Centre Européen
de Recherche et d'Enseignement
des Géosciences de l'Environnement

2017 ANR : CARBOTRYDH

CARBOTRYDH : 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.
 
The CarboTRYDH project 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, e.g. Miramont et al. 2011). 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 (Bard et al. 2015, Capano et al. 2017).
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).

Bard E, Tuna T, Fagault Y, Bonvalot L, Wacker L, Fahrni S, Synal H-A. AixMICADAS, the accelerator mass spectrometer dedicated to 14C recently installed in Aix-en-Provence, France. Nuclear Instruments and Methods B 361, 80-86, DOI : 10.1016/j.nimb.2015.01.075 (2015). 
 
Capano M, Miramont C, Guibal F, Kromer B, Tuna T, Fagault Y, Bard E. Wood 14C dating with AixMICADAS: methods and application to tree-ring sequences from the Younger Dryas event in the southern French Alps. Radiocarbon, 1-27, DOI : 10.1017/RDC.2017.83 (2017)
 
Miramont C, Sivan O, Guibal F, Kromer B, Talamo S, Kaiser KF. L'étalonnage du temps du radiocarbon par les cernes d'arbre. L'apport des series dendrochronologiques du gisement de bois subfossiles du torrent des Barbiers (Alpes Françaises du sud). Quaternaire 22 (3), 261-271 (2011). 
 
Participants: 
 
CEREGE: Edouard Bard (PI), Thibaut Tuna, Yoann Fagault, Manuela Capano
 
IMBE: Cécile Miramont, Frédéric Guibal, Christian Marschal, Lisa Shindo
Photo: Examples of subfossil trees from the Durance River region.