13C enriched Lower Carboniferous fossil plants from Donegal, Ireland: carbon isotope constraints on taphonomy, diagenesis and palaeoenvironment

Abstract

Stable carbon isotopes are now being investigated by an increasing number of palaeobotany researchers. The ratio of the two stable isotopes of carbon ( 12C and 13C) is controlled by a number of biological processes, including photosynthesis, which took place in the original plant. These isotopic signals are also increasingly being used as indicators to interpret the composition of deposits containing organic carbon. A fundamental requirement therefore is that either the isotopic ratio should not significantly change after burial, or any changes must be understood and preferably quantifiable. Preliminary new investigations have been undertaken into a formation, rich in fossil plants, in the Lower Carboniferous of Donegal, Ireland. The bulk of the fossil plants is preserved as fusain (fusinite and semi-fusinite) or fossil charcoal. A series of isotopic measurements of the material were taken and these revealed the fossils to be anomalously isotopically heavy when compared with other known deposits of similar age, plant assemblages and history. There are three possible explanations, or combinations of explanations, for these anomalous results: (1) the Donegal fossils represent evidence of photosynthetic pathways that were previously generally believed to have evolved much later; (2) the fossils have undergone significant isotopic fractionation as a result of, as yet unknown, taphonomic and diagenetic processes; (3) palaeoenvironmental conditions were such that C3 plants acquired isotopically heavy ratios during life, and this has been preserved in the fossil record.