Heavy carbon travertine related to methane generation: A case study of the Big Tarkhan cold spring, Kerch Peninsula, Crimea

The Big Tarkhan cold spring is located within the Bulganak zone of mud volcanism in the northern Kerch Peninsula (Crimea). The spring waters mainly have Cl-HCO3-Na-Ca chemistry and temperatures from 18 to 23 °C. Active travertine deposition at the site produces abundant calcite and minor amounts of siderite, halite, tincalconite, trona, gaylussite, northupite and amorphous iron hydroxides. Calcite contains the impurities of 0.26-2.16 wt.% MgO, up to 0.87 wt.% FeO, 0.15-0.73 wt.% SrO, 0.28-0.98 BaO, up to 0.43 wt.% MnO, and 0.09-0.60 Na2O. Active travertines are depleted in REE (ΣREE = 2.6-4.8 ppm) compared to the inactive ones. The Kerch travertine calcite shows unusual carbon and oxygen isotope compositions (+ 8.1 ÷ + 12.5‰ VPDB δ13C and + 10.1 ÷ + 12.9‰ VPDB δ18O). Their isotopic and trace element signatures (including REE patterns) suggest their relation to basinal waters and origin from the organic-rich clayey Maikop Fm., which is the principal source rock of the area. The Big Tarkhan travertine deposits are associated with thermogenic methane production.