Marianne Baas

Carbon-isotope stratigraphy recorded by the Cenomanian–Turonian Oceanic Anoxic Event: correlation and implications based on three key localities

We present new, detailed carbon-isotope records for bulk carbonate, total organic carbon (TOC) and phytane from three key sections spanning the Cenomanian–Turonian boundary interval (Eastbourne, England; Gubbio, Italy; Tarfaya, Morocco), with the purpose of establishing a common chemostratigraphic framework for Oceanic Anoxic Event (OAE) 2. Isotope curves from all localities are characterized by a positive carbon-isotope excursion of c. 4‰ for TOC and phytane and c. 2.5‰ for carbonate, although diagenetic overprinting appears to have obliterated the primary carbonate carbon-isotope signal in at least part of the Tarfaya section. Stratigraphically, peak δ13C values for all components are followed by intervals of high, near-constant δ13C in the form of an isotopic plateau. Recognition of an unambiguous return to background δ13C values above the plateau is, however, contentious in all sections, hence no firm chemostratigraphic marker for the end-point of the positive isotopic excursion can be established. The stratigraphically consistent first appearance of the calcareous nannofossil Quadrum gartneri at or near the Cenomanian–Turonian boundary as established by ammonite stratigraphy, in conjunction with the end of the δ13C maximum characteristic of the isotopic plateau, provides a potentially powerful tool for delimiting the stratigraphic extent and duration of OAE 2. This Oceanic Anoxic Event is demonstrated to be largely, if not wholly, confined to the latest part of the Cenomanian stage.

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.

Biomarkers as proxies for plant inputs to peats: an example from a sub-boreal ombrotrophic bog

Lipid distributions in modern peat-forming plants were determined and compared to biomarkers recovered from a ca. 2800 year old peat deposited in a Dutch ombrotrophic bog. The peat section spans a well-constrained shift in local hydrology and vegetation, and potential molecular proxies for plant inputs were directly tested against the macrofossil record. Based on the analyses of modern plants, low-molecular-weight (LMW) n-alkanes (C23, C25) and high-molecular-weight n-alkanes (C33) are proposed as potential biomarkers for Sphagnum and non-Sphagnum inputs, respectively. Consistent with this, in the Bargerveen peat core, LMW n-alkane abundances increase and HMW n-alkane abundances decrease coincident with the shift to Sphagnum dominated peat. Additional potential proxies include the abundances of C22 α,ω-alkanedioic acid and phytenyl phytenoate as Sphagnum indicators and the abundances of the triterpenoids taraxer-14-ene and taraxast-20-ene as biomarkers for Ericaceae rootlets.

A comparative study of lipids in Sphagnum species

Abstract The free lipid compositions of twelve species of Sphagnum were determined by capillary gas chromatography/mass spectrometry as part of a study to identify characteristic lipids for Sphagnum in peat bogs. Complex mixtures of lipids, comprised of C28–C29 sterols, C30 triterpenoids, C16–C30 fatty acids, C22–C30 fatty alcohols, C21–C33n-alkanes and isoprenoid and straight-chain wax esters, were identified and quantified. Sterols are dominated by the C29 sterols, 24-ethylcholesta-5,22-dien-3β-ol and 24-ethylcholest-5-en-3β-ol, whilst in some species C28 sterols are also abundant. Summed concentrations of triterpenoids varied widely (20–3500 μg/g dry weight), with only small concentrations present in the mesotrophic species, S. fimbriatum and S. palustre. Ursolic acid is always the major triterpenoid detected. Although absolute concentrations vary significantly, the carbon number distributions of fatty acids, fatty alcohols and n-alkanes are similar in all examined species. Thus, the distributions of these compounds, and especially the dominance of C23 and C25n-alkanes, are a useful chemotaxonomic fingerprint for Sphagnum species and can thus be used in compound-specific 13C and 14C studies of peat bogs.

The behaviour of esters in the presence of tetramethylammonium salts at elevated temperatures; flash pyrolysis or flash chemolysis?☆☆☆

Flash-heating experiments with model compounds such as tomato cutin, octadecyl octadecanoate, hexadecanol, triacontanol and l-α-lecithin with and without tetraalkylammonium salts, in particular tetramethylammonium hydroxide (TMAH), show that we have to discriminate between pyrolysis products sensu stricto and products resulting from bond breaking induced by chemical reagents at elevated temperatures. This discrimination is required to improve the structural elucidation of the macromolecules to be analyzed.

Mid-Cretaceous (Albian–Santonian) sea surface temperature record of the tropical Atlantic Ocean

Paleoclimate records of geologic time periods characterized by extreme global warmth such as the mid-Cretaceous are important for a better understanding of the Earth9s climate system operating in an exceptionally warm mode. Here we applied an organic geochemical proxy (TEX 86 ) on organic matter–rich Albian–Santonian sediments, recovered from Ocean Drilling Program Leg 207 Sites 1258 and 1259 on Demerara Rise, to reconstruct sea surface temperatures (SSTs) in the western equatorial Atlantic. Preceded by a stepwise Cenomanian warming trend (∼31–35 °C), the onset of the Cretaceous thermal maximum coincided here with the Cenomanian-Turonian boundary event. Once established, this extreme warm climate regime, characterized by averaged tropical SSTs close to 35 °C, lasted up to the Turonian-Coniacian transition. Two pronounced cooler intervals (∼2–3 °C) interrupt this otherwise remarkably stable record, providing the first δ 18 O independent evidence for middle Turonian cooling that previously has been attributed to glacioeustatic sea-level lowering. Coniacian SSTs decline stepwise, reaching a minimum in the Santonian (∼32–33 °C), where cooling is most pronounced, presumably concomitant with the first progressive opening of a deep-water passage through the equatorial Atlantic gateway.

Intact Membrane Lipids of "Candidatus Nitrosopumilus maritimus," a Cultivated Representative of the Cosmopolitan Mesophilic Group I Crenarchaeota

ABSTRACT In this study we analyzed the membrane lipid composition of “Candidatus Nitrosopumilus maritimus,” the only cultivated representative of the cosmopolitan group I crenarchaeota and the only mesophilic isolate of the phylum Crenarchaeota. The core lipids of “Ca. Nitrosopumilus maritimus” consisted of glycerol dialkyl glycerol tetraethers (GDGTs) with zero to four cyclopentyl moieties. Crenarchaeol, a unique GDGT containing a cyclohexyl moiety in addition to four cyclopentyl moieties, was the most abundant GDGT. This confirms unambiguously that crenarchaeol is synthesized by species belonging to the group I.1a crenarchaeota. Intact polar lipid analysis revealed that the GDGTs have hexose, dihexose, and/or phosphohexose head groups. Similar polar lipids were previously found in deeply buried sediments from the Peru margin, suggesting that they were in part synthesized by group I crenarchaeota.

Stable Carbon Isotopic Fractionations Associated with Inorganic Carbon Fixation by Anaerobic Ammonium-Oxidizing Bacteria

ABSTRACT Isotopic analyses of Candidatus “Brocadia anammoxidans,” a chemolithoautotrophic bacterium that anaerobically oxidizes ammonium (anammox), show that it strongly fractionates against 13C; i.e., lipids are depleted by up to 47‰ versus CO2. Similar results were obtained for the anammox bacterium Candidatus “Scalindua sorokinii,” which thrives in the anoxic water column of the Black Sea, suggesting that different anammox bacteria use identical carbon fixation pathways, which may be either the Calvin cycle or the acetyl coenzyme A pathway.

Isorenieratane record in black shales from the Paris Basin, France: Constraints on recycling of respired CO2 as a mechanism for negative carbon isotope shifts during the Toarcian oceanic anoxic event

The prominent negative stable carbon isotope excursion in both carbonate and organic carbon recorded in organic-rich sediments deposited during the Toarcian oceanic anoxic event (OAE) has commonly been explained by recycling of 13C-depleted CO2 (the so-called Kuspert model). More recently, the massive release of 13C-depleted methane or other forms of 13C-depleted carbon was also proposed to account for the observed negative δ13C excursions in organic carbon of terrigenous as well as of marine origin. The occurrence of diagenetic products of the carotenoid isorenieratene (isorenieratane and other aryl isoprenoids) in Toarcian black shales has been regarded as supporting evidence for the Kuspert hypothesis as they point to strong stratification of the epicontinental seas. A section of a drill core straddling the Toarcian of the Paris Basin (Cirfontaineen- Ornois) contained intact isorenieratane, providing evidence that photosynthetic green sulphur bacteria were present at the time of deposition, even prior to the OAE. However, the isorenieratane abundances are very low in the section where the negative δ13C excursion in organic carbon and phytane, a chemical fossil derived from chlorophyll, occurs. The abundance of the isorenieratene derivatives increases, once the δ13C records have shifted to more positive values. The δ13C of isorenieratane (generally circa -13.1 ± 0.5‰) indicates that the respired CO2 contribution at the chemocline was low and is thus not likely to be the main cause of the prominent up to 7‰ negative δ13C shift recorded in Toarcian organic carbon records.

Variations in abundances and distributions of isoprenoid chromans and long-chain alkylbenzenes in sediments of the Mulhouse Basin: a molecular sedimentary record of palaeosalinity☆

Abstract The abundances and distribution patterns of mono-, di- and trimethylated 2-methyl-2-(4,8,12-trimethyltridecyl) chromans (MTTCs) and long-chain alkylbenzenes in extracts of marl (66 samples), anhydrite (15) and halite (1) strata of the Salt IV Formation of the Oligocene Mulhouse Basin are reported. The distributions of the methylated MTTCs indicate salinity changes of the upper part of a density stratified water column of the basin. These variations are explained by a tectonically or climatically induced change in the supply of water of relative lower salinity to form a layer overlying the deeper water brine. Hence, it is suggested that mesohaline (3.5–15%) conditions in the surface waters were established as a result of periodic incursions of marine water and subsequent evaporation. Conversely, during periods when the surface water was derived mainly from fresh water from the hinterland, lower average salinity in the surface layer resulted. The distributions of long-chain alkylbenzenes also appear to record these changes.