Physical properties of shallow ice cores from Antarctic and sub-Antarctic islands

The sub-Antarctic is one of the most data sparse regions on earth. A number of glaciated Antarctic and subAntarctic islands have the potential to provide unique ice core records of past climate, atmospheric circulation and sea ice. However, very little is known about the glaciology of these remote islands or their vulnerability to 20 warming atmospheric temperatures. Here we present ground penetrating radar (GPR), melt histories and density profiles from shallow ice cores (14 to 24 m) drilled on three sub-Antarctic islands and two Antarctic coastal domes. This includes the first ever ice cores from Bouvet Island (54 o 26’0 S, 3 o 25’0 E) in the South Atlantic, from Peter 1 st Island (68 o 50’0 S, 90 o 35’0 W) in the Bellingshausen Sea and from Young Island (66°17′S, 162°25′E) in the Ross Sea sector’s Balleny Islands chain. Despite their sub-Antarctic location, surface melt is 25 low at most sites (melt layers account for ~10% of total core), with undisturbed ice layers in the upper ~40 m, suggesting minimal impact of melt water percolation. The exception is Young Island, where melt layers account for 47% of the ice core. Surface snow densities range from 0.47 to 0.52 kg m 3 , with close-off depths ranging from 21 to 51 m. Based on the measured density, we estimate that the bottom ages of a 100 m ice core drilled on Peter 1 st Island would reach ~1836 AD and ~1743 AD at Young Island. 30

insulated ice core storage boxes for transportation. During the voyage, the ice was stored in a -25 o C freezer and later transported to the ice core laboratories at the British Antarctic Survey (BAS). The Mertz Glacier extends into the ocean from coastal King George V land, with a floating ice tongue. The tongue traps pack ice upstream forming the Mertz Glacier Polynya along its western flank during winter, the third most productive polynya in Antarctica, (Lacarra et al., 2014). In 2010, the impact from the B9B iceberg 130 caused this tongue to calve off producing a ~80 km long iceberg. This event had a profound impact on local sea ice conditions and the formation of dense shelf water (Campagne et al., 2015).

Young Island
A 17 m ice core was drilled on Young Island (66°17′S, 162°25′E), the northernmost island in the Balleny Island 135 chain (245 km 2 ), off the coast of Adélie Land (Figure 2b). The Balleny Islands comprise three major dormant volcanic islands, Young, Buckle and Sturge (Hatherton et al., 1965), which sit in the Antarctic seasonal sea ice zone at the boundary of the polar westerlies and Antarctic coastal easterlies. Young Island is characterized by a thick ice cover, marine-terminating piedmont-glacier tongues, steep coastal cliffs, and is therefore described as "among the most inaccessible places in the world" (Hatherton et al., 1965). The core was drilled 238 m above 140 sea level.
The former shield volcano (154 km 2 ) is almost completely covered by a heavily crevassed ice cap and sits within the seasonal sea ice zone. The core was drilled on a ridge (Midtryggen) at 730 m above sea level, in a small saddle on the eastern side of the island overlooking the main glacier Storfallet. The annual average 160 temperature at this site is -9.5°C (ERA-5, 1979(ERA-5, -2017, with summer maximum below zero (-2.7°C).

South Georgia
Two ice cores were drilled on South Georgia (54 o 17'0 S, 36 o 30'0 W), the largest SAI (3755 km 2 ). A 2.2 m ice core from the Nordenskjold Glacier, Cumberland Bay, and a 1.8 m core from Heany Glacier in St Andrews Bay 165 ( Figure 4a), both on the eastern coast of the island. South Georgia was the warmest island visited, with annual average temperatures of 1.6°C recorded from a near-continuous AWS record from Grytviken, located in Cumberland bay (1905-present). The average summer temperature exceeds 5°C, with a maximum temperature of 8.4°C recorded in February 1907.

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The South Georgia ice cores are from glacial terminus sites and do not provide contemporary climate information. Drilling at this site was difficult due to the high temperatures, with evidence of water in the borehole. These cores are therefore not included in this study, but they will provide estimated bottom ages and an evaluation of proxy preservation required for future drilling campaigns at higher-altitude sites (future study).

Bouvet Island (Bouvetøya)
A 14 m ice core was drilled from the volcanic island of Bouvet in the central South Atlantic (54 o 26'0 S, 3 o 25'0 E, 50 km 2 ), also known as Bouvetøya (Figure 4b). This was the most northerly site but the islands location within the polar front ( Figure 1) classifies it as sub-Antarctic. Despite its northerly location the average yearly temperature from ERA-5 (elevation corrected) is -2.9 °C, with a maximum monthly temperature of -0.9°C. The 180 maximum value recorded by the AWS at 42 m above sea level was 6.7 °C (1997-2005, elevation corrected).
Bouvet is almost entirely ice covered (~50 km 2 ) with the exception of a few rocky outcrops around the coast. Ice flows down the flanks of the volcano, with no visible crevassing, giving way to shear ice-cliffs and near-vertical icefalls into the sea. The island is the southernmost extension of the mid-Atlantic ridge, located at the triple 185 junction between the African, South American and Antarctic plates. These pronounced sea floor features drive the cold Antarctic Circumpolar Current close to the island, keeping surface temperature cold and allowing sea ice to extent north of the island.
The last known volcanic eruption on Bouvet was 50 BC; however, visible ash and dust layers suggest eruptions 190 may be more frequent. At a number of locations, the ice edge has broken vertically away (Figure 4c) revealing horizontal bands of clean and dirty layers in the vertical stratigraphy. The islands remote location, and absence of significant local dust sources, suggests a local volcanic source from either Bouvet or the South Sandwich Islands. The 3.5 km wide Wilhelmplataet caldera appears entirely ice filled. This potentially offers the deepest coring location, however it is unclear if this is instead an ice bridge formed since the last eruption.

Ground Penetrating Radar
Ground Penetrating Radar (GPR) measurements were performed around each ice core site. We used a SIR3000 unit equipped with a 400 MHz central frequency antennae (GSSI Inc.). The system was pulled on a sledge while walking in parallel and transversal lines of 100 m to 500 m depending on the site surface conditions and latitudes. This GPR was intended to obtain data from the near surface to complement the ice-core observations and to better characterise the site spatially.
Data collected is observed in-situ as a 'radargram' that represents the number of traces received (x-axis) and the two-way travel time of the wave in nano seconds (ns) (y-axis), which is the time the signal takes from the 210 transmitter to the receiver when reflected from the internal ice discontinuities. During data collection the maximum time window was set to a value between 400 -600 ns, according to the expected maximum depth of signal propagation at the SAI sites. GPR data was monitored in-situ for calibration and stored. Data processing was done using a commercial software (ReflexW) and generally included: removal of repetitive traces (same position), correction of the surface position and distance covered, frequency filters, gain function adjustments,

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stacking and other visual enhancements to improve the interpretation of the reflecting layers. Each collected file was analysed independently, and layers were picked manually in full resolution. Thus, if layers were not sufficiently clear and continuous, they were not picked.
In order to obtain the corresponding depth (m) for the y-axis, we used the density profile (see section 3.3) of the 220 ice core to obtain the average velocity of the wave (m/ns) in the ice based on the Looyenga model (Looyenga, 1965) for each site.

Ice core analysis
Ice-core processing was carried out in the -20°C cold laboratories at BAS. The section length, diameter and weight were measured to provide a density record and the visible melt layers logged and measured (only layers 225 > 1mm). Melt layer thickness was corrected for ice thinning based on the Nye model (Nye, 1963), that assumes a vertical strain rate and thinning that is proportional to burial.

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Grids of parallel and transversal lines traced for all sites are summarized in Table 2. Given the scope of this paper, one example of a representative profile taken in the area nearby the ice core borehole is shown in the following section.
The Mertz 1 site (Cape Hurley) had a flat surface, consistent with observed internal layering. Layers are not distinguished in the upper ~7 m of snow, below this depth reflectors were not strong, however eleven distinct layers were identified, some of which were discontinuous. Layers were visible down to a depth of 62 m, which we estimated to be the approximate limit of signal propagation of the GPR system at this site. Figure 5 shows a section of a profile taken in north westerly direction. Bedrock was not detected.

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Data quality for the Mertz fast ice site was very poor ( Figure 6). A single continuous layer at 6.8 m depth was the only one identified. This site is fundamentally different in character from the others presented in this study.
While we believe the surface snow to be meteoric, it does not sit atop grounded ice, but rather a large wedge of multiyear sea ice, held fast between the AAE and Mertz Glacier tongues ( Figure 4a). The top-most layers of snow and firn appeared typical. However, at a drill depth of 6.23 meters the recovered ice was saturated with 245 liquid seawater (a strong attenuator of the radar signal). Ice recovered below this depth was different in character from the surface, being solid ice containing bubbles and interstitially saturated with saltwater. There was a standing water table in the borehole at this same depth. These observations, together with the clear radar reflector at just over 6.8 m (Figure 6), the absence of reflectors below this, and the sites elevation of ~6 m above sea level, lead to the conclusion that the fast ice is saturated with seawater below sea level.

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If the fast ice is floating (i.e. supported buoyantly), our measurement of the dry freeboard thickness allows us to estimate the total thickness of the fast ice wedge. The average density of the top 6 m is 0.55 +/-0.05 kg m -3 , increasing to 0.85 +/-0.05 kg m -3 below this. Assuming a seawater density of 1.0275 ± 0.004 kg m -3 , freeboard thickness of 6.2 m and a mean density of the ice below the water-table of 0.85 +/-0.05 kg m -3 , the total ice-255 equivalent thickness of the ice at this site is ~30 m. However, there are large uncertainties in this calculation.
The radargram indicates that the fast ice increases in thickness away from the drill site, by several meters. While this site is not likely to provide typical geochemical proxy records, it may be of interest to future studies of multiyear sea ice.

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The Young Island site was flat with a compact snow surface. Multiple layers can be identified but they are not traceable through the full profile length. The small distance between layers resulted in merging. As an example, on Figure 7, one strong layer (~4 m depth) has been traced along the profile. The maximum estimated depth of detected layers was ~36 m (Table 2) and bedrock was not detected.

Mount Siple
The surface of the studied area at Mount Siple site was relatively smooth (max. 4° slope). Figure 8 shows a full profile taken in an east-westerly direction rising from 678 m a.s.l. to 685 m a.s.l. Multiple layers are clearly interpreted with minor discontinuities, for the full depth of the profile (~36 m). Bedrock was not detected. pack. The maximum time window was set to reach an estimated depth of ~43 m (Table 2) and bedrock was not detected. Figure 9 shows a section of a profile crossing the ice core position.

Melt records
Surface melting occurs in response to a positive energy balance at the snow surface and is strongly correlated with surface air temperature. The Antarctic ice sheet experiences little melt, due to consistently low temperatures, however the coastal margins and areas of the Antarctic Peninsula are subject to surface melting (van Wessem et al., 2016). The relationship between surface melt and surface temperatures has been exploited 290 to reconstruct past climate in Antarctic ice cores (Abram et al., 2013). However, the presence of too much surface melt can damage the climate proxies they contain.
The influence of melt-water on ice core proxy preservation has been explored for arctic ice cores (Koerner, 1997). Here, melt-water percolation can allow insoluble micro particles to migrate to the melting surface, 295 causing a spike in concentrations, and influence the stable water isotope record, a commonly used proxy for past surface temperatures. Seasonal melting can cause run-off of near-surface snow and melt and refreezing at the base of the annual snowpack. This can redistribute the stable water isotope profile, resulting in a warm or coldbiased record. The influence on both stable water isotopes and chemistry can have a major impact on the ability to date ice cores using annual layer counting.

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Given their location, we expect all our sites to experience some degree of melt. Surprisingly, the site least effected by melt is Bouvet, the warmest and most northerly location. The average melt layer thickness in the Bouvet core is 0.3 cm, observed at a frequency of five layers per meter; with the largest measured melt layer just 3.98 cm ( Table 2). The average melt layer thickness at all sites (except Young Island) is considerably lower than has not yet been completed, however based on our estimates (section 3.4), it is a reasonable assumption 310 that the snow accumulation at these maritime islands will be equal to or more than the snow accumulation at James Ross Island (0.62 +/-0.14 m water equivalent). Thus, the potential for proxy preservation is promising. https://doi.org/10.5194/tc-2020-110 Preprint. Discussion started: 24 June 2020 c Author(s) 2020. CC BY 4.0 License.
The site most affected by melt is Young Island, which has frequent melt layers averaging 6.57 cm and the largest single layer of 61 cm (58 cm before thinning applied). Young Island sits within the Polar Front and the sea ice minimum (Figure 1), with average temperatures 6.4 degrees colder than Bouvet. The average summer 315 temperature (December-February) at Young Island is -2.18C. The maximum recorded temperature from an Automatic Weather Station (AWS) deployed at 30 m elevation was 4.2°C (1991)(1992)(1993)(1994)(1995)(1996)(1997), equivalent to 2.8C at the ice core site. It is very likely that the resolution of ERA-5 is not sufficient to capture local surface temperatures, however, given the islands location south of the Polar Front and the seasonal sea ice zone it seems unlikely that surface temperature alone can explain the observed melt layers.

Density
The change in firn density with depth is dependent on the snow accumulation rate and temperature at the site.

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The first stage of densification relates to grain settling and packing and occurs below the "critical density" of about 0.55 kg m −3 (Herron and Langway, 1980). A linear relationship exists between the 2-m temperature from ERA-5 and the critical depth (r 2 = 0.57, p>0.05), which is reached first at Bouvet, the most northerly location and the warmest site (Table 1; Table 2).

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The second stage of densification (0.55-0.83 kg m -3 ), occurs when the firn air passages become closed off to form individual bubbles. The density at the bottom depth of all cores remains below this value, with the exception of Young Island, which exceeds this limit at 16.6 m. This suggests that pore close off has been achieved below this depth, when air can no longer be excluded and further densification takes place by compression of the bubbles. However, the presence of a large melt layer at this depth suggests this may be the the influence of temperature, the rapid densification on the islands may be caused by layer stretching and compression related to ice flow that is not well understood at these locations.

Estimating the ice core bottom ages
Based on the fitted density curve and assuming a constant rate of snow accumulation, we can estimate the 360 expected bottom ages of the ice cores drilled at each of our sites. We estimate the (water-equivalent) accumulation rate at the annual average precipitation minus evaporation (P-E) value from ERA-5. We note that the resolution of ERA-5 may not be adequate to capture P-E at these sites and that snow accumulation may vary considerably at these island locations. However, the annual average P-E value for Bouvet, the only site that has been annual layer counted, is identical to the calculated snow accumulation in meters water equivalent.

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The estimated ice core bottom ages range from 2001 (+/-2 years), at both the Peter 1 st and Bouvet ice cores, to 1992 (+/-3 years) at the Mertz 1 (Table 3). It is difficult to estimate a potential age limit for a core drilled to bedrock at the Young, Peter 1st and Mertz 1 sites since the ice thickness was beyond the penetration depth of the GPR. For example at Mertz 1 a bottom age of 1919 AD (+/-5 years) is estimated based on a GPR bottom depth 370 of 62 m (Table 3), although the ice could feasibly be much deeper than this. In Table 5 we estimate the potential ice ages at the signal penetration depth of the GPR at Young, Peter 1 st and Mertz as 1967, 1948and 1919 respectively. At the Bouvet site, where we interpret the layer at 41 m as bedrock, the maximum potential age is 1962 AD (+/-5 years). However, considerably deeper (and therefore older) ice may be present at higher elevations. These results suggest that the SUBICE ice-core records will be suitable for obtaining sub-annual 375 resolution climate records with the potential to capture climate variability over multi-decadal timescales.
For the sites where bedrock was not detected, we estimate the bottom age of an ice core drilled to100 m depth (Table 3). This suggests that the oldest ice would be reached at Mertz 1 (1742 +/-10 years) and the youngest ice at Peter 1 st (1836 +/-10 years).

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The estimated ice core bottom age at Bouvet Island, together with the observed visible ash layers (Figure 4c), offer a glimpse at the volcanic history of this remote island. The last known eruption was 50 BCE; however, visible layers in the upper ~20 m suggest the island has been volcanically active as recently as ~ 2012. The regularity of the layers in the ice cliffs, visible all around the island, indicate frequent volcanic activity has 385 occurred during the 20 th century. This is consistent with the persistent volcanic activity observed on the near-by South Sandwich Islands, with visible ash clouds identified in the satellite records and even visible from the ship during the ACE (March 2017). We note that the visible ash layers at Bouvet may have been deposited from the South Sandwich Islands but further analysis will help establish the source.

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Initial results from five shallow ice cores drilled in the sub-Antarctic islands and coastal Antarctica suggest that these locations may be suitable for short-term climate reconstructions and up to centennial-scale reconstructions at some sites if deeper cores could be retrieved. Evidence that these ice cores span the 20 th century, a period of significant global climate change, is exciting. The GPR surveys suggest relatively uniform layering at most sites, at least to the depth of the ice cores, suitable for ice core proxy reconstructions. However, evidence of 395 crevassing at some locations (Young and Peter 1 st ) demonstrates the importance of a thorough geophysical survey before contemplating deeper drilling at these sites. Evidence from Bouvet Island reveals regular volcanic ash deposits during the 20 th century, suggesting the island is still volcanically active.
The impact of melt is less severe than expected at some locations, especially Bouvet Island, but more severe at 400 others. Young Island, part of the Balleny Island chain off the coast of Adѐlie Land, is the most susceptible to melt. However, the observed melt layer thickness at the other sites is less than that observed from the James Ross Island ice core, which yielded paleoclimate reconstructions (Abram et al., 2013). Proxy preservation was not a concern in this ice core, suggesting that melt will also not adversely influence the climate record contained in the sub-Antarctic ice cores. The observed increase in melt intensity at James Ross Island since the mid-20 th 405 century was linked to warming surface temperatures. Thus the comparable melt intensity observed at some of the SAIs may be evidence that the 20 th century warming at these locations was analogous to that on the Antarctic Peninsula.
Based on the measured density profile and the P-E from ERA-5, the estimated bottom ages of the SUBICE cores 410 range from 2001 (Peter 1 st and Bouvet) to 1992 (Mertz 1), suggesting that these records should contain a multidecadal record of climate variability in this data sparse region. We were unable to obtain ice thickness estimate for all sites, with the exception of Bouvet, however visible layers were identified in the GPR records to depths of 60 m. Even with a conservative estimate, it is possible that deeper ice core drilled on these SAIs would have the potential to capture climate variability during the 20 th century, but most likely considerably longer.

Data availability
All data will be stored at the UK Polar Data Centre (https://www.bas.ac.uk/data/uk-pdc/) or by directly contacting Liz Thomas (lith@bas.ac.uk). DOI to be provided following paper acceptance.

Author contributions
ERT lead the project; ERT, GG, JP, ACFK, BM, and MP collected the data in the field; ERT, JP, ACFK and DEM processed the ice core data; GG and MP processed the GPR data; all authors contributed to writing and editing the paper.        https://doi.org/10.5194/tc-2020-110 Preprint. Discussion started: 24 June 2020 c Author(s) 2020. CC BY 4.0 License.