J.R. Maxwell

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.

BACTERIAL POPULATIONS AND PROCESSES IN SEDIMENTS CONTAINING GAS HYDRATES (ODP LEG 146: CASCADIA MARGIN)

Abstract Bacterial populations and activity were quantified at three sites in the Cascadia Margin accretionary wedge, off the West Canadian/American coast (ODP Leg 146). At two sites sediments contained gas hydrates, Site 889/890 had a discrete zone of hydrate approximately 10 m above a bottom simulating reflector (BSR) at 225 m below sea floor (mbsf) and Site 892 had disseminated hydrate in the upper 20 mbsf and a BSR at 74 mbsf. Site 888 was a control site without gas hydrates. The control site (888) and top approximately 90 mbsf of Site 889/890 had bacterial distributions similar to previous Pacific Ocean sites. In the upper approximately 30 m of Site 892, however, bacterial populations were much lower, suggesting inhibition by the high concentrations of H 2 S within the hydrate zone. Below this depth bacterial populations rose to concentrations consistent with other sites. The control site was dominated by SO 4 reduction and rates of CH 4 oxidation in the top 90 m were low (0.002–0.033 nmol cm −3 d −1 ). At Site 889/890 bacterial populations and activity were stimulated in the discrete hydrate zone. CH 4 oxidation rates increased in the middle of this zone to 134.5 nmol cm −3 d −1 (ca. 9 times the average rate at other depths), resulting in a significant (× 10) increase in the total bacterial population. The anaerobic process(es) responsible for this oxidation remain unclear, despite SO 4 -reducing bacteria, previously associated with CH 4 oxidation, also being stimulated in this zone. Fluid flux into accretionary wedge sediments may be an important process in providing electron acceptors to maintain these relatively high rates of CH 4 oxidation. This first microbiological study of gas hydrates indicates that bacterial processes are influenced by gas and fluid venting, and they play a major role in geochemical changes within these deep (> 200 mbsf) sediments.

Microbial degradation of the marine prymnesiophyte Emiliania huxleyi under oxic and anoxic conditions as a model for early diagenesis: long chain alkadienes, alkenones and alkyl alkenoates

To investigate the effect of bacterial diagenesis on the distributions of algal lipids, the marine haptophyte Emiliania huxleyi was incubated with estuarine sediment slurries under defined conditions (oxic, sulfate reducing and methanogenic). Rapid initial degradation of lipids corresponded to increased bacterial populations, both of which subsequently declined. Under oxic conditions the algal C31 dienes were rapidly and completely degraded within 178 days. Extensive degradation of the characteristic C37 methyl alkenones occurred under all conditions (up to 85% under oxic conditions); however, the U37K′ index remained essentially constant, except for a slight increase at the longest oxic incubation time. Under anoxic conditions the alkyl alkenoates were preferentially degraded relative to the alkenones and changes in the AA36 index occurred. These results demonstrate that lipids are part of the “labile” organic matter; however, they were generally degraded at multiple rates which slowed during incubation, resulting in their partial preservation. Hence, laboratory experiments conducted over only short periods cannot predict the preservation potential of lipid components. Furthermore, preservation differed between sulfate reducing and methanogenic conditions, so the roles of anaerobic processes need to be considered individually with regard to oxic vs. anoxic preservation.

Characterisation of alkenone temperature indicators in sediments and organisms

Abstract Recent literature reports have suggested the use of long chain ( n -C 37 ) alken-2-ones in Quaternary marine sediments as a palaeoclimate indicator. They occur as di-, tri- and tetra-unsaturated components whose relative abundances are measured as an internal ratio (U 37 k ). Since the precise structures of the compounds were unknown, both the all- Z and all- E isomers of the gross structures assigned previously have been synthesised. GC coinjection studies indicate that the natural components in the alga Emiliania huxleyi and in marine sediments have the unusual and unexpected E configuration. The synthesised compounds offer promise as standards for the development of more sensitive and specific methods for measurement of U 37 k ratios.

Isomerisation, rearrangement and aromatisation of steroids in distinguishing early stages of diagenesis

Abstract The distributions of proposed reactants and products of stages of early diagenesis of steroid hydrocarbons have been studied by GC and GC-MS in a variety of downhole sequences of DSDP cores of various ages, which include the San Miguel Gap, Walvis Ridge, Gulf of California and Falkland Plateau. The compounds comprise aliphatic and aromatic hydrocarbons such as diasterenes, monoaromatic anthrasteroids and spirosterenes. From the study of the progress of proposed reactions and of presumed product/reactant relationships a pattern emerges that extractable lipid distributions can provide a means of distinguishing levels of early diagenesis which cannot be recognised readily from other methods. A number of diagentic trends are evident such as progressive isomerisation at C 20 in diasterenes, which occurs at different relative rates and temperature ranges in sediment sequences with different heat fluxes. Such studies allow investigation of the changes in biological marker compounds in the part of the continuum between changes during sediment deposition and the advent of catagenesis.

Evidence for sulphide links in a crude oil asphaltene and kerogens from reductive cleavage by lithium in ethylamine

Abstract Reduction of a sulphur-rich asphaltene from a crude oil (Rozel Point) with lithium in ethylamine (Li/EtNH2) affords an alkane fraction similar to that obtained from desulphurisation with Raney nickel, but in higher yield. Using Li/EtND2, deuterium-labelled alkanes were obtained, thereby confirming the presence of sulphide linkages in the asphaltene fraction. Reduction of two sulphur-rich kerogens with similar sulphur contents, Serpiano shale (M. Triassic) and Jurf Ed Darawish (JED, U. Cretaceous) afforded significant quantities of polar solvent-extractable material. Both kerogens also yielded low amounts of hydrocarbons although the amount released from JED kerogen was significantly higher. The results indicate that, whereas the kerogens contain hydrocarbon moieties intermolecularly linked by sulphide bridges, the distributions of the precursors lipids which were incorporated were different. For example, GC-MS analysis of the labelled products from JED kerogen revealed them to be dominated by steranes of algal origin, with the position(s) of deuterium labelling providing evidence of sulphide linking and cross-linking. On the other hand, the labelled products from Serpiano kerogen were dominated by hopanes of bacterial origin which were linked terminally via the side chain.

Crude oil biodegradation under simulated and natural conditions—II. Aromatic steroid hydrocarbons☆

Abstract The distributions of the major ring C monoaromatic and triaromatic steroid hydrocarbons of a number of oils, biodegraded under natural conditions to differing extents, have been examined by combined gas chromatography-mass spectrometry. Three major effects are observed: (i) loss of low molecular weight triaromatics, possibly from preferential water solubility; (ii) preferential degradation of mono- and triaromatics with the original 20R biological configuration; (iii) resistance of low molecular monoaromatics to biodegradation in comparison with their high molecular weight counterparts. In the laboratory, aerobic biodegradation of aromatic steroid hydrocarbons has also been demonstrated. Some of the effects are similar to those observed in the field studies.

Unusual long chain ketones of algal origin

Abstract Recent reports have suggested the use of the relative abundances of long chain (n-C37) alken-2-ones (1, 2, and 3) in Quaternary marine sediments as a palaeoclimate indicator1. Here we report the synthesis of these ketones with all-E and all-Z stereochemistry. Use of the synthesised compounds has shown that the natural counterparts in the alga Emiliania huxleyi and in marine sediments have the unusual and unexpected E configuration.

High molecular weight chlorins in a lacustrine shale

Abstract The major esterified tetrapyrrole in a Miocene oil shale (Marau, Brazil) is a free base chlorin, most likely pyrophaeophorbide a, esterified to 24-ethyl-4α-methyl-5α(H)-cholestan-3β-ol, the structure being assigned by FAB-MS and 1H NMR, and by GC-MS analysis of the sterol liberated by hydrolysis. FAB-MS and electronic absorption spectrophotometric data for the second most abundant component are consistent with the mesopyrophaeophorbide counterpart. Only one other minor esterified chlorin was detected, implying a highly specific and, hence, biologically-mediated esterification. It is not clear whether the reaction took place during biosynthesis of an unknown precursor chlorophyll or resulted from esterification of a hydrolysis product(s) from a known chlorophyll (most likely chlorophyll a) after cellular disruption, although the second possibility is considered more likely.

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.