Paul Farrimond

Massive dissociation of gas hydrate during a Jurassic oceanic anoxic event

In the Jurassic period, the Early Toarcian oceanic anoxic event (about 183 million years ago) is associated with exceptionally high rates of organic-carbon burial, high palaeotemperatures and significant mass extinction. Heavy carbon-isotope compositions in rocks and fossils of this age have been linked to the global burial of organic carbon, which is isotopically light. In contrast, examples of light carbon-isotope values from marine organic matter of Early Toarcian age have been explained principally in terms of localized upwelling of bottom water enriched in 12C versus 13 C (refs 1,2,5,6). Here, however, we report carbon-isotope analyses of fossil wood which demonstrate that isotopically light carbon dominated all the upper oceanic, biospheric and atmospheric carbon reservoirs, and that this occurred despite the enhanced burial of organic carbon. We propose that—as has been suggested for the Late Palaeocene thermal maximum, some 55 million years ago—the observed patterns were produced by voluminous and extremely rapid release of methane from gas hydrate contained in marine continental-margin sediments.

Analysis of intact bacteriohopanepolyols from methanotrophic bacteria by reversed-phase high-performance liquid chromatography-atmospheric pressure chemical ionisation mass spectrometry.

Direct detection of most intact biohopanoids is not possible using conventional GC-MS techniques due to their highly functionalised and amphiphilic nature. Here we report the application of a new reversed-phase high-performance liquid chromatography method for the direct analysis of acetylated, intact bacteriohopanepolyols in solvent extracts of methanotrophic bacteria. Atmospheric pressure chemical ionisation mass spectrometric detection provides structural information relating to the number and types of functional groups present in the four biohopanoids detected: bacteriohopanetetrol, aminobacteriohopanetriol, -tetrol and -pentol. The method should facilitate the assessment of hopanoid composition of both bacteria and environmental samples.

Preservation and diagenesis of hopanoids in recent lacustrine sediments of Priest Pot, England

Abstract Absolute concentrations of various bio- and geohopanoids have been determined in a 24 cm long core from anoxic Recent sediments of Priest Pot, a small highly productive lake in the English Lake District. Samples were Soxtherm extracted, derivatised by methods targeted at specific hopanoid groups, and quantified by GC-MS. Periodic acid/sodium borohydride treatment was used to cleave the side-chains of highly functionalised hopanoids, producing products more amenable to our detection methods. Biohopanoids are extremely abundant (totalling up to 3000 μg/g dry sediment), particularly those with composite or highly (penta- and hexa-) functionalised side-chains, suggesting that these are important precursors to sedimentary hopanoids in Priest Pot. Downcore biohopanoid abundance profiles show a general diagenetic decline with a marked dip at 5–8 cm, which we interpret as a change in bacterial input. Since this is most marked in the composite hopanoids rather than bacteriohopanetetrol, we suggest that these two hopanoid groups have at least partly different sources, and may thus have potential as environmental markers. The geohopanoids are also abundant (totalling up to 1700 μg/g dry sediment) being dominated by the ββ and αβ bishomohopanoic acids, ββ and αβ bishomohopanol and hop-17(21)-ene. Downcore plots show no apparent conversion of free bio- to geohopanoids with depth, which suggests that incorporation into kerogen may be commencing at an early diagenetic stage in the sediments.

SORPTION BY MINERAL SURFACES : REBIRTH OF THE CLASSICAL CONDENSATION PATHWAY FOR KEROGEN FORMATION?

Abstract What are the consequences for organic matter diagenesis of the observation in two recently published articles of a strong correlation between surface area and organic content in marine sediments? The findings suggest that the typical mode of occurrence of organic matter in marine sediments is as a monolayer (or equivalent concentration) sorbed to the surface of mineral grains. This comment considers the theoretical factors which may influence adsorption and propagation of polymeric organic matter on mineral surfaces, and looks at the likely diagenetic fate of adsorbed material. Both adsorption and condensation have been suggested as possible mechanisms for the preservation of labile biopolymers, but neither process is satisfactory as a stand-alone mechanism; adsorption of monomers can merely retard their biodegradation, and condensation is not favoured in solution. However, if the two processes operate in concert, the criticisms levelled against each process considered in isolation are cancelled out, adsorption promoting condensation and condensation enhancing the strength of adsorption of the products. We suggest that the diagenetic modifications of surface adsorbed organic molecules will tend to strengthen their binding to the mineral surface, such that the geomacromolecules will evolve on the mineral surface towards strongly bound monolayers. The hypothesis overcomes many of the objections to the so-called classical condensation pathway of kerogen formation.

Diverse biohopanoid compositions of non-marine sediments

Abstract Biohopanoid compositions were determined for modern sediments from 27 lacustrine environments and 1 near coastal lagoon. GC–MS analysis of existing hopanols and those produced by a side-chain cleavage reaction (periodic acid and sodium borohydride) of the bacteriohopanepolyols provided quantitative data for bacteriohopanetetrol (BHT), bacteriohopanepentol, and composite tetra-, penta- and hexafunctionalised biohopanoids. Additional analysis of acetylated total extracts by HPLC–APCI–MS has allowed us to identify up to seven intact bacteriohopanepolyols in these environments, including the first direct observation of 35-aminobacteriohopanepentol in environmental samples. This method is complementary to the periodic acid approach in providing more specific compositional information. The lacustrine environments exhibit a wide range of compositions, many with high inputs of hexafunctionalised biohopanoids (up to 60% of total tetra-, penta- and hexafunctionalised biohopanoids), indicative of major contributions to the sediment from Type I methanotrophic bacteria. Several methylated biohopanoids were also detected in most of the samples with 2β as the dominant configuration of the methyl group; 2β-methylbacteriohopanetetrol was by far the most abundant and most commonly occurring compound of this type and comprised the most abundant bacteriohopanepolyol in one saline lake (Laguna Grande, Spain). Such compounds are believed to be derived from cyanobacteria, currently the only known quantitatively-significant source of 2β-methylated biohopanoids. Biohopanoids containing a methyl group at C-3 were also detected in a limited number of environments. These data demonstrate the wide variability in bacteriohopanepolyol compositions among depositional environments, and the potential of these compounds to preserve bacterial source information and, thereby, to record environmental change.

Environmental influence on the biohopanoid composition of recent sediments

Abstract Bacteriohopanepolyols are the biological precursors of extended hopane biomarkers which are ubiquitous in the geosphere. The abundance and composition of these biohopanoids were determined for modern sediments from eight depositional environments by GC-MS analysis of hopanols derived from a side-chain cleavage reaction (periodic acid and sodium borohydride) of the bacteriohopanepolyols. We report quantitative data for bacteriohopanetetrol (BHT), composite tetrafunctionalised biohopanoids, pentafunctionalised biohopanoids and hexafunctionalised biohopanoids. BHT was found in concentrations higher than previously reported (up to ca. 1500 μg/g TOC), but comprised only a relatively small proportion (0–26%) of the total biohopanoids in all except one sample. Composite tetrafunctionalised biohopanoids were the dominant group of bacteriohopanepolyols in all other sediments, and are likely to be a major source of the geologically occurring extended hopanes. Hexafunctionalised biohopanoids were most abundant in lake sediments, particularly two small highly productive stratified lakes (where they comprised 20–40% of total biohopanoids). This feature is interpreted to be due to greater contribution to the organic matter of these sediments from Type I methanotrophic bacteria (which produce dominantly hexafunctionalised biohopanoids), consistent with the environmental settings. In the marine environments where sulphate reduction is dominant, hexafunctionalised biohopanoids are relatively minor constituents (2–6% of total biohopanoids). These data comprise the first clear demonstration that bacteriohopanepolyols vary in composition between depositional environments, and have the potential to preserve specific bacterial source information in sedimentary organic matter.

Lipid stratigraphy of a Flandrian peat bed (Northumberland, UK) : comparison with the pollen record

A Flandrian peat bed near Low Hauxley (Northumberland, UK) was studied both optically and geochemically to compare the pollen and lipid records of changing plant input during its accumulation. A progressive change in the predominant plant inputs from woodland trees to a more diverse assemblage with herbs, shrubs and Sphagnum is recorded in the pollen data, and associated lipid records. However, the pollen and lipid records do not correlate in detail; samples with near-identical lipid distributions can have markedly different pollen assemblages and vice versa. This apparent inconsistency between data sets is interpreted to be the result of different modes of incorporation of the signals. While the lipids will be largely derived from plant debris in situ, the pollen may have been transported over considerable distances, and will record an average signal of flora over a wider area. The records of local plant input (lipid data) and regional vegetation cover (pollen data) are complementary in the reconstruction of past environmental conditions.

Water as an oxygen source for the production of oxygenated compounds (including CO2 precursors) during kerogen maturation

Abstract In order to study the generation of oxygen-rich species which may act as CO 2 precursors, the interaction of sedimentary organic matter and water during hydrous pyrolysis has been investigated using 18 O labelled water. Model compound experiments were conducted with a series of compounds (3,4-dimethylphenol, dodecanoic acid and xanthene) in order to assess their levels of simple homogeneous exchange of oxygen with hot water. While dodecanoic acid underwent extensive oxygen exchange (98%), dimethylphenol underwent restricted oxygen exchange (13%) and xanthene showed no exchange. A comparison of the dimethylphenol model compound experiments in H 2 18 O with dimethylphols generated by Kimmeridge Clay kerogen, under identical conditions, showed marked differences in levels of 18 O incorporation (approx. 13% for the model compound and 40% for the kerogen-derived products) suggesting neogenic phenol formation during hydrous pyrolysis. The implications of this inferred water oxygen addition to sedimentary organic matter during kerogen degradation will undoubtedly affect mass balance calculations of the generation of oxygen-rich species from sedimentary organic matter and thus have a potential impact on mass balance assessments of the generation of CO 2 and its precursors in petroleum basins. The results also have major implications for oxygen mobility between organic matter and water in hydrothermal systems and possibly for lower temperature reactions in petroleum systems.

Compositional variations in sedimentary lacustrine organic matter and their implications for high Alpine Holocene environmental changes : Lake St. Moritz, Switzerland

Abstract Organic matter accumulation and preservation in lakes with different geomorphological and climatic regimes, is directly influenced not only by local but also by regional and more global climatic conditions. Thus, compositional variations in lacustrine organic matter can be used as good indicators of paleoenvironmental conditions of deposition. Relatively high average TOC (1.0–6.5 wt%) and good lamination characterize most of the Holocene sequence of Lake St. Moritz, an Alphine lake located in SE Switzerland. However, variations in the type and quality of the organic matter at discrete intervals have been distinguished using bulk parameters and biomarker distributions. The most prominent change occurs at the Late Glacial-Holocene boundary, where δ 13 C (OM) , Hydrogen Index and lipid concentrations indicate a substantial increase in primary productivity. Organic matter production and preservation appear to dominate from the early Holocene until approximately 3.8 Ka BP, although variations in sterol contents and fatty acid distributions indicate superimposed fluctuations, such as between 8.3 and 8.1 Ka BP and 5.1 to 4.8 Ka BP. These variations in the lake trophic-state can be correlated with other proglacial lakes in the region, as well as with similar lacustrine records on both sides of the Alps. The result of this comparison indicates a synchronous character of major to medium-amplitude climatic shifts on a European scale.

Three series of rearranged hopanes in Toarcian sediments (northern Italy)

Abstract A suite of 36 shales and marls from the Toarcian Oceanic Anoxic Event section at three localities in northern Italy was found to contain variable contents of three series of rearranged hopanes: 17α-diahopanes (C 27 and C 29–34 ; and trace C 35 ), 18α-neohopanes (C 27 and C 29–30 ) and an unidentified early-eluting series (C 27 and C 29–35 ). Principal component analysis (PCA) shows the 17α-diahopanes and the early-eluting rearranged hopanes to closely covary, suggesting a related diagenetic mechanism for the formation of their rearranged skeletons. A general covariance with diasteranes is also noted. The 18α-neohopanes show no covariance with diasteranes or the other two series of rearranged hopanes. We also report 2α-methyl analogues of the 17α-diahopanes and early-eluting rearranged hopanes. We propose that the neohopanes may be at least partly derived from different biohopanoid precursors than the other two series of rearranged hopanes, and that the diagenetic origin of the neohopane structure differs from that of the 17α-diahopanes and the unidentified early-eluting series