The measurement of the ratio of the carbon isotopes 12C and 13C in sedimentary deposits to determine the extent of plant life at the time. Plants metabolize 12C in preference to 13C, so that when plants are abundant, the atmosphere is enhanced in 13C. Residues in ancient deposits contain resistant organic molecules known as biomarkers. Determination of the residual isotope ratios in such biomarkers shows that there were dramatic changes in the abundance of land and oceanic plants at the times of the mass-extinctions at the end of the Permian and Triassic, possibly caused by the abrupt, catastrophic release of hydrogen sulphide from the oceans.
Subjects: Meteorology and Climatology.