The ABW site is situated in northern Ellsworth Land at the intersection of the Amundsen, Bellingshausen, and Weddell ice divides (74°34^′37^′′ S, 86°54^′16^′′ W), where ice is 1220 m deep and ice-surface elevation is 1350 m (Fig. ). The site sits astride a topographic high along the triple-ice-divide on otherwise relatively flat bedrock. Ice flow diverges in three directions away from the divide at a speed of less than 10 m yr⁻¹ . Shallow ice core data show that surface accumulation rates increased by 27 % between 2000 and 2010 from a previously stable annual average snow accumulation of 33 cm yr⁻¹ before 1900 . An automatic weather station was located at the site from December 2009 to December 2010 and showed the monthly average 2 m temperature to range from -15 °C in summer to -30 °C in winter and the annual average wind speed to be around 9.4 m s⁻¹ .

In December 2010, a 136 m ice core (hereafter termed F10) was drilled at ABW . The F10 core was dated back to 1702, providing a 308-year record of climate variability . Snow accumulation was calculated for the entire record using the Nye model , and showed a semi-annual cycle, peaking in the autumn and spring , with a dramatic increase in accumulation since the onset of the 1900s . We use the F10 dataset as a constraint for our inverse model, described in Sect. , to assess the longer-term changes in accumulation rates.

To assess the suitability of a new ice core drilling site, we use a 1-dimensional (1-D) ice flow model, that we will refer to as the forward model, constrained by shallow ice core and dated IRH data to extract surface accumulation records and estimate an age scale for ice at this site. The following section outlines the main principles; provide the complete numerical derivations.

Previous work using the ground-based DELORES survey data identified the presence of a deep subglacial trough running along the centre of Ferrigno Ice Stream . The DELORES survey also follows the ice divide and images continuous IRHs and a relatively flat ice-bed interface at the ABW site. The bedrock relief away from the dome at ABW is around 700 m. Coherent layers are imaged in the DELORES data at ABW to around 1080 m depth, approximately 89 % of the total ice thickness (1220 m), with layers tending to conform to the bed topography. Although we could only trace two dated IRHs to ABW, there are other traceable layers present above and below the 4.86 ka IRH which, with additional age-depth constraints, could be dated. The elevated accumulation rates at ABW, relative to more inland sites, are likely to produce well-defined annual layers that are effectively imaged by RES, and would provide high-resolution layers within ice-core data.

analysed RES data alongside ice-core data at 16 ice-core sites, noting that locations with laterally continuous layers imaged to significant depths in the RES data often provided the most continuous climate records, while sites with larger sections of incoherent scattering were often associated with more disturbed ice-core records. In the DELORES data at ABW, continuous layering is present to 89 % depth with a small region of absent layering, suggesting promising drilling conditions that could allow the recovery of a continuous and undisturbed climate record.

Figure  shows the reconstructed accumulation rates for Scenario 1 derived from the inverse model in Sect.  which assumes ∂H∂t=0. We include accumulation rate data from the WAIS Divide and Dome C ice cores alongside for comparison and to place the ABW site in the context of other deep ice cores across the Antarctic Ice Sheet. We indicate on Fig.  which regions of the solution are constrained by age-depth observations from the F10 core and dated-IRHs, and which regions are scaled to the WAIS Divide accumulation rate record.

There is a peak accumulation rate of ∼ 0.7 m yr⁻¹ around 5 ka. The magnitude of this peak accumulation is around 20-30 % higher than the modern accumulation rate estimates from the F10 shallow-ice-core data . Comparisons with the WAIS Divide record suggest that the accumulation rates at ABW could be up to three times those measured at WAIS Divide (Fig. ).

Figure  shows the optimised history of the rate of change of ice thickness at ABW from Scenario 4, and the integrated history of ice thickness from solutions to Eq. (). Here we see around a 200 m drop in ice thickness since 2.38 ka to the present-day ice thickness (1220 m). Initially, we ignored changes in ice thickness (Fig. ); here, we assume that accumulation has been in a steady-state and that there were no changes in ice thickness beyond 4.86 ka (Fig. ). Neither of these assumptions is perfect, but they together give a better indication of what may have happened at ABW since the onset of the Holocene. In reality, the accumulation regime has not been in steady-state and the ice thickness has likely evolved constantly; however, the results of Figs.  and show that there have been significant changes at ABW during the Holocene, which is an exciting prospect for ice-core drilling.

Figure  shows the modelled age-depth profiles, with associated uncertainties, for the proposed ABW ice core for different basal melt rate and thinning scenarios (Table ). For our scenarios that do not consider the effects of basal melting (Scenarios 1 and 4), predicting the age of the ice close to the ice-bed interface is uncertain since the solution to Eq. () tends to infinity (Fig. ). Furthermore, the model outputs in the deepest parts of the ice column become unrealistic due to their dependence on input parameters. As a result, for all scenarios we take a conservative estimate of the age of the oldest ice at 95 % depth (1159 m; around 60 m above the ice-bed interface; see Table ).

Our results at this depth for Scenario 1 (solid pink line in Fig. ), imply that ice ages may be older than the Last Glacial Maximum , spanning between 22.1 and 38.4 ka for different flank flow scenarios. The oldest ice averaged across all “flank” flow simulations at this depth is 29.1 ka. Incorporating thinning into the model in Scenario 4 decreases the age of the deepest ice, but still suggests the recovery of ice up to 25.2 ka at 95 % of the ice thickness (Table ).

Figure  shows the resolution and layer thickness of the ABW ice core. Assuming that annual layers can be resolved down to the centimetre scale , the model output suggests that annual layers at ABW could be resolved beyond 7 ka. Additionally, at 95 % of the ice thickness, the resolution is ∼ 0.64 ka m⁻¹, suggesting that the ice at this depth could be both high resolution and highly resolvable, potentially containing a climate record that extends well into the LGP.

Incorporating basal melting into model simulations results in greater drawdown of layers in the lowest third of the ice column and, in turn, the age-scale becomes more linear (Fig. ). This reduces the estimated retrievable climate records, at 95 % ice thickness, to 13.1 and 10.7 ka under moderate (10 mm yr⁻¹: 67 mW m⁻²) and extremely high (20 mm yr⁻¹: 140 mW m⁻²) GHF scenarios (Fig.  and Table ). These melt rates are likely overestimated by derivations from an overestimate of the true GHF at ABW. This will in turn underestimate the oldest ice at ABW and hence we treat these simulations as “worse-case” scenarios for basal melting at ABW. Regardless of the actual basal melting values, the results from Scenarios 2 and 3 show that a climate record extending beyond the Holocene could still be recovered even if there was some melting at the ice-bed interface.

Figure  displays the modelled age outputs at 95 % ice thickness at ∼ 350 m intervals along the ice divide separating Ferrigno Ice Stream from Pine Island Glacier and Evans Ice Stream. These outputs assume that (1) modern accumulation rates are identical to those at the ABW site , (2) flank flow conditions (p=3) to account for divergence of the RES traverse from the true divide, and (3) no basal melting. The model is then optimised using new ice thickness and dated-IRHs at intervals along the DELORES divide traverse in Fig. . In general, we observe older ice in regions of deeper bed topography, but the thickest regions of the divide do not always exhibit the oldest ice (see central divide region of Fig. ). Regions with relatively low ice thickness and older ice, such as the ABW site, would be more optimal for ice core drilling.

The results of modelling along the DELORES divide traverse highlight the possibility of recovering older ice at locations that offer a thicker ice column. However, comparing the modelled age to the drill depth (ice thickness) suggests that the ABW site offers a valuable climate record for its relatively low drill depth. This makes the ABW site attractive, logistically and financially, as it captures a comprehensive Holocene climate record while minimising drilling time and/or costs, and hence the subsequent environmental impact.

One of the goals of drilling a deep ice core at ABW is to recover a record of ice that can provide information on the onset of ice thinning through the Holocene.

Our initial scenarios (1–3) assume constant ice thickness at the site (∂H∂t=0 in Eq. ). As a consequence, our model interprets an increase in ice thickness as an increase in accumulation. This is because the ice particles travel faster to their present locations as the ice thickness increases and, with the assumption of constant geometry, the increase in speed is inverted as an increase in accumulation. This raises the question whether the observed changes in accumulation reconstruction (Fig. ) are the result of changes in surface accumulation, ice thinning, or a combination of both, as our initial model scenarios cannot differentiate between them. Although recent changes at ABW have been interpreted as a result of a recent increase in accumulation , there have also been reports of ice thinning in the Weddell Sea sector and even re-advance and thickening around the Ross Ice Shelf area during the Holocene . In an area currently experiencing significant changes in ice mass and ice-thinning in the contemporary period , the ABW site, which lies in the coastal area adjacent to the Bellingshausen and Amundsen Seas, is particularly important for reconstructing climatic changes and glacial retreat during the Holocene, and addressing the uncertainties surrounding whether the interior WAIS is thinning or thickening.

In an attempt to understand the thickness history at ABW in more detail, we introduce a normalised elevation term into our age-depth model (Sect. ). Including this scenario (4) allows for a quantitative estimate of thickness changes at ABW since 4.86 ka, and gives an indication of the magnitude of thickness changes at ABW over this period. As observed with previous scenarios (Fig. ), Fig.  indicates significant changes at ABW between 4.86 ka and present-day. The thickening observed between 4.86 ka and 2.38 ka in Fig.  is the result of assuming ∂H∂t=0 beyond 4.86 ka. Furthermore, the model initially underestimates the ice thickness at ABW before 4.86 ka, which leads to an instantaneous increase in the rates of thickness change at this time marker, causing a drastic change in ice thickness as the model compensates for the initial underestimation. Where the solution is constrained by time markers, we observe over 200 m of thinning between 2.38 ka and present-day, as ABW transitions from a thickening to thinning regime. This magnitude of Holocene ice thinning is consistent with other studies in West Antarctica , but we cannot precisely pinpoint the timing of this transition due to the temporal discontinuities between our limited time markers. Additionally, the absence of current constraints beyond 4.86 ka means that we cannot resolve the history of ice thickness from the mid-to-late Holocene, a period over which both thinning and thickening have been observed in West Antarctica. An ice core at ABW would help to resolve these uncertainties and improve our understanding of the state of the interior WAIS during the Holocene.

Our reconstructions in Fig.  also show elevated accumulation relative to other more inland ice cores in West Antarctica , which implies that the ice recovered could have high resolution and highly resolvable layers . Combining this with the drastic change observed between 5–2 ka makes this site attractive for ice-core drilling as the model for each scenario suggests a significant change occurred during the mid-Holocene at ABW. In West Antarctica, both elevated accumulation rates using radar stratigraphy and ice thinning signals from geomorphological, cosmogenic nuclide, and ice-core data have been observed since the mid-Holocene. Given the magnitude of the observed decrease in accumulation in Fig.  and the changes in ice thickness observed in Fig. , it can be inferred that this signal in the model output was likely caused by a combination of both extensive ice thinning and a change in the accumulation regime at ABW.

To resolve climatic changes accurately at ABW, and confirm the signal as an ice-thinning, accumulation rate decrease, or somewhere in between, we need an ice core. Recent work by at Skytrain Ice Rise demonstrated the ability to resolve similar signals using ice-core data. In their study, they identified a 450 m ice-sheet thinning signal between 8.2 and 8.0 ka and substantial retreat of the ice front between 7.7 and 7.3 ka due to ice-flow changes induced by ungrounding. The maximum modelled change in ice thickness at ABW between 4.86 and 2.38 ka, if we assume that the accumulation rate has remained constant, is around 250 m. This suggests that the order of magnitude of the potential ice thinning estimated by the model at ABW is consistent with data from previous deep-ice-core drilling projects in West Antarctica .

The data from the Skytrain Ice Core provided new temporal constraints for ice-sheet models and reinforce the importance of these data for resolving major changes to ice-sheet configurations . Given the vulnerability of this region to future retreat under the MISI hypothesis , an ice core at ABW would greatly improve our understanding of the evolution of the northern Ellsworth Land region of the Antarctic Ice Sheet throughout the Holocene, and provide a more precise timing for the onset of ice-sheet thinning currently observed in satellite data , giving insight into the future stability of the northern Ellsworth Land region.

Another goal of drilling a deep ice core at ABW is to recover a comprehensive high-resolution Holocene record of ice (at least 11.7 ka), and preferably one that observes the Last Glacial Maximum, that would provide new age-depth constraints for the northern Ellsworth Land region.

Uncertainties in the GHF observations and models at ABW significantly hinder our ability to precisely constrain the age of ice at depth. Instead, we have presented how sensitive our age-depth solution is to basal melting at ABW by testing a wide range of GHF scenarios (50–140 mW m⁻²). The solution between 4.86 ka and the present-day is not significantly affected by the addition of basal melting; the age-depth profiles only begin to deviate beyond 6 ka in the lowest regions of the ice column when the GHF exceeds 67 mW m⁻². Even when we prescribe an extremely high GHF in Scenario 3 (GHF = 140 mW m⁻²), the model still predicts a full-Holocene climate record to be retrievable at ABW.

Under lower basal melt rate scenarios (0–10 mm yr⁻¹), the potential recoverable climate record at ABW is between 30–15 ka, showing promise for extending beyond the Holocene and into the LGP. The ice recovered at ABW would provide crucial data for resolving the onsets of the dynamic thinning and retreat currently observed along the Bellingshausen Sea coastline and placing recent thinning observations from the Skytrain Ice Rise ice core in the context of the wider northern Ellsworth Land region.

While ABW hence presents an opportunity to recover a climate record that exceeds the onset of the Holocene (Fig. ), the ice layers must be interpretable and coherent for analysis . Figure  suggests that annual layers could be recovered to around 7 ka (assuming that layers of 5 cm thickness can be resolved), and that the ABW core would have a high layer resolution of 0.58 ka m⁻¹ at 95 % depth. In addition, DELORES data at ABW show coherent layers present to around 90 % depth. Through recent work performed by evaluating RES data at ice-core sites, our observations of coherent layers spanning most of the ice column suggest that the ice-core record could be undisturbed to within 100 m of the ice-bed interface.

A possible hindrance to the quality of this ice core could be the presence of a brittle ice zone in the lower ice column. In this brittle zone, recovered samples can experience extensive fracturing as the ice is brought to the surface, caused by decompression of trapped air bubbles . Brittle ice zones have been reported in various lengths at many sites in West Antarctica between depths of 400 and 1300 m The site at ABW has an ice thickness of 1220 m, suggesting a brittle ice zone could exist from around 400 m, which may have implications for the analysability of ice from the early Holocene. Although this zone presents a challenge to recovering a continuous Holocene record, specific core handling procedures can be implemented and followed to effectively mitigate the impacts of a brittle zone .

Achieving an undisturbed Holocene climate record at ABW would provide the longest climate record in the Bellingshausen Sea Sector and also provide a unique set of constraints for ice-sheet models. Better constraints in ice-sheet models improve reconstructions of past ice-sheet configurations , which in turn benefits modelling future projections of ice mass loss in northern Ellsworth Land and our understanding of the long-term (in)stability and interglacial variability of marine basins in West Antarctica.

The novelty of our modelling approach lies in its ability to integrate modern accumulation constraints from shallow-ice-core data with new age-depth observations from dated IRHs. This constrains the optimisation procedure, allowing us to find optimal records of the long-term accumulation and ice-thinning rates at ABW that extend beyond the limits of estimates from contemporary datasets. Consequently, the model can be easily applied to new sites near ice divides, where horizontal ice flow is negligible and where we have age-depth constraints spanning the whole ice column, offering exciting prospects for the ice coring community.

To improve the performance of the model, precise age observations at depth and accurate estimates of modern accumulation rates are essential. In West Antarctica, dating constraints are often limited to the WAIS Divide and Byrd ice cores , resulting in fewer dated IRHs to constrain model outputs in regions far from these ice cores . To obtain new constraints for dating new layers in the lower ice column, we need new strategically located deep ice cores. These ice cores should ideally (1) provide climatic records preceding the Holocene and the LGP to allow dating of layers in the lower ice column, and (2) be situated in areas with high layer continuity. The latter is progressively being informed by the AntArchitecture initiative's steady build-up of a 3-D age-depth structure for the Antarctic Ice Sheet .

An increased number of age-depth observations would provide more data to constrain the optimisation routine in Sect.  and, in turn, reduce the temporal discontinuities in the accumulation-rate and ice-thinning reconstructions observed in Figs.  and . This would improve our ability to constrain the timing of this inferred change in the accumulation rate observed between 4.86 ka and present day, and help to better resolve the onset of the inferred dramatic ice thinning in northern Ellsworth Land. Understanding the longer-term changes in more detail is essential to understand whether the contemporary observations of ice thinning are part of recurring periods of thinning, retreat and advance, or whether there is a significant concern of rapid irreversible retreat in the future. An ice core at ABW would provide a new high-resolution climate record essential for answering questions on the long-term history of the northern Ellsworth Land region of the Antarctic Ice Sheet, and how this region may contribute to future sea-level rise.