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Abstract. The basal unit near the base of the Antarctic Ice Sheet (AIS) plays a critical role in AIS dynamics and the preservation of old ice, yet its structure and origin remain poorly understood. Using a new airborne ice-penetrating radar dataset collected by the NSF Center for Oldest Ice Exploration (NSF COLDEX), we investigate the radar characteristics of the basal unit at the southern flank of Dome A, East Antarctica. We combine manual mapping with Delay-Doppler analysis to characterize the spatial distribution of incoherent scattering and to distinguish between two types of radar-apparent basal unit top boundaries: a sharp transition from specular to scattering reflections (type I) and a gradual disappearance of specular reflections due to radar signal attenuation (type II). We find that incoherent scattering is widespread upstream and decreases downstream, correlating with both subglacial topographic roughness and a shift from type I to type II boundaries. These patterns are interpreted as resulting from spatial variability in englacial temperature, with warmer ice downstream enhancing signal attenuation and obscuring radar features. Although incoherent scattering is not visible in the downstream region, its absence may reflect radar detection limits rather than true absence of scattering reflectors in the basal unit. Moreover, the observed correlation between scattering and subglacial roughness suggests deeper geological controls in which subglacial lithology influences both basal temperature and subglacial geomorphology. 

Abstract. Obtaining a continuous ice core record to 1.5 Ma (million years), which spans the Mid-Pleistocene Transition (MPT, 1.2 to 0.7 Ma) is a goal of multiple international efforts in Antarctica. Ice of such age is likely to be highly thinned and located in warm ice near the bed, conditions which promote diffusion of the stored atmospheric gases. Here, we assess the preservation of CO2 and the O2 / N2 ratio in the ice sheet between South Pole and Dome A where the NSF Center for Oldest Ice Exploration has surveyed with airborne radar. We employ two models: (1) a 1D, steady state ice and heat flow model to calculate the temperature and age of ice with respect to depth, and (2) a vertical gas diffusion model for clathrate ice. We analyze the preservation of CO2 signals with a period of 40 kyr to match pre-MPT glacial cycles and the preservation of O2 / N2 signals with a period of 20 kyr to match precession cycles. 1.5 Ma ice is lost to basal melt in much of the study area where ice thickness exceeds 3000 m. In locations that preserve 1.5 Ma ice, vertical gas diffusion is most sensitive to accumulation rate; high accumulation rate sites have more highly thinned old ice, and the steeper gas concentration gradients enhance diffusion. The most promising region for recovering 1.5 Ma ice is approximately 400 km from both South Pole and Dome A, a region we call the “Foothills”, due to low accumulation rates and moderate ice thickness. CO2 signals lose on average 14 % of their amplitude, while O2 / N2 signals lose on average 95 % for 1.5 Ma ice, suggesting precession cycles may not be identifiable. Unknown geothermal heat flow is a large uncertainty for both ice loss from basal melt and gas signal preservation. 

Abstract. Deposition of sulfuric acid in ice cores is important both for understanding past volcanic activity and for synchronizing ice core timescales. Sulfuric acid has a low eutectic point, so it can potentially exist in liquid at grain boundaries and veins, accelerating chemical diffusion. A high effective diffusivity would allow post-depositional diffusion to obscure the climate history and the peak matching among older portions of ice cores. Here, we use records of sulfate from the European Project of Ice Coring in Antarctica (EPICA) Dome C (EDC) ice core to estimate the effective diffusivity of sulfuric acid in ice. We focus on EDC because multiple glacial–interglacial cycles are preserved, allowing analysis for long timescales and deposition in similar climates. We calculate the mean concentration gradient and the width of prominent volcanic events, and analyze the evolution of each with depth and age. We find the effective diffusivities for interglacial and glacial maximums to be 5±3×10-9 m2 a−1, an order of magnitude lower than a previous estimate derived from the Holocene portion of EDC (Barnes et al., 2003). The effective diffusivity may be even smaller if the bias from artificial smoothing from the sampling is accounted for. Effective diffusivity is not obviously affected by the ice temperature until about −10 ∘C, 3000 m depth, which is also where anomalous sulfate peaks begin to be observed (Traversi et al., 2009). Low effective diffusivity suggests that sulfuric acid is not readily diffusing in liquid-like veins in the upper portions of the Antarctic Ice Sheet and that records may be preserved in deep, old ice if the ice temperature remains well below the pressure melting point. 

Radio-echo sounding (RES) has revealed an internal architecture within both the West and East Antarctic ice sheets that records their depositional, deformational and melting histories. Crucially, RES-imaged internal-reflecting horizons, tied to ice-core age–depth profiles, can be treated as isochrones that record the age–depth structure across the Antarctic ice sheets. These enable the reconstruction of past climate and ice dynamical processes on large scales, which are complementary to but more spatially extensive than commonly used proxy records (e.g. former ice limits constrained by cosmogenic dating or offshore sediment sequences) around Antarctica. We review the progress towards building a pan-Antarctic age–depth model from these data by first introducing the relevant RES datasets that have been acquired across Antarctica over the last 6 decades (focussing specifically on those that detected internal-reflecting horizons) and outlining the processing steps typically undertaken to visualise, trace and date (by intersection with ice cores or modelling) the RES-imaged isochrones. We summarise the scientific applications for which Antarctica's internal architecture has been used to date and present a pathway to expanding Antarctic radiostratigraphy across the continent to provide a benchmark for a wider range of investigations: (1) identification of optimal sites for retrieving new ice-core palaeoclimate records targeting different periods; (2) reconstruction of surface mass balance on millennial or historical timescales; (3) estimation of basal melting and geothermal heat flux from radiostratigraphy and comprehensive mapping of basal-ice units to complement inferences from other geophysical and geological methods; (4) advancement of the knowledge of volcanic activity and fallout across Antarctica; and (5) refinement of numerical models that leverage radiostratigraphy to tune time-varying accumulation, basal melting and ice flow, firstly to reconstruct past behaviour and then to reduce uncertainties in projecting future ice-sheet behaviour. 

Abstract. Reconstructions of past temperature and precipitation are fundamental to modeling the Greenland Ice Sheet and assessing its sensitivity to climate. Paleoclimate information is sourced from proxy records and climate-model simulations; however, the former are spatially incomplete while the latter are sensitive to model dynamics and boundary conditions. Efforts to combine these sources of information to reconstruct spatial patterns of Greenland climate over glacial–interglacial cycles have been limited by assumptions of fixed spatial patterns and a restricted use of proxy data. We avoid these limitations by using paleoclimate data assimilation to create independent reconstructions of mean-annual temperature and precipitation for the last 20 000 years. Our method uses oxygen isotope ratios of ice and accumulation rates from long ice-core records and extends this information to all locations across Greenland using spatial relationships derived from a transient climate-model simulation. Standard evaluation metrics for this method show that our results capture climate at locations without ice-core records. Our results differ from previous work in the reconstructed spatial pattern of temperature change during abrupt climate transitions; this indicates a need for additional proxy data and additional transient climate-model simulations. We investigate the relationship between precipitation and temperature, finding that it is frequency dependent and spatially variable, suggesting that thermodynamic scaling methods commonly used in ice-sheet modeling are overly simplistic. Our results demonstrate that paleoclimate data assimilation is a useful tool for reconstructing the spatial and temporal patterns of past climate on timescales relevant to ice sheets. 

Abstract. Radio-echo sounding (RES) has revealed an internal architecture within Antarctica’s ice sheets that records their depositional, deformational and melting histories. Crucially, spatially-widespread RES-imaged internal-reflecting horizons, tied to ice-core age-depth profiles, can be treated as isochrones that record the age-depth structure across the Antarctic ice sheets. These enable the reconstruction of past climate and ice-dynamical processes on large scales, which are complementary to but more spatially-extensive than commonly used proxy records across Antarctica. We review progress towards building a pan-Antarctic age-depth model from these data by first introducing the relevant RES datasets that have been acquired across Antarctica over the last six decades (focussing specifically on those that detected internal-reflecting horizons), and outlining the processing steps typically undertaken to visualise, trace and date (by intersection with ice cores, or modelling) the RES-imaged isochrones. We summarise the scientific applications to which Antarctica’s internal architecture has been applied to date and present a pathway to expanding Antarctic radiostratigraphy across the continent to provide a benchmark for a wider range of investigations: (1) Identification of optimal sites for retrieving new ice-core palaeoclimate records targeting different periods; (2) Reconstruction of surface mass balance on millennial or historical timescales; (3) Estimates of basal melting and geothermal heat flux from radiostratigraphy and comprehensively mapping basal-ice units, to complement inferences from other geophysical and geological methods; (4) Advancing knowledge of volcanic activity and fallout across Antarctica; (5) The refinement of numerical models that leverage radiostratigraphy to tune time-varying accumulation, basal melting and ice flow, firstly to reconstruct past behaviour, and then to reduce uncertainties in projecting future ice-sheet behaviour. 

Abstract. The last deglaciation, which occurred from 18 000 to 11 000 years ago,is the most recent large natural climatic variation of global extent. Withaccurately dated paleoclimate records, we can investigate the timings ofrelated variables in the climate system during this major transition. Here,we use an accurate relative chronology to compare temperature proxy data andglobal atmospheric CO2 as recorded in Antarctic ice cores. In addition tofive regional records, we compare a δ18O stack, representingAntarctic climate variations with the high-resolution robustly dated WAISDivide CO2 record (West Antarctic Ice Sheet). We assess the CO2 and Antarctic temperature phaserelationship using a stochastic method to accurately identify the probabletimings of changes in their trends. Four coherent changes are identified forthe two series, and synchrony between CO2 and temperature is within the95 % uncertainty range for all of the changes except the end of glacial termination 1 (T1). During the onset of the last deglaciation at 18 ka and the deglaciationend at 11.5 ka, Antarctic temperature most likely led CO2 by several centuries (by 570 years, within a range of 127 to 751 years, 68 %probability, at the T1 onset; and by 532 years, within a range of 337 to 629years, 68 % probability, at the deglaciation end). At 14.4 ka, the onsetof the Antarctic Cold Reversal (ACR) period, our results do not show a clearlead or lag (Antarctic temperature leads by 50 years, within a range of−137 to 376 years, 68 % probability). The same is true at the end of the ACR(CO2 leads by 65 years, within a range of 211 to 117 years, 68 %probability). However, the timings of changes in trends for the individualproxy records show variations from the stack, indicating regional differencesin the pattern of temperature change, particularly in the WAIS Divide recordat the onset of the deglaciation; the Dome Fuji record at the deglaciationend; and the EDML record after 16 ka (EPICA Dronning Maud Land, where EPICA is the European Project for Ice Coring in Antarctica). In addition, two changes – one at 16 ka in the CO2 record and one after the ACR onset in three of theisotopic temperature records – do not have high-probability counterparts in the other record. The likely-variable phasing we identify testify to thecomplex nature of the mechanisms driving the carbon cycle and Antarctictemperature during the deglaciation. 

Abstract. In 2013 an ice core was recovered from Roosevelt Island, an ice dome between two submarine troughs carved by paleo-ice-streams in the Ross Sea, Antarctica. The ice core is part of the Roosevelt Island Climate Evolution (RICE) project and provides new information about the past configuration of the West Antarctic Ice Sheet (WAIS) and its retreat during the last deglaciation. In this work we present the RICE17 chronology, which establishes the depth–age relationship for the top 754 m of the 763 m core. RICE17 is a composite chronology combining annual layer interpretations for 0–343 m (Winstrup et al., 2019) with new estimates for gas and ice ages based on synchronization of CH4 and δ18Oatm records to corresponding records from the WAIS Divide ice core and by modeling of the gas age–ice age difference. Novel aspects of this work include the following: (1) an automated algorithm for multiproxy stratigraphic synchronization of high-resolution gas records; (2) synchronization using centennial-scale variations in methane for pre-anthropogenic time periods (60–720 m, 1971 CE to 30 ka), a strategy applicable for future ice cores; and (3) the observation of a continuous climate record back to ∼65 ka providing evidence that the Roosevelt Island Ice Dome was a constant feature throughout the last glacial period. 

Abstract. Here we present Antarctic snow accumulation variability at the regional scale over the past 1000 years. A total of 79 ice core snow accumulation records were gathered and assigned to seven geographical regions, separating the high-accumulation coastal zones below 2000 m of elevation from the dry central Antarctic Plateau. The regional composites of annual snow accumulation were evaluated against modelled surface mass balance (SMB) from RACMO2.3p2 and precipitation from ERA-Interim reanalysis. With the exception of the Weddell Sea coast, the low-elevation composites capture the regional precipitation and SMB variability as defined by the models. The central Antarctic sites lack coherency and either do not represent regional precipitation or indicate the model inability to capture relevant precipitation processes in the cold, dry central plateau. Our results show that SMB for the total Antarctic Ice Sheet (including ice shelves) has increased at a rate of 7 ± 0.13 Gt decade−1 since 1800 AD, representing a net reduction in sea level of ∼ 0.02 mm decade−1 since 1800 and ∼ 0.04 mm decade−1 since 1900 AD. The largest contribution is from the Antarctic Peninsula (∼ 75 %) where the annual average SMB during the most recent decade (2001–2010) is 123 ± 44 Gt yr−1 higher than the annual average during the first decade of the 19th century. Only four ice core records cover the full 1000 years, and they suggest a decrease in snow accumulation during this period. However, our study emphasizes the importance of low-elevation coastal zones, which have been under-represented in previous investigations of temporal snow accumulation. 

Abstract. We present a 2700-year annually resolved chronology and snow accumulationhistory for the Roosevelt Island Climate Evolution (RICE) ice core, Ross IceShelf, West Antarctica. The core adds information on past accumulationchanges in an otherwise poorly constrained sector of Antarctica. The timescale was constructed by identifying annual cycles inhigh-resolution impurity records, and it constitutes the top part of theRoosevelt Island Ice Core Chronology 2017 (RICE17). Validation by volcanicand methane matching to the WD2014 chronology from the WAIS Divide ice coreshows that the two timescales are in excellent agreement. In a companionpaper, gas matching to WAIS Divide is used to extend the timescale for thedeeper part of the core in which annual layers cannot be identified. Based on the annually resolved timescale, we produced a record of past snowaccumulation at Roosevelt Island. The accumulation history shows thatRoosevelt Island experienced slightly increasing accumulation rates between700 BCE and 1300 CE, with an average accumulation of 0.25±0.02 mwater equivalent (w.e.) per year. Since 1300 CE, trends in the accumulationrate have been consistently negative, with an acceleration in the rate ofdecline after the mid-17th century. The current accumulation rate atRoosevelt Island is 0.210±0.002 m w.e. yr−1 (average since 1965 CE, ±2σ), and it is rapidly declining with a trend corresponding to0.8 mm yr−2. The decline observed since the mid-1960s is 8 times fasterthan the long-term decreasing trend taking place over the previouscenturies, with decadal mean accumulation rates consistently being belowaverage. Previous research has shown a strong link between Roosevelt Islandaccumulation rates and the location and intensity of the Amundsen Sea Low,which has a significant impact on regional sea-ice extent. The decrease inaccumulation rates at Roosevelt Island may therefore be explained in termsof a recent strengthening of the ASL and the expansion of sea ice in the easternRoss Sea. The start of the rapid decrease in RICE accumulation ratesobserved in 1965 CE may thus mark the onset of significant increases inregional sea-ice extent.