Simon C. Brassell

Applications of Biomarkers for Delineating Marine Paleoclimatic Fluctuations during the Pleistocene

The Earth’s climate is a dynamic system governed by many factors including the complex, constantly changing interactions between the atmosphere and the ocean, the configuration of the continent, and external variables such as solar luminosity and periodicities in perturbations of the Earth’s orbit. Evidence of increasing levels of CO2 (Keeling et al., 1976) attributed to anthropogenic burning of fossil fuels and the effects of deforestation has heightened concerns about future global warming of a few degrees, together with an associated reduction in ice volume and a consequent rise in sea level. However, investigations of the stratigraphic record have revealed dramatic changes in the Earth’s climate of comparable or greater magnitude in the geological past (e.g., Lamb, 1977; Tarling, 1978; Frakes, 1979; Barron, 1989a, b; Crowley and North, 1991; Bowen, 1991). Such variation in the ancient climatic history of the Earth has been unraveled and elucidated using a diverse range of approaches to which new methodologies continue to be added (e.g., Hecht, 1985).

Geochemical and biological marker assessment of depositional environments using Brazilian offshore oils

Abstract A combined geochemical and molecular characterization of a wide selection of oils from the major Brazilian offshore basins has been undertaken. The elemental (sulphur, nickel and vanadium) and bulk (oAPI and δ13C) properties of each sample been considered, together with its molecular composition determined using liquid and gas chromatography, and quantitative biological marker investigations using gas chromatography-mass spectrometry for alkanes. The results reveal significant differences in the chemical features of the various oils which enable them to be divided into five groups. The distinction of the groups appears to reflect differences in the depositional environment of the source rocks of the oils. Each group is correlated tentatively with source rocks laid down in a specific depositional regime, namely lacustrine freshwater, lacustrine saline water, marine evaporitic, marine carbonate or marine deltaic. The diagnostic features that allow this classification are: the relative abundance and carbon number distributions of n-alkanes; pristane/phytane ratios; sulphur, nickel and vanadium contents; carbon isotope data; the absolute concentrations of hopanes and steranes, and their abundance relative to 4-methylsteranes and, also the occurrence and abundance of several specific biological markers, including 18α(H)-oleanane, gammacerane, β-carotane, tricyclic terpanes, higher acyclic isoprenoids, 28, 30-bisnorhopane and 25, 28, 30-trisnorhopane. This investigation shows the value of a combined geochemical and molecular approach in the assessment of the palaeoenvironment of deposition of the source rocks which gave rise to the oils.

Organic geochemical characterisation of depositional palaeoenvironments of source rocks and oils in Brazilian marginal basins

Abstract An investigation of bulk, isotopic and molecular features of a wide selection of source rocks and oils from Brazilian marginal basins, ranging from Lower Cretaceous to Tertiary in age, provides a number of organic geochemical criteria that characterise and distinguish their depositional palaeo-environments. The methods employed include evaluation of organic carbon contents, Rock-Eval pyrolysis, vitrinite reflectance measurements, determination of carbon isotope ratios, elemental and visual kerogen analysis, assessment of oils and solvent extracts by liquid and gas chromatography, and quantitative biological marker investigations using GC-MS for aliphatic hydrocarbons and UV/Vis spectrophotometry for metalloporphyrins. Selected data were evaluated using principal component analysis. The results enable the recognition and differentiation of seven depositional regimes: lacustrine freshwater; lacustrine saline water; marine evaporitic; marine carbonate; marine deltaic; marine highly anoxic with a predominance of calcareous mudstone lithology, and marine anoxic with predominance of siliciclastic lithology. Diagnostic features in the biological markers from these depositional environments include: the carbon number distributions of n -alkanes, pristane/phytane ratios; absolute concentrations of β-carotane, higher acyclic isoprenoids, C 30 αβ hopane and steranes, and the abundance of the latter relative to 4-methylsteranes; the occurrence and abundance of particular biological markers such as gammacerane, tricyclic and tetracyclic terpanes, 18α (H)-oleanane, 28,30-bisnorhopane and 25,28,30-trisnorhopane, and the relative abundances of nickel and vanadyl porphyrins.

Mid-chain branched mono- and dimethyl alkanes in hot spring cyanobacterial mats: A direct biogenic source for branched alkanes in ancient sediments?

Hot spring cyanobacterial mats from Yellowstone National Park, U.S.A. and the Orakei Korako thermal area, New Zealand have been found to contain suites of monomethyl alkanes, compounds frequently reported in cultured and natural populations of cyanobacteria. The complete range of possible structural isomers for mid-chain branching for C17/C18 and C16/C18 alkanes occurs in the two mats, respectively. Several C19 dimethyl and C20 multibranched alkanes were also observed as major hydrocarbons of the Orakei Korako mat. The recognition of series of mid-chain branched alkanes in modern cyanobacterial mats suggests that their occurrence in ancient sediments may reflect direct biogenic contributions, rather than formation by diagenetic processes.

Late Cretaceous anoxic events in the Brazilian continental margin

Abstract A combined geochemical and micropalaeontological study of Cenomanian to Maastrichtian pelitic sediments from the Brazilian continental margin has been undertaken. The organic carbon contents, Rock-Eval pyrolysis results and biological marker distributions, together with the taxonomic selection and distribution patterns of microfossil assemblages indicate that intermittent anoxic events occurred in the Brazilian marginal basins during the Cenomanian-Santonian with deposition taking place in progressively deeper waters from neritic to bathyal. Such events are characterised by the following: 1. (i) a predominance among the benthonic foraminifera of low-diversity, small-sized specimens (mainly composed of gavelinellids, buliminids and nodosariids) indicating “dysaerobic” bottom conditions; 2. (ii) levels with virtually no benthonic foraminifera and a well-developed planktonic biota (foraminifera, radiolarians and diatoms) indicating “anaerobic” bottom conditions with oxygenated surface waters; 3. (iii) the occurrence of organic rich calcareous mudstone sediments; 4. (iv) features in the biological marker distributions such as high concentrations of 28,30-bisnorhopane and 25,28,30-trisnorhopane (up to 130 ppm), a predominance of phytane over pristane, and medium to high concentrations of nickel and vanadyl porphyrins (up to ca 5700 ppm). The widely recognized Cenomanian-Turonian and Santonian “oceanic anoxic events” (Schlanger and Jenkyns, 1976; Ryan and Cita, 1977 and Jenkyns, 1980) are therefore manifest in the Cassipore, Ceara, Sergipe/Alagoas and Campos basins within the Brazilian continental margin, as is the presence of such events in the Coniacian. In contrast, the Campanian-Maastrichtian appears to be a time interval when deposition under oxygenated conditions produced sediments with low organic carbon contents and poor hydrocarbon source potential, containing highly diversified and abundant calcareous and agglutinated benthonic foraminifera.

Biogeochemistry of hot spring environments 2. Lipid compositions of Yellowstone (Wyoming, U.S.A. ) cyanobacterial and Chloroflexus mats

Abstract The apolar, glycolipid and phospholipid components of four hot spring microbial mats were compared. The complexity among lipid components decreased as temperature and hydrogen sulphide concentration in these hot spring settings became more extreme. The major apolar lipids in all mats were wax esters typical of Chloroflexus which comprised nearly half of the lipids of a mat built solely by this photosynthetic bacterium. Among other apolar lipids some were distinctive of either the cyanobacterial mats (e.g., mid-chain branched monomethylalkanes) or the Chloroflexus mat (e.g., hentriacontatriene). Methanolysis of glycolipid and phospholipid fractions yielded mainly fatty acid methyl esters (FAMEs) ( n -C 16:0 , n -C 16:1 , n -C 18:0 , n -C 18:1 predominating) with moderate amounts of branched FAMEs, alkane -1,2-diols and 1-O-alkylglycerols and traces of phytanyl and nonisoprenoid 1,2-di-O-dialkylglycerols. These lipid occurrences compare favourably with known components of bacteria of various physiological groups which have been cultivated from hot spring mats. The relative abundance of lipids seems to correlate with the trophic structure of the mat communities. Higher water temperature paralleled increased chain length and decreased degree of unsaturation of lipids in the mats.

Biogeochemistry of hot spring environments: 3. Apolar and polar lipids in the biologically active layers of a cyanobacterial mat☆

Zeng, Y.B., Ward, D.M., Brassell, S.C. and Eglinton, G., 1992. Biogeochemistry of hot spring environments, 3. Apolar and polar lipids in the biologically active layers ofa cyanobacterial mat. Chem. Geol., 95: 347-360. The apolar lipid, glycolipid and phospholipid components of the biologically active top 5-mm surface layers of a hot spring cyanobacterial mat were investigated. Most of the major components could be associated with bacteria isolated from the mat; the vertical distribution of lipids followed the known or presumed vertical distribution of these organisms. For example, hydrocarbons (e.g., 7-methylheptadecane), phytadienes (methanolysis products from chlorophyll a) and polar lipid fatty acids typical of mat-forming cyanobacteria maximize in the top 0-1 mm and decrease in concentration with depth. Wax esters and octadecanol (produced upon methanolysis of bacteriochlorophyll cs) typical of Chloroflexus aurantiacus maximized in the 1-2- and 2-4-mm intervals. Long-chain diols derived mainly from glycolipids and typical of the aerobic heterotroph Thermomicrobium roseum maximize in the 1-2- and 2-4-mm intervals. 1-O-Alkylglycerols derived from polar lipids and typical of anaerobic fermentative or sulphate-reducing bacteria, increase in concentration with depth and maximize in deeper layers. The relative abundances of lipids appear to reflect the trophic structure of the microbial community.

Molecular stratigraphy of the Santa Barbara basin: comparison with historical records of annual climate change

Abstract The molecular record of 20th Century climatic variability in laminated sediments of the Santa Barbara basin has been compiled. Distributions and abundance profiles of alkenones and selected sterols reveal annual oscillations in sea surface temperatures and phytoplankton productivity. Positive excursions in alkenone temperature profiles coincide with all major 20th Century El Nino events recording oceanic warming. Downcore abundance profiles for dinosterol, brassicasterol, cholesterol and alkenones reflect productivity trends in phytoplankton and illustrate the comparative resilience of their molecular records. Series of subsurface concentration maxima for dinosterol, a dinoflagellate-derived sterol, appear synchronous with regional historical reports of dinoflagellate blooms.

Petroleum migration and mixing in the Potiguar basin, Brazil

Regional studies of petroleum samples from both offshore and onshore reservoirs of the Potiguar basin, Brazil, reveal compositional differences, with discernible landward trends, attributable to oil mixing. These chemical discrepancies can be related to a combination of source rocks, maturity, and migration history. The source rocks are classified in two groups based on their geochemical and molecular characteristics: (1) Neocomian and Aptian lacustrine freshwater shales, and (2) Aptian marine evaporitic shales and marls. Both lie in an offshore structural low. The geochemically contrasting oils derived from these source beds migrated updip, constrained by a monoclinal structure, through Aptian carrier beds to thermally immature strata. Evaluation of compositional heterog neities in the oils and an investigation of migration effects on their compositions reveal trends related to the amount of mixing which, in turn, reflects the timing of both oil generation and migration, and the direction of migration. The more migrated oils contain greater contributions from freshwater sources, whereas the less migrated oils display more pronounced contributions from the younger and shallower hypersaline source beds. Biomarker maturity parameters indicate that the most migrated oils are less mature, consistent with their early generation from the deeper lacustrine freshwater source beds and migration to fill the shallower reservoirs first. The results suggest that drainage of the basin evolved from lateral to vertical, and enable prediction of the distribution of petrol um types within the basin.

Stepwise chemical degradations of a UK bituminous coal

Abstract Organic materials extracted from a UK high volatile bituminous coal, its solvent extracted O-alkylation product and its solvent extracted O-alkylated, LiAlH4 reduction product have been analysed using g.c.-m.s. to determine changes in the distributions of extractable components with increasing extent of solubilization. Changes in the distribution of functional groups in the insoluble residues have been followed after each step using FT-i.r. Although the range of compounds released at each step is similar, the distribution of extractable lipid classes and their individual components varies. Hopanes (αβ and βα), acyclic isoprenoids, n-alkanes and alkyl phenanthrenes are present in all extracts, with n-alkanes predominating. Maturity parameters based on hopane, phenanthrene and n-alkane distributions give conflicting results for the relative maturities of the three extracts, possibly due to the differential association of individual components within the coal matrix. The organic components extracted after O-alkylation and after LiAlH4 reduction are likely to arise from the release of physically trapped material as a consequence of cleavage of specific bonds within the coal macromolecular structure.