of the Lincoln Sea and SOF.
... The Northern Hemisphere summer insolation peaked at ca. 11 ka, followed by a steady decrease to the current level of 9% less than the early Holocene maximum (Fig. 5;Berger & Loutre 1991). Proxy-based reconstructions from the Northern Hemisphere show, however, that the timing and magnitude of the warming differ between regions, suggesting that the early Holocene climate was highly sensitive to small changes in ice-sheet configuration (Kaufman et al. 2004;Renssen et al. 2009;Briner et al. 2016). Sweden is no exception, and most proxy-based reconstructions show that the highest temperatures were reached here some 3000-4000 years into the Holocene ( Fig. 5; e.g., Seppä et al. 2005). ...
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This paper presents a review on more than hundred years of palaeoenvironmental research in Sweden; from early descriptions of peat and tufa deposits in the late nineteenth and early twentieth centuries to multiproxy transfer function studies in the late twentieth and early twenty-first centuries CE. Research on Holocene climate variability has a long history in Sweden and many ideas and concepts about changes in temperature and precipitation during the Holocene originated in Fennoscandia. The Holocene climate evolution in Sweden follows a pattern in common for many northern latitude records with a rapid warming starting at the Pleistocene–Holocene boundary at c. 11 650 cal a BP, followed by the middle Holocene thermal maximum between c. 8000 and 5000 cal a BP. A change to colder and wetter conditions starts c. 4000 cal a BP and lasts until the late 1800 s CE. There is evidence for climatic anomalies such as the 8.2 and 4.2 ka BP events and the Little Ice Age (LIA) but only inconclusive evidence for other events, such as the 10.3 ka BP event. The main pattern of Holocene climate and environmental evolution is well known for most parts of Sweden, but the present review also shows that several research questions remain to be addressed.
... If this is the case, then an increase in the water content of the river ~4070 ka looks quite probable as this time corresponds to the onset of frequent events of cold and snowy winter seasons of the Neoglacial period and there is much paleoclimatic evidence indicating cooling and higher humidification in the North Atlantic for a time period of ~4.4-4.0 ka (Le Roy et al., 2017;Persoiu et al., 2019), the advance of glaciers in the Southeast Altai mountains ~4.9-4.2 ka (Agatova et al., 2012), and the neoglacial growth of the Greenland Ice Sheet (Briner et al., 2016). In the East Sayan mountains, the accumulation of snow could cause severe spring floods, which increased the water content of rivers and eroded the lowland peat beds. ...
Article
—Bottom sediments of lakes are regarded as one of the most informative continental facies used to reconstruct environmental changes in lake catchments. They usually contain a high-resolution and continuous record of geologic events, the knowledge of which is important for understanding the evolution of sedimentary processes and landscapes under the influence of climatic changes and the impact of short-lived catastrophic processes. The southwestern Baikal region (the Tunka rift and its East Sayan mountain framing) is replete with lakes of different geneses and morphologies. Recent fieldworks have allowed a multiproxy study of the lacustrine sediments of several lakes of the Oka and Tunka regions. The results demonstrate that there were no glaciers in the foothill plain of the Tunka rift valley earlier than 14,800 cal yr BP. On the Oka plateau, within the East Sayan high-mountain zone, deglaciation started somewhat earlier than 13,800 cal yr BP. The grain size, petromagnetic, and physical properties of the lacustrine sediments point to a general weak hydrodynamics throughout the 13,800 yr sedimentation history in Lake Khikushka. However, a gradual reduction in fine fractions from base to top of the lake section suggests a gradual drawdown of the water level following the change of the water source from glacial melt waters to atmospheric precipitation. Biogenic silt accumulation in Lake Khikushka started parallel with the onset of the Holocene ~11,700 yr BP, suggesting a quick response of the natural environment of the lake basin to major global climatic changes. The short-term episodes of enhanced hydrodynamics in Lake Khikushka ~9000, 8200, 6500, and 2000 yr BP confirm the high sensitivity of the lake geosystem to climatic changes, not only on a millennial but also on a shorter-time scale. The characteristics of the sediments of Lake Engarginskoe are evident of lacustrine-alluvial sedimentation in the reservoir in the Early and Middle Holocene. New data on the grain size and petromagnetic parameters of the lacustrine sediments from two lakes located within different climatic belts of East Sayan show their high potential for a wide range of paleoecological reconstructions.
... To predict future sea level rise and other consequences of accelerated GrIS melting, an accurate monitoring of modern ice sheet mass balance as well as a better comprehension of GrIS past dynamics are essential. Geomorphology 401 (2022) 108125 Over the last several decades, ice cores obtained from the interior of the GrIS, terrestrial climate records from ice-free areas, and marine sediment records collected from the adjacent sea floor have generated an accurate picture of the Late Quaternary climatic evolution and associated environmental changes (Briner et al., 2016). Together with Antarctica, the GrIS is one of the only ice sheets that survived the last deglaciation during Termination-1 (T-1;~19-11 ka).The GrIS persisted during the Last Interglacial period (~130-116 ka) when global temperatures were significantly higher than present, although it was particularly reduced in its NE sector (Vasskog et al., 2015). ...
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The Greenland Ice Sheet (GrIS) is a key component of the global climate system. However, our current understanding of the spatio-temporal oscillations and landscape transformation of the GrIS margins since the last glacial cycle is still incomplete. The objective of this work is to study the deglaciation of the Zackenberg Valley (74°N, 20°E), NE Greenland, and the origin of the derived landforms. Based on extensive fieldwork and high-detail geomorphological mapping we identified the different types of landforms, from which those of glacial and paraglacial origin were used to understand the processes driving regional environmental evolution. We applied cosmic-ray exposure (CRE) dating to 32 samples taken from erosive and depositional glacial landforms distributed across the valley. Geomorphological evidence shows that >800-m-thick Late Quaternary glacier filled the valleys and fjords and covered mountain summits. In subsequent phases, as ice thickness decreased, the glacier was limited to the interior of the valley, leaving several lateral moraines. The deglaciation of the Zackenberg Valley that started by ~13.7–12.5 ka also accelerated slope paraglacial processes. Many blocks from lateral moraines were remobilized and fell, reaching the valley floor where they covered the thinning glacier tongue; transforming it into a debris-covered glacier that subsequently melted gradually. By ca. 10.5 ka, the last remnants of glacial ice disappeared from the Zackenberg Valley floor, a chronology of deglaciation that is similar to that observed in other sites across NE Greenland. The results of this work must be considered in similar studies, reinforcing the need to support CRE ages of the different geomorphological phases with paleoclimatic data from other sedimentary records.
... In polar lakes, light availability throughout most of the year is controlled by ice and snow cover; warming causes thinner ice, longer open water periods and therefore greater light penetration. The majority of the Greiner Lake core encompasses the prevailing Late-Holocene cooling trend (Briner et al., 2016), which would typically be expected to lead to shorter openwater period and reduced nutrient delivery from terrestrial environments (Vincent et al., 2013). Given that Greiner Lake productivity does not appear to follow these regional climatic trends, other drivers are needed to explain its changes over time, and the evidence suggests that the principal drivers of productivity were shifting nutrient inputs and salinity changes caused by the changing level of marine influence. ...
Article
Global sea level rise and increased storm activity triggered by climate change pose a serious threat to low-lying Arctic coastal landscapes, which commonly host numerous freshwater lakes. Marine inundation of these ecosystems would result in significant ecological changes. We elucidated effects of changing levels of marine influence to lake ecosystems by exploring the ontogeny of coastal Greiner Lake in Arctic Canada. Using paleolimnological methods on a sediment core, we explored functional assemblage compositions of aquatic biota (diatoms and chironomids), biomarkers (pigments and fatty acids) and geochemistry (stable isotopes and organic matter) throughout the isolation history of the lake. The biological and geological data collectively indicated a paleoenvironmental shift from marine embayment to brackish lagoon in the Late-Holocene and to a freshwater lake ca. 500 BP. We observed that the lake was most productive during a phase of mixed freshwater and marine influence, based on higher chlorophyll-a and β,β-carotene concentrations, omega-3 fatty acids (ω-3) as well as sediment organic content. We suggest that, in addition to reorganization of aquatic communities and altered ecosystem services provided by coastal lakes, coastal freshwaters may become more productive under increasing marine influence caused by climate change. Global sea level rise may thus trigger a new pathway to increasingly greener Arctic.
... Holocene archives are thus particularly interesting, not just to establish a pre-industrial base-line of environmental conditions, but also to shine a light on paleoenvironmental conditions under a warmer climate (MacDonald, 2010). Although the HTM may be a close parallel to predicted atmospheric temperatures, major differences arise from the fact that the world was transitioning out of the LGM, with deglacial conditions persisting in the High Arctic well into the Holocene, particularly in the western CAA (Briner et al., 2016). During the LGM, the CAA was covered by the Innuitian Ice Sheet (IIS), cutting off the connection between the Arctic Ocean and Baffin Bay (e.g., England et al., 2006). ...
Thesis
Nares Strait is one of three channels of the Canadian Arctic Archipelago (CAA) which connect the Arctic Ocean to Baffin Bay. The CAA throughflow is a major component of ocean circulation in western Baffin Bay. Nares Strait borders the CAA to the east, separating Ellesmere Island from Greenland, and is 80% covered in sea ice 11 months of the year. The heavy sea ice cover is constituted of (1) Arctic (multi-year) sea-ice having entered the strait by the north, and (2) locally formed first year sea ice, which consolidates the ice cover. The hydrological history of the area is intimately linked to the formation of land-fast sea ice in the strait, constituting ice arches. The seaice cover in Nares Strait regulates freshwater (liquid and solid) export towards Baffin Bay, and is integral to the formation of an area of open water in northernmost Baffin Bay: The North Water polynya.Nares Strait has been at the heart of major geomorphological changes over the past 10,000 years. Its deglacial and post-glacial history is marked by (1) rapid retreat of the Greenland and Innuitian ice-sheets which coalesced along Nares Strait during the Last Glacial Maximum, (2) post-glacial shoaling associated to isostatic rebound, and (3) variable multi-year and seasonal sea ice conditions. Little is known about the evolution of these three environmental components of the Nares Strait history, and they are poorly constrained in terms of chronology and synchronism with other regional changes. Nares Strait and its eventful Holocene history provide a unique case study of the response of the marine and continental cryosphere to rapid climate change, such as that affecting Arctic regions in modern times.The marine sediment archives that were retrieved during the ANR GreenEdge and ArcticNet (2014 and 2016) cruises of CCGS Amundsen offer a unique opportunity to investigate the Deglacial to Late Holocene history of Nares Strait. Our reconstructions are based on a multi-proxy study of these cores, including sedimentologic (grain size and lithofacies), geochemical (XRF), mineralogical (q-XRD), micropaleontological (planktic and benthic foraminiferal assemblages), and biogeochemical (sea ice biomarkers IP25 and HBI III).Our results include an age for the Deglacial opening of Nares Strait between 9.0 and 8.3 cal. ka BP, with the event likely occurring closer to the later bracket of the timeframe (i.e., ca 8.5-8.3 cal. ka BP). This event established the throughflow from the Arctic Ocean towards northernmost Baffin Bay. Environmental conditions were highly unstable in the Early Holocene, and marine primary productivity was limited. A period of minimum sea-ice cover occurred from ca 8.1 to 7.5 cal. ka BP, during the Holocene Thermal Maximum, when atmospheric temperatures were higher than today in Nares Strait. Sea-ice cover became more stably established as a seasonal feature around 7.5 cal. ka BP and primary productivity related to ice edge blooms increased. Eventually, the duration of the ice arches increased and they were present in spring and into the summer from 5.5 to 3.7 cal. ka BP, which allowed the inception of the North Water polynya. The North Water reached its maximal potential between 4.5 and 3.7 cal. ka BP, when warmer Atlantic-sourced water upwelled in the polynya, providing nutrients for primary productivity. The establishment of a near-perennial ice arch in northern Nares Strait prevented export of multi-year sea ice into Nares Strait and hindered the formation of the southern ice arch, ultimately resulting in a less productive polynya over the past ca 3.0 cal. ka BP.
... Most terrestrial temperature proxies are sensitive to either mean annual and/or summer temperatures (Bradley, 2014;Briner et al., 2016;Kaufman et al., 2004;Sundqvist et al., 2014). However, the d 18 O of ice wedges is used as a proxy of winter air temperature and combined with other paleoclimate estimates, can enhance our understanding of seasonal fluctuation of past climates. ...
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Over the past few decades, the δ¹⁸O records of ice wedges have been used to estimate late Quaternary winter air temperatures. This study first reviews the development of ice wedges and the potential isotopic fractionation that takes place from snow deposition to formation of ice veinlets. Then, based on the high resolution analysis of ice wedges from the Eureka Sound Lowland (Ellesmere and Axel Heiberg Islands), we evaluate the effect of sampling depth and edge effects on the δ¹⁸O records and compare a composite δ¹⁸O time-series of ice wedges to the Holocene 25-yr annual and 20-yr winter δ¹⁸O records of the nearby Agassiz Ice Cap. Radiocarbon ages of dissolved organic carbon in ice wedges showed that cracking occurs mostly near the center of the wedge, but age reversals were observed. Covariance analysis showed that two δ¹⁸O profiles collected at the same depth had high noise (c. 70–80 % uncorrelated parts). Additionally, ice wedge mean δ¹⁸O records showed a decreasing trend with depth, likely due Rayleigh fractionation during freezing of residual water in the crack. Finally, the composite δ¹⁸O time-series of ice wedges and δ¹⁸O records of the Agassiz Ice Cap showed a similar Holocene cooling trend, however the wedges δ¹⁸O records had a higher degree of variability throughout the Holocene (∼4 ‰ versus ∼2 ‰ in the Agassiz records). The higher variability in the wedge is attributed to the timing of meltwater infiltrating the wedge crack over the growth period, which from the onset to termination of snowmelt, is in the order of 3–6‰. The study highlights that, if to be used as a centennial-millennial scale proxy, ice wedges should be sampled near the surface where δ¹⁸O records would be less affected by in situ freezing and plugging, and veinlets should be dated directly and smoothed to remove the local random variance.
... Investigations of carbon cycling across the transition between the cooler Little Ice Age (LIA) and warmer recent conditions in the Arctic provide a way of understanding how Arctic systems respond to climate change. Lacustrine records from close to the Jakobshavn Isfjord, near Ilulissat, east of Disko suggest that the lowest Holocene temperatures occurred here in the 19th century (LIA) (Axford et al., 2013;Briner et al., 2016), rising in the early 20th century (Box et al., 2009;Yamanouchi, 2011) and accelerating with recent warming (Hanna et al., 2012). ...
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The Arctic is rapidly changing, disrupting biogeochemical cycles and the processing, delivery and sedimentation of carbon (C), in linked terrestrial–aquatic systems. In this investigation, we coupled a hydrogeomorphic assessment of catchment soils, sediments and plants with a recent lake sediment sequence to understand the source and quality of organic carbon present in three Arctic upland lake catchments on Disko Island, located just south of the low–high Arctic transition zone. This varied permafrost landscape has exposed soils with less vegetation cover at higher altitudes, and lakes received varying amounts of glacial meltwater inputs. We provide improved isotope and biomarker source identifications for palaeolimnological studies in high-latitude regions, where terrestrial vegetation is at or close to its northerly and altitudinal range limit. The poorly developed catchment soils lead to lake waters with low dissolved organic carbon (DOC) concentrations (≤1.5 mg L−1). Sedimentary carbon/nitrogen (C/N) ratios, the C isotope composition of organic matter (δ13Corg) and biomarker ratios (n-alkanes, n-alkanols, n-alkanoic acids and sterols) showed that sedimentary organic matter (OM) in these lakes is mostly derived from aquatic sources (algae and macrophytes). We used a 210Pb-dated sediment core to determine how carbon cycling in a lake–catchment system (Disko 2) had changed over recent centuries. Recent warming since the end of the Little Ice Age (LIA∼1860 CE), which accelerated after ca. 1950, led to melt of glacier ice and permafrost, releasing nutrients and DOC to the lake and stimulating pronounced aquatic algal production, as shown by a >10-fold increase in β-carotene, indicative of a major regime shift. We also demonstrate that recent increases in catchment terrestrial vegetation cover contributed to the autochthonous response. Our findings highlight that in Arctic lakes with sparsely developed catchment vegetation and soils, recent Anthropocene warming results in pronounced changes to in-lake C processing and the deposition of more reactive, predominately autochthonous C, when compared with extensively vegetated low-Arctic systems.
... During this period, changes of the natural environment and climate play a key role in the development and progress of human civilisation. Thus, effects of Holocene climate change and geoenvironmental evolution on human have been received more and more attention (Zhou et al., 2005;Prokopenko et al., 2007;Wallinga, 2010;Ashley et al., 2011;Briner et al., 2016;Marks et al., 2017;Guo et al., 2018). Recently, a high-resolution and accurate geochronological framework of flu-lacustrine sediments, which are important records of paleo-environment and paleoclimate, allows us to understand these phenomena better. ...
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Wuhan area located middle Yangtze River Basin, is dominated by East Asian Monsoon. Widely developed Holocene flu-lacustrine sediments are ideal materials for the reconstruction of paleoclimate change and geo-environment evolution, for which the chronology is a key issue. In this study, 20 luminescence dating samples were collected from a flu-lacustrine sequence and the reliability of the quartz OSL dating to these samples were checked by using luminescence characteristics of dose recovery test and thermos transfer test. Our results indicate that different grain size fraction of 4~11 μm, 38~63 μm, and 90~125 μm were well-bleached before burial. Dating results show that all these ages range from 0.8 ± 0.1 to 7. 9 ± 0.7 ka between 4.25 and 38.55 m, and most of them follow the stratigraphic sequence and other climatic recorder within the normal range of error. Disordered OSL ages may be caused by complicated transport-deposit processes. As a consequence, OSL dating method of SAR-SGC protocol could provide a significant chronology for Holocene flu-lacustrine sediment in large river depo-system.
... For this purpose, we therefore collected reworked marine molluscs from the surface of and within diamictic sediments at the southern margin of the Humboldt Glacier (Fig. 1c). From the sample site, 15 molluscs were chosen, pre-treated following the procedure of Brock et al. (2010), and radiocarbon dated at the Be g −1 ) was used for preparation of sample GL1701-GL1703 and GL1706-GL1710. All other samples were prepared using carrier Phe1603 ...
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Determining the sensitivity of the Greenland Ice Sheet (GrIS) to Holocene climate changes is a key prerequisite for understanding the future response of the ice sheet to global warming. In this study, we present new information on the Holocene glacial history of the GrIS in Inglefield Land, north Greenland. We use 10Be and in situ 14C exposure dating to constrain the timing of deglaciation in the area and radiocarbon dating of reworked molluscs and wood fragments to constrain when the ice sheet retreated behind its present-day extent. The 10Be ages are scattered ranging from ca. 92.7 to 6.8 ka, whereas the in situ 14C ages range from ca. 14.2 to 6.7 ka. Almost half of the apparent 10Be ages predate the Last Glacial Maximum and up to 89 % are to some degree affected by nuclide inheritance. Based on the few reliable 10Be ages, the in situ 14C ages and existing radiocarbon ages from Inglefield Land, we find that the deglaciation along the coast commenced at ca. 8.6–8.3 ka cal BP in the western part and ca. 7.9 ka in the central part, following the opening of Nares Strait and arrival of warm waters. The ice margin reached its present-day position at ca. 8.2 ka at the Humboldt Glacier and ca. 6.7 ka in the central part of Inglefield Land. Radiocarbon ages of reworked molluscs and wood fragments show that the ice margin was behind its present-day extent from ca. 5.8 to 0.5 ka cal BP. After 0.5 ka cal BP, the ice advanced towards its Little Ice Age position. Our results emphasize that the slowly eroding and possibly cold-based ice in north Greenland makes it difficult to constrain the deglaciation history based on 10Be ages alone unless they are paired with in situ 14C ages. Further, combining our findings with those of recently published studies reveals distinct differences between deglaciation patterns of northwest and north Greenland. Deglaciation of the land areas in northwest Greenland occurred earlier than in north Greenland, and periods of restricted ice extent were longer, spanning the Middle and Late Holocene. Overall, this highlights past ice sheet sensitivity to Holocene climate changes in an area where little information was available just a few years ago.
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This study presents the application of an interactive downscaling in Europe using iLOVECLIM (a model of intermediate complexity), increasing its atmospheric resolution from 5.56 to 0.25∘ kilometric. A transient simulation using the appropriate climate forcings for the entire Holocene (11.5–0 ka BP) was done for both the standard version of the model and with an interactive downscaling applied. Our results show that simulations from downscaling present spatial variability that agrees better with proxy-based reconstructions and other climate models as compared to the standard model. The downscaling scheme simulates much higher (by at least a factor of 2) precipitation maxima and provides detailed information in mountainous regions. We focus on examples from the Scandes mountains, the Alps, the Scottish Highlands, and the Mediterranean. The higher spatial resolution of the downscaling provides a more realistic overview of the topography and gives local climate information, such as precipitation and temperature gradient, that is important for paleoclimate studies. With downscaling, we simulate similar trends and spatial patterns of the precipitation changes reconstructed by other proxy studies (for example in the Alps) as compared to the standard version. Our downscaling tool is numerically cheap, implying that our model can perform kilometric, multi-millennial simulations and is suitable for future studies.
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The Greenland Ice Sheet (GrIS) responds rapidly to the present climate, therefore, its response to the predicted future warming is of concern. To learn more about this, decoding its behavior during past periods of warmer than present climate is important. However, due to the scarcity of marine studies reconstructing ice sheet conditions on the Northeast Greenland shelf and adjacent fjords including the position of the ice sheet over marine regions, the timing of the deglaciation, and its connection to forcing factors including the Holocene Thermal Maximum (HTM) on NE Greenland remain poorly constrained. This paper aims to use bathymetric data and the analysis of sediment gravity cores to enhance our understanding of ice dynamics of the GrIS near the southern outlet of the Northeast Greenland Ice Stream (NGIS), as well as give insight into the timing of deglaciation and provide a palaeoenvironmental reconstruction of southwestern Dove Bugt and Bessel Fjord since the Last Glacial Maximum (LGM). The swath bathymetry data displayed in this study is the first time the bathymetry for Bessel Fjord has become available. North–south oriented glacial lineations, and the absence of pronounced moraines in southwest Dove Bugt, an inner continental shelf embayment (trough), suggests the southwards and offshore flow of the southern branch of the NGIS, Storstrømmen. Sedimentological data suggests that an ice body, theorized to be the NGIS, may have retreated from the region slightly before ~11.2 ka BP (in the Preboreal period). The seabed morphology of Bessel Fjord, a fjord terminating in southern Dove Bugt, includes numerous basins, separated by thresholds. The position of basin thresholds, which include some recessional moraines, suggest that the GrIS had undergone multiple halts or readvances during deglaciation. A minimum age of 7.2 ka BP is proposed for the retreat of ice to or west of its present-day position in the Bessel Fjord catchment area. This suggests that the GrIS retreated from the marine realm in early Holocene, around the time of the onset of the Holocene Thermal Maximum in this region, a period when the mean July temperature according to Bennike et al., (2008) was at least 2–3 °C higher than at present, and remained at or west of this onshore position for the remainder of the Holocene. The transition from predominantly mud to muddy sand layers in a mid-fjord core at ~4 ka BP may be the result of increased sediment input from nearby and growing ice caps. This shift may suggest that in late Holocene (Meghalayan), a period characterized by a temperature drop to modern values, ice caps in Bessel Fjord fluctuated with greater sensitivity to climatic conditions than the NE sector of the GrIS.
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Ice core records have long indicated that the Younger Dryas began and ended with large, abrupt climate shifts over Greenland. Key climatic features remain unknown, including the magnitude of warming during the Younger Dryas-Holocene transition along with the seasonality and spatial variability of Younger Dryas climate changes across Greenland. Here, we use geochemical and paleoecological proxies from lake sediments at Lake N14 in south Greenland to address these outstanding questions. Radiocarbon dating and diatom assemblages confirm early deglaciation and isolation of Lake N14 before ∼13,600 cal yr BP, consistent with previous work. Oxygen isotope ratios (δ¹⁸O) of chironomid head capsules, bulk aquatic moss, and aquatic moss-derived cellulose are used to reconstruct oxygen isotopes of past lake water and annual precipitation. Oxygen isotope proxies indicate annual precipitation δ¹⁸O values increased by 5.9–7.7‰ at the end of the Younger Dryas. Following the Younger Dryas, moss and cellulose δ¹⁸O values show a clear decline in precipitation δ¹⁸O values of 2–3‰ from ∼11,540–11,340 cal yr BP that may correspond with the Preboreal Oscillation. Reconstructed precipitation δ¹⁸O values then gradually increased from 11,300–10,100 cal yr BP. All three aquatic organic materials register similar shifts in precipitation δ¹⁸O values over time, and they closely parallel the δ¹⁸O shifts observed in ice cores. This evidence strongly supports the utility of these methods for reconstructing lake water δ¹⁸O, and furthermore precipitation δ¹⁸O values where lake water reflects precipitation. The relatively large shift in isotopic composition of precipitation at Lake N14 suggests that shifts in temperature, precipitation seasonality, and/or moisture sources at the end of the Younger Dryas were even larger in south Greenland than they were in central Greenland, most likely because of the proximity to major changes in North Atlantic Ocean circulation. The annual air temperature change estimated at Lake N14 at the end of the Younger Dryas is also very large (∼18 ± 7 °C) compared to the summer warming previously inferred from chironomid species assemblages there (∼6 °C). This indicates that the strongest warming at the end of the Younger Dryas occurred in the winter season, consistent with past observations of intensified Younger Dryas seasonality at Lake N14 and elsewhere in Greenland.
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We present chironomid species assemblage data from 402 lakes across northern North America, Greenland, Iceland, and Svalbard to inform interpretations of Holocene subfossil chironomid assemblages used in paleolimnological reconstruction. This calibration-set was developed by re-identifying and taxonomically harmonizing chironomids in previously described surface sediment samples, with identifications made at finer taxonomic resolution than in original publications. The large geographic coverage of this dataset is intended to provide climatic analogues for a wide range of Holocene climates in the northwest North Atlantic region and North American Arctic, including Greenland. For many of these regions, modern calibration data are sparse despite keen interest in paleoclimate reconstructions from high latitudes. A suite of chironomid-based temperature models based upon this training set are evaluated here and the best statistical model is used to reconstruct late glacial (Allerød and Younger Dryas) and Holocene paleotemperatures at five non-glacial lakes representing a wide range of climate zones across Greenland. The new continent-scale training set offers more analogues for the majority of Greenland subfossil assemblages than existing smaller training sets, with many in Iceland and northern Canada. We find strong agreement between chironomid-based reconstructions derived from the new model and independent glacier-based evidence for multi-millennial Holocene temperature trends. Some of the new Holocene reconstructions are very similar to published data, but at a subset of sites and time periods we find improved paleotemperature reconstructions attributable both to the new model's finer taxonomic resolution and to its expanded geographic/climatic coverage, which resulted in improved characterization of species optima. In the late glacial, the new model's finer taxonomic resolution yields a unique ability to resolve temperatures of the Allerød from colder temperatures of the Younger Dryas, although the magnitude of that temperature difference may be underestimated. This study demonstrates the value of geographically and climatically broad paleoecological training sets. The large, taxonomically harmonized dataset presented here should be useful for a wide range of future investigations, including but not limited to paleotemperature reconstructions across the Arctic.
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The Arctic is warming faster than any other region on Earth. Putting this rapid warming into perspective is challenging because instrumental records are often short or incomplete in polar regions and precisely-dated temperature proxies with high temporal resolution are largely lacking. Here, we provide this long-term perspective by reconstructing past summer temperature variability at Yamal Peninsula – a hotspot of recent warming – over the past 7638 years using annually resolved tree-ring records. We demonstrate that the recent anthropogenic warming interrupted a multi-millennial cooling trend. We find the industrial-era warming to be unprecedented in rate and to have elevated the summer temperature to levels above those reconstructed for the past seven millennia (in both 30-year mean and the frequency of extreme summers). This is undoubtedly of concern for the natural and human systems that are being impacted by climatic changes that lie outside the envelope of natural climatic variations for this region.
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Numerical simulations of the Greenland Ice Sheet (GrIS) over geologic timescales can greatly improve our knowledge of the critical factors driving GrIS demise during climatically warm periods, which has clear relevance for better predicting GrIS behavior over the upcoming centuries. To assess the fidelity of these modeling efforts, however, observational constraints of past ice sheet change are needed. Across southwestern Greenland, geologic records detail Holocene ice retreat across both terrestrial-based and marine-terminating environments, providing an ideal opportunity to rigorously benchmark model simulations against geologic reconstructions of ice sheet change. Here, we present regional ice sheet modeling results using the Ice-sheet and Sea-level System Model (ISSM) of Holocene ice sheet history across an extensive fjord region in southwestern Greenland covering the landscape around the Kangiata Nunaata Sermia (KNS) glacier and extending outward along the 200 km Nuup Kangerula (Godthåbsfjord). Our simulations, forced by reconstructions of Holocene climate and recently implemented calving laws, assess the sensitivity of ice retreat across the KNS region to atmospheric and oceanic forcing. Our simulations reveal that the geologically reconstructed ice retreat across the terrestrial landscape in the study area was likely driven by fluctuations in surface mass balance in response to Early Holocene warming – and was likely not influenced significantly by the response of adjacent outlet glaciers to calving and ocean-induced melting. The impact of ice calving within fjords, however, plays a significant role by enhancing ice discharge at the terminus, leading to interior thinning up to the ice divide that is consistent with reconstructed magnitudes of Early Holocene ice thinning. Our results, benchmarked against geologic constraints of past ice-margin change, suggest that while calving did not strongly influence Holocene ice-margin migration across terrestrial portions of the KNS forefield, it strongly impacted regional mass loss. While these results imply that the implementation and resolution of ice calving in paleo-ice-flow models is important towards making more robust estimations of past ice mass change, they also illustrate the importance these processes have on contemporary and future long-term ice mass change across similar fjord-dominated regions of the GrIS.
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Lakes fed by Greenlandic mountain glaciers and ice caps (GICs) contain important archives of Arctic palaeoenvironmental change. GIC proglacial lake records have been increasingly used to reconstruct Holocene glacier behaviour, largely focusing on macrostratigraphy. However, despite the wide range of topographic settings and catchment characteristics, there has been little systematic analysis of the ways that catchment conditions are registered in the clastic sediments of GIC lakes. Such signals provide valuable insights into landscape processes and palaeoenvironmental conditions that are not routinely captured in other Quaternary glacial morphosedimentary archives. This review synthesises sedimentological and geochemical evidence from existing Holocene GIC proglacial lake records to establish: how catchment‐wide conditions have been recorded in the lacustrine sequences; and our ability to isolate these signals to enhance palaeoenvironmental reconstruction. Our review shows that with careful sedimentological and targeted (bio)geochemical analyses coupled with a clear process‐based understanding, catchment and in‐lake signals can be effectively identified in the microstratigraphic and mineral grain record. Such signals include wind patterns, mass wasting, precipitation events and seasonal lake ice cover, that can complement broader palaeoclimatic proxy evidence. The approaches collated here, if more widely applied, could considerably enhance environmental reconstructions not only in Greenland, but in glaciated catchments elsewhere.
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The Arctic temperature changes are closely linked to midlatitude weather variability and extreme events, which has attracted much attention in recent decades. Syntheses of proxy data from poleward of 60° N indicate that there was asymmetric cooling of -1.54 °C and -0.61 °C for Atlantic Arctic and Pacific Arctic during the Holocene, respectively. We also present a similar consistent cooling pattern from an accelerated transient Holocene climate simulation based on the Community Earth System Model. Our results indicate that the asymmetric Holocene Arctic cooling trend is dominated by the winter temperature variability with -0.67 °C cooling for Atlantic Arctic and 0.09 °C warming for Pacific Arctic, which is particularly pronounced at the proxy sites. Our findings indicate that sea ice in the North Atlantic expanded significantly during the Late Holocene, while a sea ice retreat is seen in the North Pacific, amplifying the cooling in the Atlantic Arctic by the sea ice feedback. The positive Arctic dipole pattern, which promotes warm southerly winds to the North Pacific, offsets parts of the cooling trend in Pacific Arctic. The Arctic dipole pattern also causes sea ice expansion in the North Atlantic, further amplifying the cooling asymmetry. We found that the temperature asymmetry is more pronounced in a simulation driven only by orbital forcing, indicating that the orbital modulation of the Pacific Decadal Oscillation, which in turn links to the Arctic dipole pattern, further affects the temperature asymmetry.
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The recent retreat of nearly all glaciers and ice caps (GICs) located in Arctic regions is one of the most clear and visible signs of ongoing climate change. This paper synthesizes published records of Holocene GIC fluctuations from lake archives, placing their recent retreat into a longer-term context. Our compilation includes 66 lake-based GIC records (plus one non-lake-based record from the Russian Arctic) from seven Arctic regions: Alaska, Baffin Island in northeastern Canada, Greenland, Iceland, the Scandinavian peninsula, Svalbard, and the Russian high Arctic. For each region and for the full Arctic, we summarize evidence for when GICs were smaller than today or absent altogether, indicating warmer-than-present summers, and evidence for when GICs regrew in lake catchments, indicating summer cooling. Consistent with orbitally driven high boreal summer insolation in the early Holocene, the full Arctic compilation suggests that the majority (50 % or more) of studied GICs were smaller than present or absent by ∼10 ka. We find the highest percentage (>90 %) of Arctic GICs smaller than present or absent in the middle Holocene at ∼ 7–6 ka, probably reflecting more spatially ubiquitous and consistent summer warmth during this period than in the early Holocene. Following this interval of widespread warmth, our compilation shows that GICs across the Arctic began to regrow and summers began to cool by ∼6 ka. Together, the Arctic records also suggest two periods of enhanced GIC growth in the middle to late Holocene from ∼ 4.5–3 and after ∼2 ka. The regional records show variability in the timing of GIC regrowth within and between regions, suggesting that the Arctic did not cool synchronously despite the smooth and hemispherically symmetric decline in Northern Hemisphere summer insolation. In agreement with other studies, this implies a combined response to glacier-specific characteristics such as topography and to other climatic forcings and feedback mechanisms, perhaps driving periods of increased regional cooling. Today, the direction of orbital forcing continues to favor GIC expansion; however, the rapid retreat of nearly all Arctic GICs underscores the current dominance of anthropogenic forcing on GIC mass balance. Our review finds that in the first half of the Holocene, most of the Arctic's small GICs became significantly reduced or melted away completely in response to summer temperatures that, on average, were only moderately warmer than today. In comparison, future projections of temperature change in the Arctic far exceed estimated early Holocene values in most locations, portending the eventual loss of most of the Arctic's small GICs.
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Declining temperature has been thought to explain the abandonment of Norse settlements, southern Greenland, in the early 15th century, although limited paleoclimate evidence is available from the inner settlement region itself. Here, we reconstruct the temperature and hydroclimate history from lake sediments at a site adjacent to a former Norse farm. We find no substantial temperature changes during the settlement period but rather that the region experienced a persistent drying trend, which peaked in the 16th century. Drier climate would have notably reduced grass production, which was essential for livestock overwintering, and this drying trend is concurrent with a Norse diet shift. We conclude that increasingly dry conditions played a more important role in undermining the viability of the Eastern Settlement than minor temperature changes.
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Cosmogenic nuclide techniques have advanced the geosciences by providing tools for exposure age dating, burial dating, quantification of denudation rates and more. Advances in geochemistry, accelerator mass spectrometry and atom trap trace analyses are ushering in a new cosmogenic nuclide era, by improving the sensitivity of measurements to ultra-trace levels that now allow new applications of these techniques to numerous Earth surface processes. The advances in cosmogenic nuclide techniques have equipped the next generation of geoscientists with invaluable tools for understanding the planet, but addressing pressing needs requires rising to an even greater challenge: imbuing within the cosmogenic community, and the geosciences as a whole, a commitment to justice, equity, diversity and inclusion that matches our dedication to scientific research. In this Primer, we review the state of the art and recent exciting breakthroughs in the use of cosmogenic nuclide techniques, focusing on erosion factories over space and time, and new perspectives on ice sheet stability. We also highlight promising ways forward in enhancing inclusion in the field, as well as obstacles that remain to be overcome. Cosmogenic nuclide dating techniques provide tools for age dating, burial dating and denudation rates. Advances in accelerator mass spectrometry and atom trap trace analyses are improving measurement sensitivity. In this Primer, Schaefer et al. outline how to use cosmogenic nuclide dating across a range of applications.
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In the Northern Hemisphere, an insolation driven Early to Middle Holocene Thermal Maximum was followed by a Neoglacial cooling that culminated during the Little Ice Age (LIA). Here, we review the glacier response to this Neoglacial cooling in Greenland. Changes in the ice margins of outlet glaciers from the Greenland Ice Sheet as well as local glaciers and ice caps are synthesized Greenland-wide. In addition, we compare temperature reconstructions from ice cores, elevation changes of the ice sheet across Greenland and oceanographic reconstructions from marine sediment cores over the past 5,000 years. The data are derived from a comprehensive review of the literature supplemented with unpublished reports. Our review provides a synthesis of the sensitivity of the Greenland ice margins and their variability, which is critical to understanding how Neoglacial glacier activity was interrupted by the current anthropogenic warming. We have reconstructed three distinct periods of glacier expansion from our compilation: two older Neoglacial advances at 2,500 – 1,700 yrs. BP (Before Present = 1950 CE, Common Era) and 1,250 – 950 yrs. BP; followed by a general advance during the younger Neoglacial between 700-50 yrs. BP, which represents the LIA. There is still insufficient data to outline the detailed spatio-temporal relationships between these periods of glacier expansion. Many glaciers advanced early in the Neoglacial and persisted in close proximity to their present-day position until the end of the LIA. Thus, the LIA response to Northern Hemisphere cooling must be seen within the wider context of the entire Neoglacial period of the past 5,000 years. Ice expansion appears to be closely linked to changes in ice sheet elevation, accumulation, and temperature as well as surface-water cooling in the surrounding oceans. At least for the two youngest Neoglacial advances, volcanic forcing triggering a sea-ice /ocean feedback, could explain their initiation. There are probably several LIA glacier fluctuations since the first culmination close to 1250 CE (Common Era) and available data suggests ice culminations in the 1400s, early to mid-1700s and early to mid-1800s CE. The last LIA maxima lasted until the present deglaciation commenced around 50 yrs. BP (1900 CE). The constraints provided here on the timing and magnitude of LIA glacier fluctuations delivers a more realistic background validation for modelling future ice sheet stability.
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Sediment core ER11-16 from Køge Bugt in Southeast Greenland is used to assess early Holocene palaeoceanographic changes and sediment rafting from icebergs calved from the large outlet glaciers in the area. Diatom analysis reconstructs variability in surface water temperature, salinity and sea-ice concentrations, and benthic foraminiferal assemblages is used to reconstruct subsurface ocean conditions. We report Holocene Thermal Maximum in Southeast Greenland during the early Holocene (at least since onset of the record 9100 cal yr BP) until around 4500 cal yr BP, which contrasts with a delay until the mid-Holocene of the Holocene Thermal Maximum in South and Southwest Greenland. The early Holocene warming in Southeast Greenland was caused by a combination of high solar insolation and a weakened subpolar gyre, both of which served to warm the Irminger Current waters subducting onto the shelf. At the same time, the surface temperature was relatively high and sea-ice cover in the polar surface waters of the East Greenland Current was relatively low. High levels of iceberg rafting occurred in Køge Bugt during the early Holocene, synchronously with these warm oceanic temperatures. This is attributed to an increase in iceberg production from the extensive, but retreating, Greenland Ice Sheet. The warm surface conditions were interrupted by a marked and short-lived increase in sea ice around 8200 years ago, providing the first evidence of this global cold episode in Southeast Greenland. After 4500 cal. yr BP, sea-ice cover increased with an expansion of the East Greenland Current, suppressing the inflow of warmer subsurface Irminger Current water to the Southeast Greenland shelf. We relate this oceanic shift to the decreased Northern Hemisphere summer solar insolation. Multi-centennial variability is observed in the grain size spectrum of iceberg rafted debris; a finding we interpret in the context of palaeoceanographic changes.
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The Holocene shows rapid climatic changes associated with alternating intervals of glacier advances and retreats. The coastal regions, where beach ridges constitute common preserved landforms, are highly sensitive to register such changes and bring light into past littoral environments. Excellent marine Argentinean Holocene deposits associated with the last climate optimum are preserved at the Río de la Plata estuary (Argentina). In this beach ridge, fourteen detailed sedimentary logs were studied in order to generate an palaeoenvironmental model. The analysis of major hierarchy surfaces of the ridge was performed combined field work and Virtual-Outcrop analysis. The ridge presents internal clinoforms that subdivide it into eleven clinothems. The first two clinothems correspond to the development of a sand ridge as a response to the erosion and reworking of previous aeolian Late Pleistocene sediments. Subsequently, a bioclastic ridge was amalgamated with the sand ridge (clinothems III to XI). Simultaneously, in a landward position, a lagoon and washover deposits were developed. Finally, after the stabilization of the beach ridge the continuous sea level fall generated the actual coastal plain deposits. Six ¹⁴C ages were obtained at the base of selected clinothems, dated between 5,240 ± 110 cal BP and 3,900 ± 90 cal BP; meaning that in 1,340 years the beach prograded 290 m. Stable isotope analyses (δ¹³C and δ¹⁸O) allowed to infer that the temperature during the evolution of the ridge has two maximum values of 22.5 °C and a minimum of 18.5 °C. The salinity range between 32.53‰ and 33.16‰, showing short variability. The sudden change in the composition of the ridge, from sandy to carbonate sediments, may be interpreted as a combined result of an increase of carbonate productivity along with a decrease of siliciclastic supply at the coast. This stage would have been developed approximately around 5ka B.P., in coincidence with the Holocene Climatic Optimum (as recorded in Argentina) when propitious climatic conditions may have led to the proliferation of large communities of warm and warm temperate benthic organisms. We stress that, the strong activity of the Brazilian current during the Mid-Holocene thermal maximum enabled excellent conditions for the development of a carbonate warm beach similar to that occurring at tropical and subtropical areas nowadays. This study provides an example of the strong changes occurred in coastal environments as a result of climate change, particularly in the context of global warming episodes which characterized interglacial periods of the Late Quaternary in eastern South America.
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Reconstructions of sea‐surface conditions during the Holocene were achieved using three sediment cores from northeastern Baffin Bay (GeoB19948‐3 and GeoB19927‐3) and the Labrador Sea (GeoB19905‐1) along a north–south transect based on sea‐ice IP25 and open‐water phytoplankton biomarkers (brassicasterol, dinosterol and HBI III). In Baffin Bay, sea‐surface conditions in the Early Holocene were characterized by extended (early) spring sea ice cover (SIC) prior to 7.6 ka BP. The conditions in the NE Labrador Sea, however, remained predominantly ice‐free in spring/autumn due to the enhanced influx of Atlantic Water (West Greenland Current, WGC) from 11.5 until ~9.1 ka BP, succeeded by a period of continued (spring–autumn) ice‐free conditions between 9.1 and 7.6 ka BP corresponding to the onset of Holocene Thermal Maximum (HTM)‐like conditions. A transition towards reoccurring ice‐edge and significantly reduced SIC conditions in Baffin Bay is evident in the Middle Holocene (~7.6–3 ka BP) probably caused by the variations in the WGC influence associated with the ice melting and can be characterized as HTM‐like conditions. These HTM‐like conditions are predominantly recorded in the NE Labrador Sea area shown by (spring–autumn) ice‐free conditions from 5.9–3 ka BP. In the Late Holocene (last ~3 ka), our combined proxy records from eastern Baffin Bay indicate low in‐situ ice algae production; however, enhanced multi‐year (drifted) sea ice in this area was possibly attributed to the increased influx of Polar Water mass influx and may correlate with the Neoglacial cooling. The conditions in the NE Labrador Sea during the last 3 ka, however, continued to remain (spring–autumn) ice‐free. Our data from the Baffin Bay–Labrador Sea transect suggest a dominant influence of meltwater influx on sea‐ice formation throughout the Holocene, in contrast to sea‐ice records from the Fram Strait area, which seem to follow predominantly the summer insolation trend.
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Paleo water isotope records can elucidate how the Arctic water cycle responded to past climate changes. We analyze the hydrogen isotope composition (δ²H) of plant-derived n-alkanoic acids (waxes) from Lake Qaupat, Baffin Island, Nunavut, Canada, to assess moisture sources and seasonality during the past 5.8 ka. We compare this record to a sedimentary ancient DNA (sedaDNA)-inferred vascular plant record from the same lake, aiming to overcome the uncertainty of plant community impacts on leaf waxes. As the sedaDNA record reveals a stable plant community after the colonization of Betula sp. at 6.1 ka, we interpret plant wax δ²H values to reflect climate, specifically mean annual precipitation δ²H. However, the distributions of n-alkanoic acid homologs suggest that aquatic mosses, which are not represented in the sedaDNA record, may become more abundant towards the present. Therefore, we cannot exclude the possibility that changes in the plant community cause changes in the plant wax δ²H record, particularly long-chain waxes, which become less abundant through this record. We find that Lake Qaupat mid-chain plant wax δ²H is enriched coincident with high Labrador Sea summer surface temperature, which suggests that local moisture sources in summer and early autumn have the greatest impact on precipitation isotopes in this region.
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The Petermann 2015 expedition to Petermann Fjord and adjacent Hall Basin recovered a transect of cores, extending from Nares Strait to underneath the 48 km long ice tongue of Petermann glacier, offering a unique opportunity to study ice–ocean–sea ice interactions at the interface of these realms. First results suggest that no ice tongue existed in Petermann Fjord for large parts of the Holocene, raising the question of the role of the ocean and the marine cryosphere in the collapse and re-establishment of the ice tongue. Here we use a multi-proxy approach (sea-ice-related biomarkers, total organic carbon and its carbon isotopic composition, and benthic and planktonic foraminiferal abundances) to explore Holocene sea ice dynamics at OD1507-03TC-41GC-03PC in outer Petermann Fjord. Our results are in line with a tight coupling of the marine and terrestrial cryosphere in this region and, in connection with other regional sea ice reconstructions, give insights into the Holocene evolution of ice arches and associated landfast ice in Nares Strait. The late stages of the regional Holocene Thermal Maximum (6900–5500 cal yr BP) were marked by reduced seasonal sea ice concentrations in Nares Strait and the lack of ice arch formation. This was followed by a transitional period towards Neoglacial cooling from 5500–3500 cal yr BP, where a southern ice arch might have formed, but an early seasonal breakup and late formation likely caused a prolonged open water season and enhanced pelagic productivity in Nares Strait. Between 3500 and 1400 cal yr BP, regional records suggest the formation of a stable northern ice arch only, with a short period from 2500–2100 cal yr BP where a southern ice arch might have formed intermittently in response to atmospheric cooling spikes. A stable southern ice arch, or even double arching, is also inferred for the period after 1400 cal yr BP. Thus, both the inception of a small Petermann ice tongue at ∼ 2200 cal yr BP and its rapid expansion at ∼ 600 cal yr BP are preceded by a transition towards a southern ice arch regime with landfast ice formation in Nares Strait, suggesting a stabilizing effect of landfast sea ice on Petermann Glacier.
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The northern sector of the Greenland Ice Sheet is considered to be particularly susceptible to ice mass loss arising from increased glacier discharge in the coming decades. However, the past extent and dynamics of outlet glaciers in this region, and hence their vulnerability to climate change, are poorly documented. In the summer of 2019, the Swedish icebreaker Oden entered the previously unchartered waters of Sherard Osborn Fjord, where Ryder Glacier drains approximately 2 % of Greenland's ice sheet into the Lincoln Sea. Here we reconstruct the Holocene dynamics of Ryder Glacier and its ice tongue by combining radiocarbon dating with sedimentary facies analyses along a 45 km transect of marine sediment cores collected between the modern ice tongue margin and the mouth of the fjord. The results illustrate that Ryder Glacier retreated from a grounded position at the fjord mouth during the Early Holocene (> 10.7±0.4 ka cal BP) and receded more than 120 km to the end of Sherard Osborn Fjord by the Middle Holocene (6.3±0.3 ka cal BP), likely becoming completely land-based. A re-advance of Ryder Glacier occurred in the Late Holocene, becoming marine-based around 3.9±0.4 ka cal BP. An ice tongue, similar in extent to its current position was established in the Late Holocene (between 3.6±0.4 and 2.9±0.4 ka cal BP) and extended to its maximum historical position near the fjord mouth around 0.9±0.3 ka cal BP. Laminated, clast-poor sediments were deposited during the entire retreat and regrowth phases, suggesting the persistence of an ice tongue that only collapsed when the glacier retreated behind a prominent topographic high at the landward end of the fjord. Sherard Osborn Fjord narrows inland, is constrained by steep-sided cliffs, contains a number of bathymetric pinning points that also shield the modern ice tongue and grounding zone from warm Atlantic waters, and has a shallowing inland sub-ice topography. These features are conducive to glacier stability and can explain the persistence of Ryder's ice tongue while the glacier remained marine-based. However, the physiography of the fjord did not halt the dramatic retreat of Ryder Glacier under the relatively mild changes in climate forcing during the Holocene. Presently, Ryder Glacier is grounded more than 40 km seaward of its inferred position during the Middle Holocene, highlighting the potential for substantial retreat in response to ongoing climate change.
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Two major climatic phenomena that occurred during the Holocene are interpreted from the resonance in subharmonic modes of long-period Rossby waves winding around the North Atlantic gyre, the so-called gyral Rossby waves (GRWs). These are, on the one hand, the change in atmospheric circulation that occurred in the North Atlantic in the middle Holocene, and, on the other hand, the occurrence of abrupt cooling events more frequently than what is generally accepted. The amplitude of GRWs is deduced by filtering, within bands characteristic of various subharmonic modes, climate records from the Greenland ice sheet, pollen, and tree rings in northern Fennoscandia, and from two Norwegian glaciers in northern Folgefonna and on the Lyngen peninsula. While the subharmonic modes reflect the acceleration/deceleration phases of the western boundary current, an anharmonic mode is evidenced in the 400–450 year band. Abrupt cooling events of the climate are paced by this anharmonic mode while the western boundary current is decelerating, and the northward heat advection of air favors the melting of the pack ice. Then, the current of the northernmost part of the North Atlantic gyre cools before branching off to the north, which alters its buoyancy. On the other hand, according to high subharmonic modes, high-pressure systems prevailed over the North Atlantic in the first half of the Holocene while low-pressure systems resulted from baroclinic instabilities of the atmosphere dominate during the second half, favoring the growth of glaciers in Scandinavia by a better snowfall in winter and cooler summers.
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Griffiths et al. (2017) analyzed several ponds and lakes from the Cape Herschel region of Ellesmere Island in order to “… explicitly examine the role of ice cover as the dominant driver of diatom assemblage change …”. I reanalyze their data and suggest that their classification scheme, that they propose is due to differences in ice cover seasonality (“warm”, “cool”, “cold”, and “oasis”), is confounded with other morphological and chemical variables that better explain the differences between the groups. The “cold” sites are the deepest (lakes) and differ from the small, shallow ponds that occasionally dry, which would therefore have different diatom assemblages and histories. The “oasis” sites are nutrient enriched and probably have more stable water supplies, thereby enabling an aquatic flora providing habitats for diatoms. A key part of their interpretation is that “warm” sites have responded more rapidly to recent climate change than “cold” or “cool” sites, but their chronologies do not allow for such a conclusion. There is no clear difference between “cool” and “warm” sites, and problems in dating the sequences means inferences about their histories are not supported by data. Their results, which are restricted to the past century, are contradicted by a Holocene sequence from the region.
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We report on stable oxygen and carbon isotope data obtained on two different morphotypes of polar planktic foraminifers, i.e., fully encrusted and minor encrusted Neogloboquadrina pachyderma, from a sediment core taken on the NE Greenland continental margin. These morphotypes are supposed to live at different water depths of the upper water column in the area which today is strongly stratified, with a low-saline, cold-water layer at the surface. The paired isotopic data sets inform on temporal variations of past water salinity and temperature in the preferred water depth ranges of the investigated morphotypes and allow conclusions on the stratification of the upper water column. The radiocarbon-dated sediment core covers the time interval from 21 to 4 cal-ka, but the early part of the deglacial interval (18.5–12.7 cal-ka) is not represented, probably due to intense erosion by bottom currents. In sediments from the late last glacial maximum, oxygen isotope differences between thin-shelled and thick-shelled N. pachyderma are low and point at a weaker stratification with less freshwater than today near the surface. The carbon isotopes indicate a dense, perennial sea ice cover, very limited bioproduction, and the presence of a subsurface Atlantic Water layer. In the late deglaciation until ∼10.3 cal-ka, the stable isotope values of both analyzed morphotypes are considerably lower, with significantly stronger amplitudes in the record of thin-shelled specimens than later on. The high-amplitude record stems from a laminated sediment sequence whose older part was deposited within just a few decades. The data are evidence of a strong freshwater event in the research area that probably started before 12.7 ka and may have reduced sea surface salinities by 4–5 practical salinity units. As freshwater sources we discuss both the disintegration of NE Greenland shelf ice and export from the Arctic Ocean interior. The event may have contributed to the weakening of the Atlantic meridional overturning circulation during the Younger Dryas cold event. For the early and mid-Holocene (10-4 cal-ka), the isotope data suggest a structure of the upper water column similar to today, with a well-developed halocline separating low-saline near-surface waters from the underlying Atlantic Water layer. A seasonally disintegrated sea ice cover allowed for a considerable planktic bioproduction.
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The landscape of northeast Ontario, Canada, is dominated by glacial and postglacial landforms including numerous boreal lakes that formed following the retreat of the Laurentide Ice Sheet (LIS) and Glacial Lake Ojibway. This study uses micro-X-Ray Fluorescence elemental analysis to examine changes in sediment cores from two boreal lakes from northeast Ontario with the goal of inferring changes in terrestrial inputs and redox status of these lakes over the middle to late Holocene. These headwater lakes were selected as they have small watersheds that minimizes the complexity of hydrological responses, and as they are relatively proximal (∼130 km distance between lakes) to allow the detection of regional signals related to changes in climate over the Holocene. Strong correlations between Si, K, Ca, and Ti in Hogback Lake and Green Lake cores suggest that these elements represent proxies of terrestrial input. Standardized composite trends of these elements were very similar between the two lakes and were combined to represent a regional signal, with the highest terrestrial inputs between ∼6000 and ∼4000 cal yr BP, that subsequently declined through the Holocene. These results suggest that regional patterns in terrestrial input are linked to changes in hydroclimate and likely increased terrestrial stability over the middle to late Holocene. These composite trends are compared to similar geochemical composite trends from Iceland suggesting climate teleconnections between northeast North America and the North Atlantic. Cool conditions in the North Atlantic result in increased soil erosion in Iceland and correspond to cool and dry conditions in boreal northeast Ontario with corresponding low inputs of terrestrial material, and vice versa, over multidecadal time scales over the middle to late Holocene.
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Descriptions of regional climate expression require data from multiple lakes, yet little is known of how variation in records within morphometrically complex lakes may affect interpretations. In northeast Ontario (Canada), this issue was addressed using records of pollen, pigments, and diatoms in three sediment cores from two small boreal lakes spanning the last ~6000 years. Pollen analysis suggested warm conditions between ~6000 and ~4000 cal yr BP, coherent with previous assessments from boreal eastern Ontario and western Quebec. Analysis of phototrophic communities from fossil pigments and diatom valves suggested relatively eutrophic conditions with lower lake-levels during this interval. Generalized additive model trends identified significant regional changes in pollen assemblages and declines in pigment concentrations after ~4000 cal yr BP consistent with cooler and wetter climate conditions that resulted in regional lake oligotrophication and increased lake levels during the late-Holocene. Despite contemporaneous changes in pollen and pigment biomarkers across lakes, cores collected from adjacent basins of the same lake (Green Lake) did not show similar trends in fossil pigments likely reflecting preferential deposition of clay-rich allochthonous material in the deeper central basin and suggesting that regional signals in climate may be complicated by lake- or basin-specific catchment processes.
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The arid region of Central Asia is one of the world's major sources of dust and exerts a significant influence on marine ecosystems, atmospheric carbon dioxide concentrations, the global radiation budget, and thus global climate change. Recent global warming has considerably reduced mid-latitude net precipitation by decreasing the latitudinal temperature gradient between the Equator and the Arctic; however, the influence of ice sheet and solar insolation on moisture evolution in Central Asia during the Holocene remains uncertain. Here we show that the relative wet conditions during the early Holocene in northern Central Asia (NCA) were controlled principally by the southern position of the mid-latitude Westerlies under the negative phase of the North Atlantic Oscillation (NAO), a pattern that was influenced by the substantial remnants of the Laurentide and Fennoscandian ice sheets. Subsequent northward migration of the mid-latitude Westerlies under a positive NAO phase resulted in persistent drought conditions during the middle Holocene thermal maximum, due to the combined effects of relatively higher summer insolation and Arctic amplification as well as sea ice loss. In contrast, southward migration of the mid-latitude Westerlies since approximately 3.6 cal kyr BP, driven by declining summer insolation and coincident with the negative NAO phase, increased regional precipitation towards to persistent relatively wet conditions in Central Asia. This reconstructed pattern of Holocene moisture availability contrasts markedly with the increase in precipitation over Central Asia under the current anthropogenically forced warming, thereby justifying further investigation into the multiple forcing mechanisms driving natural and anthropogenic climate change.
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The northern sector of the Greenland ice sheet is considered to be particularly susceptible to ice mass loss arising from increased glacier discharge in the coming decades. However, the past extent and dynamics of outlet glaciers in this region, and hence their vulnerability to climate change, are poorly documented. In the summer of 2019, the Swedish icebreaker Oden entered the previously unchartered waters of Sherard Osborn Fjord, where Ryder Glacier drains approximately 2 % of Greenland's ice sheet into the Lincoln Sea. Here we reconstruct the Holocene dynamics of Ryder Glacier and its ice tongue by combining radiocarbon dating with sedimentary facies analyses along a 45 km transect of marine sediment cores collected between the modern ice tongue margin and the mouth of the fjord. The results illustrate that Ryder Glacier retreated from a grounded position at the fjord mouth during the Early Holocene (>10.7 ± 0.4 cal ka BP) and receded more than 120 km to the end of Sherard Osborn Fjord by the Middle Holocene (6.3 ± 0.3 cal ka BP), likely becoming completely land-based. A re-advance of Ryder Glacier occurred in the Late Holocene, becoming marine-based around 3.9 ± 0.4 cal ka BP. An ice tongue, similar in extent to its current position was established in the Late Holocene (between 3.6 ± 0.4 and 2.9 ± 0.4 cal ka BP) and extended to its maximum historical position near the fjord mouth around 0.9 ± 0.3 cal ka BP. Laminated, clast-poor sediments were deposited during the entire retreat and regrowth phases, suggesting the persistence of an ice tongue that only collapsed when the glacier retreated behind a prominent topographic high at the landward end of the fjord. Sherard Osborn Fjord narrows inland, is constrained by steep-sided cliffs, contains a number of bathymetric pinning points that also shield the modern ice tongue and grounding zone from warm Atlantic waters, and has a shallowing inland sub-ice topography. These features are conducive to glacier stability and can explain the persistence of Ryder’s ice tongue while the glacier remained marine-based. However, the physiography of the fjord did not halt the dramatic retreat of Ryder Glacier under the relatively mild changes in climate forcing during the Holocene. Presently, Ryder Glacier is grounded more than 40 km seaward of its inferred position during the Middle Holocene, highlighting the potential for substantial retreat in response to ongoing climate change.
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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.
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We address here the specific timing and amplitude of sea‐surface conditions and productivity changes off SW Greenland, northern Labrador Sea, in response to the high deglacial meltwater rates, the Early Holocene maximum insolation and Neoglacial cooling. Dinocyst assemblages from sediment cores collected off Nuuk were used to set up quantitative records of sea ice cover, seasonal sea‐surface temperature (SST), salinity (SSS), and primary productivity, with a centennial to millennial scale resolution. Until ~10 ka BP, ice‐proximal conditions are suggested by the quasi‐exclusive dominance of heterotrophic taxa and low dinocyst concentrations. At about 10 ka BP, an increase in species diversity and abundance of phototrophic taxa marks the onset of interglacial conditions at a regional scale, with summer SST reaching up to 10 °C between 8 and 5 ka BP, thus in phase with the Holocene Thermal Maximum as recorded in the southern Greenlandic areas/northern Labrador Sea. During this interval, low SSS but high productivity prevailed in response to high meltwater discharge and nutrient inputs from the Greenland Ice Sheet. After ~5 ka BP, a decrease in phototrophic taxa marks a two‐step cooling of surface waters. The first started at ~5 ka BP, and the second at ~3 ka BP, with a shift toward colder conditions and higher SSS suggesting reduced meltwater discharge during the Neoglacial. This second step coincides with the disappearance of the Saqqaq culture. The gap in human occupation in west Greenland, between the Dorset and the Norse settlements from 2000 to 1000 years BP, might be linked to high amplitude and high frequency variability of ocean and climate conditions.
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An overview of climate reconstructions considering summer air temperatures and effective precipitation is provided for northernmost Fennoscandia. During the earliest part of the Holocene (11,700–10,000 cal. BP), temperatures rose rapidly and were followed by mild, wet and variable conditions. An early major warming peaked around 9500 cal. BP, although many records indicate that the main Holocene warming first occurred about c. 8000 cal. BP. The sub-regional pattern of climate change suggests a defining influence of the westerlies and the North Cape Current. Non-analog climatic conditions and lags in vegetation responses to climate change may explain some of the discrepancies seen in the early Holocene between proxies. In contrast to the perceivable variable onset of the main Holocene warm period, maximum temperatures are relatively consistent between the records, indicating temperatures 1.5±0.5°C above present. Precipitation was generally high from 10,000 cal. BP but decreased towards 8000 cal. BP when dry climatic conditions became predominant. After a stable period 8000–6000 cal. BP a gradual cooling was initiated, with a more abrupt period of change 4500–3800 cal. BP when the warm and dry climate of the mid-Holocene changed into the cool, wet and unstable climate of the late Holocene. Modern conditions were reached c. 2800 cal. BP. The Holocene Thermal Maximum may be defined several different ways: as temperatures distinctly above modern delimited to 9500–4000 cal. BP; as peak temperatures 9500–6000 cal. BP; and/or as climax vegetation in the period 8000–4000 cal. BP. Prior to 8000 cal. BP vegetation probably lagged behind the warming, whereas in the period 8000–4000 cal. BP an equilibrium between climate and vegetation was established.
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The Petermann 2015 Expedition to Petermann Fjord and adjacent Hall Basin recovered a transect of cores from Nares Strait to under the 48 km long ice tongue of Petermann glacier, offering a unique opportunity to study ice-ocean-sea ice interactions at the interface of these realms. First results suggest that no ice tongue existed in Petermann Fjord for large parts of the Holocene, raising the question of the role of the ocean and the marine cryosphere in the collapse and re-establishment of the ice tongue. Here we use a multi-proxy approach (sea-ice related biomarkers, total organic carbon and its carbon isotopic composition, and benthic and planktonic foraminiferal abundances) to explore Holocene sea-ice dynamics at OD1507-03TC-41GC-03PC in outer Petermann Fjord. Our results are in line with a tight coupling of the marine and terrestrial cryosphere in this region and, in connection with other regional sea-ice reconstructions, give insights into the Holocene evolution of ice arches and associated landfast ice in Nares Strait. The late stages of the regional Holocene Thermal Maximum (5,500–6,900 cal yrs BP) were marked by reduced seasonal sea-ice concentrations in Nares Strait and the lack of ice arch formation. This was followed by a transitional period towards neoglacial cooling from 3,500–5,500 cal yrs BP, where a southern ice arch might have formed, but an early seasonal break-up and late formation likely caused a prolonged open water season and enhanced pelagic productivity in Nares Strait. Between 1,400 cal yrs BP and 3,500 cal yrs BP, regional records suggest the formation of a stable northern ice arch only, with a short period from 2,100–2,500 cal yrs BP where a southern ice arch might have persisted in response to atmospheric cooling spikes. A stable southern ice arch, or even double arching, is also inferred for the period after 1,400 cal yrs BP. Thus, both the inception of a small Petermann ice tongue at ~2,200 cal yrs BP and its rapid expansion at ~600 cal yrs BP are preceded by a transition towards a southern ice arch regime with landfast ice formation in Nares Strait, suggesting a stabilizing effect of landfast sea ice on Petermann Glacier.
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This study evaluates the dependence of simulated surface air temperatures on model resolution and orography for the mid-Holocene. Sensitivity experiments with the atmospheric general circulation model ECHAM5 are performed with low (∼3.75°, 19 vertical levels) and high (∼1.1°, 31 vertical levels) resolution. Results are compared to the respective preindustrial runs. It is found that the large-scale temperature anomalies for the mid-Holocene (compared to preindustrial) are significantly different in the low- and high-resolution versions. For boreal winter, differences are mainly related to circulation changes caused by the response to thermal forcing in conjunction with orographic resolution. For summer, shortwave cloud radiative forcing emerges as an important factor. The anomaly differences (low minus high resolution version) in the Northern Hemisphere are regionally as large as the anomalous mid-Holocene temperature signals. Furthermore, they depend on the applied surface boundary conditions. We conclude that the resolution matters for the Northern Hemisphere response in mid-Holocene simulations, which should be taken into account in model-model and data-model comparisons.
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The Arctic is rapidly changing, disrupting biogeochemical cycles and the processing, delivery and sedimentation of carbon (C), in linked terrestrial-aquatic systems. In this investigation, we coupled a hydrogeomorphic assessment of catchment soils, sediments and plants with a recent lake sediment sequence to understand the source and quality of organic carbon present in three Arctic upland lake catchments on Disko Island, located just south of the Low-High Arctic transition zone. This varied permafrost landscape has exposed soils with less vegetation cover at higher altitudes, and all lakes received varying extent of glacial meltwater inputs. We provide improved isotope and biomarker source identifications for palaeolimnological studies in high latitude regions, where terrestrial vegetation is at or close to its northerly and altitudinal range limit. The poorly developed catchment soils lead to lake waters with low dissolved organic carbon (DOC) concentrations (≤ 1.5 mg L−1). Sedimentary Carbon / Nitrogen (C / N) ratios, the C isotope composition of organic matter (δ13Corg) and biomarker ratios (n-alkanes, n-alkanols, n-alkanoic acids and sterols) showed that sedimentary organic matter (OM) in these lakes is mostly derived from aquatic sources (algae and macrophytes). We used a 210 Pb dated sediment core to determine how carbon cycling in a lake-catchment system (Disko 2) had changed over recent centuries. Recent warming since the end of the Little Ice Age (LIA ~1860 AD), which accelerated after ca. 1950, led to melt of glacier ice and permafrost releasing nutrients and DOC to the lake, stimulating pronounced aquatic algal production, as shown by a > 10 fold increase in β-carotene, indicative of a major regime shift. Our findings highlight that in Arctic lakes with sparsely developed catchment vegetation and soils, recent Anthropocene warming results in pronounced changes to in-lake C processing and the deposition of more reactive, predominately autochthonous C, compared with extensively vegetated low Arctic systems.
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The earliest Quaternary event represented is the Kap Bryant glacial stade of probable Saalian age, in which an ice sheet covered the area out to the shelf. In contrast, in the Late Weichselian Kap Fulford stade, the area was only partly glaciated, with outlet lobes from an ice sheet occupying the outer fjords. There is some evidence of an earlier (Early Weichselian?) slightly more extensive glaciation (Kap Sumner stade). In the early Holocene, the ice margin had retreated to the middle regions of the fjords and extensive glacial lakes were formed in intervening areas. The Late Holocene Steensby stade produced a readvanee of the ice sheet at the head of the fjords and the growth of local ice caps. Reworked marine fossils in glacial sediments define the Hall Land marine event of Eemian and/or Early Weichselian age. The Late Weichselian/Holocene marine event (Nyeboe Land event) is abundantly represented by deposits whose distribution shows that the limit of transgression varied over the area, with a maximum at about 125 m above sea level. Regression from this limit occurred during the Holocene, initially at a slow rate. The fossil biota are described from the marine sediments and from a small number of terrestrial occurrences.
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In southernmost Greenland near Narsarsuaq, the terminal Narsarsuaq moraine was deposited well outside of a historical Little Ice Age (LIA) moraine adjacent to the modern ice margin. Using 10Be surface exposure dating, we determine Narsarsuaq moraine abandonment at 1.51 ± 0.11 ka. A second set of 10Be ages from a more ice-proximal position shows that ice has been within or at its historical (i.e., LIA) extent since 1.34 ± 0.15 ka. Notably, Narsarsuaq moraine abandonment was coincident with climate amelioration in southern Greenland. Southern Greenland warming at ∼1.5 ka was also concurrent with the end of the Roman Warm Period as climate along the northern North Atlantic sector of Europe cooled into the Dark Ages. The warming of southern Greenland and retreat of ice from the Narsarsuaq moraine is consistent with studies suggesting possible anti-phase centennial-scale climate variability between northwestern Europe and southern Greenland. Other southernmost Greenland ice-margin records do not preclude a pre-LIA ice-margin maximum, potentially concurrent with a Narsarsuaq advance prior to ∼1.51 ka, but also lack sufficient ice-margin control to confirm such a correlation. We conclude that there is a clear need to further determine whether a late-Holocene pre-LIA maximum was a local phenomenon or a regional southern Greenland ice maximum, and if this advance and retreat reflects a regional fluctuation in climate.
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We present a systematic compilation of previously published Holocene proxy climate records from the Arctic. We identified 167 sites from north of 58° N latitude where proxy time-series extend back at least to 6 cal ka, are resolved at sub-millennial scale (at least one value every 400 ± 200 yr) and have age models constrained by at least one age every 3000 yr. In addition to conventional metadata for each proxy record (location, proxy type, reference), we include two novel parameters that add functionality to the database. First, "climate interpretation" is a series of fields that logically describe the specific climate variable(s) represented by the proxy record. It encodes the proxy-climate relation reported by authors of the original studies into a structured format to facilitate inter-comparison with climate model output. Second, "geochronology accuracy score" (chron score) is a numerical rating that reflects the overall accuracy of 14C-based age models from lake and marine sediments. Chron scores were calculated using the original author-reported 14C ages, which are included in this database. The database contains 315 records (some sites include multiple records) from six regions covering the circumpolar Arctic; Fennoscandia is the most densely sampled region (30% of the records), whereas only five records from the Russian Arctic met the criteria for inclusion. The database contains proxy records from lake sediment (60%), marine sediment (32%), glacier ice (5%), and other sources. Most (60%) reflect temperature (mainly summer warmth) and are primarily based on pollen, chironomid, or diatom assemblages. Many (15%) reflect some aspect of hydroclimate as inferred from changes in stable isotopes, pollen and diatom assemblages, humification index in peat, and changes in equilibrium-line altitude of glaciers. This comprehensive database can be used in future studies to investigate the spatial-temporal pattern of Arctic Holocene climate changes and their causes. The Arctic Holocene dataset is available from NOAA Paleoclimatology.
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Pollen analysis and radiocarbon dating of lake sediment cores from four localities in the Frederikshåb district, South-West Greenland, provide a chronology of vegetation changes in the area since c. 10000 B.P. An initial pioneer phase with the early development of some heath communities is followed by amid postglacial phase in which there is the successive appearance of woody scrub species. A final phase shows the decline of some of these. This is interpreted as a general response to a broad amplitude fluctuation in climate, with its optimal period lying between c. 7600 and 3200 B.P. The detailed composition of the vegetation however is influenced by immigration phenomena, with the appearance of many species lagging behind the attainment of their climatic thresholds. Detailed consideration is given to the possibility of Alnus crispa being present as a scrub component during the climatic optimum.
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The climatic mechanisms driving the shift from the Medieval Warm Period (MWP) to the Little Ice Age (LIA) in the North Atlantic region are debated. We use cosmogenic beryllium-10 dating to develop a moraine chronology with century-scale resolution over the last millennium and show that alpine glaciers in Baffin Island and western Greenland were at or near their maximum LIA configurations during the proposed general timing of the MWP. Complimentary paleoclimate proxy data suggest that the western North Atlantic region remained cool, whereas the eastern North Atlantic region was comparatively warmer during the MWP-a dipole pattern compatible with a persistent positive phase of the North Atlantic Oscillation. These results demonstrate that over the last millennium, glaciers approached their eventual LIA maxima before what is considered the classic LIA in the Northern Hemisphere. Furthermore, a relatively cool western North Atlantic region during the MWP has implications for understanding Norse migration patterns during the MWP. Our results, paired with other regional climate records, point to nonclimatic factors as contributing to the Norse exodus from the western North Atlantic region.
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The evidence relating to the Neoglacial history of the western sector of the Greenland Ice Sheet is critically examined. It is concluded that glacierisation began to increase about 3000-3500 BP from a significantly smaller extent than at present, but that in general this does not appear to have reached its maximum until the recent post-1000 BP advances. What little reliable evidence there is of greater early Neoglacial events can be considered to represent local, anomalous developments. -from AuthorQUATERNARY - OCEANS
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Nioghalyfjerdsfiorden in North-East Greenland is at present covered by a floating glacier. Raised marine deposits in the surrounding area contain shells of marine molluscs, bones of marine mammals and piece, of driftwood. A fairly systematic sampling of such material has been conducted, followed by extensive radiocarbon dating. We suggest that the Greenland ice sheet extended onto the shelf offshore North-East Greenland during isotope stage 2, perhaps even reaching the shelf break. During the subsequent recession of the ice sheets the entrance of Nioghalvfjerdsfjorden had become ice-free by 9.7 cal. ka FIR The recession culminated between 7.7 and 4.5 cal, ka BP. during A which time the fjord was glacier-free along its entire 80 km length. No dates younger than 4.5 cal. ka BP are available on marine material from the fjord, and it seems probable that the fjord has been continuously cohered by the floating glacier since this time. The maximum glaciation was attained around AD 1900. after which thinning and recession took place. The marine limit increases front c. 40 m above sea level near the present margin of the Inland Ice to c. 65 m above sea level at the outer coast. These figures fit into the regional pattern of the marine limit for areas both to the south and north, The marine fauna comprise two bivalves, Macoma (calcarea and Serripes groenlandicus. that may represent a Southern element present during the Holocene temperature optimum. Remains of three taxa of southern extralimital terrestrial and limnic plants were dated to 5.1 cal. kit BP, and remains of another extralimital plant were dated to 8.8 and 8.5 cal. ka BP. The known Holocene time ranges of the willow Salix arctica and the lemming Dicrostonyx torquatus ha c been extended back to 8.8 and 6.4 cal. ka BP. respectively, providing minimum date, for their immigration to Greenland.
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We reconstruct the history of the Greenland Ice Sheet margin on the Nuussuaq Peninsula in central West Greenland through the Holocene using lake sediment analysis and cosmogenic 10Be exposure dating of the prominent Drygalski Moraines. Erratics perched on bedrock outboard of the Drygalski Moraines constrain local deglaciation to ~9.9±0.6ka (n=2). Three Drygalski Moraine crests yield mean 10Be ages of 8.6±0.4ka (n=2), 8.5±0.2ka (n=3), and 7.6±0.1ka (n=2) from outer to inner. Perched erratics between the inner two moraines average 7.8±0.1ka (n=2) and are consistent with the moraine ages. Sediments from a proglacial lake with a catchment area extending an estimated 2km beneath (inland of) the present ice sheet terminus constrain an ice sheet minimum extent from 5.4ka to 0.6ka. The moraine chronology paired with the lake sediment stratigraphy reveals that the ice margin likely remained within ~2km of its present position from ~9.9 to 5.4ka. This unexpected early Holocene stability, preceded by rapid ice retreat and followed by minimum ice extent between ~5.4 and 0.6ka, contrasts with many records of early Holocene warmth and the Northern Hemisphere summer insolation maximum. We suggest ice margin stability may instead be tied to adjacent ocean temperatures, which reached an optimum in the middle Holocene.
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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.