High resolution study of radiocarbon in Tree-Ring sequences from the Younger Dryas event and early Holocene in the southern French Alps: A window into the past to document and understand rapid variations of the carbon cycle and of the solar activity – CARBOTRYDH

In a few years only, the radiocarbon unit of CEREGE inaugurated in 2015 has achieved similar or better performances than laboratories established since several decades. In parallel, intensive fieldwork allowed to augment the collection of subfossil trees stored in the dendrotheque of IMBE. After reviewing a large body of literature on wood pretreatments, we carried out different techniques in order to optimize a protocol adapted to the main types of wood samples (i.e. modern and subfossil wood from coniferous and broadleaf trees). For this purpose, we performed numerous tests on known-age woods. The acid-base-acid-bleaching pretreatment (ABA-B) is selected as an optimal choice based on its limited duration and complexity, and because of the excellent and reproducible analytical results (as shown by 14C results, 13C/12C ratios, carbon % and overall mass yield %) that it ensures. We then used our selected method to date the wood samples of the Sixth International Radiocarbon Intercomparison (SIRI), in order to confirm the chosen method by comparing the results of our measurements with the consensus values obtained in the framework of the SIRI intercomparison. In addition, we participated to an international intercomparison on single-year tree-ring samples. This intercomparison showed that the CEREGE is part of the group of the eight most accurate and precise radiocarbon laboratories in the world. These laboratories have all been involved in the establishment of the new calibration in the frame of the IntCal working group.

Our 14C results on tree-rings and other archives were used for constructing the new radiocarbon calibration in the frame of the IntCal working group, that will be used by the entire community of 14C users over the next five years. The new data measured and their integration into the IntCal20 database have allowed to revise and update significantly the use of 14C as a geochronometer and as a tracer of the environment. For example, the new record of the interhemispheric 14C gradient shows significant variations, suggesting periods characterized by a release of relatively old CO2, into the Northern Hemisphere atmosphere. The timing corresponds to the start of the rise in atmospheric CO2 that occurred at the beginning of the Younger Dryas climatic event.
The new IntCal20 radiocarbon calibration curve also allowed us to calculate the gradient of the relationship between 14C age and calendar age over the past 55 thousand years (ka). The new gradient curve exhibits a prolonged and prominent maximum between 48 and 40 cal ka BP during which the radiocarbon clock runs almost twice as fast as it should. This radiocarbon time dilation is due to the increase in the atmospheric 14C/12C ratio caused by the 14C production rise linked to the transition into the Laschamps geomagnetic excursion centered around 41 cal ka BP. This discovery will have far-reaching impacts for scientific communities, such as prehistory and paleoclimatology.

The new radiocarbon IntCal20 calibration will be used by the entire community of 14C users over the next five years. The IntCal working group is already preparing the next iteration. Ongoing and future work is and will be performed at CEREGE & IMBE to go beyond the IntCal20 calibration, notably with subfossil trees corresponding to the Bølling and Allerød periods.

The CARBOTRYDH project is acknowledged in fourteen papers published in peer-reviewed journals. The impact of this work is illustrated by the number of citations of the IntCal20 papers and by the publications in high-profile journals such as Science and PNAS.
The CARBOTRYDH project is a fundamental research project coordinated by Edouard Bard at CEREGE (UMR Aix-Marseille University, CNRS, IRD, INRAE, Collège de France) in association with Cécile Miramont from IMBE (UMR Aix-Marseille University, CNRS, IRD, Avignon University). The project started in October 2017 and lasted 45 months. It received funding of € 255,000 from the ANR, for a total cost of around € 800,000.

The CarboTRYDH project (High resolution study of radiocarbon in Tree-Ring sequences from the Younger Dryas event and early Holocene in the southern French Alps: A window into the past to document and understand rapid variations of the carbon cycle and of the solar activity) has two main objectives: the first one is to reconstruct and use a high-resolution atmospheric 14C/12C record throughout prominent climatic events and to identify the respective roles of cosmogenic isotopes production changes and of global carbon-cycle changes. The second objective is a crucial contribution to improving the accuracy of the radiocarbon geochronometer by strengthening its calibration during the last deglaciation period.
Over the recent years, we discovered many subfossil woods in the Southern French Alps (Middle Durance River system). In parallel, we installed a compact accélérateur mass spectrometry (AMS) system (AixMICADAS) and implemented an efficient purification method to 14C date wood samples at very high precision.
For the CarboTRYDH project, we will analyze subfossil wood samples from the existing IMBE collection and future field work with a focus on three time windows: 1/ transition into the Younger Dryas (YD) cold event, 2/ end of the YD cold event, 3/ so-called 8.2K event around 8200 cal. BP.
Specific comparisons will be made with existing Northern Hemisphere and Southern Hemisphere 14C sequences (e.g. Kauri tree from New-Zealand) and with 10Be records measured in polar ice cores in order to distinguish the production component (heliomagnetic modulation, solar energetic particles events) from the variations linked to carbon cycle changes (paralleled in pCO2 and pCH4 records in high accumulation ice cores). With numerical models of the global carbon cycle, we will take into account the specific modes of mixing and deposition of 14C (processes coupling atmosphere, ocean and biosphere).
The other scientific outcome of the project will be to bridge the YD gap and to link the absolute Holocene dendrocalibration with older floating chronologies (Bölling & Alleröd periods). The objective is to contribute to the next iterations of 14C calibration curve prepared in the frame of the IntCal working group and applied by all 14C users.
As a feasibility test for the CarboTRYDH project, we analyzed a two century long tree-ring sequence from two subfossil pines collected in the site of Barbiers. The new high-resolution record (every 3 yr) confirmed the great potential of the project for both its objectives. Our new but preliminary triennial record from Southern France can already be compared to that from New Zealand in order to reassess the 14C interhemispheric gradient and its abrupt variations during a time window belonging to the YD event (Capano et al. 2017).