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The occurrence in sediments of chlorophyll-derived tetrapyrroles provides evidence for primary photosynthetic communities in palaeo water columns. In ancient sedimentary rocks the components occur mainly as nickel or vanadyl complexes of alkyl porphyrins, and to a lesser extent carboxylic acids. Although extensive loss of functional groups has occurred, the structures of a number of the components reveal that chlorophyll carbon skeletons can survive intact or virtually intact, although in other cases the skeletons have been modified by rearrangement. The structures of a few components indicate an origin from the chlorophyllsc, and thus an algal input, whereas an origin from photosynthetic bacterial chlorophylls is apparent from the carbon skeletons of other components. Such studies, taken with other distributional features (e.g. ratio of nickel to vanadyl components, extent of tetrapyrrole preservation) can provide information about the productivity and preservation of organic matter and presence of water column anoxia at the time of deposition. Studies of the tetrapyrrole components in one recent and two highly immature older sediments have provided further evidence for the pathway of defunctionalization, through the identification of the minimum number of components necessary to link a chlorophyll, such as chlorophylla, to the major alkyl porphyrin in sedimentary organic matter. Additionally, evidence for an early and unexpected transformation pathway comes from the recent identification in sediments of chlorin acids esterified with sterols.

Computerized gas chromatography - mass spectrometry (g.c.-m.s.) is now an essential tool in the analysis of the complex mixtures of lipids (geolipids) encountered in aquatic sediments, both Recent (less than 1 Ma (10 6 years) old) and ancient. Most geolipid studies have been performed in the e.i. mode at low resolution but the techniques now being applied include c.i. and h.r.m.s. The large quantities of data acquired from capillary g.c.-m.s. runs necessitate fast data acquisition and data processing, including the capability for the automatic selection and refinement of key spectra. Even so, the chemist is faced with the identification and/or recognition of at least several hundred good quality spectra from a single run. Fast routine search procedures are useful here, especially for known compounds, while classification routines based on established rules for manual interpretation can be of assistance even with novel compounds. Examples from recent studies (at Bristol) of contemporary, Recent and ancient sediments, are presented. Geolipids show abundance patterns of homologous series which, while related to those of known organisms, display many novel features, including extensive carbon number ranges and stereospecific distributions. Additionally, certain carbon skeleton types, commonly thought to be rare as natural products, are major components of geolipid fractions, presumably reflecting the composite inputs and early microbial diagenetic activity.

Recently it has been shown that the measured extent of reactions in sedimentary biological marker compounds during catagenesis (for example, steroid aromatization, sterane isomerization) can assist in providing information about the extent of maturation of sedimentary organic matter before and during oil generation and also the thermal history of sediments. In the present study, aromatization of a C 27 C-ring aromatic steroid and isomerization at the chiral centres of an isoprenoid alkane, 6( R ),10( S )-pristane, have been brought about under laboratory conditions, elemental sulphur being used as the source of radicals. Precise rate laws have been determined and rate coefficients measured at different temperatures. The pre-exponential factors and activation energies were found to be 6.7 x 10 12 s -1 and 145 kJ mol -1 for the aromatization, and 2.1 x 10 7 s -1 and 120 kJ mol -1 for the configurational isomerization of pristane. These values reflect the relative behaviour of the two types of reactions in sediments, the aromatization being the more temperature-dependent reaction.