The calcium carbonate content and δ¹³Corg were measured at Biogeosciences laboratory, Université de Bourgogne/Franche-Comté in Dijon, France. The δ¹³Corg was measured on carbonate-free residues. Sample powders were reacted with HCl (2 N) at room temperature for 24 h to remove carbonate phases. Residues were rinsed with deionized distilled water until neutral, centrifuged (4500 rpm for 15 min), and dried at 50 °C overnight. Aliquots of dried decarbonated samples (~ 7–50 mg) were then weighed in tin capsules. The δ¹³Corg measurements were performed on a Vario MICRO cube elemental analyzer (Elementar, Hanau, Germany) coupled in continuous flow mode to an IsoPrime stable isotope ratio mass spectrometer (Isoprime, Manchester, UK). USGS40 L-Glutamic acid (C = 40.8 wt%; δ¹³CVPDB = -26.39 ± 0.04 ‰) and IAEA-600 Caffeine (δ¹³CVPDB = -27.77 ± 0.04 ‰). The carbon isotopic composition is expressed in delta notation and reported in per mil (‰) relative to the Vienna Pee Dee Belemnite (V-PDB) standard; external reproducibility based on duplicate analyses of samples is better than ± 0.2 ‰ (1σ).

Further Projects MOPGA (Make Our Planet Great Again) Campus France Web: https://www.campusfrance.org/en/mopga-make-our-planet-great-again-funding-programs * Défis Scientifiques program (Rennes 1 University) Web: https://pia.ac-paris.fr/portail/jcms/p2_1634474/defis-scientifiques SAD METOX (Conseil Régional de Bretagne)