D/H ratios of coals and the palaeolatitude of their deposition

Abstract

Various interpretations of D/H ratios in coals have been suggested^1–4. The factors most likely to influence these ratios seem to be: (1) prevailing climatic conditions and the related deuterium contents of available fresh water (precipitation) during plant growth; and (2) the fractionation of stable hydrogen isotopes during both plant growth and subsequent sediment maturation. On theoretical grounds, the global distribution of the hydrogen isotopes in precipitation is expected to favour an increased deuterium concentration at the Equator relative to the poles. Published measurements of the D/H ratio in present day precipitation at different latitudes, as shown in Fig. 1⁵, follow this trend. Clearly the well-established global variation in the isotopic composition of precipitation with regard to both climate and latitude provides a reasonable explanation for the differences observed in D/H ratios for coal from different continents as has been proposed for coals from Germany and South Africa¹, and for isotopic variations seen in peat². The D/H ratios for Australian coals reported here are consistent with their respective latitudes of deposition predicted from continental drift and hence the implied dependence on palaeoclimate.