Glycolaldehyde, methyl formate and acetic acid adsorption and thermal desorption from interstellar ices

We have undertaken a detailed investigation of the adsorption, desorption and thermal processing of the astrobiologically significant isomers glycolaldehyde, acetic acid and methyl formate. Here, we present the results of laboratory infrared and temperature programmed desorption (TPD) studies of the three isomers from model interstellar ices adsorbed on a carbonaceous dust grain analogue surface. Laboratory infrared data show that the isomers can be clearly distinguished on the basis of their infrared spectra, which has implications for observations of interstellar ice spectra. Laboratory TPD data also show that the three isomers can be distinguished on the basis of their thermal desorption behaviour. In particular, TPD data show that the isomers cannot be treated the same way in astrophysical models of desorption. The desorption of glycolaldehyde and acetic acid from water-dominated ices is very similar, with desorption being mainly dictated by water ice. However, methyl formate also desorbs from the surface of the ice, as a pure desorption feature, and therefore desorbs at a lower temperature than the other two isomers. This is more clearly indicated by models of the desorption on astrophysical time-scales corresponding to the heating rate of 25 and 5 M⊙ stars. For a 25 M⊙ star, our model shows that a proportion of the methyl formate can be found in the gas phase at earlier times compared to glycolaldehyde and acetic acid. This has implications for the observation and detection of these molecules, and potentially explains why methyl formate has been observed in a wider range of astrophysical environments than the other two isomers

Trapping and desorption of complex organic molecules in water at 20 K

The formation, chemical and thermal processing of complex organic molecules (COMs) is currently a topic of much interest in interstellar chemistry. The isomers glycolaldehyde, methyl formate and acetic acid are particularly important because of their role as pre-biotic species. It is becoming increasingly clear that many COMs are formed within interstellar ices which are dominated by water. Hence the interaction of these species with water ice is crucially important in dictating their behaviour. Here we present the first detailed comparative study of the adsorption and thermal processing of glycolaldehyde, methyl formate and acetic acid adsorbed on and in water ices at astrophysically relevant temperatures (20 K). We show that the functional group of the isomer dictates the strength of interaction with water ice, and hence the resulting desorption and trapping behaviour. Furthermore, the strength of this interaction directly affects the crystallization of water, which in turn affects the desorption behaviour. Our detailed coverage and composition dependent data allow us to categorize the desorption behaviour of the three isomers on the basis of the strength of intermolecular and intramolecular interactions, as well as the natural sublimation temperature of the molecule. This categorization is extended to other C, H and O containing molecules in order to predict and describe the desorption behaviour of COMs from interstellar ices

Cochleatina: an enigmatic Ediacaran–Cambrian survivor among small carbonaceous fossils (SCFs)

Conspicuously few body-fossil taxa are known to span the Ediacaran-Cambrian boundary, a pattern usually taken to signal either a terminal Proterozoic mass extinction, or taphonomic failure. We draw attention to the emerging record of small carbonaceous fossils (SCFs), which exhibit continuous preservation spanning this critical interval. Here we focus on the enigmatic SCF Cochleatina, a morphologically complex coil-shaped problematicum that ranges across the Ediacaran-Cambrian divide, and potentially among the oldest fossil occurrences of metazoans. We report new material of Cochleatina canilovica from the Ediacaran of Estonia and Ukraine, which offers new characters for assessing its palaeobiology. Significantly, new specimens include sets of three-alike triplets of Cochleatina adhering to organic sheets, suggesting a clustering habit, or grouping of elements within an individual during life; an important step in constraining the morphology and ecology of this Ediacaran-Cambrian problematicum. We present revised systematic descriptions for Cochleatina and C. canilovica, and critically evaluate previous biological interpretations, drawing comparisons with metazoan, algal and protistan analogues. We reject hypotheses supporting Cochleatina as a metazoan mouthpart, and suggest new grounds for viewing Cochleatina as a potential multicomponent predator that trapped protists among microbial mats. Most occurrences are from Baltica, but we synthesise sporadic reports of Cochleatina from other palaeocontinents, pointing to its global distribution during the latest ~10 Myr of the Ediacaran and majority of the earliest Cambrian Fortunian Stage. As a rare example of an ‘Ediacaran survivor’, Cochleatina highlights the broader significance of SCFs as a novel means of tracking evolutionary patterns through the Proterozoic-Phanerozoic transition

Geometric morphometrics of macro- and meiofaunal priapulid pharyngeal teeth provides a proxy for studying Cambrian “tooth taxa”

Priapulids are marine, benthic ecdysozoan worms that feed using a distinctive toothed pharynx. While only a handful of lineages have survived to the present day, the Cambrian priapulid stem group left behind a rich record of articulated body fossils and characteristic trace fossils in the form of burrows. Recently, the fossil record of isolated priapulid cuticular elements including pharyngeal teeth has gained increased attention as a means of revealing cryptic priapulid taxa otherwise unknown among macrofossils. In this study, we focus on the ecological implications of shape variation in the teeth of extant and extinct priapulids, which display substantial morphological differences between taxa and life stages. We define a landmarking scheme to capture shape variation in priapulid teeth and apply it to our dataset, which includes a breadth of tooth specimens from extant macrofaunal and meiofaunal lineages alongside numerous Cambrian priapulid teeth preserved as isolated small carbonaceous fossils. Through ordination of the principal components of shape, we explore the priapulid tooth morphospace and find evidence that its occupancy has expanded since Cambrian times, indicating a corresponding expansion of the group’s ecological niche. We also employ our geometric morphometric approach to make linear discriminant analysis-based taxon assignments based on tooth morphology, which can be helpful for classifying enigmatic “tooth taxa” known solely from fossil teeth. Finally, we use discriminant analysis to study tooth shapes from a functional perspective, considering known ecologies to characterize the ecological functions of unclassified isolated teeth

Animal–plant interactions in a Middle Permian permineralised peat of the Bainmedart Coal Measures, Prince Charles Mountains, Antarctica

AbstractEvidence for invertebrate feeding on glossopterid gymnosperms is documented from Middle Permian silicified peats of the Prince Charles Mountains, Antarctica, in the form of coprolites occurring both free in the peat matrix and clustered within excavations in roots, aerial wood and leaves. Observations of coprolites in thin-sections of the peats and from scanning electron microscopy of examples extracted via bulk maceration reveal nine morphotypes distinguished by size, shape, surface texture and contents. These include coprolites with coarse plant debris, spirally ornamented coprolites, coprolites containing spore/pollen remains and fern sporangia, coprolites within Glossopteris leaves, an ellipsoidal morphotype within a fern sporangium, large isolated coprolites between matted leaves, clustered forms filling galleries inside Vertebraria roots and Australoxylon wood, forms with coarse indeterminate constituents and others with fungal contents. Other faunal evidence is limited to indeterminate arthropod exoskeleton fragments. Collectively, the coprolites within the permineralised peat from the Prince Charles Mountains document the presence of diverse feeding behaviours including stem feeding, sporangial feeding, palynivory, root feeding and mycophagy. The first evidence of invertebrate feeding traces in Vertebraria (glossopterid) roots is identified. These findings indicate that herbivory by invertebrates in the high-latitude Permian forest-mire ecosystems of Antarctica was more intense and diverse than previous studies have reported, and affected all parts of the Glossopteris plant, together with components of associated herbaceous taxa

Oxygen minimum zones in the early Cambrian ocean

The relationship between the evolution of early animal communities and oceanic oxygen levels remains unclear. In particular, uncertainty persists in reconstructions of redox conditions during the pivotal early Cambrian (541-510 million years ago, Ma), where conflicting datasets from deeper marine settings suggest either ocean anoxia or fully oxygenated conditions. By coupling geochemical palaeoredox proxies with a record of organic-walled fossils from exceptionally well-defined successions of the early Cambrian Baltic Basin, we provide evidence for the early establishment of modern-type oxygen minimum zones (OMZs). Both inner-and outer-shelf environments were pervasively oxygenated, whereas mid-depth settings were characterised by spatially oscillating anoxia. As such, conflicting redox signatures recovered from individual sites most likely derive from sampling bias, whereby anoxic conditions represent mid-shelf environments with higher productivity. This picture of a spatially restricted anoxic wedge contrasts with prevailing models of globally stratified oceans, offering a more nuanced and realistic account of the Proterozoic-Phanerozoic ocean transition.This work was funded by NERC (NE/K005251/1). SWP acknowledges support from a Royal Society Wolfson Research Merit Award

Predicting vapor liquid equilibria using density functional theory: a case study of argon

Predicting vapor liquid equilibria (VLE) of molecules governed by weak van der Waals (vdW) interactions using the first principles approach is a significant challenge. Due to the poor scaling of the post Hartree-Fock wave function theory with system size/basis functions, the Kohn-Sham density functional theory (DFT) is preferred for systems with a large number of molecules. However, traditional DFT cannot adequately account for medium to long range correlations which are necessary for modeling vdW interactions. Recent developments in DFT such as dispersion corrected models and nonlocal van der Waals functionals have attempted to address this weakness with a varying degree of success. In this work, we predict the VLE of argon and assess the performance of several density functionals and the second order Møller-Plesset perturbation theory (MP2) by determining critical and structural properties via first principles Monte Carlo simulations. PBE-D3, BLYP-D3, and rVV10 functionals were used to compute vapor liquid coexistence curves, while PBE0-D3, M06-2X-D3, and MP2 were used for computing liquid density at a single state point. The performance of the PBE-D3 functional for VLE is superior to other functionals (BLYP-D3 and rVV10). At T = 85 K and P = 1 bar, MP2 performs well for the density and structural features of the first solvation shell in the liquid phase