The Pleistocene epoch was characterized by global cooling and an increase in the intensity and duration of glacial cycles. Regional surface and subsurface ocean temperature records follow distinct trends over this interval, suggesting dynamic changes in zonal and meridional heat transport and ocean circulation. These differing trends also complicate efforts to determine the evolution of total ocean heat content. Here we provide a record of mean ocean temperature over the past 3 million years from noble gas (Xe/Kr) measurements in shallow ice cores recovered in the Allan Hills blue ice area, Antarctica1. The stratigraphically complex records preclude reconstruction of individual glacial cycles and probably represent a weighted averaging of glacial and interglacial conditions2. Nonetheless, we find pronounced cooling roughly coincident with the Plio-Pleistocene Transition (around 2.7 million years ago), and steady temperatures across the Mid-Pleistocene Transition (1.2 to 0.8 million years ago). Comparisons with a recent global sea surface temperature compilation3 show broad consistency in long-term cooling but important differences at the Plio-Pleistocene and Mid-Pleistocene transitions. We suggest that the different trends in surface temperature and mean ocean temperature during these intervals are related to a redistribution of heat between the surface and subsurface via changes in deep water formation and upwelling. Our temperature record also permits an estimate of global ice volume changes between 3 and 0.5 million years ago through a deconvolution of the benthic foraminiferal δ18O record and points to a period of enhanced ice sheet growth around the time of the Mid-Pleistocene Transition.

The Center for Oldest Ice Exploration (COLDEX) is a US initiative funded to search for climate records over the last 5 million years, including locating sites for an accessible continuous ice core going back 1.5 million years. As part of this effort, COLDEX has mapped the southern flank of Dome A, East Antarctica using an instrumented Basler, including dual frequency radar observations of the ice sheet and ice bed, as well as potential fields measurements (see presentation by Kerr in EGU session G4.3) across two field seasons from Amundsen-Scott South Pole Station. The aerogeophysical system included both the UTIG VHF MARFA radar system operating at 52.5-67.5 MHz, as well as a new large high resolution UHF array from CReSIS operating at 670-750 MHz operating simultanously. A goal of this project was to obtain airborne repeat interferometry for segments of the survey, as well as directly feed ice sheet models using englacial isochrons (see Singh presentation in EGU session CR5.6). These goals lead to a survey explicitly designed around ice sheet flow lines. While prior work had sampled the region at lithospheric scales, the COLDEX survey had two components - the first was to map the region at crustal scales (line spacing of 15 km), and the second was to map subareas at ice sheet scales (line spacing of 3 km). Immediate observations include an extensive basal unit and strong discontinuity in englacial stratigraphy that runs across the survey area and appears correlated with changes in bed interface properties. The airborne campaign will be used to inform follow up ground campaigns to understand processes relevant for old ice preservation.

Hyperspectral imaging (HSI) technology has been increasingly used in Earth and planetary sciences. This imaging technique has been successfully tested on ice cores using VNIR (visible and near-infrared, 380-1000 nm) (Garzonio et al., 2018) and near-infrared (900 - 1700 nm) (McDowell et al, 2023) line-scan cameras. Results show that HSI data greatly expand ice core line-scan imaging capabilities, previously used with gray or RGB cameras (see summary in Dey et al., 2023). Combinations of selected HSI bands from the hyperspectral data cube improve feature detection in ice core stratigraphy, and map distribution of volcanic material, dust, air bubbles, fractures, and ice crystals in ice cores. Captured spectral information provides unique fingerprints for specific materials present in ice cores. This method helps to guide ice core sampling because it provides non-destructive, rapid visualization of microstructural properties, layering, bubble contents, increases in dust, or presence of tephra material. Precise identification of these atmospheric components is important for understanding past climate drivers reconstructed from ice cores. As part of the COLDEX project (Brook et al., this meeting) we adapted the SPECIM SisuSCS HSI system for ice core imaging. The ice core scanning system is housed inside the ca. -20ºC main NSF ICF freezer, and externally computer-controlled. The operator monitors scanning operations and communicates with personnel inside of the freezer via radio. The system is equipped with a SPECIM FX10 camera that measures up to 224 bands in the VNIR range. We modified the ice core holder tray and installed a heated enclosure for the camera. The system uses SCHOTT DCR III Fiber Optic light sources with an OSL2BIR bulb from Thorlabs. IR filters are removed to extend the light spectral range beyond the 700 nm limit without heating the ice core surface during rapid (

Ice core reconstructions of atmospheric methane (CH4) and its stable carbon isotope ratio (δ13CH4) provide important constraints for understanding the links between human activity, methane and climate. However, uncertainties in existing δ13CH4 records since the preindustrial (~1850 CE), reconstructed from measurements of polar firn air and a small number of high-accumulation ice core sites, limit the precise determination of the timing and rate of recent changes in source/sink evolution. To re-assess methane dynamics over the last two centuries, we present continuous multi-core records of atmospheric CH4 and carbon monoxide (CO) between 1824 and 1994 CE reconstructed from high snow accumulation Antarctic sites and supplement these data with new bubble ice measurements of δ13CH4 spanning 50-years from 1938 to 1988 CE at a < 5-year resolution. Across the 50-year record, atmospheric CH4 mixing ratios increase by > 580 ppb and each δ13CH4 measurement therefore requires a considerable correction for diffusive fractionation resulting from a sustained growth in the overlying atmospheric methane burden during firn transport. An overlap with direct atmospheric observations is used to validate corrections for this phenomenon. Source/sink dynamics necessary to drive the simultaneous temporal trends observed in CH4, CO and δ13CH4 since 1850 CE are then inferred using a 6-troposphere, multi-tracer box model. Isotopic corrections, their implications and subsequent modelling results will be discussed.

Extending ice core records beyond 800 thousand years (kyr) is a pivotal goal in paleoclimate research. Allan Hills, East Antarctica, provides a unique opportunity to evaluate old ice and reconstruct climate well beyond 800 kyr with preliminary research uncovering ice ages up to 4.5 million years. Although old ice has been found and proven to be valuable, the ice in this area demonstrates several peculiarities—such as strong layer thinning, folding, and non-atmospheric CO2 – that warrant an in-depth investigation of the ice at this site and the climate record it holds. To address these challenges, we aim to initially conduct a high-resolution continuous flow analysis (CFA) of dust, water stable isotopes, water chemistry, and methane on the upper 70m of an ice core drilled in the 2022-2023 field season at the Allan Hills. A new CFA system has been developed at Oregon State University to analyze old ice, consisting of separate laser spectrometers for water stable isotopes and methane, an Abakus particle sensor for dust, and a fraction collector for sample analysis of melt-water chemistry. Here, we aim to present preliminary data on dust and methane and demonstrate the newly developed CFA system. Preliminary analyses on this ice have revealed the surface ice to be ~400 kyr old, with the majority of the upper 70m likely in stratigraphic order. This enables meaningful comparisons to other Antarctic ice cores and strengthens our comprehension of the climate-recording behavior of the ice. A high-resolution investigation of this ice is a critical step in understanding the discrete records from the Allan Hills that extend beyond the Mid-Pleistocene Transition and into the Pliocene, pushing our ice core records into unique and enigmatic parts of Earth’s climate history.

Understanding the causes of past atmospheric methane (CH4) variability is important for characterizing the relationship between CH4, global climate and terrestrial biogeochemical cycling. Ice core records of atmospheric CH4 contain rapid variations linked to abrupt climate changes of the last glacial period known as Dansgaard-Oeschger (DO) events and Heinrich events (HE)1,2. The drivers of these CH4 variations remain unknown but can be constrained with ice core measurements of the stable isotopic composition of atmospheric CH4, which is sensitive to the strength of different isotopically distinguishable emission categories (microbial, pyrogenic and geologic)3-5. Here we present multi-decadal-scale measurements of δ13C-CH4 and δD-CH4 from the WAIS Divide and Talos Dome ice cores and identify abrupt 1‰ enrichments in δ13C-CH4 synchronous with HE CH4 pulses and 0.5‰ δ13C-CH4 enrichments synchronous with DO CH4 increases. δD-CH4 varied little across the abrupt CH4 changes. Using box models to interpret these isotopic shifts6 and assuming a constant δ13C-CH4 of microbial emissions, we propose that abrupt shifts in tropical rainfall associated with HEs and DO events enhanced 13C-enriched pyrogenic CH4 emissions, and by extension global wildfire extent, by 90-150%. Carbon cycle box modelling experiments7 suggest that the resulting released terrestrial carbon could have caused from one-third to all of the abrupt CO2 increases associated with HEs. These findings suggest that fire regimes and the terrestrial carbon cycle varied contemporaneously and substantially with past abrupt climate changes of the last glacial period.

Currently, chronologically discontinuous ice cores from the Allan Hills Blue Ice Area (BIA), Antarctica, are our only direct insight into the atmospheric composition of periods beyond the continuous ice core record (800 ka BP). An accurate and precise greenhouse gas history beyond 800 ka would aid understanding of the mechanisms involved in the climatic transitions across the late Pliocene and early Pleistocene. Here we present carbon dioxide (CO2) and methane (CH4) results from a new core from the Allan Hills BIA (ALHIC1901). The bottom 25 m of ALHIC1901 contain 52 sampled depths with co-registered 40Aratm dates (Shackleton et al. in prep), measurements of δD of ice, δ18Oatm, and concentrations of CO2 and CH4 in trapped air. Of these samples, 25 are older than the continuous ice core record, with ages from 821 ± 80 ka to 2700 ± 270 ka. The bottom meter contains ice from the Pliocene with ages from 2700 ± 270 ka to 4000 ± 400 ka. The carbon isotope ratio of CO2 (δ13C-CO2) was measured on 18 samples to examine the possibility of input of non-atmospheric CO2 from oxidation of organic matter. Our results indicate that CO2 and CH4 levels were similar in the early Pleistocene to those found for the last 800 ka. A small decline of approximately 20 ppm is seen in CO2 across the Pleistocene, and no secular trend is observed in CH4. Pliocene-aged samples appear to contain a mixture of atmospheric CO2 and CO2 derived from respiration of organic matter at the glacier bed. Using an isotope mixing model we estimate that atmospheric CO2 was lower than 350 ppm at ~3.1 Ma,

During the transition from the Last Glacial Maximum (LGM) to the Holocene, the atmospheric N2O mole fraction increased by 80 nmol mol-1. Using ice core measurements of N2O isotopomer ratios, we show that this increase was driven by increases in both nitrification and denitrification, with the relative partitioning between both production pathways depending on the assumed isotopic end-member source signatures. Similarly, we also attribute a 35 nmol mol-1 N2O mole fraction increase during the Heinrich Stadial 4/Dansgaard Oeschger 8 (HS4/DO8) millennial-scale event to increases in both N2O production pathways. In contrast, the 25 nmol mol-1 N2O mole fraction decrease during the Younger Dryas was driven almost exclusively by a decrease in nitrification. The deglacial and HS4/DO8 increases in N2O production occurred in both marine and terrestrial environments, with the terrestrial source responding faster to warming by about two centuries. Constraints on changes in nitrification and denitrification emissions are robust and consistent with previous studies showing the sensitivity of N2O emissions to abrupt Northern Hemisphere warming. This study demonstrates for the first time the importance of both denitrification and nitrification pathways in driving source changes. Absolute emissions are more uncertain due to uncertainty about source isotopomer signatures. For instance, the contribution of denitrification to emissions at the LGM shifts from (65 +/- 10) % to (91 +/- 6) % when factoring in isotope enrichment due to partial reduction of N2O to N2 during denitrification. Reducing uncertainty in source signatures will increase the power of ice core N2O isotope records in deducing environmental change.

The Center for Oldest Ice Exploration (COLDEX) is a US initiative to search for climate records covering the last 5 million years, including cores from blue ice regions where very old ice has been identified. Ice cores from the Allan Hills, Antarctica contain discontinuous ice sections that date to as old as 4.6 Ma, and numerous samples with ages between 1 and 3 Ma, all dated with the 40Aratm technique. These samples provide constraints on a variety of past environmental variables, including greenhouse gases (Marks Peterson et al., this meeting) and mean ocean and Antarctic surface temperature (Shackleton et al., this meeting), and create opportunities to explore other properties of climate and the environment beyond the 800 ka limit of the existing ice core record (for example, Hudak et al., this meeting).The Allan Hills cores and glaciological setting are unusual. Ice flow, likely from a relatively local depositional area, traps old ice at shallow depths near the ice margin, albeit in a poorly understood manner. In most locations drilled so far, ice younger than 1 Ma is underlain by a relatively thin layer (20-40 m) of older material. In the “Cul-de-sac” region, ice older than 1 Ma is found within 15 m of the surfaceDating Allan Hills cores clearly shows age reversals indicative of folding. Deformation of dust and tephra bands at the surface, and deformation of bubbles at depth, also indicate complex ice flow. Dust mass concentrations are lower than expected for glacial periods, with anomalously high values at greater depth indicating incorporation of basal sediment. δ15N of N2 measurements indicate a relatively shallow firn column in the original deposition site. Three-dimensional mapping of electrical conductivity and isotopic measurements in large, 24-cm diameter cores clearly shows inclined layers and folding. Phase-sensitive radar is being used to measure spatial variations in vertical velocity (with some repeat measurements completed), and polarimetry profiles. Temperature measurements in Allan Hills boreholes suggest heating related to shear between the old ice and shallower layers.Results from sections that date to 500-800 ka reproduce the long-term mean values of various parameters (CO2, CH4, δ18Oatm, MOT, δ18Oice) but not the entire glacial-interglacial range. Small-scale folding, diffusion and hiatuses are all possible explanations for the muted variability. Detailed studies of large diameter cores are currently investigating these possibilities.So far, five cores (ALHIC1502, 1503, 1901, 1902 and 2201) sample ice older than 1 million years. ALHIC1902 contains the oldest ice dated, at 4.6 Ma. Drilling in the 23/24 field season partly completed a 24-cm diameter core (2302) at the 1902 site, intended to provide large volumes of very old ice, A 90 m core in the Cul de Sac, (2301), at the location where old ice was found near the surface, was also completed.COLDEX drilling will continue in the Allan Hills in 2024-25 and possibly in later seasons. Future work may also include an ~1250 m ice core in a region where modelling predicts continuous stratigraphy for ~1 Ma.

Ice cores represent the only direct paleo-atmospheric archive that allow the reconstruction of greenhouse gas concentrations such as N2O. However, processes in the ice can alter the atmospheric information stored in air bubbles, for example by adding extra N2O by in situ production. This in situ production of N2O is especially severe in mineral dust-rich ice core sections corresponding to glacial periods. Understanding the production process and its link to the mineral dust content is key to systematically detecting altered samples and correcting for the in situ contribution. Isotope analysis is particularly useful for characterizing these processes and thus isolating the paleoclimatic signal from archived data. We measured the bulk nitrogen and oxygen isotopic composition of N2O in Antarctic and Greenland ice cores from glacial periods. The isotopic signatures of N2O produced in situ, calculated using a mass balance approach, differ from that of the atmospheric N2O. In addition, enrichment or depletion in 15N and/or 18O relative to atmospheric values varies with drilling site, snow accumulation rate, and properties of the snow-ice transition. Interestingly, isotopic signatures of nitrate (NO3-) exhibit similar dependencies. It is well established that NO3- is drastically altered by post-depositional processes in low accumulation areas. Joint isotopic analysis of N2O and NO3- in samples from the EDC and EDML ice cores revealed a correlation between δ15N values of NO3- and in situ N2O, pointing to NO3- as a potential precursor for in situ production. While being linearly correlated, the nitrogen isotopic signature of NO3- is twice as enriched as in situ produced N2O. This suggests that the two N atoms of N2O originate from two distinct sources and only one is likely derived from nitrate.We additionally measured the site preference of 15N in N2O in ice core samples (SP = δ15Nα - δ15Nβ, where α is the central and β the terminal N atom in the N2O molecule). Previous work on SP suggests that SP might be indicative of the N2O formation pathway provided both N atoms are derived from the same N precursor. The SP signature in Vostok samples ranges from +57 to +242 ‰, and the δ15Nα values from +92 to +234 ‰, which is comparable to the δ15N values of NO3- at Vostok. Although similar reaction pathways were expected in different ice cores, in situ N2O from Taylor Glacier samples exhibits very different SP values from -17 to -7 ‰, with δ15Nα values from -45 to -32 ‰. Given that the difference in δ15N of NO3- is also up to 200 ‰ between these two locations, our findings suggest that the center-position nitrogen (α) of in situ N2O comes from NO3- and the terminal-position nitrogen (β) from another N-bearing compound. Thus, the SP signature seems to reflect not the N2O formation pathway but the difference in δ15N of the two nitrogen pools involved in the reaction.Gaining a thorough understanding of the N2O production in ice marks a significant advancement towards interpretation of the N2O record and possibly correction for in situ production.

Abstract Nitrous oxide (N2O) is an important greenhouse gas which destroys the ozone in the stratosphere. Primary sources of atmospheric N2O are nitrification and denitrification in terrestrial soils and the ocean, and the main sink is photolysis in the stratosphere. Studies have mostly focused on the climate‐related response of N2O during glacial‐interglacial periods. However, its mechanism of variation during the Holocene remains unclear. We present a high‐resolution N2O record from the South Pole Ice (SPICE) core covering the Holocene epoch. The millennial‐scale N2O trend agrees with existing records. We constructed a Holocene composite consisting of the new N2O measurements in SPICE and existing records from other ice core sites. The N2O composite reveals four distinct periods of N2O variation during 11.5–10.0 ka, 10.0–6.2 ka, 6.2–2.2 ka, and 2.2–1.4 ka, including two maxima in 11.0–10.0 ka and 3.0–2.2 ka and minima in 8.8–6.2 ka and approximately 1.4 ka. Apart from these, our new high‐resolution record from SPICE shows a short‐term N2O decrease around 2.8 ka which is not observed in other records possibly due to lower sample resolution and/or higher age smoothing. Comparison of our new Holocene N2O composite with the paleo‐proxy records suggests the plausible linkage of major monsoon (Asian, North African, South and North American, and Australian‐Indonesian monsoon) and upwelling (Arabian Sea and Eastern Tropical South Pacific) regions in regulating the atmospheric N2O during the Holocene.

AbstractHolocene temperature evolution remains poorly understood. Proxies in the early and mid‐Holocene suggest a Holocene Thermal Maximum (HTM) where temperatures exceed the pre‐industrial, whereas climate models generally simulate monotonic warming. This discrepancy may reflect proxy seasonality biases or errors in climate model internal feedbacks or dynamics. Using seasonally unbiased ice core reconstructions at NEEM, NGRIP, and Greenland Ice Sheet Project 2, we identify a Greenland HTM of ∼2°C above pre‐industrial, in agreement with other Northern Hemisphere proxy reconstructions. The firn‐based reconstructions are verified through borehole thermometry, producing a multi‐core, multi‐proxy reconstruction of Greenland climate from the last glacial to pre‐industrial. HTM timing across Greenland is heterogenous, occurring earlier at high elevations. Total air content measurements suggest a temperature contribution from elevation changes; regional oceanographic conditions, a weakened polar lapse rate, or variable near‐surface inversions may also be important sensitivities. Our reconstructions support climate simulations with dynamic Holocene vegetation, highlighting the importance of vegetation feedbacks.

The role greenhouse gases play in the evolution of Earth's climate over the last 3 million years is uncertain beyond the continuous ice core record (800,000 years). Here, we present new snapshots of carbon dioxide (CO2) and methane (CH4) between 3.1 and 0.4 million years ago (Ma) from shallow ice cores drilled in the Allan Hills Blue Ice Area (BIA). In the oldest ice (>1 Ma), mixing and thinning have attenuated the glacial-interglacial variability, and we reconstruct long-term averages. The data indicate that CO2 and CH4 levels in the early Pleistocene are within the range of variability observed for the last 0.8 Ma. Across the Pleistocene, no significant change in mean CH4 is observed but we find a small, 25 ppm decline in CO2 from 2.9 to 1.2 Ma followed by stable mean CO2 across the mid-Pleistocene Transition. In late Pliocene samples (ranging from 2.8-3.1 Ma), trapped air is impacted by the addition of CO2 from respired organic matter. Corrections using the stable carbon isotopes of CO2 indicate that atmospheric CO2 in these late Pliocene samples is within the range measured in the early Pleistocene (<300 ppm). Observed changes in greenhouse gases are small relative to both local and global cooling observed in the same ice cores (Shackleton et al., 2024a, b) and independent records from marine sediments, suggesting that other components of Earth’s climate system contributed to global cooling over the last 3 million years.

The last glacial period was punctuated by cold intervals in the North Atlantic region that culminated in extensive iceberg discharge events. These cold intervals, known as Heinrich Stadials, are associated with abrupt climate shifts worldwide. Here, we present CO 2 measurements from the West Antarctic Ice Sheet Divide ice core across Heinrich Stadials 2 to 5 at decadal-scale resolution. Our results reveal multi-decadal-scale jumps in atmospheric CO 2 concentrations within each Heinrich Stadial. The largest magnitude of change (14.0 ± 0.8 ppm within 55 ± 10 y) occurred during Heinrich Stadial 4. Abrupt rises in atmospheric CO 2 are concurrent with jumps in atmospheric CH 4 and abrupt changes in the water isotopologs in multiple Antarctic ice cores, the latter of which suggest rapid warming of both Antarctica and Southern Ocean vapor source regions. The synchroneity of these rapid shifts points to wind-driven upwelling of relatively warm, carbon-rich waters in the Southern Ocean, likely linked to a poleward intensification of the Southern Hemisphere westerly winds. Using an isotope-enabled atmospheric circulation model, we show that observed changes in Antarctic water isotopologs can be explained by abrupt and widespread Southern Ocean warming. Our work presents evidence for a multi-decadal- to century-scale response of the Southern Ocean to changes in atmospheric circulation, demonstrating the potential for dynamic changes in Southern Ocean biogeochemistry and circulation on human timescales. Furthermore, it suggests that anthropogenic CO 2 uptake in the Southern Ocean may weaken with poleward strengthening westerlies today and into the future.

Constraining the causes of past atmospheric methane variability is important for understanding links between methane and climate. Abrupt methane changes during the last deglaciation have been intensely studied for this purpose, but the relative importance of high-latitude and tropical sources remains poorly constrained. The methane interpolar concentration difference reflects past geographic emission variability, but existing records suffered from subtle but considerable methane production during analysis. Here, we report an ice-core-derived interpolar difference record covering the Last Glacial Maximum and deglaciation, with substantially improved temporal resolution, chronology and a critical correction for methane production in samples from Greenland. Using box models to infer latitudinal source changes, we show that tropical sources dominated abrupt methane variability of the deglaciation, highlighting their sensitivity to abrupt climate change and rapidly shifting tropical rainfall patterns. Northern extratropical emissions began increasing ~16,000 years ago, probably through wetland expansion and/or permafrost degradation induced by high-latitude warming, and contributed at most 25 Tg yr −1 (45% of the total emission increase) to the abrupt methane rise that coincided with rapid northern warming at the onset of the Bølling–Allerød interval. These constraints on deglacial climate–methane cycle interactions can improve the understanding of possible present and future feedbacks.

Ice core records of carbon dioxide (CO 2 ) throughout the last 2000 years provide context for the unprecedented anthropogenic rise in atmospheric CO 2 and insights into global carbon cycle dynamics. Yet the atmospheric history of CO 2 remains uncertain in some time intervals. Here we present measurements of CO 2 and methane (CH 4 ) in the Skytrain ice core from 1450 to 1700 CE. Results suggest a sudden decrease in CO 2 around 1610 CE in one widely used record may be an artefact of a small number of anomalously low values. Our analysis supports a more gradual decrease in CO 2 of 0.5 ppm per decade from 1516 to 1670 CE, with an inferred land carbon sink of 2.6 PgC per decade. This corroborates modelled scenarios of large-scale reorganisation of land use in the Americas following New World-Old World contact, whereas a rapid decrease in CO 2 at 1610 CE is incompatible with even the most extreme land-use change scenarios.

Abstract Constraining the causes of past atmospheric methane variability is important for understanding links between methane and climate. Abrupt methane changes during the last deglaciation have been intensely studied for this purpose, but the relative importance of high-latitude and tropical sources remains poorly constrained. The methane interpolar concentration difference (IPD) reflects past geographic source variability, but existing records suffered from subtle but significant methane production during analysis. Here, we report a new, ice-core derived IPD record covering the last glacial maximum and deglaciation, with substantially improved temporal resolution, chronology, and a critical correction for methane production in samples from Greenland. Using box-models to examine latitudinal source changes, we show that tropical sources dominated abrupt methane variability of the deglaciation, highlighting their sensitivity to abrupt climate change and rapidly shifting tropical rainfall patterns. Northern extratropical sources began increasing at ~16 kyr, likely through wetland expansion and/or permafrost degradation induced by high-latitude warming, and contributed at most 20 Tg yr -1 (40% of the total source increase) to the abrupt methane rise that coincided with rapid northern warming at the onset of the Bølling Allerød interval. These new deglacial methane budget constraints can be used to improve the understanding of climate-methane cycle feedbacks.

The total air content (TAC) of polar ice cores has long been considered a potential proxy for past ice sheet elevation. Recent work, however, has shown that a variety of other factors also influence this parameter. In this paper we present a high-resolution TAC record from the South Pole ice core (SPC14) covering the last 54 000 years and discuss the implications of the data for interpreting TAC from ice cores. The SPC14 TAC record shows multiple features of interest, including (1) long-term orbital-scale variability, (2) millennial-scale variability in the Holocene and last glacial period, and (3) a period of stability from 35 to 25 ka. The longer, orbital-scale variations in TAC are highly correlated with integrated summer insolation (ISI), corroborating the potential of TAC to provide an independent dating tool via orbital tuning. Large millennial-scale variability in TAC during the last glacial period is positively correlated with past accumulation rate reconstructions as well as δ15N-N2, a firn thickness proxy. These TAC variations are too large to be controlled by direct effects of temperature and too rapid to be tied to elevation changes. We propose that grain size metamorphism near the firn surface explains these changes. We note, however, that at sites with different climate histories than the South Pole, TAC variations may be dominated by other processes. Our observations of millennial-scale variations in TAC show a different relationship with accumulation rate than observed at sites in Greenland.

Here we present a newly developed ice core gas-phase proxy that directly samples a component of the large-scale atmospheric circulation: synoptic-scale pressure variability. Surface pressure changes weakly disrupt gravitational isotopic settling in the firn layer, which is recorded in krypton-86 excess (Kr-86(xs)). The Kr-86(xs) may therefore reflect the time-averaged synoptic pressure variability over several years (site "storminess "), but it likely cannot record individual synoptic events as ice core gas samples typically average over several years. We validate Kr-86(xs) using late Holocene ice samples from 11 Antarctic ice cores and 1 Greenland ice core that collectively represent a wide range of surface pressure variability in the modern climate. We find a strong spatial correlation (r=-0.94, p < 0.01) between site average Kr-86(xs) and time-averaged synoptic variability from reanalysis data. The main uncertainties in the analysis are the corrections for gas loss and thermal fractionation and the relatively large scatter in the data. Limited scientific understanding of the firn physics and potential biases of Kr-86(xs) require caution in interpreting this proxy at present. We show that Antarctic 86Krxs appears to be linked to the position of the Southern Hemisphere eddy-driven subpolar jet (SPJ), with a southern position enhancing pressure variability.We present a Kr-86(xs) record covering the last 24 kyr from the West Antarctic Ice Sheet (WAIS) Divide ice core. Based on the empirical spatial correlation of synoptic activity and Kr-86(xs) at various Antarctic sites, we interpret this record to show that West Antarctic synoptic activity is slightly below modern levels during the Last Glacial Maximum (LGM), increases during the Heinrich Stadial 1 and Younger Dryas North Atlantic cold periods, weakens abruptly at the Holocene onset, remains low during the early and mid-Holocene, and gradually increases to its modern value. The WAIS Divide Kr-86(xs) record resembles records of monsoon intensity thought to reflect changes in the meridional position of the Intertropical Convergence Zone (ITCZ) on orbital and millennial timescales such that West Antarctic storminess is weaker when the ITCZ is displaced northward and stronger when it is displaced southward. We interpret variations in synoptic activity as reflecting movement of the South Pacific SPJ in parallel to the ITCZ migrations, which is the expected zonal mean response of the eddy-driven jet in models and proxy data. Past changes to Pacific climate and the El Nino-Southern Oscillation (ENSO) may amplify the signal of the SPJ migration. Our interpretation is broadly consistent with opal flux records from the Pacific Antarctic zone thought to reflect wind-driven upwelling.We emphasize that Kr-86(xs) is a new proxy, and more work is called for to confirm, replicate, and better understand these results; until such time, our conclusions regarding past atmospheric dynamics remain speculative. Current scientific understanding of firn air transport and trapping is insufficient to explain all the observed variations in Kr-86(xs). A list of suggested future studies is provided.

Abstract High-resolution ice core records from coastal Antarctica are particularly useful to inform our understanding of environmental changes and their drivers. Here, we present a decadally resolved record of sea-salt sodium (a proxy for open-ocean area) and non-sea salt calcium (a proxy for continental dust) from the well-dated Roosevelt Island Climate Evolution (RICE) core, focusing on the time period between 40–26 ka BP. The RICE dust record exhibits an abrupt shift towards a higher mean dust concentration at 32 ka BP. Investigating existing ice-core records, we find this shift is a prominent feature across Antarctica. We propose that this shift is linked to an equatorward displacement of Southern Hemisphere westerly winds. Subsequent to the wind shift, data suggest a weakening of Southern Ocean upwelling and a decline of atmospheric CO2 to lower glacial values, hence making this shift an important glacial climate event with potentially important insights for future projections.