Jochen Schmitt

Aerosol-derived impurities in deep Antarctic ice cores provide high-resolution records of past climate and atmospheric variability. However, post-depositional englacial geochemical processes driven by impurity remobilization through ice metamorphism can perturb the originally deposited signals, challenging the interpretation of deep ice records. To address this, we investigate englacial mineral alterations by analyzing the elemental composition of 18 ice-core sections of the EPICA Dome C (EDC) ice core (ranging from 281.6–3137.1 m depth) using single-particle inductively coupled plasma time-of-flight mass spectrometry (sp-ICP-TOFMS) coupled to a continuous flow analysis (CFA) system. This reveals a deep-ice environment dominated by pervasive acid dissolution, leaving behind refractory mineral phases. We document the progressive neoformation of potassium-rich alunite-supergroup minerals (jarosite, alunite, and mixed phases) and the probable formation of Fe-(oxyhydr)oxide coatings. These secondary phases concurrently immobilize trace elements (iodine, arsenic, lead) via surface adsorption and structural substitution. These transformations occur within highly localized microenvironments and are accelerated by increasing in situ temperatures with depth. They are further enabled by the old age of deep ice, which provides hundreds of thousands of years for these reactions to occur. These findings underscore the importance of accounting for the effects of post-depositional geochemical transformation when interpreting impurity records from EDC and other old ice cores. The colder thermal regime of the Beyond EPICA Little Dome C is expected to lead to slower geochemical transformation, potentially providing a higher-fidelity impurity record for the epochs currently covered by EDC.

Atmospheric methane (CH ₄ ) plays a central role in Earth’s climate, yet the drivers of its decline during the high-dust conditions of glacial periods, such as the Last Glacial Maximum (LGM), remain uncertain. Previous explanations imply source-driven changes, assuming an atmospheric lifetime comparable to that of present day. Recent work shows that interactions between mineral dust and sea salt aerosols produce CH ₄ -removing chlorine radicals. In this work, we show that during the LGM, CH ₄ lifetime shortened to 7.8 years, 20% lower than that of present day. Chlorine contributed ~15% of global CH ₄ loss, fourfold that of present day. Our results reproduce ice core CH ₄ isotopic evidence, demonstrating that stronger-than-assumed atmospheric sinks can explain CH ₄ variability without invoking substantial source changes, highlighting the overlooked role of chlorine chemistry in the glacial CH ₄ budget.

Aerosol-derived impurities in deep Antarctic ice cores provide high-resolution records of past climate and atmospheric variability. However, post-depositional englacial geochemical processes driven by impurity remobilization through ice metamorphism can perturb the originally deposited signals, challenging the interpretation of deep ice records. To address this, we investigate englacial mineral alterations by analyzing the elemental composition of 18 ice-core sections of the EPICA Dome C (EDC) ice core (ranging from 281.6–3137.1 m depth) using single-particle inductively coupled plasma time-of-flight mass spectrometry (sp-ICP-TOFMS) coupled to a continuous flow analysis (CFA) system. This reveals a deep-ice environment dominated by pervasive acid dissolution, leaving behind refractory mineral phases. We document the progressive neoformation of potassium-rich alunite-supergroup minerals (jarosite, alunite, and mixed phases) and the probable formation of Fe-(oxyhydr)oxide coatings. These secondary phases concurrently immobilize trace elements (iodine, arsenic, lead) via surface adsorption and structural substitution. These transformations occur within highly localized microenvironments and are accelerated by increasing in situ temperatures with depth. They are further enabled by the old age of deep ice, which provides hundreds of thousands of years for these reactions to occur. These findings underscore the importance of accounting for the effects of post-depositional geochemical transformation when interpreting impurity records from EDC and other old ice cores. The colder thermal regime of the Beyond EPICA Little Dome C is expected to lead to slower geochemical transformation, potentially providing a higher-fidelity impurity record for the epochs currently covered by EDC.

Aerosol-derived impurities in deep Antarctic ice cores provide high-resolution records of past climate and atmospheric variability. However, post-depositional englacial geochemical processes driven by impurity remobilization through ice metamorphism can perturb the originally deposited signals, challenging the interpretation of deep ice records. To address this, we investigate englacial mineral alterations by analyzing the elemental composition of 18 ice-core sections of the EPICA Dome C (EDC) ice core (ranging from 281.6–3137.1 m depth) using single-particle inductively coupled plasma time-of-flight mass spectrometry (sp-ICP-TOFMS) coupled to a continuous flow analysis (CFA) system. This reveals a deep-ice environment dominated by pervasive acid dissolution, leaving behind refractory mineral phases. We document the progressive neoformation of potassium-rich alunite-supergroup minerals (jarosite, alunite, and mixed phases) and the probable formation of Fe-(oxyhydr)oxide coatings. These secondary phases concurrently immobilize trace elements (iodine, arsenic, lead) via surface adsorption and structural substitution. These transformations occur within highly localized microenvironments and are accelerated by increasing in situ temperatures with depth. They are further enabled by the old age of deep ice, which provides hundreds of thousands of years for these reactions to occur. These findings underscore the importance of accounting for the effects of post-depositional geochemical transformation when interpreting impurity records from EDC and other old ice cores. The colder thermal regime of the Beyond EPICA Little Dome C is expected to lead to slower geochemical transformation, potentially providing a higher-fidelity impurity record for the epochs currently covered by EDC.

Aerosol-derived impurities in deep Antarctic ice cores provide high-resolution records of past climate and atmospheric variability. However, post-depositional englacial geochemical processes driven by impurity remobilization through ice metamorphism can perturb the originally deposited signals, challenging the interpretation of deep ice records. To address this, we investigate englacial mineral alterations by analyzing the elemental composition of 18 ice-core sections of the EPICA Dome C (EDC) ice core (ranging from 281.6–3137.1 m depth) using single-particle inductively coupled plasma time-of-flight mass spectrometry (sp-ICP-TOFMS) coupled to a continuous flow analysis (CFA) system. This reveals a deep-ice environment dominated by pervasive acid dissolution, leaving behind refractory mineral phases. We document the progressive neoformation of potassium-rich alunite-supergroup minerals (jarosite, alunite, and mixed phases) and the probable formation of Fe-(oxyhydr)oxide coatings. These secondary phases concurrently immobilize trace elements (iodine, arsenic, lead) via surface adsorption and structural substitution. These transformations occur within highly localized microenvironments and are accelerated by increasing in situ temperatures with depth. They are further enabled by the old age of deep ice, which provides hundreds of thousands of years for these reactions to occur. These findings underscore the importance of accounting for the effects of post-depositional geochemical transformation when interpreting impurity records from EDC and other old ice cores. The colder thermal regime of the Beyond EPICA Little Dome C is expected to lead to slower geochemical transformation, potentially providing a higher-fidelity impurity record for the epochs currently covered by EDC.

Aerosol-derived impurities in deep Antarctic ice cores provide high-resolution records of past climate and atmospheric variability. However, post-depositional englacial geochemical processes driven by impurity remobilization through ice metamorphism can perturb the originally deposited signals, challenging the interpretation of deep ice records. To address this, we investigate englacial mineral alterations by analyzing the elemental composition of 18 ice-core sections of the EPICA Dome C (EDC) ice core (ranging from 281.6–3137.1 m depth) using single-particle inductively coupled plasma time-of-flight mass spectrometry (sp-ICP-TOFMS) coupled to a continuous flow analysis (CFA) system. This reveals a deep-ice environment dominated by pervasive acid dissolution, leaving behind refractory mineral phases. We document the progressive neoformation of potassium-rich alunite-supergroup minerals (jarosite, alunite, and mixed phases) and the probable formation of Fe-(oxyhydr)oxide coatings. These secondary phases concurrently immobilize trace elements (iodine, arsenic, lead) via surface adsorption and structural substitution. These transformations occur within highly localized microenvironments and are accelerated by increasing in situ temperatures with depth. They are further enabled by the old age of deep ice, which provides hundreds of thousands of years for these reactions to occur. These findings underscore the importance of accounting for the effects of post-depositional geochemical transformation when interpreting impurity records from EDC and other old ice cores. The colder thermal regime of the Beyond EPICA Little Dome C is expected to lead to slower geochemical transformation, potentially providing a higher-fidelity impurity record for the epochs currently covered by EDC.

Nitrous oxide (N2O) is a potent greenhouse gas involved in the destruction of stratospheric ozone. Past atmospheric mixing ratios of N2O are archived in ice cores; however, the presence of in situ N2O production in dust-rich Antarctic ice complicates their accurate reconstruction, especially during glacial periods. This production occurs in extremely cold ice and without sunlight. This study aims to understand the reaction producing N2O in Antarctic ice by identifying the precursors and the reaction pathway. We compared the oxygen and nitrogen bulk and position-specific isotope composition of in situ N2O in ice cores to the isotopic composition of nitrate (NO3-), a possible precursor of N2O. The 15N signature of NO3- is fully transferred into the central N atom (Nα) of in situ N2O, but it is not transferred into the terminal N atom (Nβ), resulting in a 50 % transfer of the 15N signature of NO3- into the bulk 15N isotopic composition. These findings suggest that the in situ N2O production involves two different nitrogen precursors present in ice: the central N atom (Nα) originates from NO3- and the terminal N atom (Nβ) from a different precursor not yet identified. Oxygen isotope analysis shows that NO3- cannot be the only reservoir for the O atom of in situ N2O. Temperature, pH, and absence of sunlight in Antarctic ice point to an abiotic N-nitrosation reaction. The limiting factor of the reaction is probably associated with mineral dust and might be Fe2+, reducing NO3- to NO2- or the precursor of the Nβ atom. The site preference (SP) values of in situ N2O are highly variable between different ice cores and depend on the bulk 15N isotopic composition of N2O, itself depending on the 15N isotopic composition of the NO3- precursor. This finding is unexpected because SP is usually determined by the production pathway through symmetric reaction intermediates that mix the N atoms in α and β positions and average out their isotopic difference. In contrast, our results provide the first evidence of a hybrid N2O production pathway involving an asymmetric intermediate that preserves the distinct 15N signatures of two different precursors – one contributing to the Nα atom and the other to the Nβ atom.

The BURNice project aims to quantify global fire emissions during important climate events from the Holocene to the last interglacial termination using ethane in polar ice cores. The project aims to combine analytical measurements and model simulations to explore the fate of ethane (C2H6) in the atmosphere in the past. Ethane is a short-lived non-methane hydrocarbon with a simple atmospheric budget. Ethane is primarily emitted into the atmosphere by fire and is removed via reaction with OH and Cl radicals, resulting in a lifetime of ~2 months.Measurements of ethane in ice cores are conducted using continuous sublimation extraction (CSE) method in tandem with GC-IRMS for parallel quantification of ethane, methane (CH4), methane isotopes (δ13CH4), and other trace gases. We perform simulations using the Community Earth System Model (CESM) with an improved representation of halogen chemistry to investigate the spatio-temporal dynamics of ethane in the paleo-atmosphere. Preliminary results focus on the effect of the Cl-sink on ethane in modern-day, which is poorly constrained.This study presents 1) advancements in measurements of ethane in NH and SH ice cores using CSE-GC-IRMS, and 2) results of sink-varied modern-day simulations of ethane in the atmosphere using CESM.

The climate evolution over the Pleistocene (last 2.6 Myr) is characterised by a sequence of cold glacials that are interrupted by warmer interglacial phases. These glacial-interglacial cycles are expressed in changes in global ice volume, ocean temperature, and other parameters that serve as proxies to infer land and ocean processes or to provide information about radiative forcing changes.The Mid-Pleistocene Transition (MPT; 1.2-0.9 Myr) is characterised by the transition from 41 kyr cycles to about 100 kyr cycles and likely a trend toward colder glacial periods. The Mid Brunhes (MB) marks a climate transition at about 450 kyr after which the interglacial intensity (e.g. temperatures and greenhouse gas concentrations) increased. Consolidated explanations of the causes of both the MPT and the MB are lacking. Internal climate dynamics or feedback mechanisms are required since frequencies and the power of orbital parameters did not systematically change over the last 1.2 Myr.For the MPT, two main classes of explanations have been put forward to account for the advent of the 100-kyr cycles. One argues that the ice sheets grew larger because the glacial climate became colder, driven by a long-term decline in glacial CO2 (GHG forcing). The other argues that NH ice sheets survived the next potential termination because land-surface properties (e.g. regolith, routing of ice sheets) made them less sensitive to meltdown. Here, we present a record of CF4 measured on ice core samples from the EPICA and Beyond EPICA ice cores, as well as some even older samples from the Alan Hills blue ice area that may shed additional light on the potential reasons for the MB and MPT. CF4 is a natural gas with a very long lifetime (on the order of 100 kyr) occurring predominantly in granitic rocks and other acidic plutonites and is released during chemical weathering and erosion of these rocks. Granites are globally distributed but have a bias towards high northern latitudes (Laurentide region, Scandinavia). Accordingly, some connection of CF4 release to ice sheet extent is to be expected. Over the Pleistocene, the surfaces of these high northern areas have been intensively eroded, and we would expect that a removal of this regolith layer would have left a sizable imprint in our CF4 record. However, our CF4 record shows a rather gradual decline from 2.6 Myr BP toward its local minimum reached shortly before the MB. In contrast, our CF4 record is consistent with the view that the CF4 release is most sensitive to chemical weathering and thus to temperature and the hydrologic regime. While we observe moderate changes in the average long-term CF4 emission flux (several glacial-interglacial cycles), the dominant variability is between glacials and interglacials, with the interglacials exhibiting about 40% higher emissions than colder times. The post-MB increase in CF4 is thus most easily understood through increased weathering during the warmer post-MB interglacials.Our work provides new constraints on the regolith hypothesis for the MPT, and the first record of Pleistocene granite weathering/erosion trends.

Ice cores provide the only direct archive of past atmospheric greenhouse gases (GHG). However, physical and chemical processes occurring before, during, and after bubble enclosure can alter the atmospheric signal recorded in ice. In particular, gas diffusion within the bubble-clathrate transition zone (BCTZ) has been shown to generate centimeter-scale, non-atmospheric variability, which we refer to as the “Lüthi effect” (Lüthi et al., 2010). Below the BCTZ, diffusive smoothing dampens these non-atmospheric signals but also atmospheric variability. While the Lüthi effect and diffusive smoothing have been documented for CO2 and the δO2/N2 ratio, their impact on N2O and CH4 remains poorly constrained. In addition, chemical reactions within the ice can alter atmospheric signals, particularly for N2O, which has been shown to be produced in situ by nitrate reduction in dust-rich Antarctic ice during glacial periods.Here we investigate diffusion and in situ production processes potentially affecting CO2, CH4, and N2O by analyzing five samples from the BCTZ of the Talos Dome ice core (TALDICE). GHG concentrations were measured at centimeter-scale resolution using a novel laser sublimation extraction system coupled to a quantum cascade laser absorption spectrometer (Mächler et al., 2023). δO2/N2 ratios were analyzed by isotope ratio mass spectrometry following the recapture of the same samples after GHG measurements with the laser spectrometer.Our results show for the first time that N2O and CH4 are also affected by the Lüthi effect in the BCTZ. The strong 1:1 correlation between CO2 and N2O variability suggests similar diffusion coefficients for these gases. These findings provide new constraints on N2O diffusion, relevant for modeling diffusive smoothing in deep and old ice such as the recently drilled Beyond EPICA ice core. Consistent with previous studies, our results indicate that the N2O record in TALDICE is not significantly affected by the aforementioned in situ production during glacial periods. The atmospheric N2O signal can therefore be retrieved when measurements are either spatially averaged to smooth the centimeter-scale variability induced by the Lüthi effect, obtained above the BCTZ, or taken well below the BCTZ where diffusive smoothing has attenuated this variability.ReferencesLüthi, D., Bereiter, B., Stauffer, B., Winkler, R., Schwander, J., Kindler, P., Leuenberger, M., Kipfstuhl, S., Capron, E., Landais, A., Fischer, H., and Stocker, T. F.: CO2 and O2/N2 variations in and just below the bubble–clathrate transformation zone of Antarctic ice cores, Earth and Planetary Science Letters, 297, 226–233, https://doi.org/10.1016/j.epsl.2010.06.023, 2010.Mächler, L., Baggenstos, D., Krauss, F., Schmitt, J., Bereiter, B., Walther, R., Reinhard, C., Tuzson, B., Emmenegger, L., and Fischer, H.: Laser-induced sublimation extraction for centimeter-resolution multi-species greenhouse gas analysis on ice cores, Atmos. Meas. Tech., 16, 355–372, https://doi.org/10.5194/amt-16-355-2023, 2023.