Subducted organic matter buffered by marine carbonate rules the carbon isotopic signature of arc emissions

• Published in: Nature communications Vol. 13; no. 1; p. 2909
• Main Authors: Tumiati, S., Recchia, S., Remusat, L., Tiraboschi, C., Sverjensky, D. A., Manning, C. E., Vitale Brovarone, A., Boutier, A., Spanu, D., Poli, S.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 25.05.2022; Nature Publishing Group; Nature Portfolio
• Subjects: 140/58; 147/135; 704/2151/209; 704/2151/431; Calcium carbonate; Carbon; Carbon dioxide; Earth Sciences; Emissions; Geochemistry; Graphite; Humanities and Social Sciences; Inorganic carbon; Isotopes; Marine sediments; Mathematical models; multidisciplinary; Organic matter; Phytoplankton; Planetology; Redox properties; Rocks; Science; Science (multidisciplinary); Sciences of the Universe; Sediments
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/s41467-022-30421-5

Ocean sediments consist mainly of calcium carbonate and organic matter (phytoplankton debris). Once subducted, some carbon is removed from the slab and returns to the atmosphere as CO 2 in arc magmas. Its isotopic signature is thought to reflect the bulk fraction of inorganic (carbonate) and organic (graphitic) carbon in the sedimentary source. Here we challenge this assumption by experimentally investigating model sediments composed of 13 C-CaCO 3  +  12 C-graphite interacting with water at pressure, temperature and redox conditions of an average slab–mantle interface beneath arcs. We show that oxidative dissolution of graphite is the main process controlling the production of CO 2 , and its isotopic composition reflects the CO 2 /CaCO 3 rather than the bulk graphite/CaCO 3 (i.e., organic/inorganic carbon) fraction. We provide a mathematical model to relate the arc CO 2 isotopic signature with the fluid–rock ratios and the redox state in force in its subarc source. The carbon isotopic signature of CO 2 released from marine sediments subducted beneath volcanic arcs does not reflect their organic/inorganic fraction, but instead the fluid-rock ratios and the redox conditions in force at the top of the slab.