Biogeochemistry - Geochemistry: An Introduction - page 177

C-C bond) or unsaturated, derived from membrane material and (b) the aromatic com-
pounds derived from the phenol rings of lignin. On land, this forms the base of humic
substances, notably humic and fulvic acids, which are heterogeneous mixtures of amino
acids, sugars, proteins, and aliphatic compounds. Fulvic acids are soluble in water under
all conditions, humic acids are insoluble at pH 2.

During diagenesis, humic compounds evolve into kerogen which, upon heating between
50 and 120 ◦ C evolves into the hydrocarbons of crude oil. Oil is normally preserved when
it seeps out from its primary site of formation towards a porous reservoir. Further heat-
ing induces the cracking of hydrocarbon chains with loss of methane (natural gas) and
eventually, at temperatures in excess of 300 ◦ C, leaves pure carbon as graphite (coal or
anthracite) in the rock. Since methane (CH 4 ) has the highest possible hydrogen/carbon
ratio, two convenient maturation indices of organic products are the O/C and H/C ratios
( Fig. 8.1 ). Normally the terms used are: gas, for C 2 to C 4 hydrocarbons; light oil, for C 4 to
C 14 ; and oil, beyond that point.

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8.7 Biomarkers

Easily degradable organic matter, such as proteins and carbohydrates, is unlikely to fare
well with recycling and diagenesis. The fate of insoluble molecules is certainly more
promising. Among those, some fatty acids, long-chain alkanes and alkenes, and terpenes
often pass diagenesis largely unharmed even if they lose some of their functional groups.
These geochemical fossils can be matched with existing organisms or their ancestors, and
used to interpret ancient microbial ecosystems. They can also have been left by unknown
organisms and reveal long-gone creatures and metabolic pathways. Biomarkers can be used
to reconstruct the evolutionary tree of a particular group of microbes. Occasionally, the dis-
covery of new biomarkers has led to the discovery of unsuspected organisms and metabolic
processes, such as the “anammox” bacteria capable of combining ammonia with nitrites
into N 2 . There is, however, much ambiguity in interpreting these complex molecules since
more than one group of bacteria or algae may produce similar substances.
Among the most successful findings, let us quote archeol as a compound produced by
methanogenic archæa and crocetane produced by methanotroph archæa, which were essen-
tial in understanding the consortia built around methanogens and methanotrophs. Aromatic
carotenoids are pigments which betray the presence of bacteria selectively catching light
at particular wavelengths to achieve the phototrophic oxidation of sulfides. Some forms
of terpenes, known as hopanes, have only recently been related to cyanobacteria. Other
well-identified but orphan compounds remain without a known progenitor. Alkenones are
produced by coccolithophores to regulate cytoplasm viscosity and the relative abundance of
related varieties is commonly used to infer ancient sea-surface temperatures and pressures
of atmospheric carbon dioxide.
Because of their remarkable preservation, biomarkers are also used for isotopic analysis:
the

13 C of alkenones is believed to be a very sensitive, and unfortunately unique, indicator
of the carbonate speciation in the surface ocean and therefore of the CO 2 pressure in the
atmosphere.

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