Radiocarbon dating of ultra-small carbonate samples from deep-sea sediments and coral reef cores: opening a can of worms

International audienceFor 14C measurements of ultra-small samples we use the gas ion source of the AixMICADAS facility (Bard et al. 2015 Nucl. Instr. Meth.) equipped with a gas interface system coupled to a carbonate handling system (CHS). Our method is based on the online sequential leaching of ca. 30 % of the sample mass with HCl using the CHS, followed bycomplete hydrolysis of the residual carbonate with phosphoric acid. The precision, accuracy and blank levels have been tested extensively with reference materials andinternal standards, including corals and foraminifera samples ranging from about 3 to 100 μg of carbon (Tuna et al. 2018 Nucl. Instr. Meth., Fagault et al. 2019 Nucl. Instr. Meth.).We will present 14C results from a high sedimentation rate core located within the present oxygen minimum zone of the northern Indian Ocean. We dated 12 different species of benthic foraminifera adapted to various dissolved oxygen conditions over several Heinrich and Dansgaard-Oeschger events. The capacity to date individual shells of foraminifera allowed us to study in detail the transitions in and out the H1 event, leading to important implications for the hypothetic release of 14C-depleted CO2 from ocean mid-waters into the atmosphere. Another illustration of the usefullness of 14C dating of ultra-small carbonate samples is its application to study sea-level changes during the last deglaciation, notably for the site of Tahiti in French Polynesia (Bard et al. 1996 Nature,Bard et al. 2010 Science, Deschamps et al. 2012 Nature). U-Th dating of aragonitic corals provides the ultimate geochronological precision, but the most precise information on past sea levels is based on calcitic organisms, which cannot be dated reliably by U-Th due totheir small size (e.g. vermetids) or high 232Th/238U ratio (crustose coralline algae, microbialite). We will present new 14C measurements performed on fossil assemblages of samples bracketing the meltwater-pulse 1A from Tahiti cores. The comparison of 14C ages of the different fossil organisms allows strengthening the estimations of amplitude and timing of MWP-1A