Geology Reference
In-Depth Information
Chapter 3
Organic Preservation of Biopolymers
in Fossil Leaves
Abstract
This chapter investigates the morphology and chemical structure of fossil
leaves from the Ardèche diatomite (Late Miocene, southeast France) and compares
them to their modern equivalents. Chemical analyses of the fossil leaves revealed
the presence of a recalcitrant (non-hydrolysable) geopolymer comprised of benzene
derivatives, lignin-derived components, pristenes and an aliphatic component; the
latter consists partly of fatty acyl subunits ranging in carbon number from C
8
to C
32
with an abundance of C
16
and C
18
units. Chemical degradation of the modern plants
failed to reveal the presence of the aliphatic biomacromolecule cutan, thereby
precluding selective preservation of this compound as the source for the aliphatic
component of the fossil leaves. In contrast, C
16
and C
18
fatty acyl units are predomi-
nant in the cutin and phospholipid fatty acid (PLFA) fractions of the modern leaves,
while C
10
to C
32
acid units are characteristic of the free fatty acid (FA) fraction of
epicuticular waxes. However, TEM and SEM investigations of the fossils revealed
no evidence for cuticle preservation, and while a contribution from cutin cannot be
excluded, the aliphatic component of the fossil polymer is possibly derived instead
from the
in situ
polymerisation of labile cell membrane lipids and free fatty acids.
Keywords
Kerogen Organic matter preservation Pyrolysis n-alkanes, leaves
Introduction
Sedimentary organic matter comprises about 90 % weight of total organic matter on
Earth (Berner
1989
); kerogen comprises 95 % of this by weight, and the remaining
5 % is attributed to extractable constituents termed bitumen. It is formed by
diagenetic alteration of biological material and is converted to petroleum products
by thermal catagenesis (Tissot and Welte
1984
). Depending on the biological source
and fi nal composition kerogen is classifi ed as type I, II or III. Types I and II are
hydrogen rich kerogens and often have a high aliphatic contribution in the form of
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