Thanh Thuy Nguyen Tu

Isotope reconstruction of plant palaeoecology. Case study of Cenomanian floras from Bohemia

Palaeoenvironments inferred from stable carbon isotope ratios of fossil plants collected from various environments of the Cenomanian of Bohemia were compared with palaeobotanical and sedimentological data to test the use of isotope ratios as a record of local palaeoenvironments and fossil-plant ecology. A number of isotope and biogeochemical patterns suggested that stable carbon isotope ratios in the Cenomanian plants from Bohemia have not been significantly affected by diagenesis. Stable carbon isotope ratios of the palaeoflora from one of the sections studied were used as a reference for a non-stressed habitat since both sedimentological and palaeobotanical evidence suggested that the fossil flora underwent no environmental stresses that could have influenced its δ13C values. Comparisons of δ13C values of the other palaeofloras with that of the reference section, allowed inference of the palaeoenvironmental stresses undergone by fossil plants. These palaeoenvironmental patterns deduced from 13C/12C ratios are in agreement with the palaeoenvironments previously reconstructed by sedimentological and palaeobotanical studies. Combining evidence from those studies with isotope data provided detailed insights into the palaeoecology of the plants studied. Finally, stable carbon isotope ratios allowed precise characterisation of the ecology of the best-represented species of the deposits studied, the ginkgoalean plant Eretmophyllum obtusum and the conifer Frenelopsis alata, which were quite common in the salt-marsh environments in Europe during the Cenomanian. Hence, stable carbon isotope ratios can help in evaluating the environmental stresses undergone by fossil plants and the combination of these results with palaeobotanical and sedimentological data can provide detailed insights into fossil-plant ecology.

Effects of fungal infection on lipid extract composition of higher plant remains: comparison of shoots of a Cenomanian conifer, uninfected and infected by extinct fungi

The lipid fraction extracted from uninfected shoots of a fossil conifer, Frenelopsis alata, was analysed by gas-chromatography‐mass-spectrometry, and compared with shoots of the same conifer infected by extinct epiphyllous fungi, so as to study the eAects of fungal infection on the chemical composition of extracts from higher plant remains. The extracts from the uninfected shoots appeared to be composed of (i) common constituents of higher plant lipids such as n-alkanes and fatty acids, (ii) elemental sulphur, and (iii) substantial amounts of terpenoids characteristic of conifers, such as cadalene, beyerane, dehydroabietane and related compounds. Comparison of this extract with that of fungalinfected shoots revealed, in addition to the aforementioned compounds, the presence of substantial amounts of hydroxysuccinic acid and functionalised benzoic compounds that were interpreted as degradation products of lignin by fungi. This study aAorded preliminary indications of the composition of extracts from higher plant remains infected by fungi. # 2000 Elsevier Science Ltd. All rights reserved.

Evolution of the chemical composition of Ginkgo biloba external and internal leaf lipids through senescence and litter formation

External and internal lipids were analysed in fresh, senescent and litter leaves of Ginkgo biloba to derive qualitative and quantitative information on the changes that occur during the very early stages of diagenesis, and to identify potential biomarkers of the only living Ginkgoale. Analysis of fresh leaves led to the identification of additional compounds, including several series of phenolic constituents, and showed differences between the external and internal lipids (absence/presence of some components, relative abundances and distributions of some series). Pronounced differences concerned with the evolution of content (as wt.% of whole leaves) were observed between the internal lipids (regular decrease from fresh to senescent and to litter leaves) and the external lipids (no significant changes in content). Molecular studies showed variations in the relative abundances and distributions of most constituents of G. biloba leaf lipids which reflect different degrees of stability during the first stages of diagenesis. It also appeared that (i) the phenolic components and the co-occurrence of α-tocopherol and nonacosan-10-ol could be useful biomarkers of G. biloba, and (ii) most of the internal lipids continue to be exported to the leaf surface where they are further degraded during senescence and litter formation.

Comparison of leaf lipids from a fossil ginkgoalean plant and its extant counterpart at two degradation stages: diagenetic and chemotaxonomic implications

Comparative characterisation of total lipids from fresh leaves and leaves from litter of Ginkgo biloba demonstrated that all lipid constituents were degraded during the early stages of diagenesis. However, as shown by calculation of degradation levels, some of these compounds were preferentially degraded and a stability scale was established for G. biloba leaf lipids. Leaf lipids preserved in Eretmophyllum andegavense, a ginkgoalean plant of Cenomanian age, appeared less complex than those of G. biloba, its extant counterpart. The presence of dimethoxyalkylcoumarins, characteristic constituents of G. biloba, confirmed on chemical grounds the phylogenetic link between both ginkgoalean plants; it also revealed a relatively high degree of preservation of some lipids of E. andegavense.

Carbon and nitrogen dynamics in decaying wood: paleoenvironmental implications

The effect of early diagenesis on carbon and, especially, nitrogen isotopes (δ13C and δ15N) of organic matter is not well understood and is of interest for accurate paleoenvironmental reconstructions. Wood samples were incubated in distilled water and river water to assess the effects of early diagenesis on carbon and nitrogen dynamics. Elemental content and isotopic composition of carbon and nitrogen as well as mass loss of wood pieces were determined. Mass loss in river water was three times greater than in distilled water. This difference was attributed to the development of two different types of fungi characterised by various degradation rates. Carbon dynamics of wood samples showed similar patterns in both type of water: (i) a sharp increase in carbon content, possibly related to carbohydrate degradation, before it slowly returned towards initial values, and (ii) no significant changes in δ13C values. In contrast, nitrogen dynamics of samples showed complex patterns: (i) N release associated with 15N depletion in distilled water, attributed to uptake of 15N-enriched pool (i.e. proteins) by fungi, and (ii) N accumulation associated with 15N enrichment in river water. The latter pattern was attributed predominantly to microbially mediated importation of 15N-enriched nitrate from river water. Although challenging, the present results suggest that early diagenesis may average an environmental signal by integrating individual signals (woods, fungi, water) and microbial processes. Considering the non-linear behaviour of early diagenesis, this integration is probably almost instantaneous on the geological time scale, which may not preclude paleoenvironmental reconstructions.