Data on modern radiocarbon activity in the old travertine formations of the Pymvashor hydrothermal system were used, in combination with 230Th/U dating of the travertine, to estimate the 14C contents of the total dissolved inorganic carbon in the ancient thermal water at the time of precipitation of the travertine (14Ccalc). With the known values of 14Ccalc and average age of the thermal water, and under the assumption that the residence time of the water in aquifer was constant over the last 13.9 ± 1.5 ka, the initial 14C contents (14C0) in the ancient thermal water were estimated. The findings in this study are that (1) both 14C0 and δ13С have decreased in young waters compared to the ancient waters; (2) although atmospheric 14C activity (14Catm) has also decreased in the same time, the decrease in 14C0 is faster than the decrease in 14Catm. Under certain assumptions, one could link changes in 14C0 and δ13C to climate change. Decrease in δ13С of soil CO2 and decrease in 14C0 was caused predominantly by warmer and wetter climate, decomposition of fossil organic matter, and decrease in 14Catm. There could be also increased dissolution of solid carbonate and increased carbon exchange between DIC and soil CO2, caused by thawing permafrost.
Keywords: Age dating; carbon-13; carbon-14; climate change; isotope geochemistry; thorium-230; travertine; uranium isotopes.