Kliti Grice

BIOSYNTHETIC EFFECTS ON THE STABLE CARBON ISOTOPIC COMPOSITIONS OF ALGAL LIPIDS : IMPLICATIONS FOR DECIPHERING THE CARBON ISOTOPIC BIOMARKER RECORD

Thirteen species of algae covering an extensive range of classes were cultured and stable carbon isotopic compositions of their lipids were analysed in order to assess carbon isotopic fractionation effects during their biosynthesis. The fatty acids were found to have similar stable carbon isotopic compositions to each other in all the algae with the exceptions of the C28 fatty acid in Scenedesmus communis and the polyunsaturated fatty acids in Ampidinium sp. and Gymnodinium simplex which are 3.5‰ more depleted in 13C and up to 8‰ more enriched in 13C, compared to the C16 fatty acid, respectively. Phytol is consistently enriched in 13C by 2–5‰ compared with the C16 fatty acid in all algae. The sterols are, however, enriched in 13C by 0–8‰ compared to the C16 fatty acid, possibly due to a different pool of isopentenyl diphosphate in the cytosol. These large ranges in carbon isotopic compositions of compounds biosynthesized by the same eukaryotes can significantly complicate the interpretation of δ13C values of sedimentary biomarkers.

Diamondoid hydrocarbon ratios as indicators of biodegradation in Australian crude oils

The effect of biodegradation on diamondoid distributions in petroleum has been investigated on a series of crude oils reservoired in two Australian sedimentary basins, the Gippsland Basin and the Carnarvon Basin. The ratio of methyladamantanes to adamantane rises with increasing biodegradation. Significant changes in the ratio occur at extreme levels, indicating that diamondoids can be indicators of petroleum biodegradation especially when most other hydrocarbons have been removed. The methyladamantane/ adamantane ratio can also be used to assess the composition of crude oils that are comprised of a mixture of severely biodegraded and non-biodegraded oil.

13C-depleted charcoal from C4 grasses and the role of occluded carbon in phytoliths

The δ13C values of plants and corresponding charred materials from wood, C3 and C4 grasses, derived from natural burning and laboratory combustion were obtained to determine whether there was a significant difference in δ13C of grass-derived char (C3 and C4) compared with wood-derived (C3) char. Our data showed that there is an up to 8‰ 13C-depletion in C4-derived chars from natural burning but there was no significant isotopic change in chars from wood or C3 grasses. We suggest that this 13C-depletion in C4-derived chars is due to protected organic matter in silicate structures (phytoliths), which were found to be depleted by up to 9‰. Analysis of this protected carbon by Py-GCMS indicated the presence of low relative amounts of n-alkanes. However, 13C-NMR data suggested that a significant portion of the phytolith-occluded material was composed of simple carbohydrates (O-alkyl carbon) and that alkyl carbon (lipid material) constituted a minor fraction. These isotopic and spectroscopic data have important implications for the calculation of the proportions of C3- versus C4-derived charred organic matter in modern as well as geological studies.

δ13C of organic matter transported from the leaves to the roots in Eucalyptus delegatensis: short-term variations and relation to respired CO2

Post-photosynthetic carbon isotope fractionation might alter the isotopic signal imprinted on organic matter (OM) during primary carbon fixation by Rubisco. To characterise the influence of post-photosynthetic processes, we investigated the effect of starch storage and remobilisation on the stable carbon isotope signature (δ13C) of different carbon pools in the Eucalyptus delegatensis R. T. Baker leaf and the potential carbon isotope fractionation associated with phloem transport and respiration. Twig phloem exudate and leaf water-soluble OM showed diel variations in δ13C of up to 2.5 and 2‰, respectively, with 13C enrichment during the night and depletion during the day. Damped diel variation was also evident in bulk lipids of the leaf and in the leaf wax fraction. δ13C of nocturnal phloem exudate OM corresponded with the δ13C of carbon released from starch. There was no change in δ13C of phloem carbon along the trunk. CO2 emitted from trunks and roots was 13C enriched compared with the potential organic substrate, and depleted compared with soil-emitted CO2. The results are consistent with transitory starch accumulation and remobilisation governing the diel rhythm of δ13C in phloem-transported OM and fragmentation fractionation occurring during respiration. When using δ13C of OM or CO2 for assessing ecosystem processes or plant reactions towards environmental constraints, post-photosynthetic discrimination should be considered.

EFFECTS OF ZOOPLANKTON HERBIVORY ON BIOMARKER PROXY RECORDS

The stable carbon isotopic compositions of cholesterol, generally the most dominant sterol in the copepod Temora, bears the δ13C “signature” of its dietary precursor sterol when fed on Isochrysis galbana and Rhodomonas sp. The δ13C of cholesterol in the faecal pellets released from Temora longicornis fed on Rhodomonas sp. is identical to the δ13C of the sterols in the diet, indicating that no significant carbon isotopic fractionation effects occur when the copepod modifies eukaryotic precursor sterols to cholesterol. Furthermore, the ratio of long-chain alkenones and their stable carbon isotopic compositions in I. galbana were identical to those egested in faecal material. Thus Zooplankton herbivory does not invalidate the use of these alkenones as a proxy for sea surface temperature and pCO2.

Molecular isotopic characterisation of hydrocarbon biomarkers in Palaeocene-Eocene evaporitic, lacustrine source rocks from the Jianghan Basin, China

Immature organic matter in lacustrine source rocks from the Jianghan Basin, eastern China, was studied for distributions and stable carbon isotopic compositions (δ 13 C) of hydrocarbon biomarkers. All of the bitumens contain isorenieratane (δ 13 C ca. -17‰) indicating the presence of Chlorobiaceae, and thus periods of euxinic conditions extended into the photic zone. Aliphatic hydrocarbon fractions are dominated by pristane and phytane and contain relatively low amounts of 4a- and 4β-methylsteranes (C 30 , δ 13 C ca.-16‰) believed to originate from 4-methylsterols in dinoflagellates. δ 13 C values of gammacerane (ca. -24‰) and C 30 17α,21β(H)-hopane (ca.-23‰) are consistent with an origin from bacterivorous ciliates that fed on Chlorobiaceae. The bitumens contain low relative amounts of organo-sulphur compounds. One rock contains C 37 and C 38 n-alkanes (δ 13 C ca. -36‰) derived from alkadienes and alkenones of prymnesiophytes. The distributions of hydrocarbon biomarkers suggest that they were initially sulphurised during early diagenesis and later released during early thermal maturation.

Unique marine Permian‐Triassic boundary section from Western Australia

A unique marine Permian‐Triassic boundary section containing rich oil source rocks has been continuously cored in a petroleum borehole from the Perth Basin of Western Australia. Such sequences, which provide a biostratigraphic and environmental record at the time of the largest extinction event of the past 500 million years, are globally rare, and this is the first to be documented in Australia. Throughout geological history there have been periods of global marine anoxia that commonly resulted in the widespread deposition of petroleum source rocks, most notably in the mid‐Cretaceous and Late Jurassic. An apparent paradox is that, previously, source rocks have not been recognised in association with the Permian‐Triassic boundary, despite widespread marine anoxia at this time. The Perth Basin source rocks contain abundant and unusual biomarkers, apparently related to the highly specialised and limited biota that flourished in the aftermath of the end‐Permian extinction event. Local conditions may have favoured source‐rock development, either due to higher productivity resulting from coastal upwelling or through enhanced preservation under strongly anoxic conditions.

Isotopically heavy carbon in the C21 to C25 regular isoprenoids in halite-rich deposits from the Sdom Formation, Dead Sea Basin, Israel

Abstract A series of Miocene/Pliocene halite deposits (with extremely low organic carbon contents) from the Sdom Formation (Dead Sea Basin, Israel) have been studied. Distributions and δ13C contents of biomarkers have been determined using GC–MS and irm-GCMS analyses, respectively. The hydrocarbon fractions consist mainly of pristane, phytane and C21 to C25 regular isoprenoids. The predominance of C21+ regular isoprenoids in the Dead Sea halites and other hypersaline deposits indicate that these components could be derived from ether-bound membrane lipids of halophilic archaea. The lack of intact ether-bound lipids in the polar fractions indirectly infers that such components have already been released at early stages of diagenesis. Their δ13C contents are enriched in 13C, by up to 7‰, compared to the biomarkers of presumed phytoplanktonic origin (i.e. steranes and hopanes) within the same sediment sample, in agreement with a source other than algae and cyanobacteria, thus, tentatively assigned as halophilic archaea. Based on biomarker distributions, δ13C contents and mineral compositions, these sediments appear to have been deposited in a salinity stratified water body with a bottom water brine. Continual evaporation and deposition of the higher salts (i.e. carnallites) are favourable conditions for the growth of halophilic archaeal communities.

A remarkable paradox : Sulfurised freshwater algal (Botryococcus braunii) lipids in an ancient hypersaline euxinic ecosystem

Abstract Two relatively immature hypersaline sediments of Miocene/Pliocene age from the Sdom Formation, Dead Sea, Israel were studied using both GC–MS and irm–GCMS analyses. A novel series of extractable organosulfur compounds (OSC) derived from functionalised lipids of freshwater Botryococcus braunii algae races B and L were tentatively identified based on their mass spectra and Raney nickel desulfurisation products. Desulfurisation of the polar fractions released high amounts of apolar components, attributed to the major part of the macromolecular matrix being comprised of multiple sulfur-linked biomarkers derived from a limited number of highly functionalised lipids of Botryococcus. Most of the components appear to be early thermal released products and are not directly formed from sulfur incorporation into functionalised precursor lipids. One sample is mainly comprised of lipids derived from race B and possibly A and L, and the other sample is comprised of lipids derived from B. braunii races B and L. Carbon isotopic signatures of the individual biomarkers derived from the three races are widely dispersed. For example, δ values of components derived from the L race are ca. −21‰ and those from the B race are significantly enriched in 13C having values in the −10 to −13‰ range. Overall, B and L race derived components are 13–20‰ and 5–7‰, respectively, more enriched in 13C than phytoplanktonic biomarkers of marine origin. Stable carbon isotopic data of the B. braunii components point to differing bloom periods for the three races. The specific structures of the biomarkers (especially OSC) provide compelling evidence for the existence of freshwater algae in ancient hypersaline environments in a stratified water body.

Distributions and stable carbon isotopic compositions of biomarkers in torbanites from different palaeogeographical locations

Abstract The relative distributions and stable carbon isotopic compositions of certain aliphatic and aromatic biomarkers from 11 Botryococcus braunii rich torbanites from Scotland, South Africa and Australia covering the Late Carboniferous to Late Permian were investigated. The data was scrutinised for any evidence of molecular features which may be characteristic of palaeogeography. The torbanites studied were selected to cover a range of age and palaeoclimatic conditions (e.g. Permian and Late Carboniferous when Gondwana was covered by an extensive ice sheet, the Late Permian when the climate warmed from glacial to cool to temperate and the Carboniferous when Laurasia was located within the equatorial zone). All torbanites are composed of abundant n-alkanes and novel macrocyclic alkanes which, based on similar δ13C values, are ascribed a common origin. All samples are also characterised by a high cyanobacterial input as indicated by abundant hopanoids. Other biomarker distributions and stable carbon isotopic compositional differences separated the torbanites into three groups, which also correlate to three different palaeogeographical/palaeoclimatic precincts: (i) Group A—southern Africa and eastern Australia (Temi) torbanites which are characterised by a high relative abundance of cyanobacterial and methylotrophic hopanoids, abundant branched hydrocarbons and 13C enriched homologous series of monomethylakanes; (ii) Group B—eastern Australia (Newnes and Glen Davis) torbanites contain relatively high amounts of drimanes and abundant 13C enriched homologous series of monomethylakanes; and (iii) Group C—Scottish torbanites (Torbane Hill and Westfield) contain high relative amounts of cyanobacterial hopanoids, methylotrophic hopanoids and abundant branched aliphatics (but no 13C enriched homologous series of monomethylakanes).