Philippe Schaeffer

Molecular indicators of palaeoenvironmental conditions in an immature Permian shale (Kupferschiefer, Lower Rhine Basin, north-west Germany) from free and S-bound lipids

A study of the distributions of free and S-bound biomarkers and their δ13C values in the extract of a bulk sample of the Permian Kupferschiefer from the Lower Rhine Basin has been carried out to provide a more detailed characterization of the biological inputs. The distributions and isotopic compositions of selected components released from nickel boride desulphurization have been compared with those of components in the free fraction. The free biomarkers occur in greater abundance than their S-bound counterparts. The distribution and carbon isotopic composition of both free and bound components in the extract indicate a major cyanobacterial and algal input. Likewise, the free and bound aromatic hydrocarbons contain components which reveal an origin from the carotenoids of Chlorobiaceae (green sulphur bacteria), providing molecular evidence for the occurrence of photic zone anoxia in the water column. Most of the components in the complex mixture of mono-, di-, tri- and tetra-aromatics in the free aromatic hydrocarbon fraction result from cyclization and aromatization reactions.

Caspian sea level from Topex‐Poseidon altimetry: Level now falling

Tide gauge measurements recorded between 1978 and 1993 indicate that the Caspian sea level (CSL) has been rising at an average rate of ∼12 cm/yr during this 15-yr time span. Decadal CSL changes are currently attributed to changes in river runoff and effective evaporation. We have analysed three and a half years (from January 1993 through August 1996) of altimeter range data from the TOPEX-POSEIDON mission over the Caspian sea to estimate temporal variations in the sea level. We show that the Caspian sea level was still rising at a rate of 18.9±0.5 cm/yr between January 1993 and July 1995 and that the northwestern Caspian in the area of the Volga delta was rising faster (by ∼3 cm/yr) than the middle and south Caspian. However, by mid-1995, the CSL started to drop abruptly, a trend still observed in 1996. The average sea level decrease recorded from mid-1995 amounts to −24.8±1.4 cm/yr.

Timing of early diagenetic sulfurization of organic matter: A precursor-product relationship in Holocene sediments of the anoxic Cariaco Basin, Venezuela

Abstract The incorporation of reduced inorganic sulfur into organic matter is a significant mechanism for the preservation of functionalized organic compounds in the sedimentary environment, but the timing of this process is not currently known. Analysis of organic matter in the Holocene and latest Pleistocene sediments of the Cariaco Basin indicates conversion of a triunsaturated tricyclic triterpenoid, (17E)-13β(H)-malabarica-14(27),17,21-triene, identified by isolation and subsequent one- and two-dimensional 1 H and 13 C NMR spectroscopy, to a monounsaturated triterpenoid thiane through incorporation of reduced inorganic sulfur species into the isoprenoid side chain within the upper 6 m of sediments. Time control provided by AMS 14 C dates was used in conjunction with measured relative abundances of the two compounds to calculate the reaction rate for the sulfurization of this triterpenoid, and empirically determine the first order rate constant (2 × 10 −4 yr −1 ) for the reaction. This is the first time that such a precursor-product relationship has been positively identified for a sulfurization reaction in sediments.

Geochemical study of macromolecular organic matter from sulfur-rich sediments of evaporitic origin (Messinian of Sicily) by chemical degradations

Abstract Selective chemical degradations have been performed on sulfur-rich macromolecular organic matter from sediments of the Messinian of Sicily (Upper Miocene, Italy). The biomarkers released upon different desulfurization experiments are much more abundant than the free lipids and display completely different distributions, which reflect the selective trapping of (poly)functionalized molecules by inorganic sulfur species at an early stage of diagenesis. The bound biomarker distributions reveal a dominant phytoplanktonic input originating from an upper oxic water layer and a moderate bacterial reworking of the organic matter. A contribution from cyanobacteria to the organic matter cannot be excluded. The presence of carotenoids typical from photosynthetic sulfur bacteria attests to a stratified water body where an anoxic photic zone occurred. LiAlH4 hydrogenolysis of sulfur-rich macromolecules revealed that polysulfide bridges contribute to a large extent in the cross-linking. The results of deuterium labelling experiments are discussed in terms of sulfur incorporation on functionalized lipids.

Large and rapid climate variability during the Messinian salinity crisis: Evidence from deuterium concentrations of individual biomarkers

During the Messinian, 6 m.y. ago, massive sea-level fall and widespread deposition of evaporites occurred in the Mediterranean Sea when it became isolated from the world oceans. Here we present the first hydrogen isotope data from individual sedimentary biomarkers, n -alkanes and isoprenoids, that tracked climatically driven hydrographic changes in response to extreme evaporation during the Messinian salinity crisis. The stable hydrogen and carbon isotope compositions of these biomarkers show a range of 160‰ in δD values and 14‰ in δ 13 C values, and roughly covary. This indicates that the source waters of the biomarkers were therefore in some cases extremely enriched in deuterium, having average δD as great as +66‰ VSMOW (Vienna standard mean ocean water). Such values are only known from desert climates today. Because the offset between the δD values of n -alkanes and isoprenoids preserved in the Miocene sedimentary rocks is similar to the offset found in modern biological samples, we conclude that diagenesis did not significantly affect the primary deuterium concentrations.

Changes in palaeoenvironmental conditions during deposition of the Permian Kupferschiefer (Lower Rhine Basin, northwest Germany) inferred from molecular and isotopic compositions of biomarker components

Abstract Biomarker distributions based on GC and GC-MS data supplemented by stable carbon isotopic compositions based on irm (isotope ratio monitoring) GC-MS data have been used to investigate changes in depositional conditions in a number of samples from a 2 m core (Rheinberger Heide) covering the entire deposition of the Permian Kupferschiefer (Lower Rhine Basin, northwest Germany). Compound classes investigated are aliphatic and aromatic hydrocarbons and maleimides (1H-pyrrole-2,5-diones). The core has been previously divided into four sections: TI, TIIA, TIIB and TIII based on lithology. The biomarker distributions and the δ13C values of components indicate contributions from cyanobacteria, algae, green sulphur bacteria and higher plants (minor) throughout Kupferschiefer deposition. All of the phytoplanktonic components show 13C enrichment in the lowest section, suggesting a greater productivity. The occurrence of components derived from green sulphur bacteria (Chlorobiaceae) indicates that the entire deposition was characterised by periods of photic zone anoxia. Such conditions were rapidly established after the initial transgression of the Zechstein Sea and may have been productivity-driven, but were less prevalent during deposition of the upper core sections.

Biomarker evidence for “Heinrich” events

The so-called Heinrich layers, found in North Atlantic sediments, have been attributed to quasi-periodic discharges of armadas of icebergs originating from the Laurentide ice sheet. Using the distribution of biomarkers in a sediment core (BOFS 5K) we have attempted to provide more information about their origin and the effect of the Heinrich events on oceanic circulation and the climatic system. Estimates of sea surface temperature changes, via U3K7 measurements, show that the surface ocean underwent significant cooling during the events, probably as a result of incoming iceberg meltwater. The low sea temperatures and the rapid variation in the accumulation of chlorophyll-derived pigments suggest that the events provoked a major change in oceanographic conditions, which may have affected the thermohaline circulation in the Atlantic Ocean. Within the most recent layers we have also found vanadyl alkyl porphyrins and aryl isoprenoid hydrocarbons, diagenetic products of photosynthetic pig- ments (chlorophylls and carotenoids, respectively). These components are not expected to occur in late Quaternary sediments, so their presence demonstrates a contribution to the layers of ancient, organic rich sedimentary material of continental origin. The presence of vanadyl porphyrins and absence of their nickel counterparts, and the distributional features of the aryl isoprenoids (e.g., presence of the C40 reduced carotenoid isorenieratane of green sulphur bacterial origin) indicate the existence of anoxic conditions in the photic zone of a marine water column in the original depositional setting of the contributed ancient sediment. These results suggest that it should be possible, using a biomarker correla- tion approach, to determine the source area of the ancient sediment eroded by the icebergs. Copyright

Sulphur and oxygen sequestration of n-C37 and n-C38 unsaturated ketones in an immature kerogen and the release of their carbon skeletons during early stages of thermal maturation

Sedimentary rock from the Gessoso-solfifera Formation (Messinian) in the Vena del Gesso Basin (northern Italy) containing immature (Ro = 0.25%) S-rich organic matter was artificially matured by hydrous pyrolysis at temperatures from 160 to 330°C for 72 h to study the diagenetic fate of n-C37 and n-C38 di- and tri-unsaturated methyl and ethyl ketones (alkenones) biosynthesised by several prymnesiophyte algae. During early diagenesis, the alkenones are incorporated into the kerogen by both sulphur and oxygen cross-linking as indicated by chemical degradation experiments with the kerogen of the unheated sample. Heating at temperatures between 160 and 260°C, which still represents early stages of thermal maturation, produces large amounts (up to 1 mg/g TOC) of S-bound, O-bound, and both S- and O-bound n-C37 and n-C38 skeletons, saturated n-C37 and n-C38 methyl, ethyl, and mid-chain ketones, C37 and C38 mid-chain 2,5-di-n-alkylthiophenes, C37 and C38 1,2-di-n-alkylbenzenes, and C37 and C38 n-alkanes. With increasing thermal maturation, three forms of the n-C37 and n-C38 skeletons are relatively stable (saturated hydrocarbons, 1,2-di-n-alkylbenzenes and saturated ketones), whereas the S- and O-bound skeletons are relatively labile. These results suggest that in natural situations saturated ketones with an n-C37 and n-C38 skeleton can be expected as well as the corresponding hydrocarbons.

Clues to early diagenetic sulfurization processes from mild chemical cleavage of labile sulfur-rich geomacromolecules

Macromolecular fractions, isolated from the solvent extract of sulfur-rich Recent (Siders Pond, USA; Lake Cadagno, Switzerland; Walvis Bay, Namibia) and immature sediments (Gibellina, Messinian of Sicily; Vena del Gesso, Messinian of Italy), were investigated by chemical degradation using sodium ethanethiolate/methyliodide. This mild reagent which cleaves polysulfide bonds to yield methylsulfides has the advantage over other methods of leaving intact other functionalities (like double bonds) and preserving sulfur atoms at their incorporation site. The method is, therefore, well-suited to the molecular level investigation of sulfur-rich macromolecules from Recent sediments containing highly functionalized polysulfide-bound subunits. In Recent anoxic sulfur-rich sediments, the release of various methylthioethers clearly demonstrates that intermolecular sulfurization of organic matter does occur at the earliest stages of diagenesis. Steroids and phytane derivatives are the major sulfurized lipids, a feature also observed in more mature sulfur-rich sediments. Several phytene derivatives, such as cis and trans 1-methylthiophyt-2-enes, as well as methylthiosteroids, including 5α- and 5β-3-(methylthio)-cholest-2-enes, were identified by comparison with synthesized standards. Steroid methylthioenolethers are released from polysulfide-bound steroid enethiols present in the macromolecular fractions. The latter, which correspond to thioketones, can be considered as intermediates in the reductive sulfurization pathway leading from steroid ketones to polysulfide-bound saturated steroid skeletons and are characterized for the first time in the present study. Thus, it could be shown that the major part of the polysulfide-bound lipids occurring in Recent sediments is apparently the result of sulfurization processes affecting carbonyls (aldehydes and ketones). The unsaturated methylthioethers obtained from Recent sediments were not present in more mature evaporitic samples, which suggests that polysulfide-bound unsaturated thiols are intermediates formed during the first sulfurization steps occurring soon after deposition and that they are rapidly transformed by various processes taking place during early diagenesis, notably to yield their saturated counterparts.

Product distributions from chemical degradation of kerogens from a marl from a Miocene evaporitic sequence (Vena del Gesso, N. Italy)

Abstract Lithium in ethylamine ( Li EtNH 2 ) treatment has been carried out on sulphur-rich kerogens from 10 sub-samples of the cycle IV marl from the Vena del Gesso evaporitic sediments (Upper Miocene, Italy). The treatment releases, after chromatography of the resulting solvent extracts, up to 30% of the kerogen total organic carbon. The biomarkers released are dominated by components of algal origin. The concentrations of the aliphatic hydrocarbons obtained are of the same order of magnitude as the bound alkanes released upon desulphurization of polar fractions from organic extracts and show similar distributions. The presence of diaromatic isoprenoids derived from Chlorobiaceae indicates that photic zone anoxia occurred during the deposition of the cycle IV marl, such conditions being more prevalent at the middle of the section. Polar material makes up by far most of the organic material released and contains in particular alcohols and amides which may result from the cleavage of ester bonds present in these immature kerogens, the amides being secondary products formed by reaction with the EtNH2. In the case of the alcohols, however, these components could also arise from cleavage of sulphide-bound moieties.