Global mapping of carbon isotope ratios in coal

- Global δ13C map values in coal from 95 coals from 10 countries.
- The δ13C in Cenozoic coals are lighter than those in the older Palaeozoic coals.
- The history of δ13C in coal indicate changes in δ13C of atmospheric CO2.

We present stable carbon isotope ratios (δ13C) in 95 coals from 10 countries on a global δ13C map and trace the relationship between δ13C and coal production over geologic time. In addition, we compare δ13C trends in coal as determined from samples and as calculated independently from trends in the δ13C of atmospheric carbon dioxide (CO2). We show that the δ13C values measured in coal display a wide range (− 27.4‰ to − 23.7‰). Furthermore, the δ13C values in Cenozoic coals are systematically lighter than those in Paleozoic coals. We made independent estimates of δ13C values in coal by tracing the derivation of carbon from atmospheric CO2, from its incorporation in plants to the resulting coals. The resulting δ13C histories in coal also show a trend toward lighter values. We suggest that the history of δ13C in coal may reflect isotopic changes in atmospheric CO2 caused by interactions of the short-term and long-term carbon cycles, in addition to factors in the photosynthetic process.