Ecological distribution of Cenomanian terrestrial plants based on 13C/12C ratios

The Cenomanian lagoonal member of the `Argiles du Baugeoisʹ (Anjou, France) yielded a rich and exceptionally well-preserved fossil leaf flora. Stable carbon isotope ratios of fossil plants were measured in order to investigate the presence of a palaeoenvironmental signal. The small intraspecies variations of δ13C values observed for most of the fossil leaves suggested that the isotopic signal had not been significantly altered. Hence, an isotopic approach was used as an attempt to assist in reconstructing the ecological distribution of the flora. The 13C-enrichment of the fossil leaves suggested that the plants underwent water or salt stress. The occurrence of the conifer Frenelopsis suggested, at least locally, a saline environment such as a lagoon. According to their decreasing δ13C values, all the plants collected could have been distributed along a decreasing salinity transect from the lagoon to a flood plain. The predominant species, Eretmophyllum andegavense, a fossil ginkgo, exhibited a wide range of δ13C values. This could indicate that this taxon grew in a wide range of habitats with varying salinities. The origin of the organic matter appeared to be terrestrial with a major contribution from water-stressed plants such as Frenelopsis, as suggested by the similarity between δ13C values of the conifer and the sedimentary organic matter. δ13C values of the sediment indicated stable palaeoenvironmental conditions during the deposition of the `Argiles du Baugeoisʹ member. Hence, while detailed palaeovegetation reconstructions are generally limited to the few deposits which present in-situ palaeoflora, the contribution of isotope geochemistry could allow reconstructions for a wider range of fossil plant deposits.