Carbonates of the Tomtor complex of ultramafic alkaline rocks and carbonatites (the northern part of the Republic of Sakha Yakutia) are distinguished by a wide range of carbon isotopic composition 13C from +2 to 59.9. The geological position, localization patterns, mineral and chemical compositions and the relationship with REE mineralization of samples with values of 13C carbonates from 25 to 59 are characterized. The formation of abnormally low 13C in carbonates is determined by the biogenic oxidation of methane from 13Cmet to 70.
