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The EPICA (European Project for Ice Coring in Antarctica) Dome C (EDC) ice core drilling in East Antarctica reaches a depth of 3260 m. The reference EDC chronology (AICC2012) provides an age vs depth relationship covering the last 800 kyr (thousands of years) with an absolute uncertainty rising up to 8,000 years at the bottom of the ice core. The origins of this relatively large uncertainty are threefold: (1) the δ18Oatm, δO2/N2 and total air content (TAC) records are poorly resolved and discontinuous over the last 800 kyr, (2) the three orbital tools are not used simultaneously and (3) large uncertainties are associated with their orbital targets. Here, we present new highly resolved δ18Oatm, δO2/N2 and δ15N measurements for EDC ice core covering the last five glacial – interglacial transitions as well as novel absolute 81Kr ages. We have compiled chronological and glaciological information including novel orbital age markers from new data on EDC ice core as well as accurate firn modeling estimates in a Bayesian dating tool to construct the new AICC2023 chronology. The average uncertainty of the ice chronology is reduced from 2,500 years to 1,800 years in AICC2023 over the last 800 kyr. The new timescale diverges from AICC2012 and suggests age shifts reaching 3,800 years towards older ages over Marine Isotopes Stages (MIS) 5, 11 and 19. But, the coherency between the new AICC2023 timescale and independent chronologies of other archives (Italian Lacustrine succession from Sulmona Basin, Dome Fuji ice core and northern Alpine speleothems) is improved by 1,000 to 2,000 years over these time intervals.

Exchange of carbon between the ocean and the atmosphere is a key process that influences past climates via glacial–interglacial variations of the CO2 concentration. The melting of ice sheets during deglaciations induces a sea level rise which leads to the flooding of coastal land areas, resulting in the transfer of terrestrial organic matter to the ocean. However, the consequences of such fluxes on the ocean biogeochemical cycle and on the uptake and release of CO2 are poorly constrained. Moreover, this potentially important exchange of carbon at the land–sea interface is not represented in most Earth system models. We present here the implementation of terrestrial organic matter fluxes into the ocean at the transiently changing land–sea interface in the Max Planck Institute for Meteorology Earth System Model (MPI-ESM) and investigate their effect on the biogeochemistry during the last deglaciation. Our results show that during the deglaciation, most of the terrestrial organic matter inputs to the ocean occurs during Meltwater Pulse 1a (between 15–14 ka) which leads to the transfer of 21.2 Gt C of terrestrial carbon (mostly originating from wood and humus) to the ocean. Although this additional organic matter input is relatively small in comparison to the global ocean inventory (0.06 %) and thus does not have an impact on the global CO2 flux, the terrestrial organic matter fluxes initiate oceanic outgassing in regional hotspots like in Indonesia for a few hundred years. Finally, sensitivity experiments highlight that terrestrial organic matter fluxes are the drivers of oceanic outgassing in flooded coastal regions during Meltwater Pulse 1a. Furthermore, the magnitude of outgassing is rather insensitive to higher carbon-to-nutrient ratios of the terrestrial organic matter. Our results provide a first estimate of the importance of terrestrial organic matter fluxes in a transient deglaciation simulation. Moreover, our model development is an important step towards a fully coupled carbon cycle in an Earth system model applicable to simulations at glacial–interglacial cycles.

To understand the causal relationship between forcing (orbital parameters, greenhouse gas concentration…) and the climate change, dating climate archives is crucial. Ice cores are unique archives because they provide a direct record of greenhouse gas concentration. However, dating ice cores is particular since they require two chronologies: one for the ice and one for the younger air trapped in bubbles inside the core. The coherent AICC2012 chronology was established for five ice cores: EPICA Dome C (EDC), EPICA Dronning Maud Land (EDML), North Greenland Ice core Project (NGRIP), Vostok (VK) and TALos Dome Ice CorE (TALDICE). A sedimentation model was used to reconstruct past variations of three parameters: accumulation of snow at surface, ice layer thinning in depth and Lock-In-Depth (LID), the depth where air is trapped. Ice and gas ages along the core are estimated from these parameters. Then, a Bayesian tool optimised the age scale by constraining the chronology to respect chronological observations (orbital tuning, stratigraphic links between cores, tephra layers…) and by fitting the three parameters to background scenarios (accumulation deduced from ice isotopes, LID from δ15N, …). The AICC2012 chronology is associated with an uncertainty which arises up to 6 kyrs due to the discontinuity of the ice core composition records and to the poor knowledge when it comes to choose an optimised target for orbital tuning.Since AICC2012, many new data have been obtained to improve the ice core chronology and it is the right period to produce an updated coherent chronology which could also be extended to other ice cores. Here, we present a first step toward the construction of the next coherent ice core chronology by including new dating constraints from recent data on the EDC ice core: 1) air isotopes (δ18Oatm , δO2 /N2) and air content used as orbital dating constraints, 2) the δ15N signal used to estimate the background scenario for LID. In addition, we make use of the East Asian stalagmite δ18Ocalcite signal as an alternative synchronisation target for the δ18Oatm (Extier et al. 2018).This new dating experiment on EDC ice core aims to lower uncertainty of the chronology while providing a critical look on former hypotheses considered to establish AICC2012. For example, δ15N record was discontinuous at the time and it has been reconstructed based on its correlation with δD. Now that we have a continuous δ15N signal, we can evaluate the relevance of this reconstruction. Following this work, we will use new tie point constraints resulting from volcanic synchronisation which has recently been undertaken between Greenland and Antarctica (Svensson et al. 2020) and the ice cores Dome Fuji and WAIS Divide will be further studied to be included in the chronology.

Atmospheric abundance of oxygen (O2) has been co-evolved with different aspects of the Earth system since appearance of oxygenic photosynthesis by cyanobacteria around 2.4 109 years before present (Ga). Therefore, much attention has been paid to understand the changes in O2 and the underlying mechanisms over the Earth’s history. The pioneering work by Stolper et al. (2016) revealed the long-term decreasing trend of O2 mixing ratios over the last 800,000 years using the ice-core composite record of molar ratios of O2 and nitrogen (δ(O2/N2)), implying a slight imbalance between sources and sinks. Over geological time scale, O2 is mainly controlled by burial and oxidation of organic carbon and pyrite, but also by oxidation of volcanic gases and sedimentary rocks. Nevertheless, the O2 cycle of the late Pleistocene has not been well understood, partly due to the lack of knowledge about the individual sources and sinks. Since then, Kölling et al. (2019) proposed a simple model to estimate the O2 release/uptake fluxes due to the pyrite burial/oxidation that predicts up to ~70% of the O2 decrease of the last 800,000 years could be explained by pyrite burial/oxidation.Building on this, we present here our preliminary, tentative attempt for reconstruction of the net organic carbon burial flux over the last 800,000 years by combining available information (including new δ(O2/N2) data) and assuming constant O2 fluxes associated with volcanic outgassing and rock weathering. The long-term organic carbon burial flux trend obtained with our new calculations is similar to the global ocean δ13C records but also to simulations using a conceptual carbon cycle model (Paillard, 2017). These results partly support the geomorphological hypothesis that the major sea-level drops during the earlier period of the last 800,000 years lead to enhanced organic carbon burial, and that significant changes in the net organic carbon happen around Marine Isotopic Stage (MIS) 13. In addition, we present the long-term decreasing trend of the global biosphere productivity, or gross photosynthetic O2 flux, reconstructed from new measurements of triple-isotope composition of atmospheric O2 trapped in ice cores. As the largest O2 flux, the observed decrease in gross photosynthesis requires to be compensated by parallel reduction of global ecosystem respiration.

The Last Deglaciation (21-10 ka) is the most recent transition from a glacial to interglacial state. It is characterized by a pronounced sea level change of 95 m resulting in flooding of land areas and changes of coastlines. This period is also marked by several millennial events like the Heinrich Event 1 with diverse effects on sea level, oceanic circulation, climate and carbon cycle. In case of flooding of land surfaces during periods of sea level rise, carbon and nutrients stored in terrestrial organic matter in vegetation and soils are transferred to the ocean, potentially impacting the global ocean biogeochemical cycle and the uptake/release of CO2 once being remineralized. Changes in the ocean biogeochemical cycles are also indirectly related to the poorly constrained stoichiometry and remineralization time-scales of terrestrial organic matter, which both differ from the well-known parameters for marine organic matter.We present here the first coupled transient simulation over the Last Deglaciation using the global ocean biogeochemical model HAMOCC (HAMburg Ocean Carbon Cycle) as part of the paleo-version of the MPI-ESM (Max Planck Institute Earth System Model) to study the impact of terrestrial organic matter input on the ocean biogeochemical cycle and oceanic CO2 fluxes during large sea level variations. This model version combines (1) a fully interactive adaptation of the ocean bathymetry with corresponding changes of the land-sea distribution, (2) a transient river routing and (3) the land-sea terrestrial organic matter transfer after flooding. Our simulation provides new insights on the land carbon inputs to the ocean carbon inventory (water column and sediment) due to flooding, with 170 GtC between 21-10 ka, of which 21.1 GtC and 36.8 GtC are within two 1000 years large freshwater discharge events (between 15-14 ka and 12-11 ka). These inputs of carbon rich material to the ocean during flooding events have however only a local effect on ocean CO2 outgassing, the global ocean remaining a sink of CO2. To infer the response of CO2 fluxes in this context, sensitivity experiments can be performed during the type of Heinrich event (15-14 ka) to evaluate and better constrain the terrestrial organic matter remineralization parameters.

The Mid-Pleistocene transition (MPT) took place between 1,200 Ma and 800 ka (still debated). During this transition, the Earth’s orbitally paced ice age cycles intensified, lengthened from ∼40 000 (∼40 ky) to ∼100 ky, and became distinctly asymmetrical while Earth’s orbital variations remained unchanged. Although orbital variations constitute the first order forcing on glacial-interglacial oscillations of the late Quaternary, they cannot explain alone the shifts in climatic periodicity and amplitude observed during the MPT. In order to explain the MPT, long-term evolution of internal mechanisms and feedbacks have been called upon, in relation with the global cooling trend initiated during the Cenozoic, the expansion of Antarctic and Greenland Ice Sheet and/or the long-term decline in greenhouse gases (particularly CO2). A key point is therefore to accurately reconstruction of oceanic temperatures to decipher the processes driving climate variations.In the present work, we studied the marine sediment core MD96-2048 taken from south Indian Ocean (26*10’482’’ S, 34*01’148’’ E) in the region of the Agulhas current. We compared 5 paleothermometers: alkenone, TEX86, foraminiferal- transfer function, Mg/Ca and clumped isotope. Among these approaches, carbonate clumped-isotope thermometry (∆47) only depends on crystallization temperature, and the ∆47 relationship with planktonic foraminifer calcification temperature is well defined. Since Mg/Ca is not only controlled by temperature but is also affected by salinity and pH. The classical d18O in planktic is dependent on SST and d18Osw, which is regionally correlated with the salinity in the present-day ocean. Assuming that the present-day d18Osw-salinity relation was the same during the MPT, we are able to separate changes in d18Osw from temperature effects and reconstruct past salinity. Combining d18O, Mg/Ca and ∆47 on planktonic foraminifera allow in theory to reconstruct SST, SSS and pH.Here, we measured d18O, Mg/Ca and ∆47 on the shallow-dwelling planktonic species Globigerinioides ruber ss. at the maximal of glacial and interglacial periods over the last 1.2 Ma. Our set of data makes it possible to estimate the long-term evolution of SST, salinity and pH (and thus have an insight into the atmospheric CO2 concentration) across the MPT. Frist, strong differences are observed between the 5 derived-SST: the alkenone and TEX86 recorded the higher temperatures than the other SST proxies. Alkenone derived-SST do not show glacial-interglacial variations within the MPT. The Mg/Ca and transfer function derived-SST show a good agreement each other, while the clumped-isotope derived-SST are systematically colder than the other derived-SST. Then, our ∆47-SST, salinity and pH results clearly show that amplitude of glacial-interglacial variations was insignificant between 1.2 and 0.8 Ma (within the MPT) and increased after the MPT. Finally, we also discussed the potential to use this unique combination of proxies to reconstruct changes of atmospheric CO2 concentration.

The primary production, or oxygenic photosynthesis of the global biosphere, is one of the main source and sink of atmospheric oxygen (O2) and carbon dioxide (CO2), respectively. There has been a growing number of evidence that global gross primary productivity (GPP) varies in response to climate change. It is therefore important to understand the climate- and/or environment controls of the global biosphere primary productivity for better predicting the future evolution of biosphere carbon uptake. The triple-isotope composition of O2 (Δ17O of O2) trapped in polar ice cores allows us to trace the past changes of global biosphere primary productivity as far back as 800,000 years before present (800 ka). Previously available Δ17O of O2 records over the last ca. 450 ka show relatively low and high global biosphere productivity over the last five glacial and interglacial intervals respectively, with a unique pattern over Termination V (TV) - Marine Isotopic Stage (MIS) 11, as biosphere productivity at the end of TV is ~ 20 % higher than the four younger ones (Blunier et al., 2012; Brandon et al., 2020). However, questions remain on (1) whether the concomitant changes of global biosphere productivity and CO2 were the pervasive feature of glacial periods over the last 800 ka, and (2) whether the global biosphere productivity during the “lukewarm” interglacials before the Mid-Brunhes Event (MBE) were lower than those after the MBE.Here, we present an extended composite record of Δ17O of O2 covering the last 800 ka, based on new Δ17O of O2 results from the EPICA Dome C and reconstruct the evolution of global biosphere productivity over that time interval using the independent box models of Landais et al. (2007) and Blunier et al. (2012). We find that the glacial productivity minima occurred nearly synchronously with the glacial CO2 minima at mid-glacial stage; interestingly millennia before the sea level reaches their minima. Following the mid-glacial minima, we also show slight productivity increases at the full-glacial stages, before deglacial productivity rises. Comparison of reconstructed interglacial productivity demonstrates a slightly higher productivity over the post-MBE (MISs 1, 5, 7, 9, and 11) than pre-MBE ones (MISs 13, 15, 17, and 19). However, the mean difference between post- and pre-MBE interglacials largely depends on the box model used for productivity reconstruction.

Variations in ocean-atmosphere carbon exchange, in response to varying physical and biogeochemical ocean states, is one of the major causes of the glacial-interglacial atmospheric CO2 changes. Most of the existing modelling studies use time-slice simulations with Earth System Models to quantify the proposed mechanisms, such as the impact of a weakened Southern Ocean westerlies and a massive discharge of freshwater from ice sheet melting on the deglacial atmospheric CO2 rise. We present the variations of ocean biogeochemistry in a transient deglaciation (21 – 10 kB.P.) simulation using the Max Planck Institute Earth System Model. We force the model with reconstructions of atmospheric greenhouse gas concentrations, orbital parameters, ice sheet and dust deposition. In line with the physical ocean component, we account for the automatic adjustment of all marine biogeochemical tracers in response to changing bathymetry and coastlines that relate to deglacial melt water discharge and isostatic adjustment. We include a new representation of the stable carbon isotope (13C) in the ocean biogeochemical component to evaluate the simulation against δ13C records from sediment cores.The model reproduces several proposed oceanic CO2 outgassing mechanisms. First, the net primary production (NPP) in the North Atlantic Ocean dramatically decrease (by 40 – 80%) during the first melt water pulse (15 – 14 kB.P.) which is caused by the weakening in the strength of the Atlantic Meridional Overturning Circulation from 21 to 3 Sv. However, globally the oceanic NPP only slightly decreases by 8% as oceanic NPP in the South Hemisphere increases during the same period. Second, during the melt water pulse in the Southern Ocean the ventilation of intermediate waters, which has high DIC content and low alkalinity concentration, is slightly enhanced. Third, the surface alkalinity decreases due to dilution and due to episodic shifts between CaCO3 production and opal production by phytoplankton. Lastly, CO2 solubility decreases with increasing deglacial sea surface temperature. The increase of surface pCO2 caused by the above mechanisms is, however, smaller than that of the prescribed atmospheric CO2. Thus, the ocean is a weak carbon sink in this deglacial simulation.

Significant changes in atmospheric CO2 over glacial-interglacial cycles have mainly been attributed to the Southern Ocean through physical and biological processes. However, little is known about the contribution of global biosphere productivity, associated with important CO2 fluxes. Here we present the first high resolution record of Δ17O of O2 in the Antarctic EPICA Dome C ice core over Termination V and Marine Isotopic Stage (MIS) 11 and reconstruct the global oxygen biosphere productivity over the last 445 ka. Our data show that compared to the younger terminations, biosphere productivity at the end of Termination V is 10 to 30 % higher. Comparisons with local palaeo observations suggest that strong terrestrial productivity in a context of low eccentricity might explain this pattern. We propose that higher biosphere productivity could have maintained low atmospheric CO2 at the beginning of MIS 11, thus highlighting its control on the global climate during Termination V., Biosphere productivity is an important component of the CO2 cycle, but how it has varied over past glacial-interglacial cycles is not well known. Here, the authors present new data that shows that global biosphere productivity was 10 to 30% higher during Termination V compared to younger deglaciations.

During the last 800,000 years, atmospheric CO2 concentrations have varied with an amplitude of more than 100 ppm, with the fastest increases registered during deglaciations. The mechanisms behind the increases of CO2 are still discussed since several parameters are involved. Biological productivity on land and in the ocean played a major role in the variations of atmospheric CO2. Particularly, productivity variations in the Southern Ocean along deglaciations are key because changes in the efficiency of the Soft Tissue Pump (STP) and the Carbonate Counter Pump (CCP) in the Subantarctic Zone significantly impact the exchanges between ocean and atmospheric reservoirs. As calcifying organisms, coccolithophores and planktonic foraminifera represent the major producers of CaCO3 and are therefore good tools to reconstruct past variations of CCP.Among the last 9 deglaciations, Termination V registers the strongest global productivity (20% higher) compared to the other 8 interglacial periods. Associated with the Mid-Brunhes event, it is followed by the warm MIS 11, the longest interglacial (~ 30 ka). MIS 11 also registers a strong carbonate production in the ocean, most probably favoured by the low eccentricity during this period. Studying the variations of the CCP during this specific period of time is therefore important to better understand its relation with biospheric productivity changes and its impact on atmospheric CO2.Here we present micropaleontological (coccoliths and foraminifera) and geochemical (CaCO3) data from marine core MD04-2718, located in the Indian sector of the Southern Ocean (48°53 S; 65°57 E) throughout Termination V and MIS 11, that we compared with other productivity data from the Southern Ocean as well as reconstruction of global biospheric productivity data (Δ17O of O2). Results show that coccolith and foraminifera abundances and masses increase during Termination V and MIS 11. The good correlation between variations of CaCO3 in the sediment and calcite mass from coccoliths and foraminifera shells proves that exported CaCO3 is essentially of planktonic origin and reveals that CCP significantly increases over this period.We suggest that the strengthening of CCP through the increase in production and export of calcite associated to coccolith and foraminifera in the Southern Ocean may have contributed to increase the atmospheric CO2 during Termination V and MIS 11, while the strong biological productivity registered during this period would have permitted to maintain the CO2 level relatively low