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Ice CORe DAting tools revisited to infer the dynamic of glacial – interglacial transitions over the last 1.5 million years

Periodic Reporting for period 2 - ICORDA (Ice CORe DAting tools revisited to infer the dynamic of glacial – interglacial transitions over the last 1.5 million years)

Reporting period: 2021-06-01 to 2022-11-30

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 an ice core to be drilled in the coming years and covering the last 1500 ka.
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 aims at rethinking the way ice core chronology is built for decreasing drastically the associated uncertainties. This is currently done by (1) developing a mechanistic approach for the interpretation of isotopic tracers used for ice core dating and (2) combining numerous low to mid latitude ice core tracers to provide a global picture of climate change during terminations. The strategy involves interdisciplinarity between climate, geochemistry, ecophysiology and innovative instrumental developments as well as field, laboratory experiments and modeling.
Our first objective is to learn about the quantitative interpretation of oxygen isotopes used to date the ice cores and make the link with climate in polar regions (ice core) and climate at lower latitudes. we developed a new interdisciplinary collaboration between physician, geochemist and ecologists to measure continuously the oxygen concentration and isotopic composition in closed biological chambers where plants are grown. This required the development of a new optical spectrometer able to measure continuously and with a high precision the isotopic composition of oxygen. In parallel, we developed closed and controled biological chambers with a fully automated control to perform systematic triplicate measurements for each plant in order to determine the fractionation coefficients associated with respiration and photosynthesis. An important finding is the evidence of oxygen fractionation during photosynthesis of terrestrial plants which modifies the current interpretation of the isotopic composition of dioxygen in ice cores.
In addition to the development of biological experiments, we aim at understanding the modification of the elemental and isotopic composition of gases used to date ice cores during the formation of air bubbles in the ice. This is done by new measurements on the field. In particular, one new year-long firn air pumping is being successfully performed at the Concordia station in Antarctica and 2 new firn air pumping experiments will be performed in austral summer 2022-2023. A huge preparation of instruments and tests for this coming field mission have been performed over the previous year and the containers are now on their way for the coming summer season.
A strong challenge for this project is to be able to date ice with smaller quantity of ice since the Beyond EPICA ice core will be very thinned at high depth. A new experimental line has thus been developed to provide absolute dating constraints from measurements of the isotopic composition of argon (100 g of ice instead of 500g). This method has already been applied on two polar ice cores (TALDICE and EPICA Dome C) to demonstrate its feasibility.
The last workpackage of this project aims at providing climatic sequences over the glacial - interglacial transitions of the Quaternary through a huge data acquisition of numerous ice core proxies. Numerous samples from the EPICA Dome C ice cores have been measured covering the last 800 000 years and two main scientific results have been obtained and published as explained below.
First, we presented the 800 ka deuterium excess record from the East Antarctica EPICA Dome C ice core, tracking sea surface temperature in evaporative regions of the Indian sector of the Southern Ocean from which
moisture precipitated in East Antarctica is derived. We found that low obliquity leads to surface warming in evaporative moisture source regions during each glacial inception, although this relative temperature increase is counterbalanced by global cooling during glacial maxima. Links between the two regions during interglacials depends on the existence of a temperature maximum at the interglacial onset. In its absence, temperature maxima in the evaporative moisture source regions and in East Antarctica were synchronous. For the other interglacials, temperature maxima in the source areas lag early local temperature maxima by
several thousand years, probably because of a change in the position of the evaporative source areas.
Second, using the ancient air enclosed in polar ice cores, we present the first 800,000-year record of triple isotopic ratios of atmospheric oxygen, which reflects past global biosphere productivity. We observe that global biosphere productivity in the past eight glacial intervals was lower than that in the preindustrial era and that, in most cases, it starts to increase millennia before deglaciations. Both variations occur concomitantly with CO2 changes, implying a dominant control of CO2 on global biosphere productivity that supports a pervasive negative feedback under the glacial climate.
First, the controlled biological experiments are now producing many results which will refine our understanding and interpretation of isotopic and elemental composition of oxygen in ice core. We are starting the work with the aquatic biosphere.

Second, the firn air pumping experiments to be performed next austral summer in Antarctica will provide a wealth of new data to better constrain the modification of isotopic and elemental composition of air when it is trapped in ice.

Third, many data have been aquired covering the last 800 000 years on the EPICA Dome C ice core to improve the sequences of events over the deglaciation. Ongoing work is performed on:
(1) the determination of the precise temporal link between CO2 and temperature increases using new measurements of d15N of N2.
(2) the improved dating of the EPICA Dome C ice core using new O2/N2 measurements, total air content and d18O of O2 measurements.
(3) the determination of the mid vs high latitude sequences of climatic events during the last climatic cycles using d-excess, 17O-excess and d18O of O2 .

Finally, the delivery of a new chronology of the EPICA Dome C ice core and the measurements in continuous flow of d18O of O2 in the ice core using the optical spectrometer developed in the first half of the project will be developed in the second half of the project.
Closed and controled biological chamber
Link between temperature of the evaporative source and Antarctic temperature over the last 800 000 y
Evolution of the gloal biosphere productivity over the last 800 000 years