Other
Scientific paper
Sep 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994gecoa..58.3703d&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 58, Issue 17, pp.3703-3711
Other
Scientific paper
Hydrocarbon identification led to the recognition of three distinct chemical races in the green microalga Botryococcus braunii . These three races ( A , B , and L ) also show pronounced differences in the chemical structure of the algaenans, i.e., the insoluble, highly aliphatic, nonhydrolysable macromolecular constituents (termed PRB) of their outer walls. PRB A and PRB B are based upon polymethylenic chains, while PRB L is based on C 40 isoprenoid moieties. Previous studies demonstrated that the selective preservation of PRB A and/or PRB B played a major role in the genesis of various Botryococcus -derived kerogens; in sharp contrast, the above kerogens did not show any indices of a PRB L contribution. The main purpose of this study was therefore to examine, via the screening of a number of Botryococcus -derived kerogens, if some contribution of PRB L , i.e., of an "isoprenoid algaenan", could be observed. To this end, the pyrolysis products of the three PRB and of seven Botryococcus -rich kerogens were examined. Parallel study of the algaenans of the three races of B . braunii indicated that a clear-cut distinction can be made, by analysis of low polarity pyrolysis products, between PRB A and PRB B , on the one hand, and PRB L , on the other hand. Thereafter, identification of the low polarity pyrolysis products of Botryococcus -rich kerogens revealed some contribution of the L race, along with the A or B race, in three samples out of seven: a Pliocene Torbanite and two Recent sediments. These observations thus provided the first example of the formation of kerogen fractions by the selective preservation of an algaenan based on isoprenoid chains. But they also revealed that important transformations of such chains can take place during kerogen early diagenesis. In sharp contrast, examination of the four Palaeozoic Torbanites revealed a complete lack of isoprenoid moieties, and hence no evidence of PRB L contribution. Comparison of n -alkane/ n -alk-1-ene doublet distribution in the pyrolysate of these four kerogens, derived from the A and/or B races, showed some differences in their degree of maturation and in the contribution of higher plant materials to their formation.
Behar Françoise
Derenne Sylvie
Largeau Claude
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