Reply on RC1

Precision, accuracy, and temporal resolution are key to make full use of atmospheric trace gas records in ice cores. These aspects will become especially crucial for ice cores that aim to extend the ice core record to the last 1.5 Myr, i.e., across the Mid Pleistocene Transition (as currently drilled within the European project Beyond EPICA – Oldest Ice Core (BE-OIC)). The ice from this period is expected to be close to bedrock and, due to glacier flow, extremely thinned with 15,000 years of climate history contained in only one meter of ice. Accordingly, for a century-scale resolution, the sample vertical extent must be reduced to a few cm containing only about 1−2 mL air STP. We present a novel combined system for the extraction and the simultaneous measurement of CO2, CH4, and N2O concentrations, as well as δ13CO2, which achieves a vertical resolution of 1−2 cm with precisions of 0.4 ppm, 3 ppb, 1 ppb and 0.04 ‰, respectively. This is accomplished by employing a directional and continuous laser induced sublimation followed by analysis of the sample gas by quantum cascade laser absorption spectroscopy (QCLAS). Besides the low sample volume requirements and the vertical resolution capabilities, the described method holds additional advantages over previous methods, including the immunity of the highly specific QCLAS analysis to drilling fluid contamination as well as the non-destructive nature of the spectroscopic gas analysis. The combined extraction and analysis system was extensively tested by sublimating gas-free ice with introduction of a standard gas to determine the accuracy and characterize potential artefacts. Moreover, Antarctic ice samples were measured to confirm the measurement performance, covering the range of variability expected in Pleistocene ice and to highlight the vertical resolution capabilities critical for its application within BE-OIC.

<strong class="journal-contentHeaderColor">Abstract.</strong> Precision, accuracy, and temporal resolution are key to making full use of atmospheric trace gas records in ice cores. These aspects will become especially crucial for ice cores that aim to extend the ice core record to the last 1.5 Myr, i.e., across the Mid-Pleistocene Transition (as currently drilled within the European project Beyond EPICA – Oldest Ice Core (BE-OIC)). The ice from this period is expected to be close to bedrock and, due to glacier flow, extremely thinned with 15 000 years of climate history contained in only 1 m of ice. Accordingly, for a century-scale resolution, the sample vertical extent must be reduced to a few centimeters containing only about 1–2 mL air STP. We present a novel combined system for the extraction and the simultaneous measurement of <span class="inline-formula">CO₂</span>, <span class="inline-formula">CH₄</span>, and <span class="inline-formula">N₂O</span> concentrations, as well as <span class="inline-formula"><i>δ</i>¹³CO₂</span>, which achieves a vertical resolution of 1–2 cm (<span class="inline-formula">3.5×3.5</span> cm cross section) with precisions of 0.4 ppm, 3 ppb, 1 ppb, and 0.04 ‰, respectively, in sublimation tests with standard gas over gas-free ice. This is accomplished by employing a directional and continuous laser-induced sublimation followed by analysis of the sample gas by a quantum cascade laser absorption spectrometer (QCLAS). Besides the low sample volume requirements and the vertical resolution capabilities, the described method holds additional advantages over previous methods, including the immunity of the highly specific QCLAS analysis to drilling fluid contamination as well as the non-destructive nature of the spectroscopic gas analysis. The combined extraction and analysis system was extensively tested by sublimating gas-free ice with introduction of a standard gas to determine the accuracy and characterize potential artifacts. Moreover, Antarctic ice samples were measured to confirm the measurement performance, covering the range of variability expected in Pleistocene ice and highlighting the vertical resolution capabilities critical for its application within BE-OIC.

The largest share of greenhouse gas (GHG) emissions related to livestock originates from methane (CH4) and nitrous oxide (N2O) which have a far higher influence on global warming, it is therefore necessary to accurately monitor CH4 and N2O emissions to provide theoretical and practical basis for further estimating and regulating GHG emissions from livestock and improving livestock production performance. For the purpose of sensing CH4 and N2O emissions during livestock living process in real time, an optical sensor based on continuous-wave (CW) external cavity quantum cascade laser (EC-QCL) operating at room temperature was developed. CH4 and N2O absorption lines, located around 8 μm, of the ν4 and ν1 fundamental vibrational bands, respectively, were chosen for direct absorption spectroscopy, which allows for sensitive, selective and simultaneous measurement of CH4 and N2O concentrations. Use of a Herriot multi-pass cell with an effective path-length of 100 m, 1σ (SNR = 1) limits of detection of 26.8 ppbv, 20.3 ppbv and 0.01 % for CH4, N2O and H2O vapor were achieved, respectively. Field measurement of CH4 and N2O emissions from horses has been carried out in a stable over two weeks at the Vernaelde farm in Couderkerque Branche city, France. Concentrations of CH4 and N2O up to 10 times and 1.5 times higher than their levels in the local ambient air (∼ 2.12 ppmv and ∼ 427 ppbv) were observed, respectively.

During the Mid-Pleistocene Transition (MPT), ca. 1250–800 kya, the Earth’s glacial cycles changed from 41 ky to 100 ky periodicity. The emergence of this longer ice-age periodicity was accompanied by higher global ice volume in glacial periods and lower global ice volume in interglacial periods. Since there is no known change in external orbital forcing across the MPT, it is generally agreed that the cause of this transition is internal to the earth system. Resolving the climate–carbon cycle–cryosphere dynamics processes responsible for the MPT remains a major challenge in ice core and climate science. To address this challenge, the international ice core community has prioritized recovery of an ice core record spanning the MPT interval. The results from such ‘oldest ice’ projects are still several years away. Our objective here it to make an advanced prediction of atmospheric CO2 out to 1.5 my. Our prediction utilizes existing records of atmospheric carbon dioxide (CO2) from Antarctic ice cores spanning the past 800 ky along with the existing benthic water stable isotope (ẟ18O) record from marine sediment cores. Our predictions assume that the relationship between CO2 and benthic ẟ18O over the past 800 thousand years can be extended over the last one and a half million years. The implied null hypothesis is that there has been no fundamental change in the global climate–carbon cycle–cryosphere feedback systems across the MPT. We find that our predicted CO2 record is significantly lower during glacial intervals than the existing blue-ice and boron isotope-based estimates of CO2 that pre-date the continuous 800 ky CO2 record. Our predicted glacial CO2 concentrations are ~9 ppm below glacial CO2 concentrations observed in blue ice data at ca. 1 mya and ~19 ppm below glacial CO2 concentrations reconstructed from boron isotopic data over ca ~1.1–1.25 mya. These results support rejection of our null hypothesis and provide quantitative evidence of a fundamental shift in the global climate–carbon cycle–cryosphere feedback systems across the MPT. However, the definitive test of the various theories explaining the MPT will be comparison of our predicted records with the forthcoming oldest ice core records.

The Mid-Pleistocene Transition (MPT; 1.25 – 0.8 Myr) marks a transition from the “41-kyr world”, in which the Earth alternated between cold and warm periods about every 41,000 years, to the “100-kyr world” in which the Earth remained predominantly under glacial conditions but was punctuated every 80,000 to 120,000 years by interglacial periods. Variations in orbital forcing, the “pacemaker of the ice ages”, are stable across the MPT and thus cannot be invoked as a driver of this transition. Thus, most hypotheses call upon a forcing that drives a secular change, a feedback in the Earth system that changes/emerges, or a combination of the two.One hypothesis for the MPT suggests that a long-term decline in (glacial) atmospheric CO2 levels led to a cooling, facilitating the formation of extensive ice sheets in North America and a sea-level drop of approximately 70 m (Bintanja &amp; van de Wal, 2008). Accordingly, decreasing atmospheric CO2 concentration may have played a central role in driving this global cooling. Despite recent advances in marine and ice core CO₂ reconstructions (Nuber et al., 2025; Marks Peterson et al., 2025) the change of greenhouse gas forcing across the MPT remains uncertain for CO2.In order to investigate the role of atmospheric CO2 across the MPT, greenhouse gases and the stable carbon isotopic composition of CO2 (δ13C-CO2) were measured on discrete ice core samples from the Beyond EPICA ice core. For this purpose, a coupled Laser induced sublimation extraction – Quantum Cascade laser Absorption Spectrometer (LISE-QCLAS) was used, allowing the simultaneous and semi-continuous extraction and measurement of CO2, CH4 and N2O as well as δ13C-CO2 on air samples of only 1 – 2 mL, corresponding to 10 – 15 g of ice.This talk will present the first ice core data capable of capturing glacial–interglacial variations in atmospheric CO₂ across the MPT. Additionally, we will present the first unconditionally pristine measurements of δ13C-CO2 during the 41-kyr world. These data will allow us to explore the underlying biogeochemical processes that may be responsible for the new modes of atmospheric CO2 variability we have observed.

Assessments of soil carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) emissions are critical for determination of the agricultural practices' potential to mitigate global warming. This study evaluated the photoacoustic spectroscopy (PAS) for the assessment of soil greenhouse gases (GHG) fluxes in comparison to the standard gas chromatography (GC) method. Two long-term experiments with different tillage and cropping systems over a Paleudult were evaluated using static chambers. PAS measurements of CO2 and N2O concentrations showed good relationship and linearity (R2=0.98 and 0.94, respectively) with GC results. However, CH4 measurements were significantly affected by air sample moisture which interfered on CH4 detection by PAS. Overestimation of CO2 and N2O concentrations in air samples determined by PAS (14.6 and 18.7%, respectively) were also related to sampling moisture. CO2 and N2O fluxes showed good agreement between methods (R2=0.96 and 0.95, respectively), though PAS overestimated fluxes by 18.6 and 13.6% in relation to GC results, respectively. PAS showed good sensitivity and was able to detect CO2 and N2O fluxes as low as 332mg CO2 m-2 h-1 and 21µg N2O m-2 h-1. PAS analyzer should be detailed calibrated to reduce humidity interference on CO2, CH4 and N2O concentrations measurements avoiding overestimation or erroneous determination of soil GHG fluxes.

The international ice core community has a target to obtain continuous ice cores stretching back as far as 1.5 million years. This would provide vital data (including a CO2 profile) allowing us to assess ideas about the cause of the Mid-Pleistocene Transition (MPT). The European Beyond EPICA project and the Australian Million Year Ice Core project each plan to drill such a core in the region known as Little Dome C. Dating the cores will be challenging, and one approach will be to match some of the records obtained with existing marine sediment datasets, informed by similarities in the existing 800 kyr period. Water isotopes in Antarctica have been shown to closely mirror deepwater temperature, estimated from Mg / Ca ratios of benthic foraminifera, in a marine core on the Chatham Rise near to New Zealand. The dust record in ice cores resembles very closely a South Atlantic marine record of iron accumulation rate. By assuming these relationships continue beyond 800 ka, our ice core record could be synchronised to dated marine sediments. This could be supplemented, and allow synchronisation at higher resolution, by the identification of rapid millennial scale-events that are observed both in Antarctic methane records and in emerging records of planktic oxygen isotopes and alkenone sea surface temperature (SST) from the Portuguese Margin. Although published data remain quite sparse, it should also be possible to match 10Be from ice cores to records of geomagnetic palaeointensity and authigenic 10Be/9Be in marine sediments. However, there are a number of issues that have to be resolved before the ice core 10Be record can be used. The approach of matching records to a template will be most successful if the new core is in stratigraphic order, but should also provide constraints on disordered records, if used in combination with absolute radiogenic ages.

Recent ice cores from the Allan Hills, a blue ice area in Antarctica, are nearly 3 million years old. These cores extend ice core chronologies, enabling new insight into key climate periods such as the Mid-Pleistocene Transition. The interpretation of these climate records is complex because of the disturbed stratigraphy in this ice. Here, we present a new three-dimensional multitrack electrical conductivity measurement method (3D ECM) to resolve layer structure. We demonstrate this technique on a cumulative 60 m of two large-diameter (241 mm) ice cores, ALHIC2201 and ALHIC2302. Measurements were taken on the upper section of both cores due to better ice core quality in this shallow ice. We find well-defined and dipping layering in both cores, averaging 29° in ALHIC2201 and 69° in ALHIC2302 from horizontal. We observe a slight decrease in dip with depth in both cores, although it only achieves statistical significance in ALHIC2302. We discuss how this new method can be applied to enable accurate, high-resolution multi-proxy record development even in ice cores with steeply dipping layers. 3D ECM improves interpretation of blue ice area cores by providing accurate, non-destructive constraints on stratigraphy.

The polar regions are important components of the Earth's climate system, as these areas are the main sources of fresh water on Earth. Any conflicts in that system can affect people's lives in different ways. Numerous studies show that global air temperature has increased, and it can lead to a global sea level rise due to polar ice-sheet mass loss. An increase in sea level in polar regions not only provides evidence of climate change, but also acts as a complex feedback mechanism between the Earth system components. This feedback can be used to predict what can happen in the future at different spatial and temporal resolutions. The ice core is an important climate archive that can help us to understand past climate conditions and we can use this information to predict future climate change using Earth climate modelling. There are numerous methods to investigate information from the ice core, Dielectric Profiling (DEP) recoded along the ice core is one such methods. Dielectric Profiling (DEP) is related to the impurity content of the ice core, such as acidity, salt, and ammonia concentrations or impurities caused by a volcanic event. In this dissertation, I focus on measuring and processing the recorded permittivity and conductivity of DEP data and then use the data in different aspects of glaciology and paleoclimatology. This study investigates different ice core data, including The East GReenland Ice-core Project (EGRIP) as a first ice core from the upper region of the North-East-Greenland Ice Stream (NEGIS), the North Greenland Eemian (NEEM) ice core with the oldest reconstructed record from a folded core, and the North Greenland Ice Core Project (NGRIP) with the oldest undisturbed record in Greenland. This dissertation established a first chronology of the EGRIP ice core over the Holocene and late last glacial period. The depth–age relationship of an ice core plays a key role in assessing the past Earth climate system. After field measurement in Greenland and processing the data set, I rely on DEP data and electrical conductivity measurements (ECM) and tephra records for the synchronization between the EGRIP, NEEM, and NGRIP ice cores. The synchronization was mainly of volcanic events and common patterns of DEP and ECM peaks. The timescale was transferred to the EGRIP ice core from the annual-layer-counted Greenland Ice Core Chronology 2005 (GICC05) timescale from the NGRIP core. This new timescale for the EGRIP is named GICC05-EGRIP-1. Second, this dissertation investigates the internal architecture of the Greenland ice sheet by linking numerical forward modelling on the dielectric profile of ice cores and data derived from multi-channel ultra-wideband radar systems from the airborne survey. The modelling results have very good agreement with radio-echo sounding measurements. It was found that the internal reflections are mainly due to conductivity changes, and the across–-flow concentrated fabric in the deeper part of drill site (e.g., EGRIP ). Also, this study provides the possibility to determine ages for the reflectors, which can be considered as isochrone, and extend the dating from the drill site regions along the radar lines over large parts of northern Greenland. Third, this dissertation evaluates extreme climate after the massive eruption of 'Alaska's Okmok volcano and its effect on human lives. In this way, unpublished DEP data from NGRIP2 and NEEM ice have been processed for the synchronization between ice cores. This volcano was evaluated based on the DEP, ECM, volcanic tephra data, and climate proxy records in six Arctic ice cores. The Earth system model simulations indicate that temperatures in Mediterranean regions were 7°C below normal during the two-year period after this massive eruption. The global cooling due to that eruption affected the lives of people in the late Roman Republic in 43 and 42 BC.

A sensitive open-path gas sensor employing a continuous-wave (CW) distributed feedback (DFB) quantum cascade laser (QCL) and direct absorption spectroscopy (DAS) was demonstrated for simultaneously measurements of atmospheric CO and N2O. Two interference free absorption lines located at 2190.0175 cm-1 and 2190.3498 cm-1 were selected for CO and N2O concentration measurements, respectively. The Allan variance analysis technique was performed to investigate the long-term performance of the QCL sensor system. The results indicate that a detection limit of 9.92 ppb for CO and 7.7 ppb for N2O with 1-s integration time were achieved, which can be further improved to 1.5 ppb and 1.1 ppb by increasing the average time up to 80 s.

In the accumulation regions of the Greenland Ice Sheet (GrIS), not all surface meltwater contributes to runoff. A significant portion is retained through refreezing within the underlying firn layer, a process that critically moderates the overall mass loss from the GrIS. The refreezing of percolating meltwater at shallow depths leads to densification of the near-surface and the formation of ice layers. The extent of meltwater refreezing is influenced by firn density and temperature, which together govern the permeability of the near-surface ice layers. The presence of shallow, thick ice layers (&gt; 1m thick, also known as "ice slab") restricts the deeper percolation of meltwater, thereby promoting its conversion into runoff. For example, the formation of ice slab in GrIS has resulted in nearly a 30% increase in the area contributing to runoff generation since 2001. Therefore, modelling ice slab is essential for understanding the total mass loss of the GrIS, both in recent years and in future projections.In this study, we present a high vertical resolution, physically distributed model that simulates surface mass balance, refreezing, ice layer formation, and runoff. A novel temperature-dependent criterion for ice layer permeability is incorporated that has been rigorously validated against field measurements from the Devon Ice Cap in the Canadian Arctic, where it demonstrates a strong agreement with point-scale observations of surface mass balance and vertical density profiles. We applied the model to the GrIS from 1999 to 2022, using a horizontal spatial resolution of 0.25&amp;#176; &amp;#215; 0.25&amp;#176;, a vertical resolution of 1 cm, and a temporal resolution of 15 minutes. The model simulations are calibrated using the SUMup archive of surface mass balance observations and validated against shallow core measurements of vertical density profiles. The high vertical resolution of the model provides insights into the process of ice slab evolution and impacts on runoff magnitudes and spatial distribution from the accumulation area of the GrIS. We analyze the model results to examine the relationship between the formation of ice slab and the runoff limit across the GrIS exploring sensitivities to changing climate.

The future contributions of the Antarctic Ice Sheet to sea level rise will depend on the evolution of its surface mass balance (SMB), which could amplify/dampen mass losses increasingly observed at the ice sheet's edge. In situ constraints of SMB over annual-to-decadal timescales consist mostly of firn/ice cores that have a surface footprint $\sim$cm$^{2}$. SMB constraints also come from climate models, which have a higher temporal resolution but a larger surface footprint of several km$^{2}$. We use ice-penetrating radar data to obtain an intermediate spatial and temporal resolution SMB record over three ice rises along the Princess Ragnhild Coast. The co-located ice cores allow us to obtain absolute radar-derived SMB rates at a multi-annual-to-decadal temporal resolution. By comparing the ice core SMB measurements and the radar-derived SMB records, we determine that pointwise measurements of SMB are representative of a small surface area, $\sim 200-500$ m radius extending from the ice core drill site for the ice rises studied here, and that the pointwise measurements are systematically 7–15 cm w.e. a$^{-1}$ lower than the mean SMB value calculated for the whole ice rises. However, ice core records are representative of an entire ice rise's temporal variability at the temporal resolution examined.

A method has been developed for determining the N2O concentrations of air bubbles trapped in ice cores. The air is removed by cutting ice samples of about 45 cm3 with a rotating knife, under pure nitrogen. About 2 cm3 of the gas extracted from the ice is analyzed. The N2O concentrations are measured by gas chromatography, using electron capture detection with a detection limit of approximately 1 ppbv. The accuracy of the analysis is lower than 6%. This method has been used to analyze 34 Antarctic ice samples. Twelve air samples are from the D57 core and date approximately from AD 1600 and 1900. Data indicate a concentration of about 270 ppbv approximately 400 years ago, and of about 293 ppbv for the beginning of the 20th Century. The other samples have been taken from the Dome C core and date back to the time period extending from the Holocene to the Last Glacial Maximum. The results obtained for the Holocene period are in very good agreement with the concentrations measured for the pre-industrial time from the D57 core and indicate that, during the Holocene period, atmospheric N2O mixing ratios may have remained fairly constant. The value observed during the last climatic transition suggest a slight increase in the N2O concentrations when the climate was warming up. The results obtained on samples formed during the Last Glacial Maximum show high scattering which is best explained by the bad quality of this part of the core.

The Million Year Ice Core (MYIC) Project is an Australian Antarctic Program initiative to recover a continuous ice core spanning the mid-Pleistocene Transition (MPT; 700–1,250kyr). MYIC pilot drilling and borehole reaming for casing installation started in the 2024/25 austral summer at Dome C North (DCN, 75.04220S, 123.63120E, ice depth of 3064 m). DCN is 9 km NE of Concordia Station and 45 km NE of the European Beyond EPICA Oldest Ice site at Little Dome C (LDC). In the 2025/26 season, casing was installed and deep drilling commenced using a new AAD deep drill system. Completion of drilling to bedrock is scheduled for the 2028/29 season.One-dimensional ice modelling, constrained by ice penetrating radar and isochrones traced back to the original EPICA Dome C ice core site, indicate an age above the basal ice at DCN potentially reaching 2 million years (Ma) and a resolution at 1.5 Ma of 10,000 years per metre or better (Chung et al., 2023).Laboratory capabilities for MYIC are directed at measurements required to test hypotheses on the cause of the MPT. Ice core continuous flow analysis (CFA) for conductivity, particles and soluble ions are underway, with fraction-collected aliquots taken for measurement of cosmogenic 10Be. Gas and water isotope measurements on the returned ice are scheduled to start this year. The new gas laboratory developed for the project combines a small-sample sublimation extraction system coupled to a Quantum Cascade Laser spectrometer and dual inlet mass spectrometry for combined measurement of CO2, δ13C-CO2, CH4, and N2O, as well as the main air isotopes. There are opportunities for measurements of other parameters through national and international collaboration.

Nitrous oxide (N2O) is the third most important greenhouse gas in the atmosphere, and the ocean is an important source of N2O. As the Arctic Ocean is strongly affected by global warming, rapid ice melting can have a significant impact on the N2O pattern in the Arctic environment. To better understand this impact, N2O concentration in ice core and underlying seawater (USW) was measured during the seventh Chinese National Arctic Research Expedition (CHINARE2016). The results showed that the average N2O concentration in first-year ice (FYI) was 4.5 ± 1.0 nmol kg−1, and that in multi-year ice (MYI) was 4.8 ± 1.9 nmol kg−1. Under the influence of exchange among atmosphere-sea ice-seawater systems, brine dynamics and possible N2O generation processes at the bottom of sea ice, the FYI showed higher N2O concentrations at the bottom and surface, while lower N2O concentrations were seen inside sea ice. Due to the melting of sea ice and biogeochemical processes, USW presented as the sink of N2O, and the saturation varied from 47.2% to 102.2%. However, the observed N2O concentrations in USW were higher than that of T-N2OUSW due to the sea–air exchange, diffusion process, possible N2O generation mechanism, and the influence of precipitation, and a more detailed mechanism is needed to understand this process in the Arctic Ocean.