The Skytrain Ice Rise ice core adds to the handful of climate records from Antarctica that cover the whole of the last glacial period (here, we consider ~100-10 ka bp). By our count 7 records have been published covering the entire period and a further 6 cover a substantial part of it. Using the synchroneity of signals across the continent (for example in components of dust and in methane) we can tie the records together temporally. The precision of such ties is good enough to allow comparison of the timing and shape of particular events across the continent.Many of the differences between sites will derive from local changes of elevation that certainly occur at ice rise sites. We first will discuss the glacial water isotope record from Skytrain Ice Rise (in comparison to other sites) in this context. This will supplement the work we have already done on the Holocene and the last interglacial period using the Skytrain Ice Rise core.However we primarily focus on a number of events such as the Antarctic Cold Reversal and some of the large Antarctic Isotopic Maxima (eg AIM 12). We will present records from the different sectors of Antarctica. We will investigate whether any sectors of Antarctica led in such events, and determine the relative amplitude of such events around the continent. This information will offer diagnostic tests to ideas about the causes and process of millennial scale variability across the glacial period.

Precise synchronization of paleoclimate records is essential for inferring the dynamics and past evolution of the climate system. For the last glacial period, the time scales of ice cores from the Greenland and Antarctic ice sheets have been synchronized by the use of cosmogenic radionuclides, atmospheric gas concentrations, and traces of large volcanic eruptions. Here we identify the sulfate deposition signatures of the same 300 volcanic eruptions in different Greenland and Antarctic ice cores to obtain an inter-hemispheric volcanic ice-core synchronization of the entire last glacial period and the early Holocene (10-110 ka). Compared to earlier bipolar volcanic synchronizations, we close a gap in the period 16.5-24.5 ka and extend the synchronization to cover the 10-12 ka and 60-110 ka intervals. Furthermore, we increase the density of bipolar match points and make updates and corrections of the existing bipolar and unipolar synchronizations. The volcanic synchronization is in agreement with existing bipolar synchronizations from independent 10Be and methane matching. The bipolar volcanic synchronization allows us to determine the precise phasing of interhemispheric abrupt climate events throughout the last glacial period, particularly those associated with Dansgaard-Oeschger (D-O) events. Our improved synchronization and extended time period allow us to show that at the time of the D-O warming transitions, the average Antarctic temperature reaches a maximum within decades after the Greenland temperature maximum. This rapid Antarctic warming is superimposed on the well-known millennial-scale thermal bipolar-seesaw warming in Antarctica commonly attributed to oceanic heat transport and confirms earlier work that the abrupt change observed in Greenland is associated with a direct atmospheric circulation change at a global scale. The exception to this pattern occurs for the EDML ice-coring site located in the Atlantic sector of Antarctica, potentially related to sea-ice conditions in the Weddell Sea. Comparison to state-of-the-art climate model simulations shows excellent agreement in the overall bipolar climate phasing at the warming transitions and allows for analysis of the climate-system behavior at those transitions. The model simulations suggest that the abrupt Antarctic warming response observed is connected with an interhemispheric atmospheric response involving a global scale reorganization of the zonal mean atmospheric circulation. The abrupt D-O surface warming signal in the Northern Hemisphere is teleconnected into an abrupt Antarctic surface warming through changes in the Southern Hemisphere eddy-driven jet and anomalous circulation changes in the associated Ferrel and Polar cells.

During the Last Glacial Maximum, the Antarctic ice sheet was significantly larger than today, holding an additional 6 to 14 meters sea level equivalent. Although less than 10% of the total glacial-interglacial range in eustatic sea level change, understanding the “when and where” of how Antarctica reconfigures during a deglaciation is crucial to understanding how the ice sheet will behave in the future. Some models show that many areas of the Antarctic ice sheet are inherently unstable during the Last Glacial Maximum and, when forced by increasing temperatures and rising sea level during the last deglaciation, undergo a rapid retreat to their present-day grounding line configurations (if not beyond). In particular, the Ross and Weddell Sea regions, which are now largely covered by floating ice shelves, were susceptible to this “tipping point” behaviour. Recently, evidence from the Skytrain Ice Rise ice core (~79°S, 078°W, 784m altitude) using water isotopes and total air content (a proxy for elevation) provided strong evidence that the Weddell Sea underwent such a transition about 8,000 years before present (BP) (Grieman et al., in press).Here we present new total air content data from the Fletcher Promontory ice core (~78°S, 082°W, 873m altitude) which also lies in the Weddell Sea region about 220 km from Skytrain Ice Rise site, with the fast-flowing Rutford Ice Stream situated in between. The data were measured with a novel, high-accuracy total air content system and span approximately 11,000 to 6,000 years BP with an average resolution of 150 years. The most notable feature is an 8.8 mmol/kg (+/-2.0) increase between 8,000 and 7,000 years BP. This confirms the shift observed in the Skytrain Ice Rise ice core (~6.6 mmol/kg) that has been attributed to a 430 ± 110 m drop in elevation. Using both Skytrain Ice Rise and Fletcher Promontory as the two independently derived elevation histories we will discuss the reliability of total air content as an elevation proxy as well as provide crucial constraints on state-the-art ice sheet model predictions of past “tipping point” behaviour.Grieman, M., Nehrbass-Ahles, C., Hoffmann, H., Bauska, T. K., King, A. C. F., Mulvaney, R., Rhodes, R. H., Rowell, I. F., Thomas, E. R., and Wolff, E. W.: Abrupt Holocene ice loss due to thinning and ungrounding in the Weddell Sea Embayment, Nature Geoscience, In Press.

To extract climatically relevant chemical signals from the deepest, oldest ice in the polar ice sheets, we must first understand the degree to which chemical ions diffuse within solid ice. Volcanic sulfate peaks are the ideal target for such an investigation because they have a uniform peak shape at deposition. Processes of chemical diffusion and ice sheet thinning modify sulfate peak shapes with depth/age in an ice core. Our previous work developed a forward model, which simulates sulfate peak evolution in the ice sheet, and identifies the optimum effective diffusion rate for individual peaks. Analysis of the EPICA Dome C (EDC) sulfate record over the last 450 kyr suggests that the rate of sulfate diffusion is initially relatively rapid (2.4 ± 1.7 x 10-7 m2yr-1 median for Holocene ice) and slows down over time to rates on the order of 1 x 10-8 m2 yr-1 or less. We hypothesize this may result from a switch in the mechanism of diffusion resulting from the changing location of sulfate ions within the ice microstructure. Here we apply our forward model to three other ice cores: NGRIP (Greenland), EDML (East Antarctica) and WAIS Divide (West Antarctica) to determine sulfate diffusion rates and their evolution over depth/age. These ice cores are different to each other, and to EDC, in terms of their temperature profiles, ice grain size evolution and dust loading, all factors which may influence sulfate diffusion rates.

Extremely thinned layers and possible folding make the dating of the deepest sections of ice cores especially challenging. Cosmogenic radionuclides have the potential to provide independent age estimates. The 36Cl/10Be ratio is largely independent of production rate changes that affect individual radionuclides and has an effective half-life of 384 kyr, making it an ideal tool for dating the new 1.5 Myr old ice core that the Beyond EPICA Oldest Ice Core project aims to retrieve at Little Dome C in East Antarctica. However, the loss of 36Cl through hydrogen chloride outgassing at low accumulation sites complicates its application and the long-term decay of the 36Cl/10Be ratio in ice has not been studied. Here, we show that 36Cl is preserved in glacial periods and that the 36Cl/10Be ratio decreases more slowly than expected from physical decay over the last 900 kyr. While the glacial 36Cl flux decreases at the expected rate of physical decay within the uncertainty, the 10Be flux decreases faster, which maybe linked to a post-depositional mobility of 10Be in deep ice and leads to the slower decrease of the 36Cl/10Be ratio. In addition to this long-term trend, the 36Cl/10Be ratio fluctuates around a fitted decay curve, which is likely caused by different climate sensitivities of the transport and deposition pathways of the individual radionuclides. Both effects need to be better understood and quantified to improve age estimates based on the 36Cl/10Be ratio.

An ultimate target of Quaternary climate studies is to predict the strength and timing of glacial cycles using only the Milankovic forcing as input. Here we consider just one aspect of this challenge, the intensity of interglacials. Previous work (PIGS Working Group, 2015) has identified 11 interglacials in the last 800 kyr. Are some of them globally strong or weak? Is there a step change at the mid-Brunhes (between MIS 13 and MIS11)? And what controls the observed intensity?We first discuss what we mean by intensity. Some datasets (such as mean global temperature or sea level) have a more global character and might be considered more robust indicators of interglacial strength, but are more difficult to estimate compared to simpler parameters such as CO2 concentration and Antarctic temperature. Many records show “overshoots”, temporary maxima that are followed by longer plateaus of interglacial character. Despite these complications, some patterns do emerge. In global scale records, MIS 5e, 11, 9, 1 stand out as particularly warm, with 13 and 17 particularly cold. Some terrestrial records show a different pattern with MIS 13 unusually strong in many Asian records. There is a tendency to more intense interglacials after 450 ka, but MIS 7e and 7c would sit quite happily in the pre-mid-Brunhes pattern.A first look at the astronomical/orbital context is not encouraging. We see the obvious MIS11 paradox, that weak precessional forcing leads to a strong interglacial (or the opposite, most clearly seen in MIS 15e and 7c). However two different approaches have been quite successful, and may point the way to a more satisfying conclusion. Yin and Berger (2010, 2012) predicted the strength of interglacials using Milankovic forcing plus CO2 concentration as inputs. This approach suggests that the main cause of stronger interglacials after the mid-Brunhes is higher CO2 and pushes the problem into understanding the controls on the intensity of CO2 maxima. Mitsui et al (2022) used Milankovic forcing plus the strength of the previous glacial. In this model, the tendency to stronger interglacials after the mid-Brunhes arises essentially from a tendency to higher obliquity, as part of a 1.2 Myr cycle. Neither approach views the change across the mid-Brunhes as an ”event” and we propose it should rather be termed a mid-Brunhes “Shift” (MBS).Here we discuss how we might approach a unified explanation that draws on both models, with periods of highest CO2 perhaps being related to the pattern and timing of AMOC strength during the termination. This is influenced by the size of glacial ice sheets and by orbital intensity through their influence on the amount of freshwater available and the rate at which it is delivered into the ocean. Finally we consider whether the pattern of obliquity is enough to understand the MBS, i.e. is it part of a longer term oscillation.

Productivity in the Pleistocene glacial Southern Ocean was probably enhanced owing to iron fertilization by aeolian dust. Marine sediments indicate such an increase north of the modern Antarctic Polar Front but reduced biogenic activity south of it. However, quantitative estimates for the integrated net effect are difficult to obtain. Here we use the SO42− isotopic composition and other geochemical ice core records from the Atlantic sector of the Southern Ocean to reconstruct net changes in integrated biogenic sulfur productivity in the surface ocean over the penultimate glacial termination. We show that biogenic SO42− aerosol contributes 58

The fate of the West Antarctic Ice Sheet (WAIS)1 is the largest cause of uncertainty in long-term sea-level projections. In the last interglacial (LIG) around 125,000 years ago, data suggest that sea level was several metres higher than today2, 3-4, and required a significant contribution from Antarctic ice loss, with WAIS usually implicated. Antarctica and the Southern Ocean were warmer than today5, 6, 7-8, by amounts comparable to those expected by 2100 under moderate to high future warming scenarios. However, direct evidence about the size of WAIS in the LIG is sparse. Here we use sea salt data from an ice core from Skytrain Ice Rise, adjacent to WAIS, to show that, during most of the LIG, the Ronne Ice Shelf was still in place, and close to its current extent. Water isotope data are consistent with a retreat of WAIS9, but seem inconsistent with more dramatic model realizations10 in which both WAIS and the large Antarctic ice shelves were lost. This new constraint calls for a reappraisal of other elements of the LIG sea-level budget. It also weakens the observational basis that motivated model simulations projecting the highest end of projections for future rates of sea-level rise to 2300 and beyond.

The 36Cl/10Be ratio has the potential to be a dating tool for old ice, as it decays with a combined half-life of years and is thought to be independent of production changes, which affect the individual radionuclide concentrations in ice cores. However, when EDC samples with various ages between the Holocene and 887 kyr BP were analysed, the 36Cl/10Be ratio was found to vary significantly between samples instead of decaying smoothly over time. Due to the different physical and chemical properties of 36Cl and 10Be, different sensitivities to changes in climatic parameters, such as tropopause pressure and precipitation, are potentially the cause of the observed variability. Additionally, chlorine can be lost at low accumulation sites, such as EDC, as it can turn into hydrogen chloride and gas out from the firn. We present new measurements of the 36Cl/10Be ratio from the Skytrain ice core, which should be unaffected by chlorine loss, due to the higher accumulation rate at its drilling site. The measurement series extends below the dated sections of the ice core to test the decay dating and help extend the Skytrain age scale. To analyse differences in transport and deposition between radionuclides, the 36Cl/10Be ratio will also be determined with annual resolution in samples from 1982 – 2013 and compared to several climate parameters of the NOAA/CIRES/DOE 20th century reanalysis (V3) dataset. However, the data of this project are not yet available.

Abstract. Dating the bottommost section of an ice core is often complicated by strong layer thinning and possible disturbances in the stratigraphy. The radioactive decay of atmospherically produced 36Cl and 10Be can provide age estimates, where traditional methods can no longer be used. In this study, we investigated ice from the bottom of the Skytrain ice core, which was drilled in West Antarctica next to the Ronne Ice Shelf and has previously been dated to 126 kyr BP about 24 m above bedrock. Apart from decay, radionuclide concentrations in ice can be influenced by production rate variations, atmospheric transport and deposition variations, and, at low accumulations sites, by chlorine loss through hydrogen chloride outgassing. Using the 36Cl/10Be ratio largely removes production related variations and we were able to confirm that no 36Cl loss occurs at Skytrain Ice Rise, as the nuclear weapon test caused peak in 36Cl concentrations was found at the expected depth corresponding to the 1950s and 60s. An analysis of samples with known age showed that individual radionuclide concentrations and the 36Cl/10Be ratio are negatively correlated to the δ18O signal, which was used to apply a climate correction that enabled a higher precision for age estimates of previously undated samples. The deepest analysed section of the Skytrain ice core was found to be 552 ± 112 kyr old.

Glacial export productivity in the glacial Southern Ocean may have been enhanced due to iron fertilization from aeolian dust input. Marine sediments indicate such a glacial increase north of the modern Antarctic Polar Front but reduced biogenic activity and reduced nitrogen supply by upwelled deep waters south of it. Due to the sparsity of Southern Ocean sediment data, deriving an overall estimate of marine productivity changes is, however, difficult to achieve. Due to their larger spatial footprint, additional information on basin-wide productivity changes can be obtained from marine biogenic aerosol tracers in Antarctic ice cores. We use SO42- concentrations and its sulfur isotopic composition as well as other geochemical tracers in the EPICA Dronning Maud Land (EDML) ice core in the Atlantic Sector of the Southern Ocean (AS-SO) to provide the first complete glacial/interglacial source decomposition of total SO42- from the penultimate glacial to the last glacial inception. Our isotopic source decomposition shows that despite other (e.g. terrestrial) sources being significant contributors to total SO42- during glacial times, biogenic SO42- production is always the dominant source at EDML. Using information on recent dimethylsulfide emissions and aerosol forward modeling, we can show that biogenic sulfate recorded in the EDML ice core is derived from the AS-SO south of 35°S but the major source lies south of 50°S, i.e., mainly the seasonal sea ice zone. During the penultimate glacial these sources shifted about 4° northward in parallel to sea ice expansion. Taking reduced wet deposition of biogenic sulfate aerosol during glacial times into account, we can show that the biogenic sulfate production during the Penultimate Glacial Maximum and the Last Interglacial integrated over the AS-SO may have been only slightly higher in the penultimate glacial and differed by less than 15%. We see millennial biogenic sulfur changes of the same order during the Last Interglacial, which we attribute to temporal changes in the seasonal sea ice zone. An early interglacial productivity minimum in our biogenic sulfate record parallels within age uncertainties features previously reported in the literature, i.e., a minimum in winter, thus seasonal, sea ice extent, a stagnation event in Antarctic Bottom Water and a maximum in summer surface temperature encountered during the early LIG.

The bottommost sections of ice cores are often difficult to date, due to the low temporal resolution and possible disturbances, such as folding and missing layers. One possible tool for dating this ice is the 36Cl/10Be ratio, which decays with a combined half-life of 384 kyr years. Individual radionuclides are created by galactic cosmic rays in the atmosphere, but the ratio has been modelled to remove the varying production signal. The chronology of the recently drilled Skytrain ice core from West Antarctica ends with an age of 126 kyr BP 24 m above bedrock. Our aim was to obtain age estimates for samples in the undated section below, while improving our understanding of the 36Cl/10Be ratio as a dating tool. Two datasets were measured: an annually resolved record of the last few decades and a series of older samples from the Holocene, the last interglacial and five samples from the undated section. The data from recent decades was used to test whether the Skytrain site is affected by 36Cl loss, which occurs at low accumulation sites, such as EPICA Dome C and Little Dome C in East Antarctica, where 36Cl is gassing out as HCl. By measuring anthropogenic 36Cl from nuclear bomb tests in the 50s and 60s, we were able to confirm that the peak is found at the expected depth and that no 36Cl loss occurs. In older samples, there was a marked difference between glacial and interglacial data, with higher individual 36Cl and 10Be concentrations in glacial times. This is observed at other sites as well and can most likely be attributed to a dilution effect. However, the 36Cl/10Be ratio was also found to be higher in the last glacial period and correlated with the d18O signal, which likely results from the different physical and chemical properties of 36Cl and 10Be. While 36Cl can be found in its gaseous form or attached to particles, 10Be is always attached to particles, which yields different sensitivities to changes in temperature or precipitation. Possible mechanisms include a washout en-route, which may affect one radionuclide more than the other or an increased scavenging efficiency for 36Cl in mixed-phase clouds. While not fully understood, the correlation with d18O was used to detrend the data and estimate the age of five samples below the dated section, the oldest being 541 +55-61 kyr old.

Antarctic ice cores can help determine ice mass loss from the Antarctic Ice Sheet (AIS) during past warm periods. We compile Last Interglacial (LIG) delta 18O ${\delta }<^>{18}O$ measurements from eight Antarctic cores and compare these to new isotope-enabled LIG simulations, which explore three plausible LIG AIS elevation and extent scenarios. We find that these simulations capture less than 10% of East Antarctic core-mean delta 18O ${\delta }<^>{18}O$ changes. Although our simulations do not fully explain the changes, they capture some inter-core geographical delta 18O ${\delta }<^>{18}O$ variations. Some LIG AIS configurations show higher skill than PI AIS configurations in simulating the inter-core differences. The remaining discrepancies between the simulated and observed core-mean water isotope changes suggest that LIG simulations also need to include the influences of reduced Antarctic sea ice, a warmer Southern Ocean, and resultant shifts in vapor source regions to produce a more satisfactory match to delta 18O ${\delta }<^>{18}O$ observed at ice core sites.

Volcanic supereruptions are considered among the few drivers of global and existential catastrophes, with recent hypotheses suggesting massive volcanic stratospheric sulfate injection could instigate major shifts in global climate. The absence of supereruptions during recent history as well as large uncertainties on eruption ages limits understanding of the climatic risk they impose. Polar ice cores have well-resolved continuous age models, record past temperature, and contain volcanic sulfate and cryptotephra deposits which can be geochemically fingerprinted to determine eruption timing and improve stratospheric sulfur loading estimates. Here, we provide an age of 79,500 years for the Atitl & aacute;n Los Chocoyos supereruption, one of the largest Quaternary eruptions, by identifying tephra shards in ice cores from both Greenland and Antarctica. This ice core age is supported by a revised marine sediment core stratigraphy age for the Los Chocoyos ash layer. Through comparison with well-dated ice-core temperature proxy records, our study suggests that despite being one of the largest sulfur emissions recorded in ice cores, the Los Chocoyos supereruption did not trigger a millennial-scale cold period.

Abstract. Dating the bottommost section of an ice core is often complicated by strong layer thinning and possible disturbances in the stratigraphy. The radioactive decay of atmospherically produced 36Cl and 10Be can provide age estimates, where traditional methods can no longer be used. In this study, we investigated ice from the bottom of the Skytrain ice core, which was drilled in West Antarctica next to the Ronne Ice Shelf and has previously been dated to 126 kyr BP about 24 m above bedrock. Apart from decay, radionuclide concentrations in ice can be influenced by production rate variations, atmospheric transport and deposition variations, and, at low accumulations sites, by chlorine loss through hydrogen chloride outgassing. Using the 36Cl / 10Be ratio largely removes production related variations and we were able to confirm that no 36Cl loss occurs at Skytrain Ice Rise, as the nuclear weapon test caused peak in 36Cl concentrations was found at the expected depth corresponding to the 1950s and 1960s. An analysis of samples with known age showed that individual radionuclide concentrations and the 36Cl / 10Be ratio are negatively correlated to the δ18O signal, which was used to apply a climate correction that enabled a higher precision for age estimates of previously undated samples by reducing the uncertainty of the initial ratio from 14 % to 9 %. The deepest analysed section of the Skytrain ice core was found to be 561-109+110 kyr old.

The sulfur (S) isotope composition of ice cores represents a novel proxy to quantify variability in the sources of atmospheric sulfate. Sulfate aerosols exert a crucial but highly uncertain feedback on the climate system, acting as cloud condensation nuclei and scattering incoming solar radiation. Ice cores from Antarctica indicate that the majority of sulfate aerosols are sourced from marine biogenic activity, with little variability in emissions between glacial and interglacial periods. However, there are currently no published S isotope studies from the Arctic extending beyond the Common Era, and it is unclear how processes could differ between hemispheres. We present a high-resolution S isotope record of the NEEM ice core from Greenland, covering an entire glacial cycle from 0-128 ka BP. S isotope values appear to co-vary with the climate, exhibiting far lighter isotopic compositions during the Last Glacial compared to the Holocene or Last Interglacial. Systematic variability of S isotope values across Dansgaard–Oeschger events and strong linear relationships with water isotope compositions and calcium concentrations of the ice are also observed. We interpret these trends to show climatically controlled changes in the key sources of sulfate reaching the NEEM ice core site. During peak glacial conditions, the budget is dominated by sulfate sourced from terrestrial dust and volcanic emissions, with a negligible marine biogenic component. This finding suggests that we can use S isotopes to identify time periods when the source region for marine biogenic emissions reaching Greenland may have been completely ice-covered. Overall, this study provides new insights into the processes controlling sulfate aerosols in the Arctic and how the S cycle interacts with the climate.

The extent of grounded ice and buttressing by the Ronne Ice Shelf, which provides resistance to the outflow of ice streams, moderate West Antarctic Ice Sheet stability. During the Last Glacial Maximum, the ice sheet advanced and was grounded near the Weddell Sea continental shelf break. The timing of subsequent ice sheet retreat and the relative roles of ice shelf buttressing and grounding line changes remain unresolved. Here we use an ice core record from grounded ice at Skytrain Ice Rise to constrain the timing and speed of early Holocene ice sheet retreat. Measured δ 18 O and total air content suggest that the surface elevation of Skytrain Ice Rise decreased by about 450 m between 8.2 and 8.0 kyr before 1950 ce (±0.13 kyr). We attribute this elevation change to dynamic thinning due to flow changes induced by the ungrounding of ice in the area. Ice core sodium concentrations suggest that the ice front of this ungrounded ice shelf then retreated about 270 km (±30 km) from 7.7 to 7.3 kyr before 1950 ce . These centennial-scale changes demonstrate how quickly ice mass can be lost from the West Antarctic Ice Sheet due to changes in grounded ice without extensive ice shelf calving. Our findings both support and temporally constrain ice sheet models that exhibit rapid ice loss in the Weddell Sea sector in the early Holocene.