Seawater redox variations during the deposition of the Kimmeridge Clay Formation, United Kingdom (Upper Jurassic): evidence from molybdenum isotopes and trace metal ratios

The Kimmeridge Clay Formation (KCF) and its equivalents worldwide represent one of the most prolonged periods of organic carbon accumulation of the Mesozoic. In this study, we use the molybdenum (Mo) stable isotope system in conjunction with a range of trace metal paleoredox proxies to assess how seawater redox varied both locally and globally during the deposition of the KCF. Facies with lower organic carbon contents (TOC 1–7 wt %) were deposited under mildly reducing (suboxic) conditions, while organic-rich facies (TOC >7 wt %) accumulated under more strongly reducing (anoxic or euxinic) local conditions. Trace metal abundances are closely linked to TOC content, suggesting that the intensity of reducing conditions varied repeatedly during the deposition of the KCF and may have been related to orbitally controlled climate changes. Long-term variations in ?98/95Mo are associated with the formation of organic-rich intervals and are related to third-order fluctuations in relative sea level. Differences in the mean ?98/95Mo composition of the organic-rich intervals suggest that the global distribution of reducing conditions was more extensive during the deposition of the Pectinatites wheatleyensis and lower Pectinatites hudlestoni zones than during the deposition of the upper Pectinatites hudlestoni and Pectinatites pectinatus zones. The global extent of reducing conditions during the Kimmerigidan was greater than today but was less widespread than during the Toarcian (Early Jurassic) oceanic anoxic event. This study also demonstrates that the Mo isotope system in Jurassic seawater responded to changes in redox conditions in a manner consistent with its behavior in present-day sedimentary environment

Evidence for substantial intramolecular heterogeneity in the stable carbon isotopic composition of phytol in photoautotrophic organisms

Author Posting. © Elsevier B.V., 2007. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Organic Geochemistry 39 (2008): 135-146, doi:10.1016/j.orggeochem.2007.09.002.The ubiquitous isoprenoid phytol was isolated from a range of algae, terrestrial plants and a bacterium and its two terminal carbon atoms were quantitatively removed by chemical oxidation. The product, 6,10,14-trimethylpentadecan-2-one, was depleted in 13C by 1-4‰ relative to the parent phytol. This difference is significant, and indicates that the pathway for biosynthesis of phytol induces substantial intramolecular stable carbon isotopic fractionations. The nature and magnitude of the fractionations suggest strongly that it is associated both with the biosynthesis of isopentenyl pyrophosphate via the 2-C-methylerythritol-4-phosphate pathway and with the formation of carotenoids and phytol from geranyl-geraniolphosphate. As a result of these large, intramolecular isotopic differences, diagenetic products formed by loss of C, such as pristane, may be naturally depleted in 13C by several permil relative to phytane.Shell International Petroleum Maatschappij BV is thanked for financial support for the irm- GC-MS facilit

Resonant two-site tunnelling dynamics of bosons in a tilted optical superlattice

We study the non-equilibrium dynamics of a 1D Bose-Hubbard model in a gradient potential and a superlattice, beginning from a deep Mott insulator regime with an average filling of one particle per site. Studying a quench that is near resonance to tunnelling of the particles over two lattice sites, we show how a spin model emerges consisting of two coupled Ising chains that are coupled by interaction terms in a staggered geometry. We compare and contrast the behavior in this case with that in a previously studied case where the resonant tunnelling was over a single site. Using optimized tensor network techniques to calculate finite temperature behavior of the model, as well as finite size scaling for the ground state, we conclude that the universality class of the phase transition for the coupled chains is that of a tricritical Ising point. We also investigate the out-of-equilibrium dynamics after the quench in the vicinity of the resonance and compare dynamics with recent experiments realized without the superlattice geometry. This model is directly realizable in current experiments, and reflects a new general way to realize spin models with ultracold atoms in optical lattices.Comment: 12 pages, 6 figure

Hadley circulation and precipitation changes control black shale deposition in the Late Jurassic Boreal Seaway

New climate simulations using the HadCM3L model with a paleogeography of the Late Jurassic [155.5 Ma], and proxy-data corroborate that warm and wet tropical-like conditions reached as far north as the UK sector of the Jurassic Boreal Seaway [~35oN]. This is associated with a northern hemisphere Jurassic Hadley cell and an intensified subtropical jet which both extend significantly polewards than in the modern (July-September). Deposition of the Kimmeridge Clay Formation [KCF] occurred in the shallow, storm-dominated, epeiric Boreal Seaway. High resolution paleo-environmental proxy data from the Kimmeridge Clay Formation [KCF; ~155–150 Ma], UK are used to test for the role of tropical atmospheric circulation on meter-scale heterogeneities in black shale deposition. Proxy and model data show that the most organic-rich section [eudoxus to mid-hudlestoni zones] is characterised by a positive δ13Corg excursion and up to 37 wt% total organic carbon [%TOC]. Orbital-modulation of organic carbon burial primarily in the long eccentricity power band combined with a clear positive correlation between %TOC carbonate-free and the kaolinite/illite ratio supports peak organic carbon burial under the influence of very humid climate conditions, similar to the modern tropics. This re-interpretation of large-scale climate relationships, supported by independent modelling and geological data, has profound implications for atmospheric circulation patterns and processes affecting marine productivity and organic carbon burial further north along the Boreal Seaway, including the Arctic

Characterisation of an extremely organic sulfur-rich, 150 Ma old carbonaceous rock: Palaeoenvironmental implications

An extremely organic sulphur-rich coccolithic limestone of the Upper Jurassic Calcaires en plaquettes Formation (southern Jura, France) was analysed for its molecular composition using chemical and instrumental methods. The bitumen accounts for 19% of the total organic carbon, and consists predominantly of high molecular weight sulphur-bound constituents in the asphaltene and polar fractions. n-Alkane and hopanoid carbon skeletons, together accounting for the majority of compounds in the polar and apolar fractions, occur mainly in a sulphur-bound form as thiophenes, dithiophenes, cyclic sulphides and compounds that are intermolecularly sulphur-bound. A series of C18-C32 9-methylalkanes, components of as yet unknown origin, is present exclusively in a sulphur-bound form. Sulphur incorporation into the aromatic carotenoid isorenieratene provided many thiophenic and sulphidic aryl and diaryl isoprenoids. Although steroids are only minor constituents of the bitumen, it is noteworthy that 24-methyl-5,14,17(H)-cholestanes are more abundant than their 14,17(H)-counterparts, and that a series of C27-C29 steradienes was detected with unknown positions of the double bonds. Like the bitumen, the kerogen and asphaltene pyrolysates are dominated by organic sulphur compounds. In addition to thiophenes and benzothiophenes (common in pyrolysates of organic sulphur-rich macromolecules), several novel homologous series of sulphur-containing products were found. Based on mass spectra and desulphurisation products, these were identified tentatively as alkylbithiophenes, dialkylbithiophenes, alkylphenylthiophenes, alkyl(methylphenyl)thiophenes and components possessing both a thiophene and a benzothiophene moiety. The abundant sulphurisation of organic matter and the presence of isorenieratene derivatives indicate that the depositional environment was euxinic most of the time. Since the hopanoid biomarkers far outnumber the steroids, and since the contribution of algal biopolymers to the kerogen is low, it is believed that the water column-dwelling cyanobacteria were important contributors of organic matter

Air flow over foredunes and implications for sand transport

More than 4000 hourly wind profiles measured on three topographically different foredunes are analysed and discussed. Wind flow over the foredunes is studied by means of the relative wind speed: the ratio between wind speed at a certain location and the reference wind speed at the same height. Relative wind speeds appear to be independent of general wind speed but dependent on wind direction. For perpendicular onshore winds the flow over the foredune is accelerated due to topographic changes and decelerated due to changes in surface roughness. Accelerations dominate over decelerations on the seaward slope. The pattern of acceleration and deceleration in relation to wind direction is more or less comparable for different foredunes, but the magnitudes differ. An increase in foredune height from 6 to 10m leads to an increase in speed-up near the top of the seaward slope from 110 to 150 per cent during onshore wind, but further increase of foredune height from 10 to 23m appears to have little effect, due to increased roughness and deflection of flow. Topography also influences the direction of the flow. Between beach and top, the flow deflects in the direction of the normal during onshore winds. During offshore winds the flow is deflected to the parallel. Near the dunefoot, deflection is always in the direction of the parallel, and increases with steeper topography. The maximum deflection near the dunefoot was 90°, over a 23 m high dune, observed during offshore winds. Patterns of erosion and sedimentation resulting from winds from different directions can be explained by the observed accelerations and decelerations. Owing to speed-up on the seaward front of the foredune, sand transport capacity of the wind increases, which results in erosion if vegetation is absent. During strong onshore wind, sand is lifted near the dunefoot and moves over the foredune in suspension. During weaker winds, vertical wind velocities do not exceed fall velocities of the sand grains, and most of the sand is deposited near the dunefoot

Controls on the molecular and carbon isotopic composition of organic matter deposited in a Kimmeridgian euxinic shelf sea: Evidence for preservation of carbohydrates through sulfurisation

Thirteen samples from the Kimmeridge Clay Formation (KCF) in Dorset, covering all different lithologies, were studied using bulk and molecular geochemical and microscopical techniques. Our data show that the positive correlation between TOC and 13CTOC reported for shales (Huc et al., 1992) also holds for other lithologies (e.g., limestones) if we correct for dilution by carbonate (TOC*). Despite the wide range of 13CTOC values (−26.7 to −20.7 ), the 13C values of individual biomarkers of algal and green sulfur bacterial origin and of kerogen pyrolysis products (i.e., n-alkanes) show in general only small changes (<2 ). This indicates that changes in the concentration of dissolved inorganic carbon (DIC) or 13C of DIC (13CDIC) in the palaeowater column cannot account for the 6 difference in 13CTOC. Kerogen pyrolysates indicated that with increasing TOC*, and thus increasing 13CTOC, carbon isotopically heavy C1-C3 alkylated thiophenes with a linear carbon skeleton become increasingly abundant; in the case of the Blackstone Band kerogen (TOC* = 63%) they dominate the pyrolysate. These thiophenes are probably derived from sulfur-bound carbohydrates in the kerogen. Algal carbohydrates are typically 5 10 heavier than algal lipids and differences in preservation of labile carbohydrate carbon through sulfurisation may thus explain the range in 13CTOC values without the need to invoke any change in water column conditions. The increasing dominance of thiophenes in the kerogen pyrolysate with increasing TOC* is consistent with the increasing Sulfur Index (mg S org/g TOC), the decreasing SPYRITE/STOT ratio, and the increasing dominance of orange amorphous organic matter produced by natural sulfurisation. The organic matter of all sediments was deposited under euxinic conditions as revealed by the occurrence of isorenieratene derivatives indicating (periodic) photic zone euxinia. At times of reduced run-off from the hinterland, represented by so-called condensed sections, the flux of reactive iron was relatively small compared to the flux of reactive organic matter, which resulted in the formation of relatively small amounts of pyrite and an excess of hydrogen sulfide capable of reacting with fresh organic matter. Within the condensed sections, variations in the degree of sulfurisation of organic matter are probably due to both differences in primary production and differences in the supply of reactive iron. These findings demonstrate that climatic changes, probably driven by Milankovitch cycles, can have a large impact on the molecular and carbon isotopic compositions of the sedimentary organic matter in an otherwise relatively stable stratified basin. They also show that large amounts of labile carbohydrate carbon may be preserved through sulfurisation