The discovery of more than 600 whole and fragmentary engraved stone plaques in the early third millennium BC infill from the ditches of a causewayed enclosure at Vasagård, on the Danish island of Bornholm, represents a unique find in Neolithic miniature art. Termed ‘sun stones’ in reference to the rayed images that characterise many of the plaques, the stones were deposited en masse over a short period. This article offers a fundamental classification of the rich imagery captured in the engravings and examines its potential function at a time of possible climatic crisis that impacted not just Bornholm but the wider northern hemisphere.

Pleistocene Ice Ages display abrupt Dansgaard-Oeschger (DO) climate oscillations that provide prime examples of Earth System tipping points-abrupt transition that may result in irreversible change. Greenland ice cores provide key records of DO climate variability, but gas-calibrated estimates of the temperature change magnitudes have been limited to central and northwest Greenland. Here, we present ice-core δ15N-N2 records from south (Dye 3) and coastal east Greenland (Renland) to calibrate the local water isotope thermometer and provide a Greenland-wide spatial characterization of DO event magnitude. We combine these data with existing records of δ18O, deuterium excess, and accumulation rates to create a multiproxy "fingerprint" of the DO impact on Greenland. Isotope-enabled climate models have skill in simulating the observational multiproxy DO event impact, and we use a series of idealized simulations with such models to identify regions of the North Atlantic that are critical in explaining DO variability. Our experiments imply that wintertime sea ice variation in the subpolar gyre, rather than the commonly invoked Nordic Seas, is both a sufficient and a necessary condition to explain the observed DO impacts in Greenland, whatever the distal cause. Moisture-tagging experiments support the idea that Greenland DO isotope signals may be explained almost entirely via changes in the vapor source distribution and that site temperature is not a main control on δ18O during DO transitions, contrary to the traditional interpretation. Our results provide a comprehensive, multiproxy, data-model synthesis of abrupt DO climate variability in Greenland.

Past interglacial climates with smaller ice sheets offer analogs for ice sheet response to future warming and contributions to sea level rise; however, well-dated geologic records from formerly ice-free areas are rare. Here we report that subglacial sediment from the Camp Century ice core preserves direct evidence that northwestern Greenland was ice free during the Marine Isotope Stage (MIS) 11 interglacial. Luminescence dating shows that sediment just beneath the ice sheet was deposited by flowing water in an ice-free environment 416 ± 38 thousand years ago. Provenance analyses and cosmogenic nuclide data and calculations suggest the sediment was reworked from local materials and exposed at the surface <16 thousand years before deposition. Ice sheet modeling indicates that ice-free conditions at Camp Century require at least 1.4 meters of sea level equivalent contribution from the Greenland Ice Sheet.

We present new data from the debris-rich basal ice layers of the NEEM ice core (NW Greenland). Using mineralogical observations, SEM imagery, geochemical data from silicates (meteoric 10Be, εNd, 87Sr/86Sr) and organic material (C/N, δ13C), we characterize the source material, succession of previous glaciations and deglaciations and the paleoecological conditions during ice-free episodes. Meteoric 10Be data and grain features indicate that the ice sheet interacted with paleosols and eroded fresh bedrock, leading to mixing in these debris-rich ice layers. Our analysis also identifies four successive stages in NW Greenland: (1) initial preglacial conditions, (2) glacial advance 1, (3) glacial retreat and interglacial conditions and (4) glacial advance 2 (current ice-sheet development). C/N and δ13C data suggest that deglacial environments favored the development of tundra and taiga ecosystems. These two successive glacial fluctuations observed at NEEM are consistent with those identified from the Camp Century core basal sediments over the last 3 Ma. Further inland, GRIP and GISP2 summit sites have remained glaciated more continuously than the western margin, with less intense ice-substratum interactions than those observed at NEEM.

The Younger Toba Tuff eruption similar to 74 ka ago in Indonesia, is among the largest known supereruptions in the Quaternary and its potential impact on the climate system and human evolution remains controversially debated. The eruption is dated radiometrically to 73.88 +/- 0.32 ka (1s, Storey et al., 2012) and it occurred at the abrupt cooling transition from Greenland Interstadial 20 to Greenland Stadial 20. The precise stratigraphic position of volcanic fallout detected in ice cores from both polar ice sheets has previously been narrowed down to four potential candidates. Here, we compile all available Greenland and Antarctic sulfate records, together with electrical conductivity records and recently obtained sulfur isotope records to identify, quantify and characterize these Toba candidates in terms of their likely latitudinal position of eruption, sulfur emission strength and radiative forcing. We identify that the youngest event of the four candidates is composed of two separate eruptions, both likely located in the extra-tropical Northern Hemisphere. We deem the two older events unlikely candidates for the Toba eruption because of their limited sulfur emission strengths. The second youngest event has the largest sulfur output of the Toba candidates, and it is also larger than any other volcanic event identified in ice core records over the last 60 kyr. Comparable amounts of sulfate deposits in Greenland and Antarctica strongly suggest a tropical source. We thus propose the second youngest event (74,156 years before 2000 CE) to be most likely associated with the Toba eruption. The estimated stratospheric sulfate loading of the proposed Toba eruption is 535 +/- 96 Tg, which is 3 times that of Samalas 1258 CE, 6 times that of Tambora (1815) CE and 20 times that of Pinatubo (1991) CE. We derive the continuous time-series of volcanic sulfate deposition, sulfur emission strength and radiative forcing over the 74.8-73.8 ka time window, suitable for conducting experiments with climate models that either require prescribed forcing field or interactively reproduce aerosol processes. We estimate the cumulative volcanic sulfur emission strength and the radiative forcing of the two younger events and they are found to be much stronger than those at the onset of the Younger Dryas and those preceding the Little Ice Age. Stacked Greenland water isotope records show an accelerated transition trend and abrupt shift after the proposed Toba eruption and suggest that the Greenland moisture source moved southward shortly after the Toba eruption. The Toba eruption may thus have an amplifying effect on the cooling transition leading to Greenland Stadial 20. (c) 2023 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

We report high resolution measurements of the stable isotope ratios of ancient ice ( δ 18 O, δ D) from the N orth Greenland Eem ian deep ice core (NEEM, 77.45° N, 51.06° E). The record covers the period 8–130 ky b2k (y before 2000) with a temporal resolution of ≈0.5 and 7 y at the top and the bottom of the core respectively and contains important climate events such as the 8.2 ky event, the last glacial termination and a series of glacial stadials and interstadials. At its bottom part the record contains ice from the Eemian interglacial. Isotope ratios are calibrated on the SMOW/SLAP scale and reported on the GICC05 (Greenland Ice Core Chronology 2005) and AICC2012 (Antarctic Ice Core Chronology 2012) time scales interpolated accordingly. We also provide estimates for measurement precision and accuracy for both δ 18 O and δ D.

YOUNGER DRYAS? J.-H. Seo1, C. Han2,3, S. Bharadwaj1, G. Wörner4, A. Svensson5, E. Osterberg1, J.P. Steffensen5, S. Hong2, and M. Sharma1, 1Department of Earth Sciences, Dartmouth College, Hanover, NH 03755, 2Department of Ocean Sciences, Inha University, Incheon, South Korea, 3Korea Polar Research Institute, Incheon, South Korea, 4Georg-August-Universität Göttingen, GZG, Geochemisches Institut, Goldschmidtstr.1, Göttingen D37077, Germany. 5Niels Bohr Institute, Centre for Ice and Climate, University of Copenhagen, Tagensvej 16 DK2200, Copenhagen Denmark.

SignificanceUnderstanding Greenland Ice Sheet history is critical for predicting its response to future climate warming and contribution to sea-level rise. We analyzed sediment at the bottom of the Camp Century ice core, collected 120 km from the coast in northwestern Greenland. The sediment, frozen under nearly 1.4 km of ice, contains well-preserved fossil plants and biomolecules sourced from at least two ice-free warm periods in the past few million years. Enriched stable isotopes in pore ice indicate precipitation at lower elevations than present, implying ice-sheet absence. The similarity of cosmogenic isotope ratios in the upper-most sediment to those measured in bedrock near the center of Greenland suggests that the ice sheet melted and re-formed at least once during the past million years.

The Camp Century Ice core, NW Greenland, recovered a 4.5 m basal section consisting of frozen sediments and debris-rich ice. This material was recently re-discovered in Danish ice core storage and visually logged. As part of a multi-disciplinary effort to unlock the climatic and paleo-environmental signal of this unique record, we have analysed detrital mineral composition and metamorphic ages. Bulk mineral analyses were performed at the Geological Survey of Denmark and Greenland on grain mounts from 2 core intervals using a SEM automated quantitative mineralogy (AQM) approach coupled to laser ablation ICP-MS analyses. This setup allows us to gain a full mineral description together with single-grain U-Pb dates for a large population of metamorphic components, e.g. apatite, rutile, titanite and zircon. In addition, amphibole grains were picked for 40Ar-39Ar dating performed at the LDEO Argon Isotope Lab. Mineralogical characterization was completed by X-Ray diffraction analysis of the fine fraction to determine the presence and nature of potential clay weathering products, and single-crystal X-ray diffraction was utilized to characterize the atomic arrangements of minerals that occur in solid solutions. The AQM results indicate that metamorphic minerals are present in sufficient amounts (100’s) for gaining statistically valid provenance data. Preliminary results show ages in the 1900 – 1700 Ma range (amphibole, rutile) and around 2700 Ma (zircon). This, along with the presence of swelling clays in the sediments, is consistent with weathering of the local bedrock, and/or sediments transported from the Inglefield orogenic belt north of the site. To gain information on the youngest thermal events of sediment sources, potentially revealing deep glacial incision, (U-Th-Sm)/He dating of single apatite grains is underway. Preliminary work on the 125-250 µm size fraction yield abundant subhedral-to-subrounded, euhedral apatite suitable for thermochronology. Here we report the results from the different methods and discuss the implications for understanding erosional processes and potential transport pathways of the Camp Century basal ice sediments.

As the melting of the Greenland Ice Sheet (GrIS) accelerates, it is critical to improve our knowledge of its Pleistocene history in order to better understand its sensitivity to different climate states. The study of sediment from the base of the ice sheet offers valuable insights, since this material holds useful information about its history and origin. Here, we present various mineralogical and geochemical analysis from basal sediments of the NEEM ice core from northwestern Greenland (NEEM community, 2013), a complement to the first analysis of the basal ice made by Goossens et al. (2016). In an effort to specify the provenance and characterize the sediments in the basal ice of the NEEM ice core, strontium and neodymium isotopic ratios were measured in 7 bulk till samples located into the deepest part of the core. Laser granulometry and shape characterization by SEB images of the grains suggest a mixed origin of this material. The deepest sample yield in situ cosmogenic 10Be and 26Al concentrations lower than 104 at.g-1 and 21Ne concentration in the 107-108 at.g-1 range. These preliminary cosmogenic nuclides data suggest that several cycles of waning and waxing of the GrIS had occurred over the last 10 million years. Additional sample material is being processed to reduce the uncertainty of 26Al and 10Be measurements and refine this chronology. To better characterize the origin of the basal sediment and the duration of pre-burial exposure, measurements of meteoric cosmogenic 10Be in 7 samples distributed along the basal part of the core are currently in progress. These data will be combined with the measurement of total organic carbon and nitrogen in the same samples. C and N concentrations and isotopes bring useful information about the type of soil and till material in these basal sediments (Bierman et al., 2016). Bierman, P.R., Shakun, J.D., Corbett, L.B., Zimmerman, S.R., Rood, D.H., 2016. A persistent and dynamic East Greenland Ice Sheet over the past 7.5 million years. Nature 540, 256–260. https://doi.org/10.1038/nature20147 Goossens, T., Sapart, C.J., Dahl-Jensen, D., Popp, T., El Amri, S., Tison, J.-L., 2016. A comprehensive interpretation of the NEEM basal ice build-up using a multi-parametric approach. The Cryosphere 10, 553–567. https://doi.org/10.5194/tc-10-553-2016 NEEM community, 2013. Eemian interglacial reconstructed from a Greenland folded ice core. Nature, 493. doi:10.1038/nature11789

Volcanic ash (fine-grained tephra) within Greenland ice cores can complement the understanding of past volcanism and its environmental and societal impacts. The presence of ash in sparse concentrations in the ice raises questions about whether such material represents primary ashfall in Greenland or resuspended (remobilized) material from continental areas. In this article, we investigate this issue by examining tephra content in quasi-annual samples from two Greenland ice cores during a period of ca. 20 years and considering their relationships with sulphur and particulate data from the same cores. We focus on the interval 815–835 CE as it encompasses a phase (818–822 CE) of heightened volcanogenic sulphur previously ascribed to an eruption of Katla, Iceland. We find that tephra is a frequent but not continuous feature within the ice, unlike similarly sized particulate matter. A solitary ash shard whose major element geochemistry is consistent with Katla corroborates the attribution of the 822±1 CE sulphur peak to this source, clearly showing that a single shard can signify primary ashfall. Other tephras are present in similarly low abundances, but their geochemistries are less certainly attributable to specific sources. Although these tephra shards tend to coincide with elevated sulphur and fine (<10 µm) particulates, they are not associated with increased coarse (>10 µm) particle concentrations that might be expected if the shards had been transported by dust storms. We conclude that the sparse shards derive from primary ashfall, and we argue that low tephra concentrations should not be dismissed as insignificant.

The Greenland Ice Sheet (GrIS) is melting in response to a rapidly warming climate. It is imperative to understand GrIS sensitivity to past climate, especially during periods when the ice sheet was smaller than present or possibly absent. The Camp Century ice core from NW Greenland, collected in 1966 and the first ice core to be drilled to the bed of the GrIS, revolutionized our understanding of global paleoclimate since 125 ka. However, basal sediment from the ice core was not fully explored and then sat in storage for decades – until it was re-discovered two years ago. We are now investigating these unique samples from the sub-glacial environment using modern analyses. Here, we present initial results from two samples, the upper and lower portions of >4 m of basal sediment. We applied an array of geochemical analyses to characterize paleoenvironment (lipid biomarkers, δ13C, δ15N), to infer past climatic conditions (δ18O, δD) from frozen pore water, and to determine the exposure and burial history of the sediments below the ice sheet (optically stimulated luminescence [OSL], cosmogenic 10Be, 26Al, and 21Ne). The sub-glacial sediment consists of poorly sorted, reddish-brown, quartz-rich diamict, with paleo-permafrost features in some layers. This material contains woody macrofossils, fungal sclerotia (Cenococcum geophilum), and mosses (Tomenthypnum nitens, Polytrichum juniperinum) that yield a 14C age >55 ka. Woody tissue from the upper and lower samples yield stable δ13C ratios of -26.7±0.1‰ and -29.6±0.1‰ and δ15N ratios of 2.4±0.8‰ and -2.3±0.8‰. Leaf wax (n-alkanoic acid) distributions are similar to modern Arctic shrubs. Frozen pore water yielded δ18O ratios of -23.06±0.08‰ and -21.49±0.08‰, enriched relative to all overlying ice (<-27‰). Deuterium-excess values are 4.3±0.8 ‰ and 13.4±0.4 ‰, respectively. These stable isotope measurements of pore water suggest snowfall precipitation at temperatures similar to today if the site were ice-free. OSL measurements from the lower sediment suggest a minimum depositional age >600 ka. In situ 10Be concentrations in quartz decrease with depth from 7.7±0.1 x104 atoms/g (500-850 µm) and 6.6±0.2 x104 (250-500 µm) in the upper sediment to 1.6±0.1 x104 atoms/g (500-850 µm) and 1.8±0.1 x104 (250-500 µm) in the lower sediment. The 26Al/10Be ratio also decreases with depth. In the upper sediment, 26Al/10Be ratios range between 4.2 and 4.9 indicating > 900 ka of burial. In the lower sediment, 26Al/10Be ratios range from 1.4 to 2.0 indicating >2 Ma of burial. Measured 21Ne/10Be ratios in quartz exceed 1000, which could indicate long-term burial and/or the presence of nucleogenic 21Ne. These results demonstrate that Camp Century basal sediment was exposed under ice-free conditions that supported vegetation similar to today. Cosmogenic data indicate the deeper sediment has been buried for most of the Pleistocene and the OSL date rules out surface exposure of the deeper material at MIS 11. Cosmogenic analysis indicates that the upper sample experienced less burial or more recent re-exposure. These data are consistent with a growing body of evidence indicating a dynamic Pleistocene GrIS, even under a pre-industrial climate system in which atmospheric CO2 concentrations did not exceed ~300 ppm.

The Northern Hemisphere experienced dramatic changes during the last glacial, featuring vast ice sheets and abrupt climate events, while high northern latitudes during the last interglacial (Eemian) were warmer than today. Here we use high-resolution aerosol records from the Greenland NEEM ice core to reconstruct the environmental alterations in aerosol source regions accompanying these changes. Separating source and transport effects, we find strongly reduced terrestrial biogenic emissions during glacial times reflecting net loss of vegetated area in North America. Rapid climate changes during the glacial have little effect on terrestrial biogenic aerosol emissions. A strong increase in terrestrial dust emissions during the coldest intervals indicates higher aridity and dust storm activity in East Asian deserts. Glacial sea salt aerosol emissions in the North Atlantic region increase only moderately (50%), likely due to sea ice expansion. Lower aerosol concentrations in Eemian ice compared to the Holocene are mainly due to shortened atmospheric residence time, while emissions changed little.

Atmospheric nuclear weapons testing (NWT) resulted in the injection of plutonium (Pu) into the atmosphere and subsequent global deposition. We present a new method for continuous semiquantitative measurement of (239)Pu in ice cores, which was used to develop annual records of fallout from NWT in ten ice cores from Greenland and Antarctica. The (239)Pu was measured directly using an inductively coupled plasma-sector field mass spectrometer, thereby reducing analysis time and increasing depth-resolution with respect to previous methods. To validate this method, we compared our one year averaged results to published (239)Pu records and other records of NWT. The (239)Pu profiles from the Arctic ice cores reflected global trends in NWT and were in agreement with discrete Pu profiles from lower latitude ice cores. The (239)Pu measurements in the Antarctic ice cores tracked low latitude NWT, consistent with previously published discrete records from Antarctica. Advantages of the continuous (239)Pu measurement method are (1) reduced sample preparation and analysis time; (2) no requirement for additional ice samples for NWT fallout determinations; (3) measurements are exactly coregistered with all other chemical, elemental, isotopic, and gas measurements from the continuous analytical system; and (4) the long half-life means the (239)Pu record is stable through time.

Analysis of stable oxygen isotope (δ18O) characteristics is a useful tool to investigate water provenance in glacier river systems. In order to attain knowledge on the diversity of spatio-temporal δ18O variations in glacier rivers, we have examined three glacierized catchments in Greenland with different areas, glacier hydrology and thermal regimes. At Mittivakkat Gletscher River, a small river draining a local temperate glacier in southeast Greenland, diurnal oscillations in δ18O occur with a three-hour time lag to the diurnal oscillations in runoff. Throughout the peak flow season the δ18O composition is controlled by the proportion between snowmelt and ice melt with episodic inputs of rainwater and occasional storage and release of a specific water component due to changes in the subglacial drainage system. At Kuannersuit Glacier River on the island Qeqertarsuaq, the δ18O characteristics were examined after the major 1995–1998 glacier surge event. Despite large variations in the δ18O values of glacier ice on the newly formed glacier tongue, there were no diurnal oscillations in the bulk meltwater emanating from the glacier in the post-surge years 2000–2001. In 2002 there were indications of diurnal oscillations, and in 2003 there were large diurnal fluctuations in δ18O. At Watson River, a large catchment at the western margin of the Greenland Ice Sheet, the spatial distribution of δ18O in the river system was applied to fingerprint the relative runoff contributions from sub-catchments. Spot sampling indicates that during the early melt season most of the river water (64–73 %) derived from the Qinnguata Kuussua tributary, whereas the water flow on 23 July 2009 was dominated by bulk meltwater from the Akuliarusiarsuup Kuua tributary (where 7 and 67 % originated from the Russell Glacier and Leverett Glacier sub-catchments, respectively). A comparison of the δ18O compositions from glacial river water in Greenland shows distinct differences between water draining local glaciers (between −17.4 and −13.7 ‰), large ice caps (between −23.0 and −17.8 ‰) and the Greenland Ice Sheet (between −29.9 and −23.2 ‰).