Aromatic hydrocarbons provide new insight into carbonate concretion formation and the impact of eogenesis on organic matter

Investigations of aromatic biomarkers extracted from carbonate concretions can contribute to characterization of the enhanced microbial activity that mediates carbonate concretion formation. This microbial footprint can be further inferred from the stable isotopic values of carbonate (δ13C) and pyrite (δ34S). Here, we used a combination of GC–MS and GC × GC-ToF-MS to compare the aromatic fractions of two Toarcian carbonate concretions from the H. falciferum ammonite zone of the Posidonia Shale (SW-Germany) and their host sediment. The results revealed that n-alkylated and phytanyl arenes were enhanced in the concretions, relative to the host sediment. These findings support a very early diagenetic (eogenetic) microbial source for alkylated and phytanyl arenes derived from the microbial ecosystem mediating concretion formation. In contrast, aromatic compounds formed by thermal maturation (e.g. polycyclic aromatic hydrocarbons, aromatic steroids, organic sulphur compounds) remained invariant in host rock and concretion samples. When combined with bulk sediment and concretion properties, the distribution of aromatic compounds indicates that eogenetic microbial activity upon concretion growth does not diminish organic matter quality

Root calcretes and uranium-bearing silcretes at sedimentary discontinuities in the miocene of the Madrid Basin (Toledo, Spain)

This paper reports a detailed study of the calcrete and silcrete profiles in the Miocene detrital deposits in the western area of Madrid, at the boundary of two main sedimentary units. The aims of this work were to better understand the pedogenic and diagenetic environments in which these profiles formed and to determine the cause(s) of their enrichment in uranium. Calcrete and silcrete duricrusts are characteristic features of closed continental basins in semiarid climates; this paper discusses the significance of duricrusts as indicators of important change in such basins.The detailed macromorphological, micromorphological, and geochemical study of three duricrust profiles revealed the sequence of pedogenic, vadose, and groundwater processes responsible for their formation. During the first stage of their development, carbonate laminae formed a white “grill-like” structure within the detrital parent materials. The microstructure and macrostructure of the carbonate, which includes alveolar septal structures and needle-fiber calcite, indicates the important role of roots and their associated microorganisms in calcrete formation. Early silicification occurred in the pedogenic-vadose environment affecting the detrital parent material, roots, and calcretes, forming an early silcrete defined by opaline glaebules and silica rhizoliths. The detailed preservation of the cells in the silicified roots denotes the early replacement of root organic matter.The green or green-yellowish fluorescence of the silicified root structures under short-wavelength UV shows their preferential enrichment in uranium. Calcitization and silicification coexisted in the pedogenic vadose environment, leading to several reversible replacements of calcite and silica. Later, the rise of the water table promoted silicification under phreatic conditions, as indicated by the good preservation of the texture of the detrital host rocks and calcretes. Other silcrete textures, such as ovoidal opaline accumulations, intraclasts produced by autobrecchification, and vadose silica cements, indicate later vadose environments, and consequently variations in the water table.The geochemical features of the calcretes and silcretes (major, minor, and rare earth elements) were inherited from their parent materials. The rare-earth-element patterns of some silcretes show them to have a positive Ce anomaly, suggesting that oxidizing conditions reigned during their formation. The good correlation between silica and uranium suggests that the silica phases acquired uranium through the direct silicification of roots that had fixed uranium from organic matter.This study shows that calcrete–silcrete duricrusts provide detailed information regarding the processes occurring in semiarid continental basins. In the studied basin, roots played a key role in both the development of the duricrust profiles and their enrichment in uranium. These duricrusts provide important information for understanding the overall stratigraphy of the studied basin and its large-scale sequential evolution

Comparison of tri-, tetra- and pentacyclic caged hydrocarbons in Australian crude oils and condensates

Thermally stable and biodegradation resistant, the tricyclic and pentacyclic diamondoid caged hydrocarbons are commonly used as source and maturity indicators of oils and potential source-rocks, but similar tetracyclic structures appear to have received much less attention. Using two-dimensional (2D) gas chromatography – time of flight mass spectrometry (GC × GC-TOFMS), 29 Australian crude oils and condensates were analysed for the presence of caged C12H18 tetracyclics such as ethanoadamantane and iceane. The thermodynamically more stable 2,4-ethanoadamantane was identified by comparison with a synthesised authentic standard. Three of its bridgehead methyl-substituted isomers, 6-methylethanoadamantane (6-ME), 1-ME and 2-ME, were tentatively assigned based on mass spectral comparison and relative elution order. Further series of non-bridgehead methyl isomers, plus dimethyl isomers, were also inferred based on mass spectra and 2D elution positions. The tri-, tetra and pentacyclic caged hydrocarbons and their methyl-substituted homologues were semi-quantified in the Australian oils. The potential of a novel index, the methylethanoadamantane index (MEI), based on the ratio of the more stable bridgehead isomers divided by the sum of all the methyl substituted isomers (MEI = S(6-ME + 1-ME + 2-ME)/STotal methylethanoadamantanes), was explored. A significant positive association was found between the MEI and MAI (r2 = 0.203, p < 0.05) and a significant negative association was found between MEI and MDI (r2 = 0.246, p < 0.05). Stronger relationships were found for other commonly applied diamondoid ratio indices, including S Methyl Adamantanes/S Methyl Diamantanes (SMA/SMD) versus SMA/SME (r2 = 0.781, p < 0.0001, n = 26). The relatively low volatility of the ethanoadamantanes compared to the adamantanes and their likely greater resistance to microbial attack than the ethyladamantanes, may make analysis of these compounds a useful addition to the commonly measured diamondoids

From supratidal to subtidal, an integrated characterisation of Carbla Beach shallow microbial mats (Hamelin Pool, Shark Bay, WA): Lipid biomarkers, stable carbon isotopes and microfabrics

Modern microbial mats from Shark Bay are commonly regarded as robust analogues for Precambrian stromatolites. These microbial mats are complex ecosystems that exhibit intense biogeochemical recycling. In this study, a multi-proxy approach (including lipids, compound-specific carbon isotope analysis and petrography) is used to characterise microbial communities in three different types of mats (tufted, pustular and smooth) along the shallowest section of a tidal flat gradient. Cyanobacterial lipids were present in all three mats. Petrographical (optical and scanning electron microscopy) investigations also revealed that ooids in the tufted mat were larger and more common compared to the pustular and smooth mats. Biomarkers specific to sulfate reducing bacteria were detected in all mats. The diatom-specific C25:1highly branched isoprenoid (HBI) alkene was most abundant in the smooth mat. However, imaging revealed that the smooth mat only contained rare diatoms of small size (~10 µm), whereas the pustular mat contained a variety of larger diatoms (~50 µm). The C25:1HBI alkene marker is only produced by four diatom genera, which were most likely more represented in the smooth mat. Additionally, the smooth mat contained a greater contribution from aquatic macrophytes (Paq = 0.38) compared to the shallower mats, which is corroborated by the presence of13C-enriched seagrass lipids (i.e. C29steradiene). In all the mats, a major eukaryotic contribution was revealed via imaging techniques and supported by a high sterol content. This eukaryotic component (e.g. benthic foraminifera, diatoms) can impact the cohesive structure of the mats, the lithification processes and the lipid distribution, potentially complicating comparisons with Precambrian microbialites that were not affected by eukaryotic activity. This study reemphasizes the complexity of microbial ecosystems, and therefore highlights the benefit of multi-proxy approaches to characterise these biological systems