Precambrian–Cambrian boundary interval occurrence and form of the enigmatic tubular body fossil Shaanxilithes ningqiangensis from the Lesser Himalaya of India

The affinity of the Ediacaran fossil Shaanxilithes ningqiangensis and putatively related forms has long been enigmatic; over the past few decades, interpretations ranging from trace fossils to algae to metazoans of uncertain phylogenetic placement have been proposed. Combined morphological and geochemical evidence from a new occurrence of S. ningqiangensis in the Krol and Tal groups of the Lesser Himalaya of India indicates that S. ningqiangensis is not a trace fossil, but rather an organic‐walled tubular body fossil of unknown taxonomic affinity. Specimens consist of compressed organic cylindrical structures, characterized by extended, overlapping or fragmented iterated units. Where specimens intersect, overlapping rather than branching or intraplanar crossing is observed. Lithologic comparisons and sequence stratigraphic data all suggest a late Ediacaran age for the uppermost Krol Group and basalmost Tal Group. By extending the biogeographical distribution of S. ningqiangensis, hitherto confined to the Ediacaran of China and potentially Siberia, to the Precambrian–Cambrian boundary interval of India, this new occurrence of S. ningqiangensis expands the biostratigraphic utility of this enigmatic fossil to the inter‐regional and intercontinental scale. Moreover, study of these new and exceptionally preserved samples may help to significantly constrain the long‐debated problem of Shaanxilithes' affinity, elucidating its ‘problematic’ status and shedding new light upon the ecology and taphonomy of one of the most significant intervals in early life history.     

Vindhyan Akinites: An Indicator of Mesoproterozoic Biospheric Evolution

A wide size range of rod-shaped, ellipsoidal akinites assignable to Archaeoellipsoides are reported from the Newari locality of the Mesoproterozoic Kheinjua Formation of the Semri Group, Vindhyan Supergroup. These akinites of heterocystous cyanobacteria (Archaeoellipsoides) represent the smallest of the forms reported from any other assemblage to date and are well comparable to the akinites of modern bloom forming Anabaena species. Like any other Mesoproterozoic microfossil assemblage, The Newari microfossil assemblage is also dominated by cyanobacterial population, but the presence of Archaeoellipsoides (akinites) or heterocyst forming Nostocales and Stigonematales are rather rarely reported. These fossils set a minimum date for the evolution of derived cyanobacteria, capable of marked cell differentiation, and they corroborate geochemical evidence indicating that atmospheric oxygen level was above 1% of present day level during Mesoproterozoic time. In the presence of oxygen a protected environment for nitrogenase (an oxygen sensitive nitrogen fixing enzyme) is produced by these akinites, which were abundant in coastal communities of Mesoproterozoic shallow marine carbonates. It is therefore interpreted that the presence of Vindhyan’s akinites indicate Mesoproterozoic biospheric evolution.     

Palaeoecology of a billion‐year‐old non‐marine cyanobacterium from the Torridon Group and Nonesuch Formation

A new chroococcalean cyanobacterium is described from approximately 1‐billion‐year‐old non‐marine deposits of the Torridonian Group of Scotland and the Nonesuch Formation of Michigan, USA. Individual cells of the new microfossil, Eohalothece lacustrina gen. et sp. nov., are associated with benthic microbial biofilms, but the majority of samples are recovered in palynological preparations in the form of large, apparently planktonic colonies, similar to extant species of Microcystis. In the Torridonian, Eohalothece is associated with phosphatic nodules, and we have developed a novel hypothesis linking Eohalothece to phosphate deposition in ancient freshwater settings. Extant cyanobacteria can be prolific producers of extracellular microcystins, which are non‐ribosomal polypeptide phosphatase inhibitors. Microcystins may have promoted the retention and concentration of sedimentary organic phosphate prior to mineralization of francolite and nodule formation. This has a further implication that the Torridonian lakes were nitrogen limited as the release of microcystins is enhanced under such conditions today. The abundance and wide distribution of Eohalothece lacustrina attests to the importance of cyanobacteria as oxygen‐producing photoautotrophs in lacustrine ecosystems at the time of the Mesoproterozoic–Neoproterozoic transition.     

Slides,soft-sediment deformations,and mass flows from Proterozoic Lakheri Limestone Formation,Vindhyan Supergroup,central India,and their implications towards basin tectonics

The Neoproterozoic Lakheri Limestone (LL) Member of Vindhyan Supergroup, central India, interpreted as a low-gradient homoclinal ramp, contains a wide range of signatures indicating syn-sedimentary basinal extension and compression. Whereas features like intraformational truncation (slide) surfaces of varying geometry, creep and bedding translation manifest the phases of extension, the compressional events are registered in bed-confined thrusts and outcrop-scale folds. A wide range of outcrop and microscopic deformational features are associated with the sliding events, the expressions of which vary based on their relative position with respect to the slide surface (over- or underlying) and the degree of built-up pore water pressure. The detached sediment mass often evolved in the form of mass flows with rheology varying between cohesive debris flow and low-density turbidity current. In particular, operation of reflected turbiditic flows is suggestive of irregular depositional substrate, induced by curvilinear syn-sedimentary slides in otherwise low-gradient distal shelf platformal setting. The present study intends to relate the observed extensional and compressional features of LL succession with the flexural response of early rifted Vindhyan basement under reversing in-plane stress in its post-rift depositional history. Bipolar NE–SW orientation of the slide planes is well consistent with the proposed rifted configuration of Vindhyan basement. Centimeter- to decimeter-deep slide detachments and equally thick mass-flow beds are indicative of relatively deeper level of necking during the early syn-rift phase of Vindhyan history.     

Mesoproterozoic surface oxygenation accompanied major sedimentary manganese deposition at 1.4 and 1.1 Ga

Manganese (Mn) oxidation in marine environments requires oxygen (O₂) or other reactive oxygen species in the water column, and widespread Mn oxide deposition in ancient sedimentary rocks has long been used as a proxy for oxidation. The oxygenation of Earth's atmosphere and oceans across the Archean-Proterozoic boundary are associated with massive Mn deposits, whereas the interval from 1.8–1.0 Ga is generally believed to be a time of low atmospheric oxygen with an apparent hiatus in sedimentary Mn deposition. Here, we report geochemical and mineralogical analyses from 1.1 Ga manganiferous marine-shelf siltstones from the Bangemall Supergroup, Western Australia, which underlie recently discovered economically significant manganese deposits. Layers bearing Mn carbonate microspheres, comparable with major global Mn deposits, reveal that intense periods of sedimentary Mn deposition occurred in the late Mesoproterozoic. Iron geochemical data suggest anoxic-ferruginous seafloor conditions at the onset of Mn deposition, followed by oxic conditions in the water column as Mn deposition persisted and eventually ceased. These data imply there was spatially widespread surface oxygenation ~1.1 Ga with sufficiently oxic conditions in shelf environments to oxidize marine Mn(II). Comparable large stratiform Mn carbonate deposits also occur in ~1.4 Ga marine siltstones hosted in underlying sedimentary units. These deposits are greater or at least commensurate in scale (tonnage) to those that followed the major oxygenation transitions from the Neoproterozoic. Such a period of sedimentary manganogenesis is inconsistent with a model of persistently low O₂ throughout the entirety of the Mesoproterozoic and provides robust evidence for dynamic redox changes in the mid to late Mesoproterozoic.     

Proterozoic photosynthesis – a critical review

Chlorophyll‐based photosynthesis has fuelled the biosphere since at least the early Archean, but it was the ecological takeover of oxygenic cyanobacteria in the early Palaeoproterozoic, and of photosynthetic eukaryotes in the late Neoproterozoic, that gave rise to a recognizably modern ocean–atmosphere system. The fossil record offers a unique view of photosynthesis in deep time, but is deeply compromised by differential preservation and non‐diagnostic morphologies. The pervasively polyphyletic expression of modern cyanobacterial phenotypes means that few Proterozoic fossils are likely to be members of extant clades; rather than billion‐year stasis, their similarity to modern counterparts is better interpreted as a combination of serial convergence and extinction, facilitated by high levels of horizontal gene transfer. There are few grounds for identifying cyanobacterial akinetes or crown‐group Nostocales in the Proterozoic record. Such recognition undermines the results of various ancestral state reconstruction analyses, as well as molecular clock estimates calibrated against demonstrably problematic Proterozoic fossils. Eukaryotic organisms are likely to have acquired their (stem‐group nostocalean) photoendosymbionts/plastids by at least the Palaeoproterozoic, but remained ecologically marginalized by incumbent cyanobacteria until the late Neoproterozoic appearance of suspension‐feeding animals.     

The importance of Precambrian microfossils for modern biostratigraphy

A new model of the distribution of Proterozoic microorganisms is developed, based on studies of Riphean and Vendian silicified and organic-walled microfossils from the reference sections of northern Eurasia, and on their comparison with other known microfossil assemblages. Within the interval from 2.0 to 0.535 Ga, seven successive informal global microphytological units (referred to as proterohorizons) are determined: (1) Labradorian proterohorizon occupies the upper part of the Lower Proterozoic (Paleoproterozoic), 2.0–1.65 Ga; (2) Anabarian proterohorizon, Lower Riphean-lower Middle Riphean (lower and middle Mesoproterozoic), 1.65–1.2 Ga; (3) Turukhanian proterohorizon, upper Middle Riphean (upper Mesoproterozoic), 1.2–1.03 Ga; (4) Uchuromayan proterohorizon, lower Upper Riphean (lower Neoproterozoic), 1.03–0.85 Ga; (5) Yuzhnouralian proterohorizon, upper Upper Riphean (upper Neoproterozoic without Ediacaran); (6) Amadeusian proterohorizon, Lower Vendian (Ediacaran), 0.6–0.55 Ga; and (7) Belomoryan proterohorizon, Upper Vendian (Ediacaran), 0.55–0.535 Ga.     

The Ediacaran Aspidella‐type impressions in the Jinxian successions of Liaoning Province,northeastern China

Macroscopic impression fossils from the Xingmincun Formation of the Jinxian Group, Liaoning Province of northeastern China, are identified as members of the Aspidella plexus of Ediacaran age. This is the first recognition of the taxon in the Liaoning Province, although such fossils have been previously recorded in the succession, but were referred to as new species and relegated to an earlier Neoproterozoic age. A revision of the taxonomic interpretation and relative age estimation of the previous record is provided, as well as an evaluation of abiotic vs. biotic processes that could produce similar structures to studied impressions. The mode of preservation of the fossils is considered from a biochemical point of view and along with the properties of organic matter in the integument of soft‐bodied metazoans. The selective preservation of the Ediacaran organisms, including metazoans, as impressions (moulds and casts) against the organically preserved contemporaneous cyanobacterial and algal microfossils, and an exceptionally small number of terminal Ediacaran metazoan fossils (Sabellidites, Conotubus and Shaanxilithes), demonstrates the non‐resistant characteristics and the very different biochemical constitution of the Ediacaran metazoans compared with those that evolved in the Cambrian and after. The refractory biomacromolecules in cell walls of photosynthesizing microbiota (bacterans, cutans, algaenan and sporopollenin groups) and in the chitinous body walls of Sabellidites contrast sharply with the labile biopolymers in Ediacaran metazoans known only from impressions. The newly emerging biosynthesis of resistant biopolymers in metazoans (chitin and collagen groups) initiated by the annelids at the end of Ediacaran and fully evolved in Cambrian metazoans, considered with the ability to biomineralize, made their body preservation possible. The Chengjiang and Burgess Shale metazoans show evidence of this new biochemistry in body walls and cuticles, and not only because of the specific taphonomic window that enhanced their preservation.     

Absence of biomarker evidence for early eukaryotic life from the Mesoproterozoic Roper Group: Searching across a marine redox gradient in mid‐Proterozoic habitability

By about 2.0 billion years ago (Ga), there is evidence for a period best known for its extended, apparent geochemical stability expressed famously in the carbonate–carbon isotope data. Despite the first appearance and early innovation among eukaryotic organisms, this period is also known for a rarity of eukaryotic fossils and an absence of organic biomarker fingerprints for those organisms, suggesting low diversity and relatively small populations compared to the Neoproterozoic era. Nevertheless, the search for diagnostic biomarkers has not been performed with guidance from paleoenvironmental redox constrains from inorganic geochemistry that should reveal the facies that were most likely hospitable to these organisms. Siltstones and shales obtained from drill core of the ca. 1.3–1.4 Ga Roper Group from the McArthur Basin of northern Australia provide one of our best windows into the mid‐Proterozoic redox landscape. The group is well dated and minimally metamorphosed (of oil window maturity), and previous geochemical data suggest a relatively strong connection to the open ocean compared to other mid‐Proterozoic records. Here, we present one of the first integrated investigations of Mesoproterozoic biomarker records performed in parallel with established inorganic redox proxy indicators. Results reveal a temporally variable paleoredox structure through the Velkerri Formation as gauged from iron mineral speciation and trace‐metal geochemistry, vacillating between oxic and anoxic. Our combined lipid biomarker and inorganic geochemical records indicate at least episodic euxinic conditions sustained predominantly below the photic zone during the deposition of organic‐rich shales found in the middle Velkerri Formation. The most striking result is an absence of eukaryotic steranes (4‐desmethylsteranes) and only traces of gammacerane in some samples—despite our search across oxic, as well as anoxic, facies that should favor eukaryotic habitability and in low maturity rocks that allow the preservation of biomarker alkanes. The dearth of Mesoproterozoic eukaryotic sterane biomarkers, even within the more oxic facies, is somewhat surprising but suggests that controls such as the long‐term nutrient balance and other environmental factors may have throttled the abundances and diversity of early eukaryotic life relative to bacteria within marine microbial communities. Given that molecular clocks predict that sterol synthesis evolved early in eukaryotic history, and (bacterial) fossil steroids have been found previously in 1.64 Ga rocks, then a very low environmental abundance of eukaryotes relative to bacteria is our preferred explanation for the lack of regular steranes and only traces of gammacerane in a few samples. It is also possible that early eukaryotes adapted to Mesoproterozoic marine environments did not make abundant steroid lipids or tetrahymanol in their cell membranes.     

Proterozoic carbonaceous remains from the Chorhat Sandstone: oldest fossils of the Vindhyan Supergroup, Central India

According to new radiometric determinations, the Chorhat Sandstone (Kheinjua Formation, Semri Group) is much older (1628 and 1632 Ma) than previously thought. This makes the carbonaceous fossils here described not only the oldest, but also the only reliably dated fossil record of the Vindhyan Supergroup, India. The new fossils represent a Chuaria-Tawuia assemblage with other carbonaceous remains of uncertain biologic affinities. Their size range (0.02-3.5 mm) indicates a transitional phase in the evolution from micro- to mega scopic organisms. Alternatively, low oxygen levels may have induced small size.     

Mesoproterozoic Archaeoellipsoidès: akinetes of heterocystous cyanobacteria

The genus Archaeoellipsoides Horodyski & Donaldson comprises large (up to 135 μ long) ellipsoidal and rod-shaped microfossils commonly found in silicified peritidal carbonates of Mesoproterozoic age. Based on morphometric and sedimentary comparisons with the akinetes of modern bloom-forming Anabaena species, Archaeoellipsoides is interpreted as the fossilized remains of akinetes produced by planktic heterocystous cyanobacteria. These fossils set a minimum date for the evolution of derived cyanobacteria capable of marked cell differentiation, and they corroborate geochemical evidence indicating that atmospheric oxygen levels were well above 1% of present day levels 1,500 million years ago. Akinetes, atmospheric oxygen, cyanobacteria, heterocyst, microbial fossils, nitrogen fixation, peritidal, Proterozoic.     

Microfossils from the Krol ‘A’ of the Lesser Himalaya,India: Additional supporting data for its early Ediacaran age

An assemblage of microfossils of moderate diversity, with the remarkable occurrence of ECAP acritarchs, is reported from the Ediacaran Krol ‘A’ (= the Mahi Formation) succession of Lesser Himalaya, India. Microfossils occur in the chert nodules exposed in Solan district, Himachal Pradesh. Two microfossils, Barogophycus symmetricus n. gen. n. sp. and Botominella lineata are new to the well-established Krol assemblage. The paper concentrates predominantly on fossil eukaryotic filamentous and coccoidal micro-organism of simple morphology. The assemblage is dominated by remains of prokaryotic cyanobacteria and demonstrates the diversity achieved by microorganisms at the beginning of the Ediacaran Period in the aftermath of the Marinoan glaciation. Filamentous and coccoidal microorganisms differ in taxonomic composition from the Mesoproterozoic microbiotas. The Krol microfossils assemblage has biostratigraphic potential and usefulness in broadly demarcating the different levels of the Ediacaran strata. Because of the presence of ECAP acritarchs, along with the appearance of other eukaryotic microorganisms of filamentous and coccoidal morphology, the general level of the Krol ‘A’ assemblage is considered as Ediacaran.     

Topography and tectonics of the Taconic outer trench slope as revealed through gradient analysis of fossil assemblages

Similarities in stratigraphic sequence, syndepositional topography and tectonics, and regional tectonic setting between the Mohawk Valley, New York, in the Middle Ordovician and the Australian flank of the Timor Trough today support the idea that the Taconic Foreland Basin orginated in an arc-continent collision. The Trenton Group strata studied - a roughly four million year long sequence correlated by bentonite beds along an 83 km downslope transect in the central Mohawk Valley - represent the continental shelf and outer trench slope. Bank limestones pass upward and downslope into deep water black shales in a manner reminiscent of the lateral and vertical sedimentary sequence on the Sahul Shelf and Australian flank of the Timor Trough. In studying topography, relative depth was measured through reciprocal averaging ordination of benthic macroinvertebrate fossil assemblages. Downslope bathymetric profiles show the development of a horst and graben topography coincident with regional tilting and transgression - a pattern associated with downward flexure of the Australian Plate approaching Timor. The net rate of transgression corresponds to an arc-continent convergence rate on the order of 2 cm per year.     

New ammonoid taxa from the Lower Cretaceous Giumal Formation of the Tethyan Himalaya (Northern India)

Knowledge of the Early Cretaceous ammonoids of the NW‐Himalayas was poor until recent discoveries. Intense sampling from the Giumal Formation exposed near the village of Chikkim (Spiti Valley, Himachal Pradesh, India) led to the recognition of a new Early Cretaceous ammonoid fauna. The succession consists of arenitic sandstone interbedded with shale that was deposited by turbidity currents on an unstable shelf in the Early Cretaceous. Ammonoids have been obtained only from sandstone beds in the lower one‐third and close to the top of the c. 350‐m‐thick section. Eight new ammonoid taxa (1 genus and 7 species) are described: Sinzovia franki sp. nov. (rare), Giumaliceras giumaliense gen. et sp. nov. (abundant), Giumaliceras bhargavai gen. et sp. nov. (rare), Neocomites (Eristavites) platycostatiformis sp. nov. (rare), Cleoniceras oberhauseri sp. nov. (abundant), Australiceras himalayense sp. nov. (rare) and Deshayesites fuchsi sp. nov. (rare). Sinzovia and Deshayesites are reported for the first time from the Tethyan Himalaya. According to the biostratigraphic relevance of some ammonoid taxa described here, the age of the Giumal Formation can be constrained from Berriasian (Giumaliceras assemblage) to Aptian (Cleoniceras assemblage). The discovery of the new fauna substantiates the significance of the Giumal Formation around Chikkim and facilitates comparison with faunal assemblages from other regions in the Tethys Ocean and beyond.     

Integrated sedimentological,ichnological and sequence stratigraphical studies of the Koti Dhaman Formation (Tal Group), Nigali Dhar Syncline,Lesser Himalaya,India: paleoenvironmental,paleoecological, paleogeographic significance

Abstract

Integrated ichnology, sedimentology and sequence stratigraphy of the Lower Quartzite Member to the Arkosic Sandstone Member of the Koti Dhaman Formation (Cambrian Series 2, Stage 4), Tal Group, Nigali Dhar Syncline, Lesser Himalayan lithotectonic zone are presented. Trilobite traces of Gondwanan affinity i.e., Cruziana salomonis, Cruziana fasciculata, Rusophycus dispar and Rusophycus burjensis are recorded along with Arenicolites isp. and Skolithos isp. from the Lower Quartzite Member. A rich and diverse ichnoassemblage attributed to the Cruziana ichnofacies is described for the first time from the Arkosic Sandstone Member of the same formation. Seven ichnofossil assemblages, i.e., Cruziana-Rusophycus, Planolites-Palaeophycus, Cruziana problematica, Diplichnites, Cochlichnus anguineus, Bergaueria perata and Psammichnites gigas have been recognized in the Lower Quartzite to Arkosic Sandstone members of the Koti Dhaman Formation. Seven sedimentary facies i.e., sandstone–shale facies (FT1), cross-bedded (trough and planar) sandstone (FT2), bedded sandstone facies (FT3), shale facies (FT4), shale–sandstone facies (FT5), shale-rippled sandstone facies (FT6) and planar and trough cross-laminated sandstone (FT7) and four facies associations FA1-FA4 are identified in the Koti Dhaman Formation. The formation contains shallowing upward parasequences of a tidal flat complex. Overall, two major events are recognized: i) the break in sedimentation between the Lower Quartzite Member and the overlying Shale Member probably related to forced-regressive event and ii) the facies shift from FT6 to FT7 of the Arkosic Sandstone Member represents an erosive transgressive event; the surface is interpreted as wave ravinement surface, which also serves as a sequence boundary. Integrated ichnology, sedimentology and sequence stratigraphic studies indicate that the Lower Quartzite Member was deposited in a shallow subtidal sand sheet complex and tidal flat complex; the Shale Member was deposited in a mud flat setting of a tidal flat complex, and the Arkosic Sandstone Member in a mixed-flat (tidal flat complex) to sand sheet complex front and margin (subtidal sand sheet complex). Overall, the lower to middle part of the Koti Dhaman Formation represents a tide-dominated shallow subtidal–intertidal to mud-flat subenvironments of the tidal flat complex. A palaeogeographic reconstruction of lower Cambrian (516–514?Ma) is presented based on the distribution of trilobite traces from the Lesser Himalaya and the Bikaner–Nagaur area of Peninsular India (eastern Gondwana), Egypt, Jordan, Turkey (western Gondwana) and Canada (Avalonia).     

Paleogeography and sediment dispersal patterns of the proterozoic Bhander Group,western India

Directionals, isopachs, lateral and vertical facies changes, and stratigraphic relationships were employed to reconstruct the shoreline, paleoslope and dispersal patterns of the Porterozoic sediments comprising the Bhander Group in the Mandalgarh-Singoli area of the western part of the Great Vindhyan Basin. An east-southeast—west-northwest shoreline and a north-northeast paleoslope was inferred. Sediment dispersal patterns were recognized by relating paleocurrents to the inferred shoreline and paleoslope. The onshore offshore and cross-slope/longshore dispersal patterns so obtained for the Bhander Group are in agreement with its origin in coastal environments. Earlier workers postulated a northwest regional paleoslope of the Vindhyan Basin on the assumption that the predominant paleocurrent pattern reflected the paleoslope. This study, however, suggests that the predominant paleocurrent pattern in the study area is independent of the paleoslope and represents the longshore sediment dispersal.     

Biostratigraphic control of the latest-Ordovician glaciogenic unconformity in Alnif (Eastern Anti-Atlas, Morocco), based on brachiopods

In the Alnif region of the Eastern Atlas (Morocco), seven fossiliferous horizons within the Lower-Ktaoua and Upper-Tiouririne formations (Ktaoua Group), as well as in the glaciomarine microconglomeratic shales of the Upper Formation of the Second-Bani Group have yielded biostratigraphically significant brachiopods and other taxa, such as trilobites and echinoderms. Several brachiopod species with short stratigraphic ranges, well-known in south-western Europe, allow a precise chronostratigraphic control of a succession that displays important lateral lithological and facies changes, when compared with the type sections in the Central Anti-Atlas. They have also permitted a better consensus between the macrofauna-determined age and that based on micropaleontological analyses. For the first time, the occurrence of a Hirnantia Fauna within the microconglomeratic shales of the Upper Formation of the Second Bani Group is reported. The biostratigraphic conclusions restrict the age of the Latest Ordovician glaciation to the early Hirnantian.     

Ambient Inclusion Trails (AITs) from the Neoproterozoic Gangolihat Formation,Lesser Himalaya,India

Discovery of well-preserved Ambient Inclusion Trails (AITs) is reported for the first time in India from stromatolitic dolomite unit of the Neoproterozoic Gangolihat Formation, Kumaun Lesser Himalaya. AITs are distinct microtubular structures formed by migration of a mineral in a rock substrate. They exhibit several noncrystallographic morphologies such as curved and helical types among others. Mode of field occurrence, association, and petrographic textures suggest that AITs are indigenous to the host rock. Based on the physico-chemical conditions necessary for the formation of AITs, involvement of a biological process is considered for the genesis of AITs in the Gangolihat Formation.     

Identification of lanostanes,A-ring methylated steranes and secosteranes in late Neoproterozoic crude oils by GC×GC-TOFMS: New insights into molecular taphonomy of steroids

The late Neoproterozoic marine succession (Marwar Supergroup) deposited in the Bikaner-Nagaur Basin in western India is an excellent provenance to study steroid biomarkers. Traditional one-dimensional gas chromatography mass spectrometry (GC–MS) and metastable reaction monitoring (MRM) transitions have been previously employed for routine biomarker analyses of crude oils and sediments. The present study with GC×GC-TOFMS (time-of-flight mass spectrometer) demonstrates an improved distribution of the sterane compounds segregated from the co-eluting n-alkanes, cycloalkanes and triterpanes in terminal Proterozoic crude oils. The steranes identified here offer novel insights into the molecular taphonomic alteration of eukaryotic lipids during the late Neoproterozoic. The presence of lanostane and 3β alkyl steranes is probably indicative of a depositional environment stressed by high salinity. To the best of our knowledge, this is the oldest known record of lanostane steroids found in the geosphere. Secosteranes with an open C-ring form as a result of diagenetic cleaving of carbon–carbon bonds. The concomitant presence of 2α-, 3β - and 4α-methyl steranes (A-ring methylated steranes) reflects specific biological input and a distinct palaeo-depositonal environment. The 3β - and 2α-methyl steranes probably form by migration of methyl substituents within the steroid structure. The recognition of a diverse range of steroid compounds by GC×GC-TOFMS advocates its excellent analytical potential in the study of natural products in geological samples. Hence, this state-of-the-art technology will be worth using for re-evaluating and investigating hydrocarbon biomarkers in order to minimize the gaps that exist in the understanding of biotic evolution over geological time scales.     

Post‐tectonic limitations on Early Devonian (Emsian) reef development in the Gobi‐Altai region,Mongolia

Pellegrini, A.F.A., Soja, C.M. & Minjin, C. 2011: Post‐tectonic limitations on Early Devonian (Emsian) reef development in the Gobi‐Altai region, Mongolia. Lethaia, Vol. 45, pp. 46–61. This study investigates a Lower Devonian (Emsian) carbonate sequence from the Chuluun Formation where it is exposed in the Gobi‐Altai region of southern Mongolia. Quantification of abundance patterns across guild, morphotype and general taxonomic levels was based on stratigraphical and thin‐section analyses. Comparison with other Emsian carbonate platforms allowed the factors that influenced community development in the wake of a tectonic perturbation to be determined. Our evidence reveals that potential reef‐building biotas preserved in the Chuluun Formation experienced rapid colonization of a newly submerged carbonate platform following an episode of tectonic uplift and the development of a coastal alluvial fan. Although critical reef‐building organisms were present, colonial corals and stromatoporoids exhibited limited vertical growth and showed no significant lateral expansion of individuals or biotic assemblages. Nor did those taxa experience significant increases in abundance, density, or size. We conclude that incomplete succession and the lack of reef development occurred most likely because of an unsuitable substrate, limited accommodation space and isolation that reduced colonization potential. □Community succession, Emsian, palaeoecology, reef suppression.