Occurrence of proteinaceous moieties in S-and O-rich Late Tithonian kerogen (Kashpir oil Shales, Russia)

Abstract The polar fraction, isolated from the o-line pyrolysate at 400 C of a Late Tithonian, sulphur-and oxygen-rich, kerogen was examined via Raney Nickel desulphurization and TMAH thermochemolysis. Important information on this kerogen, not accessible via conventional pyrolysis, was thus obtained: (i) its structure is not simply based on alkyl skeletons cross-linked by ether and (poly)sulphide bridges, (ii) TMAH thermochemolysis aorded direct evidence of the survival of proteinaceous moieties in this 140 million years old kerogen and (iii) encapsulation within an aliphatic organic matrix was probably the main pathway reponsible for such a conspicuous preservation, also possibly favoured by the presence of numerous sulphur links.

Kimmeridgian-Tithonian sea-level fluctuations in the Uljanovsk-Saratov Basin (Russian Platform)

Abstract The Uljanovsk-Saratov Basin, located in the southeast of the Russian Platform, presents an intriguing record of the Kimmeridgian-Tithonian sea-level fluctuations. In the Late Jurassic, this basin was a trough within the Interior Russian Sea. The data available from both outcrops and boreholes have permitted outlining a number of lithostratigraphic units and regional hiatuses in the northeastern segment of the Uljanovsk-Saratov Basin, thus permitting a precise reconstruction of transgressions/regressions and deepenings/shallowings. In total, three transgressive-regressive cycles and two deepening pulses have been established. These regionally documented changes were both related in part to global eustatic changes, and they also corresponded in part to the regional sea-level changes in some basins of Western Europe and Northern Africa, but not to those of the Arabian Platform. Differences observed between the global and regional curves as well as rapid Tithonian sea-level oscillations are explained by the influences of tectonic activity. It is hypothesized that the regional Tithonian oxygen depletion might have been a consequence from the rapid flooding of a densely vegetated land

Contact metamorphic reactions and fluid–rock interactions related to magmatic sill intrusion in the Guaymas Basin

Igneous basaltic intrusions into young organic-rich sedimentary basins have a major impact not only on the carbon cycle but also on major and trace element transfers between deep and superficial geological reservoirs. The actively rifting Guaymas Basin in the Gulf of California, which was drilled by the International Ocean Discovery Program during Expedition 385, represents the nascent stage of an ocean characterized by siliceous organic-rich sediments (diatom ooze) intruded by a very dense network of basaltic sills. This study focuses on Site U1546 where the relatively high geothermal gradient (over 200 ∘C km−1) induces early diagenetic transformations in both pore waters and sediments, involving sulfide, carbonate and silica. Geochemical and mineralogical characterizations of the sediment at sill contacts indicate that sulfides and silica polymorphs are the main phases impacted by contact metamorphism, being evident by a transition from opal-CT to quartz and pyrite to pyrrhotite, respectively. Mass balance calculations have been used to estimate mass transfers in metamorphic aureoles. In the top contact aureole, predominantly isochemical metamorphism is reflected by the presence of authigenic quartz and disseminated 20–50 µm sized pyrrhotite crystals, filling primary interstitial space, and partial dissolution of detrital feldspar grains. In the bottom contact aureole, quartz and euhedral pyrrhotite crystals occur, which are up to 4 times larger than those at the top contact. Significant metamorphism of sediments is observed in the lower contact aureole, where plagioclase recrystallizes around the detrital feldspars and locally euhedral pyroxenes are included in patches of carbonate cement; this suggests precipitation from carbon-rich fluids at temperatures (T) higher than 300 ∘C. The lower contact aureole also is more enriched in CaO, Na2O, Fe2O3 and trace elements (Cu, As, Zn, etc.) compared to the upper contact. Based on these petrological investigations, a conceptual model of magma–sediment–fluid interaction is proposed distinguishing top and bottom contact processes. Initial contact metamorphism due to sill emplacement is characterized by dehydration reactions in sediments and crystallization of new minerals. It was followed by carbonate precipitation from the released fluids. At a final stage, the temperature re-equilibrated with the geothermal gradient and the rocks were further altered by hydrothermal fluids.</p

Palynology, organic geochemistry and carbon isotope analysis of a latest Ordovician through Silurian clastic succession from borehole Tt1, Ghadamis Basin, southern Tunisia, North Africa: Palaeoenvironmental interpretation

Palynological and palynofacies analyses combined with organic carbon isotope measurements have been performed on terminal Ordovician through Silurian clastic sediments from the North African margin of Gondwana (southern Tunisia). A first carbon isotopic signal (delta(13)C(org)) from Gondwanan Silurian sedimentary sequences is presented, showing interesting correlation with existing coeval isotopic curves from other areas. Changes in lithology, palynofacies characteristics, palynomorph diversity, carbon isotope developments, and organic geochemistry parameters appear to be all causally linked, and to reflect changes in palaeoceano-graphic conditions. In particular, the detailed chronostratigraphic correlation and the observed similarities in patterns of carbon isotopic developments and (palyno- and litho-) facies changes through the study section has permitted to identify the supposedly global earliest Wenlock ("Ireviken Event") and late Ludlow ("Lau Event") isotopic excursions for the first time in high-latitude Gondwana. This confirms that these excursions were linked to global changes in the oceanic system. The present results suggest that from Rhuddanian to early Wenlock times, an extended period of black shale deposition occurred over the North African Gondwanan margin, progressively transgressing from basin palaeodepressions to basin palaeohighs. Palynofacies and organic geochemistry support a coastal upwelling-promoted productivity increase during this interval, associated to a decrease in diversity of the marine microplanktonic communities. The earliest Wenlock strong positive isotopic shift appears associated with this protracted period of massive black shale deposition, and thus of organic carbon burial, on continental platforms located in high-latitude settings. This could well explain the apparent paradox between excessive carbonate deposition simultaneous to a carbon isotopic shift towards lighter values observed in low palaeolatitude localities (Laurentia and Baltica). the strong, distinct isotope excursion occurring in late Ludlow times is possibly linked to the well know Lau Event. Evidence for extensive organic carbon burial is lacking to explain this strong excursion. Some significant changes in the marine palynomorph communities are recorded in connection to the increase in stable isotope values (delta(13)C(org)). Additionally, a strong correlation between abundance of terrestrially derived palynomorphs (miospores) and delta(13)C(org) development is recorded which seems to support increased clastic input from a terrestrial source. (C) 2008 Elsevier B.V. All rights reserved

Controls on detrital sedimentation in the Cariaco Basin during the last climatic cycle: insight from clay minerals

International audienceA high resolution analysis of the clay mineral content of sediments of the Cariaco Basin spanning the last climatic cycle was performed in order to determine how the detrital sedimentation in the basin records climatic oscillations at various timescales. At the scale of glacial interglacial cycles, the clay mineral content records changes in the geographic origin of the detrital supply, from a dominantly kaolinite-and smectite-rich southern source during glacial to a more illite-rich northerly source during interglacials. Though possibly influenced by eustatic variations, these changes mostly reflect the latitudinal fluctuations of the intertropical convergence zone (ITCZ). The Eemian shows an increased contribution from the northern source compared to the Holocene. During MIS3, seasonality modulates the effect of ITCZ position and leads to increased smectite contribution. Fluctuations of the smectite and kaolinite contents during Dansgaard/Oeschger cycles reflect changes of runoff intensity. Enhanced supplies in kaolinite during Heinrich-like events and stadials also suggest remobilizations of sediments deposited on the Unare platform resulting from rapid sea-level variations. The Younger Dryas is characterized by a significantly different clay mineral association reflecting a drastic modification of terrigenous supply. An eolian contribution would explain this peculiar mineralogy, although local contributions cannot be ruled out. A similar event is noted at the MIS6-5 transition pointing for the occurrence of a Younger Dryas like episode during Termination II

Aliphatic and aromatic biomarkers from Gondwanan sediments of Late Ordovician to Early Devonian age: An early terrestrialization approach

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 C(19) 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. (C) 2011 Elsevier Ltd. All rights reserved

Occurrence of retene in upper Silurian-lower Devonian sediments from North Africa: Origin and implications

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. (C) 2009 Elsevier Ltd. All rights reserved