Organic Preservation of Biopolymers in Fossil Leaves - Biopolymers: A Molecular Paleontology Approach - page 44

To investigate these processes and their relative signifi cance further, we have
examined fossil leaves from the Ardèche diatomite and modern representatives of
the same genera using chemical and morphological techniques. This approach is
useful because direct links between precursor organic materials and their sedimen-
tary products can be established. Previous investigations have applied this approach
successfully to both fossil arthropod cuticles and leaves (see Briggs 1999 for review),
but we have extended these analyses by employing a combination of pyrolytic,
spectroscopic, microscopic and chemical degradation techniques that allowed us to
evaluate the composition and structure of the leaves as a consequence of diagenesis
and early thermal alteration. From this, we infer the likely source of the aliphatic
component of the Ardèche leaves, propose possible processes for its formation and
derive implications for organic matter preservation in general.
Samples
Samples were collected by DEGB and NG during August 2002 (and by DEGB in a
trip before) from a freshwater lacustrine deposit of Late Miocene age (8-8.5 m.y.
old) (St. Bauzile; Ardèche, southeastern France). The Ardèche diatomite represents
the largest European deposit of diatomite and is overlain by a Pliocene basalt fl ow.
The Ardèche diatomite was never deeply buried such that, despite the basalt
fl ows, it is largely thermally immature.
At least 81 families of plants, 72 families of insects and 25 families of vertebrates
have been reported from the Ardèche diatomite (Riou 1995 ) making it an important
conservation deposit (Konservat-Lagerstätte). The carbon content of the diatomite is
low, minimizing the possibility of exogenous organic matter contamination of fossils.
For this study all samples were collected from the same horizon; thus presumably
they were subjected to the same environmental and diagenetic conditions and are
of comparable thermal maturity. Leaves of nine species of angiosperms, Acer
pseudocampestre, Castaneavesca , Quercuspalaeocerris , Q. suber , Q. hispanica,
Q. sp. ,Vitisteutonica, Tilia sp. and Populus alba , and one gymnosperm, the conifer
Pinus, were investigated for chemical analysis. Selected leaves from these were used
for microscopic investigation. These are referred to as specimen 1 (sp.1) throughout.
Additional samples of C. vesca , Q. hispanica and P. alba were used for 13 C-NMR
microscopic and chemical studies. These are referred to as specimen 2 (sp.2) through-
out. Samples of the fossil leaves were removed from the rock by lifting with a
dissecting needle or scraping with a scalpel blade. Chemical reagents (HF, etc.) were
not used to dissolve the matrix to avoid any chemical alteration of fossil leaves.
Leaves of modern representatives of these genera were collected at the University
of Bristol Botanic Garden during November 2002. Agave americana and
Cliviaminiata samples were collected from University of Nancy Botanic gardens.
These include Quercusrobur, Populushybrida, Castaneasativa, Acer campestre,
Vitisvinifera, and Pinussylvestris.
Chemical analyses were performed by NG and microscopic analyses by MEC.