Ice-core data used for the construction of the Greenland Ice-Core Chronology 2005 and 2021 (GICC05 and GICC21)

We here describe, document, and make available a wide range of data sets used for annual-layer identification in ice cores from DYE-3, GRIP, NGRIP, NEEM, and EGRIP. The data stem from detailed measurements performed both on the main deep cores and shallow cores over more than 40 years using many different setups developed by research groups in several countries and comprise both discrete measurements from cut ice samples and continuous-flow analysis data. The data series were used for counting annual layers 60 000 years back in time during the construction of the Greenland Ice-Core Chronology 2005 (GICC05) and/or the revised GICC21, which currently only reaches 3800 years back. Now that the underlying data are made available (listed in Table 1) we also release the individual annual-layer positions of the GICC05 timescale which are based on these data sets. We hope that the release of the data sets will stimulate further studies of the past climate taking advantage of these highly resolved data series covering a large part of the interior of the Greenland ice sheet

A first chronology for the East Greenland Ice-core Project (EGRIP) over the Holocene and last glacial termination

This paper provides the first chronology for the deep ice core from the East Greenland Ice-core Project (EGRIP) over the Holocene and the late last glacial period. We rely mainly on volcanic events and common peak patterns recorded by dielectric profiling (DEP) and electrical conductivity measurement (ECM) for the synchronization between the EGRIP, North Greenland Eemian Ice Drilling (NEEM) and North Greenland Ice Core Project (NGRIP) ice cores in Greenland. We transfer the annual-layer-counted Greenland Ice Core Chronology 2005 (GICC05) from the NGRIP core to the EGRIP ice core by means of 381 match points, typically spaced less than 50 years apart. The NEEM ice core has previously been dated in a similar way and is only included to support the match-point identification. We name our EGRIP timescale GICC05-EGRIP-1. Over the uppermost 1383.84 m, we establish a depth–age relationship dating back to 14 967 years b2k (years before the year 2000 CE). Tephra horizons provide an independent validation of our match points. In addition, we compare the ratio of the annual layer thickness between ice cores in between the match points to assess our results in view of the different ice-flow patterns and accumulation regimes of the different periods and geographical regions. For the next years, this initial timescale will be the basis for climatic reconstructions from EGRIP high-resolution proxy data sets, e.g. stable water isotopes, chemical impurity or dust records

Upstream flow effects revealed in the EastGRIP ice-core using a Monte Carlo inversion of a 2D ice-flow model

The North East Greenland ice-stream (NEGIS) is the largest active ice-stream on the Greenland ice-sheet and is a crucial contributor to the ice-sheet mass balance. To investigate the ice-stream dynamics and to gain information about the past climate, a deep ice-core is drilled in the upstream part of the NEGIS, termed the East Greenland ice-core project (EastGRIP). Upstream flow effects introduce non-climatic bias in ice-cores and are particularly strong at EastGRIP due to high ice-flow velocities and the location in an ice-stream on the eastern flank of the Greenland ice-sheet. Understanding and ultimately correcting for such effects requires information on the source area and the local atmospheric conditions at the time of ice deposition. We use a two-dimensional Dansgaard-Johnsen model to simulate ice-flow along three approximated flow-lines between the summit of the ice-sheet and EastGRIP. Model parameters are determined using a Monte Carlo inversion by minimizing the misfit between modeled isochrones and isochrones observed in radio-echo-sounding images. We calculate backward-in-time particle trajectories to determine the source area of ice found in the EastGRIP core today and present estimates of surface elevation and past accumulation-rates at the deposition site. The thinning function and accumulated strain obtained from the modeled velocity field provide useful information on the deformation history in the EastGRIP ice. Our results indicate that increased accumulation in the upstream area is predominantly responsible for the constant annual layer thickness observed in the upper part of the ice column at EastGRIP. Inverted model parameters suggest that the imprint of basal melting and sliding is present in large parts along the flow profiles and that most internal ice deformation happens close to the bedrock. The results of this study can act as a basis for applying upstream corrections to a variety of ice-core measurements, and the model parameters can be useful constraints for more sophisticated modeling approaches in the future

A multi-ice-core, annual-layer-counted Greenland ice-core chronology for the last 3800 years: GICC21

Ice-core timescales are vital for the understanding of past climate; hence they should be updated whenever significant amounts of new data become available. Here, the Greenland ice-core chronology GICC05 was revised for the last 3835 years by synchronizing six deep ice cores and three shallow ice cores from the central Greenland ice sheet. A new method was applied by combining automated counting of annual layers on multiple parallel proxies and manual fine-tuning. A layer counting bias was found in all ice cores because of site-specific signal disturbances; therefore the manual comparison of all ice cores was deemed necessary to increase timescale accuracy. After examining sources of error and their correlation lengths, the uncertainty rate was quantified to be 1 year per century. The new timescale is younger than GICC05 by about 13 years at 3835 years ago. The most recent 800 years are largely unaffected by the revision. Between 800 and 2000 years ago, the offset between timescales increases steadily, with the steepest offset occurring between 800 and 1100 years ago. Moreover, offset oscillations of about 5 years around the average are observed between 2500 and 3800 years ago. The non-linear offset behavior is attributed to previous mismatches of volcanic eruptions, to the much more extensive dataset available to this study, and to the finer resolution of the new ice-core ammonium matching. By analysis of the common variations in cosmogenic radionuclides, the new ice-core timescale is found to be in alignment with the IntCal20 curve (Reimer et al., 2020)

A first annual-layer-counted chronology for the EastGRIP ice core and the search for a precise dating for the Thera eruption

The development of paleoclimatic timescales is of vital importance for the understanding of climate. Ice cores are optimal tools for the construction of a timescale because they record the signal of multiple annually resolved proxies with well preserved stratigraphy. In 2018, the East GReenland Ice Coring Project (EastGRIP) reached a depth of 1760 m, corresponding to an age of approximately 21000 years BP. The newly drilled core has been matched to other Greenland ice cores to adapt the GICC05 ice-core timescale. This provides a chronological basis for the study of the core that is consistent with other Greenland cores. The techniques adopted for matching of the ice cores rely on the assumed synchronicity of deposits from volcanic eruptions, biomass burning events, and solar events [1]. These time markers are essential for the synchronization of different time records as well as for the determination of regional leads and lags occurring at the onset of climatic transitions. The measurements used for volcanic matching are electrical conductivity measurements (ECM) and dielectric profiling (DEP), which were performed directly in the field and then processed to a high precision in depth assignment. Independent matching of DEP and ECM matching was performed to assess the precision of the synchronization before the two records were merged. The strength of the volcanic matching between Greenland ice cores is increased by locating the same Northern Hemisphere volcanic ash deposits (tephra), which possess unique geochemical `fingerprints'. This challenging search is conducted along the length of each core and is particularly useful in the Last Glacial Maximum, where the presence of acidic spikes is scarce both in ECM and DEP data. The transferred timescale is complemented by automated counting of annual layers between the observed tie-points, using annually resolved proxy data measured by chemical Continuous Flow Analysis (CFA). Ultimately, these new results will feed into the revision of the GICC05 time scale and hopefully reconcile the differences between GICC05 and the timescale proposed by Sigl et al [2]. In this framework, we are trying to narrow down the dating of the Thera eruption on Santorini (around 3500 BP). The timing of this event is still debated, because of an apparent discrepancy of about 100 years between carbon-14 dating and historical dating [3]. ​ [1] S. O. Rasmussen et al. “A first chronology for the North Greenland Eemian Ice Drilling (NEEM) ice core". In: Climate of the Past 9.6 (2013), pp. 2713{2730. [2] M. Sigl et al. “Timing and climate forcing of volcanic eruptions for the past 2,500 years". In: Nature 523 (2015), pp. 543{549. [3] C. L. Pearson et al. “Annual radiocarbon record indicates 16th century BCE date for the Thera eruption". In: Science Advances 4.8 (2018)

The establishment of a depth/age relationship dating back to 14,965 b2k from the EGRIP ice core and compression synthetic radar modelling and airborne radar around EGRIP drill site

We have established the initial chronology for the EGRIP ice core over the Holocene and the late last glacial period. We rely on conductivity patterns and volcanic events determined by means of dielectric profiling (DEP), electrical conductivity measurements (ECM) and tephra records for the synchronization between the EGRIP, NEEM and NGRIP ice cores in Greenland. We have transferred the annual-layer-counted Greenland ice Core Chronology 2005 (GICC05) timescale from the NGRIP core to the EGRIP ice core by means of 373 match points. The second part of this study compares numerically modelled radargrams and the airborne radar measurements (radio-echo sounding) to understand the recorded physical properties of internal layers towards reflection mechanisms. Synthetic modelling of electromagnetic wave propagation has been applied to the EGRIP ice core based on the conductivity and permittivity, as measured at 250 kHz by DEP. For the comparison between synthetic and observed data, we have used radio-echo sounding data from AWI’s multichannel ultra-wideband radar around the EGRIP drill site, that were recorded during the 2018 field season

Physical properties of internal layers in Greenland ice sheet: measurements and modelling data analysis

The aim of our study is to analyse physical properties of internal layers of deep ice cores in Greenland (NGRIP and NEEM) and a new ice core record from the East GReenland Ice-core Project (EGRIP) on the North East Greenland Ice Stream (NEGIS). For this purpose, in the first part of this study, we have established the initial chronology for the EGRIP ice core over the Holocene and the late last glacial period. We rely on conductivity patterns and volcanic events determined by means of dielectric profiling (DEP), electrical conductivity measurements (ECM) and tephra records for the synchronization between the EGRIP, NEEM and NGRIP ice cores in Greenland. We have transferred the annual-layer-counted Greenland ice Core Chronology 2005 (GICC05) timescale from the NGRIP core to the EGRIP ice core by means of 373 match points. The second part of this study compares numerically modelled radargrams and the airborne radar measurements (radio-echo sounding) to understand the recorded physical properties of internal layers towards reflection mechanisms. Synthetic modelling of electromagnetic wave propagation has been applied to the EGRIP, NEEM and NGRIP2 ice cores based on the conductivity and permittivity, as measured at 250 kHz by DEP. For the comparison between synthetic and observed data, we have used radio-echo sounding data from AWI’s multichannel ultra-wideband radar around the EGRIP drill site, that were recorded during the 2018 field season, and the CReSIS data from the University of Kansas around the NEEM and NGRIP2 drill sites. The timescales (depth-age relation from first part of our study) have been transferred to the synthetic and observed radargrams by means of sensitivity studies. We have found that conductivity only explains a fraction of the radar signals in Greenland ice sheet and the orientated fabric is widespread and influences the radar data

Permittivity measured with the dielectric profiling (DEP) technique on the NGRIP1 ice core (down to 1371.69 m depth)

Dielectric Profiling (DEP) of the North Greenland Ice Core Project (NGRIP1) core were recorded in the field during the 1996-1998 field seasons with the DEP device described by Wilhelms et al. (1998). The permittivity and conductivity of DEP data are calculated by their respective densities and conductivities (Wilhelms, 2005). The resolution of DEP data is 5 mm. The DEP was not processed at a consistent temperature due to the varying temperature in the field seasons. For more information on the calibration procedure see Mojtabavi et al, 2020