Formation of carbonate nodules in the soils of the Darling Downs, Queensland, Australia, and the dating of the Talgai Cranium

A study of the carbon isotopic composition of soil organic matter, carbonate nodules, present day ground waters and plants has shown that: (a) the soil organic carbon formed since 10,000 years B.P. had identical 13 C/ 12C ratios C-17°/oo) as plants of the subtropical grasslands ecosystem which are still found on uncultivated portions of the flood plains; (b) soil organic matter formed prior to 10,000 years B.P. has a lower 13 C/ 12C ratio and indicates a change in the plant cover from either temperate grasses or a forest ecosystem to subtropical grasses; (c) the carbonate nodules found in the soil are in 13C equilibrium with CO2 produced from carbon of contemporary plant composition; (d) the 14C/ 12C ratio of both the soil organic carbon and the carbonate nodules decreases with depth in the soil, the soil organic matter extrapolating to zero age at the surface, and the carbonate nodules extrapolating to zero age at about 80 - 100cm depth in the soil. No nodules are found higher than lm depth. This suggests that the carbonate nodules form at about lm depth in equilibrium with soil CO2; (e) the soil horizon at which the Talgai Cranium was found corresponds to the surface at 14,000 years B.P.