The isotopic compositions of the travertine and of the hot spring solutions were studied at Main Springs and New Highland Terrace in the Mammoth Hot Springs area of Yellowstone Park. The springs issue at 74°C and a pH of 6.65 and the carbon isotopic composition of the travertine depositing at the orifice is +2%.δC¹³ (PDB). As the water travels out from the orifice, it cools and loses CO₂. The travertine depositing at lower temperature is enriched in C¹³, reaching values of +4.8%. and the solution has a pH of 8.2 at 27°C. The δC¹³ of the carbon species in solution is about −2.3%. at 74° and about +4.3 at 27°C. Therefore, the difference in δC¹³ between the solid and solution is approximately 4%. at 74° and decreases to zero at about 20°C. These differences are shown to be due to kinetic (non-equilibrium) factors. The δO¹⁸ contents of the travertine and water show that in most samples the carbonate oxygen is in equilibrium with the water O¹⁸ at the temperatures of deposition. This is especially true for travertine depositing slowly and at temperatures above about 50°C. Calculations based on pH and alkalinity titrations of the hot spring waters in situ show that at the spring orifice the water is very high in free CO₂, which is quickly lost in transit. The springs are supersaturated with respect to both aragonite and calcite during most of their travel in the open air.

The carbon isotopic composition of the travertine is similar to that in the marine carbonates that are adjacent to the springs and that are the probable source of the calcium carbonate.

The travertine from inactive prehistoric springs near Mammoth has similar δC¹³ and O¹⁸ to that from the active springs. Soda Butte, an inactive center 25 miles east of Mammoth, contains heavier carbon and oxygen than the springs near Mammoth.