Periodic Reporting for period 4 - ICORDA (Ice CORe DAting tools revisited to infer the dynamic of glacial – interglacial transitions over the last 1.5 million years)
Période du rapport: 2024-06-01 au 2025-05-31
The Quaternary period (last 2600 thousands of years, hereafter ka) is the ideal period to evaluate our understanding of climate processes with general circulation models (GCM) used for prediction of future climate as presented in the IPCC reports. During the Quaternary period, the largest climate changes are glacial – interglacial transitions, hereafter terminations, the last termination being a classical benchmark for GCM. The rhythm of terminations changed from a world associated with a 40 ka periodicity to a world associated with a 100 ka glacial – interglacial periodicity between 1250 and 700 ka. The cause for this transition is a long debated question highlighting that the causes and mechanisms of terminations are still poorly understood. The timing and amplitudes of terminations indeed result from multiple influences of insolation forcing, ice sheet size, atmospheric greenhouse gases (GHG) concentration as well as shorter (millennial) scale climate variability. The big challenge of ICORDA consists in solving major puzzles on the mechanisms of terminations by deciphering these different influences using two key Antarctic ice core records: EPICA Dome C covering the last 800 ka and the Beyond EPICA ice core with the deepest part reaching at least 1,2 Million years drilled up to January 2025.
While ice cores provide unique continuous and high resolution climatic and GHG records, they are still too poorly dated on long timescales to address the aforementioned challenge. ICORDA aimed at rethinking the way ice core chronology is built for decreasing drastically the associated uncertainties. This has been done by (1) developing a mechanistic approach for the interpretation of the elemental and isotopic composition of gases trapped in ice cores and (2) combining numerous low to mid latitude ice core tracers to provide a global picture of climate change during terminations. The strategy involved interdisciplinarity between climate, geochemistry, ecophysiology and innovative instrumental developments as well as field, laboratory experiments and modeling.
The techniques developed within ICORDA were key for the analyses and dating of the new Beyond EPICA ice core.
While ice cores provide unique continuous and high resolution climatic and GHG records, they are still too poorly dated on long timescales to address the aforementioned challenge. ICORDA aimed at rethinking the way ice core chronology is built for decreasing drastically the associated uncertainties. This has been done by (1) developing a mechanistic approach for the interpretation of the elemental and isotopic composition of gases trapped in ice cores and (2) combining numerous low to mid latitude ice core tracers to provide a global picture of climate change during terminations. The strategy involved interdisciplinarity between climate, geochemistry, ecophysiology and innovative instrumental developments as well as field, laboratory experiments and modeling.
The techniques developed within ICORDA were key for the analyses and dating of the new Beyond EPICA ice core.
The ICORDA project aimed to develop new and more accurate dating tools for deep ice cores, with the goal of improving the temporal reconstruction of climatic events during the last 800,000 years. By combining these tools with high- and low-latitude climate proxies, the project sought to establish clearer sequences of environmental change during past climate transitions.
A major focus was the development and calibration of the δ¹⁸O of O2 and δO2/N2 orbital dating proxies, measured in air bubbles trapped in ice. This involved designing a new optical spectrometry instrument capable of continuously measuring elemental and isotopic O2 composition with high precision (Piel et al., 2024). To study biological fractionation of atmospheric O2, new chambers quantified O2 fractionation during photosynthesis and respiration in various plant species (Paul et al., 2023). Continuous measurements enabled updated fractionation coefficients (Paul et al., 2025), incorporated into a box model to better interpret δ¹⁸O of O2 variations as dating tools and tracers of low-latitude hydrology. Additional modelling included incorporation of O2 isotopes into the ocean component of the iLOVECLIM model (PhD of E. Clermont), and new firn and snow models linking air transport, densification, and physical properties, guided by Antarctic firn-air pumping campaigns (PhD of R. Harris-Stuart; Harris-Stuart et al., 2024).
Parallel work focused on robust noble-gas dating, targeting argon and krypton isotopes. New extraction and purification lines accommodated various ice sample sizes for ⁴⁰Ar and ⁸¹Kr analysis. Krypton measurements were carried out at Hefei University, argon at LSCE, and both evaluated during I. Crotti’s PhD using TALDICE samples. The project also addressed limitations of the ⁴⁰Ar method by analysing deep ice from Antarctic and Greenland sites, with key results published for TALDICE and EPICA Dome C (Crotti et al., 2021; Bouchet et al., 2023).
Multi-proxy analyses of glacial terminations, based on EPICA Dome C records and Beyond EPICA material, formed the third major component. Despite COVID-19 delays, initial results on the penultimate deglaciation were delivered in early 2025. ICORDA researchers reconstructed temporal sequences linking source-region evaporation, Antarctic temperature, atmospheric CO2, and global photosynthesis over the last nine terminations (Landais et al., 2021; Grisart et al., 2022; Yang et al., 2022). A key output was the AICC2023 chronology (Bouchet et al., 2023), synthesising δO2/N2, Total Air Content, δ¹⁸O of O2, and δ¹⁵N of N2. Additional work refined the CO2–temperature timing and constrained sea-level and CO2 phasing uncertainties over the past 640,000 years (Auriol et al., in press).
To support Beyond EPICA analyses, the project developed a low-pressure optical spectrometry prototype for continuous δ¹⁸O of O2 measurements on small air volumes, building on the 2024 instrument. Full campaigns for Beyond EPICA are planned for February 2026.
A major focus was the development and calibration of the δ¹⁸O of O2 and δO2/N2 orbital dating proxies, measured in air bubbles trapped in ice. This involved designing a new optical spectrometry instrument capable of continuously measuring elemental and isotopic O2 composition with high precision (Piel et al., 2024). To study biological fractionation of atmospheric O2, new chambers quantified O2 fractionation during photosynthesis and respiration in various plant species (Paul et al., 2023). Continuous measurements enabled updated fractionation coefficients (Paul et al., 2025), incorporated into a box model to better interpret δ¹⁸O of O2 variations as dating tools and tracers of low-latitude hydrology. Additional modelling included incorporation of O2 isotopes into the ocean component of the iLOVECLIM model (PhD of E. Clermont), and new firn and snow models linking air transport, densification, and physical properties, guided by Antarctic firn-air pumping campaigns (PhD of R. Harris-Stuart; Harris-Stuart et al., 2024).
Parallel work focused on robust noble-gas dating, targeting argon and krypton isotopes. New extraction and purification lines accommodated various ice sample sizes for ⁴⁰Ar and ⁸¹Kr analysis. Krypton measurements were carried out at Hefei University, argon at LSCE, and both evaluated during I. Crotti’s PhD using TALDICE samples. The project also addressed limitations of the ⁴⁰Ar method by analysing deep ice from Antarctic and Greenland sites, with key results published for TALDICE and EPICA Dome C (Crotti et al., 2021; Bouchet et al., 2023).
Multi-proxy analyses of glacial terminations, based on EPICA Dome C records and Beyond EPICA material, formed the third major component. Despite COVID-19 delays, initial results on the penultimate deglaciation were delivered in early 2025. ICORDA researchers reconstructed temporal sequences linking source-region evaporation, Antarctic temperature, atmospheric CO2, and global photosynthesis over the last nine terminations (Landais et al., 2021; Grisart et al., 2022; Yang et al., 2022). A key output was the AICC2023 chronology (Bouchet et al., 2023), synthesising δO2/N2, Total Air Content, δ¹⁸O of O2, and δ¹⁵N of N2. Additional work refined the CO2–temperature timing and constrained sea-level and CO2 phasing uncertainties over the past 640,000 years (Auriol et al., in press).
To support Beyond EPICA analyses, the project developed a low-pressure optical spectrometry prototype for continuous δ¹⁸O of O2 measurements on small air volumes, building on the 2024 instrument. Full campaigns for Beyond EPICA are planned for February 2026.
ICORDA delivered major methodological advances in ice-core dating, isotope geochemistry, and environmental observation. Central to these innovations is the development of new laser spectrometers capable of continuous, high-precision measurements of O2 concentration and isotopic composition. Two versions were produced: one for controlled biological experiments and one adapted for continuous-flow analysis of the small air volumes extracted from ice cores. These instruments represent a significant improvement over traditional discrete sampling approaches.
The project also created fully instrumented, multiplexed biological chambers that allow precise quantification of O2 fractionation during photosynthesis and respiration. Developed through interdisciplinary collaboration, these chambers provide a unique platform for determining biological fractionation factors essential for interpreting O2-based climate proxies. In parallel, new analytical lines for noble-gas extraction and purification expanded the possibilities for ⁴⁰Ar and ⁸¹Kr dating of deep ice.
These tools, combined with firn-sampling and modeling efforts, substantially enhance the analytical framework available for reconstructing past climates and biogeochemical cycles. ICORDA also produced a new integrated chronology for the past 800,000 years, unifying key climate variables such as CO2, sea level, biosphere productivity, and polar temperatures into a coherent framework that will serve as a long-term reference for the paleoclimate community.
Beyond these technical achievements, ICORDA generated two breakthroughs that push the field clearly beyond the previous state of the art. First, by coupling new O2 measurements, biological experiments, and O2–CO2 modeling, the project revealed inconsistencies in current representations of the carbon cycle, showing that O2 and CO2 must be jointly constrained to capture biological feedbacks accurately. This unexpected result has major implications for future climate modeling and projections.
The project also created fully instrumented, multiplexed biological chambers that allow precise quantification of O2 fractionation during photosynthesis and respiration. Developed through interdisciplinary collaboration, these chambers provide a unique platform for determining biological fractionation factors essential for interpreting O2-based climate proxies. In parallel, new analytical lines for noble-gas extraction and purification expanded the possibilities for ⁴⁰Ar and ⁸¹Kr dating of deep ice.
These tools, combined with firn-sampling and modeling efforts, substantially enhance the analytical framework available for reconstructing past climates and biogeochemical cycles. ICORDA also produced a new integrated chronology for the past 800,000 years, unifying key climate variables such as CO2, sea level, biosphere productivity, and polar temperatures into a coherent framework that will serve as a long-term reference for the paleoclimate community.
Beyond these technical achievements, ICORDA generated two breakthroughs that push the field clearly beyond the previous state of the art. First, by coupling new O2 measurements, biological experiments, and O2–CO2 modeling, the project revealed inconsistencies in current representations of the carbon cycle, showing that O2 and CO2 must be jointly constrained to capture biological feedbacks accurately. This unexpected result has major implications for future climate modeling and projections.