We present a record of melt events obtained from the East Greenland Ice Core Project (EastGRIP) ice core in central northeastern Greenland, covering the largest part of the Holocene.
The data were acquired visually using an optical dark-field line scanner. We detect and describe melt layers and lenses, seen as bubble-free layers and lenses, throughout the ice above the bubble–clathrate transition. This transition is located at 1150 m depth in the EastGRIP ice core, corresponding to an age of 9720 years b2k.
We define the brittle zone in the EastGRIP ice core as that from 650 to 950 m depth, where we count on average more than three core breaks per meter.
We analyze melt layer thicknesses, correct for ice thinning, and account for missing layers due to core breaks.
Our record of melt events shows a large, distinct peak around 1014 years b2k (986 CE) and a broad peak around 7000 years b2k, corresponding to the Holocene Climatic Optimum. In total, we can identify approximately 831 mm of melt (corrected for thinning) over the past 10 000 years.
We find that the melt event from 986 CE is most likely a large rain event similar to that from 2012 CE, and that these two events are unprecedented throughout the Holocene. We also compare the most recent 2500 years to a tree ring composite and find an overlap between melt events and tree ring anomalies indicating warm summers.
Considering the ice dynamics of the EastGRIP site resulting from the flow of the Northeast Greenland Ice Stream (NEGIS), we find that summer temperatures must have been at least 3
Melt layers are commonly thought of as events with surface melt due to intense solar radiation and/or a high temperature, leading to the formation of superficial liquid melt puddles followed by their percolation into the snowpack (e.g.,
The features in the snowpack resulting from superficial melt water can be divided into horizontal melt layers and lenses
A 10 000-year melt layer record from a Greenlandic ice core (the Greenland Ice Sheet Project 2 (GISP2) ice core) was presented by
A range of techniques have been applied to investigate melt layers in ice cores from Greenland and other locations.
Studies of melt features in the ablation zone of the Greenland ice sheet have been conducted using multiple shallow ice cores
Melt, or bubble-free, layer records for the past 10 000 years have only been identified for the GISP2
More common methods to detect melt layers are to identify irregularities in the electronic conductivity measurements (ECM, Sune Rasmussen, personal communication, 2021) or anomalies in stable water isotope records
The 2012 CE melt and rain event in Greenland is very well observed and documented, e.g.,
A problem with interpreting melt layers is shown by the snowpit sampling performed during the 2012 CE melt event at NEEM (Figs.
In the Appendix, we also present the result from a simple rain/melt event experiment performed in April 1995, using cold coffee as a colored substitute for melt (Fig.
Melt layers can be found in ice cores throughout the Holocene in central Greenland. To analyze and understand these, a climatic overview is necessary.
The timing and intensity of the Holocene Climatic Optimum (HCO) is still debated: e.g.,
The East Greenland Ice Core Project (EastGRIP) ice core, based on which we create our melt layer record, is drilled through the Northeast Greenland Ice Stream (NEGIS, Fig.
Our analysis covers the upper 1090 m of the EastGRIP ice core, corresponding to the years 1965 CE to 7604 BCE, i.e., 9569 years.
We use the age scale provided by
The depth notation in this work refers to the depth below the 2017 ice sheet surface, the year in which ice core drilling began. Ice core processing started 13.75 m below the surface, which corresponds to the year 1965 CE (44 years b2k,
We terminate our investigation of bubble-free layers at a depth of 1090 m (approximately 9604 years b2k) because of the almost complete transition from air bubbles to clathrates (e.g.,
The line scanner is a well-established and powerful tool for high-resolution analysis of ice stratigraphy, making use of contrast enhancement by the optical dark-field method
The appearance of different structures in line scan images of firn (left) and ice (right).
The line scan images for firn and ice differ substantially (Fig.
At EastGRIP, the firn–ice transition is situated at around 70 m depth
In the upper 1100 m of the EastGRIP ice core, the majority of the ice contains bubbles, and thus has the “normal” appearance of firn and ice (Fig.
We define the different types as follows:
Melt layers are, in general, continuous features ranging across the entire horizontal core width (10 cm). The melt layer thickness can vary within one layer, but we define that it should always be greater than 1 mm at its narrowest point (1 mm Melt lenses have the same appearance as melt layers, yet are of smaller dimensions and are not continuous across the width of the core. The definition of layer and lens is therefore determined by the core diameter, which in the EastGRIP ice core is approximately 10 cm. Lenses can have a rounded shape, yet, in general, they show elongation along the horizontal. These disk-shaped structures point to a melt layer above and, in order not to overestimate the number of events, the lens itself should thus not be seen as a separate event (Sepp Kipfstuhl, personal communication, 2021). Crusts are very thin (around 1 mm thick) bubble-free layers that are, in general, continuous from one side of the core to the other. They have a sharp border with the bubbles around them. These thin layers can be identified reasonably well and distinguished from melt layers in the upper 250 m. Yet, as the thinning of layers proceeds, it becomes no longer possible to distinguish them from the 2D line scan images. We therefore assume that, below 250 m, all layers with the appearance of crusts are actually thinned melt layers.
Thinning is influential to such a degree that crusts eventually become no longer detectable using line scan images.
Core breaks influence the counting of melt layers and lenses. Core breaks are fractures in the core, which mainly occur for two reasons: they are produced when breaking the ice core free at the bottom of the borehole (see Drilling-related core breaks are usually approximately horizontal. During smooth drilling operations and with good ice quality, core breaks occur every few meters, depending on the length of the core barrel chamber implemented in the deployed drilling system. In the brittle zone, where the internal pressure of the trapped air bubbles is very high and exceeds the tensile strength of the ice core, the ice core samples will break up and sometimes even explode. This is an effect of pressure–temperature relaxation after core recovery at the surface. Core breaks in the brittle zone can have any orientation and thus tend to run diagonally across the core and line scan image.
During line scanning, light is introduced at an angle from below the core slab.
As the core breaks usually have a rough break surface followed by a gap and another rough break surface, the light intensity will drop when crossing the void. This intensity loss casts shadows on either side of the core break. These shadows greatly depend on the geometry of the core break and can easily be mistaken for a bubble-free layer. A rare occasion (one of two in total) is shown by Fig.
A core break casting a shadow and a melt layer have a very similar appearance in line scan images. A distinction is made based first on the proximity to the break and then on differences in brightness along the ice core's round drilling edge (yellow boxes). Core break shadows darken the edge of the sample. The minimum section not suited for analysis is indicated by the red bar.
To account for this difficulty, features close to core breaks and the edges of images are in general disregarded. This implies that the more core breaks we have, the more bubble-free events we may miss, and the more we underestimate the number of events. It is, therefore, necessary to obtain an overview of core breaks throughout the depth of interest. We estimate the chance of missing a bubble-free event by assuming a 4 cm sample loss for each break. In general, a shadow is cast 1.5 cm to either side of the break, and the break itself disturbs the image across at least 1 cm, adding up to 4 cm in total (Fig.
For the comparison of melt events to the tree ring data, we translate the EastGRIP (Greenland Ice Core Chronology 2005: GICC05) ages to the tree ring timescale (NS1-2011). We verified that there is good alignment of EastGRIP and N-Tree data, as many volcanic eruptions align with drastic cooling events to within 1 to 2 years. We refer to ages and events using the GICC05 timescale for consistency throughout the paper.
We find 561 melt events throughout the last 9700 years in the EastGRIP record (Fig.
Number of melt layers and lenses per century throughout the last 9700 years in the EastGRIP ice core. Running means are shown as solid lines.
Both melt lenses and layers follow the same trend and are most abundant during the same periods. As both features represent refrozen melt water, we can consequently group them together as melt events (Fig.
Including uncertain events, the number of events shows a slight increase towards the Early Holocene. These are melt layers and lenses that are difficult to see in the line scan data, and should thus be treated with caution.
Events older than 9000 years become difficult to detect due to progressive bubble to clathrate transformation; therefore, values gradually decrease. Slightly before 9000 years b2k, the ratio of uncertain to certain layers increases, indicating the difficulty of detecting melt layers. Also, we do not capture the most recent years, i.e., those younger than 44 years b2k (1956 CE).
We count core breaks (Fig.
As we know the number of melt events per sample, we can estimate the number of events missed due to core break shadows (Fig.
Our correction described above assumes no correlation between the locations of core breaks and melt layers. This correlation could be expected, as melt layers might affect the crystal structure or other physical properties of the core. We performed a nonquantitative visual inspection and did not find any connection of melt layers to weakening or strengthening of the ice, which would affect the initiation and location of core breaks in the brittle zone.
We have documented the thicknesses of the 137 certain melt layers (
The layer thicknesses of melt layers are shown by the yellow, orange, and red bars (smaller than 4 mm, between 4 and 8 mm, and greater than 8 mm, respectively). To distinguish events occurring within a short period, the layer thicknesses are indicated by circles (measured thicknesses
We correct the melt layer thickness for thinning, i.e., we correct the initial thickness
Thin melt layers (
We expect to miss thinner melt layers the further back we go in time, which is seen in our results (Fig.
We only find melt layers exceeding a thickness of 15 mm between 6100 and 8100 years b2k, with one exception at 1014 years b2k (Fig.
Derived from melt layer thicknesses, we present a melt layer record of the total amount of melt per century and millennium (Fig.
Millimeters of melt per century (Fig.
In both plots (centuries and millennia), there is a particularly prominent peak at around 1000 years b2k (light blue). The melt event from this period, i.e., 1014 years b2k or 986 CE, was of such an intensity that it left an unprecedented spike in the melt record of the past 10 000 years. Here, it is important to note that this is an event confined to a short period of one or a few summers, and not a signal that is representative of the entire century or millennium.
If a melt lens is behind bubbles, it becomes hard to see in our 2D images, and will probably be classified as uncertain or missed completely (Fig.
We may also miss bubble-free layers in the ice sheet's stratigraphy due to the ice core's restricted diameter.
Studies such as
The EastGRIP ice core is drilled into the NEGIS (Fig.
Adding the values from
In comparison with ice core melt layer records from southwestern Greenland
When analyzing melt layers on an annual timescale, the well-studied 2012 CE melt event in Greenland (e.g.,
Nine melt layers and 12 melt lenses (blue bars and ellipses, respectively) within just 50 cm at around 138 m depth. Vertical gray lines represent annual markers. The depth 138.05 m corresponds to the year 986 CE (GICC05) or 991 CE (GICC21).
At a depth of around 138 m, we find 9 melt layers and 12 melt lenses within just 50 cm (Fig.
According to the thinning function of
The nine melt layers might not represent nine separate events; they could have been created in one single event. It may be possible that melt water percolated approximately 1.5 m deep into the snowpack and left nine melt layers. All of these layers would thus may have formed within a few days. During the 2012 CE warm event, with rain, melt percolated 0.7 m into the snowpack at NEEM, i.e., half the depth of our event (Fig.
Assuming the melt layers around 986 CE to have formed in one event, then this must have been a long-lasting period of high temperatures and/or of intense rainfall.
Rain events are rare on the Greenland ice sheet, and melt events such as the 2012 CE event are clearly noticeable, given the many melt layer traces they leave. It is also worth noting that the 1889 CE melt event, which is present in most areas and ice cores across Greenland and therefore considered a big event, consists of only two melt layers with a total of 8.5 mm melt. The 1889 CE event must therefore not have been as intense as the 986 CE (total of 63.2 mm melt) or the 2012 CE event. The only melt event comparable to the 986 CE event – although with significantly thinner layers – happened around the year 675 BCE (2675 years b2k and 328 m depth, Fig.
In a previous version of this work, we noted a possible connection between the 986 CE event and the settlement voyages of Erik the Red from Iceland to Greenland in the same year. Applying the GICC21 timescale
We use the tree ring data
Tree-ring growth anomaly
A more recent, 2000-year, temperature reconstruction from
The melting at EastGRIP might not be synchronous with all tree ring peaks, but it still offers some insight into the correlation of melt and tree ring growth on a larger geographic scale. This is also the case for volcanic eruptions: many volcanic events do not correspond to deep cooling in the tree ring records, although local minima are often observed in correspondence. Due to the age uncertainty of melt events and difficulties in timescale translations, we cannot evaluate a more precise age offset. Moreover, even though more melting occurs during tree-ring warm decades, not every prominent peak in the tree ring record has melt events in its proximity.
The location of EastGRIP might not represent the complexity of the climatic dynamics that produces tree-ring growth anomalies at scattered locations around the Northern Hemisphere, but the occurrence of more melt in warm periods and in proximity to some of the warmest years suggests a partial correlation. We expect that future studies could improve the results we have presented, in particular for the correlation of the melt events at EastGRIP with other ice cores and with more temperature records from the Northern Hemisphere.
We have created a melt record from the EastGRIP ice core covering the largest part of the Holocene. This record is only the second one, after
The melt event that left the most melt layers in our record was the 986 CE event, followed by the 675 BCE event. The 2012 CE event is not displayed in our record but seems to have left similar traces to the 986 CE event (Fig.
In our melt event record, we distinguish between melt layers and lenses and compare the most recent 2500 years to the tree ring temperature anomaly record from
The value of a melt record from the EastGRIP ice stream ice core is due to its change of location and elevation over the past 9000 years. Today, the highly dynamic EastGRIP site is 170 km further north-northeast and 400 m lower than 9000 years ago. With a corresponding lapse rate of 0.6 to 0.9
Melt records from central Greenland deep ice cores, e.g., GISP2 or EastGRIP, are subjected to less horizontal thinning in the Early Holocene than shallower ice cores, e.g., the RECAP ice core
Our melt layer record can provide the basis to better understand summer temperatures in the Holocene, as the melt layers pinpoint warm events. The frequency or temporal distribution of these events can be incorporated into climate reconstructions or modeling studies (e.g.,
While ice core studies of melt events show the finished picture of melt layers, lenses, and pipes (see the “Methods” section) in the snowpack, the 2012 CE melt event at NEEM offered a unique chance to observe the creation of these structures in real time. The warm event in 2012 CE lasted from 12 to 15 July, with temperatures varying around 0
By 12 July, a substantial warming of the surface snowpack was observed, with the top 12 cm of the snowpack close to melting point (
Experimental simulations of melt events have been performed by
We present the results from a simple rain/melt event experiment performed in April 1995 on a traverse from the Greenland Ice Core Project (GRIP) site to the Northern Greenland Ice Core Project (NorthGRIP) site in Greenland (Fig.
The vertical melt pipes remained mostly invisible, but the horizontal expansion of the coffee into layers and lenses was very pronounced. It is worth noting that this represented one event, which created multiple layers in the snowpack. Furthermore, these melt layers were not at the surface; they penetrated 40 cm deep. It was also apparent that melt layers from the same event (coffee injection) could appear very differently, despite the fact they were only 30 cm apart, i.e., on either side of the trench wall (compare Fig.
Melt layers are not always as clear as shown in the examples in Fig.
Uncertain melt layers from 102 and 187 m depth. Scale on right.
The data were collected in a semi-automated fashion using MATLAB.
We run a script that divides the line scan image (length 165 cm) into ten equal sections with a 2 cm overlap. Thus, we display 16.5 (
Over the Holocene, the melt layer thicknesses range between 1 and 14 mm (Fig.
Histogram of melt layer thicknesses; the same color code is used as in Fig.
We see that most melt layers (
The time-averaged total melt record is corrected for thinning and for layers that are potentially missed due to core breaks (Fig.
Thinning-corrected melt layer thicknesses (circles). Black circles were removed from the record due to the cutoff at 1.54 mm (pink circle and line).
We analyze the duration between melt layers and melt lenses, representing the time from an event to the next younger event (Fig.
Time from an event to the next younger event for
In Fig.
A similar pattern is also visible in Fig.
In both records (Fig.
The long-term trend (Fig.
Our climatic interpretation of Holocene summer temperatures (Fig.
We clearly see the Medieval Warm Period at around 1000 years b2k, identified by a number of melt layers and lenses. Concerning the Roman Warm Period, only the second half (2000 to 1600 years b2k) is visible in the number of melt events and the melt layer thickness (Fig.
We find distinct cold periods around 500, 3000, 5600, and 8200 years b2k. In all our measurements (Fig.
Periods that are neither explicitly warm nor cold (e.g., the very recent past, i.e., younger than 100 years b2k) are shown in white (Fig.
We test the hypothesis that warmer periods contain more melt events than colder ones (Fig.
Histograms of melt events per decade over the last 2500 years.
To highlight the correlation between melt and Northern Hemisphere temperatures, we distinguish between cold, warm, and temperate decades (
To compare with our work, we use the only other melt layer record from a central Greenland ice core covering large parts of the Holocene.
The GISP2
At site A in Greenland (70.8
Number of bubble-free layers and lenses per century throughout the last 9700 years in the EastGRIP ice core. Indications of bubble-free areas, which are very uncertain melt layers and lenses that only hint at bubble-free areas, are shown in orange. Sloping bubble-free layers with a tilt of more than 10
Another available, but not peer reviewed, melt layer record has been assembled by
Other than melt events, we find crusts, sloping bubble-free layers, and indications of melt events (Fig.
Sloping bubble-free layers (Fig.
These layers cannot be leftovers of sloping surface structures due to their steep tilt. Resolving the initial shape of the layer, i.e., by stretching it in the vertical, would cause the layers to become even steeper – too steep for ice sheet surface structures.
Figure
Sloping layers at around 15
Primary data are available on PANGAEA (
JW provided the initial idea for the paper and acquired the data. The tree ring to melt comparison, statistics, and verification of the timescale were handled by GS; the idea for the tree ring comparison came from AS. Support regarding melt layers in ice cores in general came from AS, JF, SK, and DDJ. The coffee experiment and melt layer definition were performed by SK and JW. NEEM snowpit data and input were handled by HAK and PV. Climatic interpretations and the ice sheet evolution were derived by BV, AS, and JW. Physical properties of melt layers and their appearance were contributed by IW and SK. JW prepared the paper with contributions and revisions from all co-authors.
The contact author has declared that neither they nor their co-authors have any competing interests.
Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
EastGRIP is directed and organized by the Centre for Ice and Climate at the Niels Bohr Institute, University of Copenhagen. It is supported by funding agencies and institutions in Denmark (A. P. Møller Foundation, University of Copenhagen), the USA (US National Science Foundation, Office of Polar Programs), Germany (Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research), Japan (National Institute of Polar Research and Arctic Challenge for Sustainability), Norway (University of Bergen and Bergen Research Foundation), Switzerland (Swiss National Science Foundation), France (French Polar Institute Paul-Emile Victor, Institute for Geosciences and Environmental Research), and China (Chinese Academy of Sciences and Beijing Normal University). Julien Westhoff, Anders Svensson, Bo Vinther, Sepp Kipfstuhl, and Dorthe Dahl-Jensen thank the Villum Foundation, as this work was supported by the Villum Investigator Project IceFlow (no. 16572). Giulia Sinnl acknowledges support via the ChronoClimate project funded by the Carlsberg Foundation. Helle Astrid Kjær acknowledges the support by TiPES. This is TiPES contribution no. 157; the TiPES (Tipping Points in the Earth System) project has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement no. 820970. Paul Vallelonga acknowledges the support by ice2ice which receives funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013, ERC grant agreement no. 610055). Ilka Weikusat acknowledges HGF funding (VH-NG-802). The authors thank the reviewers (two anonymous reviewers and Elizabeth Thomas) for their comments and for greatly improving the paper throughout the process. The authors also thank the editor Denis-Didier Rousseau for handling the process.
This research has been supported by the Villum Investigator Project IceFlow (grant no. 16572), the Carlsberg Foundation (ChronoClimate project), European Union's Horizon 2020 research and innovation program (grant no. 820970), European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013, ERC grant agreement no. 610055), and Helmholtz Gemeinschaft für Forschung (VHNG-802).
This paper was edited by Denis-Didier Rousseau and reviewed by Elizabeth Thomas and two anonymous referees.