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International audience; Neutral aldoses of the upper 50 m (representing ca. 120 kyr) of sediments from the Cariaco Basin were studied using a new technique based on water extraction followed by sequential acid hydrolysis with 2N, 4N and 6N trifluoroacetic acid (TFA) to investigate the fate of carbohydrates during diagenesis. The distribution and abundance of neutral aldoses and their evolution with depth shed light on the origin and fate of carbohydrates upon burial. The carbohydrates originated mainly from phytoplanktonic production, in particular diatoms. The porewater carbohydrate fraction was invariant and its composition attested to considerable biodegradation of the source material. Bacteria in the sediment may have also contributed to this fraction. The carbohydrates in the 2N fraction represented a decreasing fraction of the total organic carbon (TOC) with increasing depth/age. This fraction likely derived from storage polysaccharides, which were progressively degraded in the sediment. The 4N fraction was dominated by hexoses, suggesting that it comprised sugars derived from structural polysaccharides and exopolymeric substances. It showed a slight decrease with depth, over the 120 kyr. The 6N fraction comprised a relatively constant fraction of TOC with sediment depth. The aldose distribution suggested that the 6N fraction showed the imprint of the initial biological signal preserved by a proteinaceous macromolecular structure and/or a mineral matrix, but not from condensation in the sediment of more labile polysaccharides

Twenty one core samples of Late Ordovician to Early Devonian age from sections in southern Tunisia, North African Platform (Gondwana) and containing marine and terrestrial organic matter with microbial input were investigated to link the aliphatic and aromatic hydrocarbon distributions with the terrestrial and marine palynomorph content (e.g. acritarchs, prasinophytes, chitinozoans, cryptospores and trilete spores). In addition to several biomarkers of algal/bacterial origin, long chain n-alkanes that might be derived from land plants, as well as the terrestrial diterpane, norabietane, were found. Several land plant-derived biomarkers, such as retene, cadalene, simonellite, tetrahydroretene and C19 isohexylalkyl naphthalene were observed in the aromatic fractions. While these terrestrial biomarkers could be clearly recognized in the middle Silurian–lower Devonian samples, their presence in the upper Ordovician–lower Silurian sediments is more doubtful, because of much lower relative abundances. The land plant biomarkers show a fairly good correlation with the occurrence and abundance of cryptospores and trilete spores, derived from bryophytes and tracheophytes, which covered the emerged areas around the Ghadamis Basin during the Silurian and Devonian. The early tracheophytes (e.g. Cooksonia, lycophytes and zosterophylls) are therefore suggested as a new terrestrial source for most of the saturated and aromatic biomarkers found in sediments of Middle Silurian to Early Devonian age.

The biomarkers retene, cadalene, tetrahydroretene and isohexylalkylnaphthalene have been identified in the extracts of Silurian–Devonian clastic sediments from southern Tunisia. The occurrence of retene is unexpected as it is generally considered to be a diagenetic product of the conifer constituent abietic acid; however, conifers evolved only after the Devonian. The retene shows a strong correlation with the abundant terrestrial signal from bryophyte-derived cryptospores in the palynofacies, suggesting that it derives from a terrestrial source. Accordingly, we propose that either abietic acid synthesis evolved prior to the appearance of conifers or retene is a diagenetic product of precursors other than abietic acid, which were synthesized by the earliest land plants. Without excluding the first possibility, it appears likely that retene derives here from the diagenesis of compounds with a kaurane-type skeleton, which were abundantly produced by the early Palaeozoic bryophytes.

Thermochemolysis experiments with tetramethylammonium hydroxide (TMAH), on three Late Jurassic kerogens with contrasting features, confirmed the major role of encapsulation into aliphatic structures for the survival of proteinaceous moieties in kerogens, probably via lipid sulphurisation. It also appeared that (i) some amino acid moieties survived even in the kerogen of a sediment deposited under oxic conditions, although in lower relative abundance compared to the other two kerogens, (ii) the survival of amino acids on geological time scales, within an organic matrix, is probably rather common in kerogens, especially for glycine and alanine, and (iii) thermochemolysis of kerogen sub-units with a lower degree of cross-linking rather than direct thermochemolysis of whole kerogens is recommended to test the presence of amino acid moieties, so as to avoid dilution problems and to increase the accessibility of the TMAH.

A kerogen, termed aBS base , from the Gorodische section (Russian Platform) was studied using a combination of microscopic, spectroscopic and pyrolytic methods in order to determine its chemical structure, source organisms and formation pathway(s). This kerogen was mainly formed via degradation-recondensation of phytoplanktonic material. Selective preservation and natural sulphurisation pathways only played a minor role, whereas a substantial contribution of ether linked lipids was noticed, revealing large oxygen cross-linking. Such observations allowed us to put forward, for the first time to the best of our knowledge, a substantial role for the oxidative reticulation pathway in the formation of a kerogen. Comparision with a previously studied sample from the same outcrop revealed contrasting features which reflect differences in preservation pathways triggered by different depositional conditions.

The kerogen of an organic-rich sample, termed f top, from the Gorodische section (Russian platform) was studied using a combination of microscopic, spectroscopic and pyrolytic methods so as to examine its chemical structure, source organisms and formation pathway(s). This kerogen, which is mainly composed of orange gel-like, nanoscopically amorphous organic matter, exhibits a relatively high aliphatic character; organic sulphur is mainly present as di(poly)sulphides and alkylsulphides. The f top kerogen was chiefly formed via intermolecular incorporation of sulphur in algal or cyanobacterial lipids and carbohydrates. However, its formation also involved oxidative condensation via ether linkages. Comparison of f top sample with other S-rich kerogens points to a closer similarity with Monterey kerogens rather than with a kerogen from the bituminous laminites of Orbagnoux.