Steven D. Andrews

Size and composition of membrane protein clusters predicted by Monte Carlo analysis

Biological membranes contain a high density of protein molecules, many of which associate into two-dimensional microdomains with important physiological functions. We have used Monte Carlo simulations to examine the self-association of idealized protein species in two dimensions. The proteins have defined bond strengths and bond angles, allowing us to estimate the size and composition of the aggregates they produce at equilibrium. With a single species of protein, the extent of cluster formation and the sizes of individual clusters both increase in non-linear fashion, showing a “phase change” with protein concentration and bond strength. With multiple co-aggregating proteins, we find that the extent of cluster formation also depends on the relative proportions of participating species. For some lattice geometries, a stoichiometric excess of particular species depresses cluster formation and moreover distorts the composition of clusters that do form. Our results suggest that the self-assembly of microdomains might require a critical level of subunits and that for optimal co-aggregation, proteins should be present in the membrane in the correct stoichiometric ratios.

Periodicity determination of lacustrine cycles from the Devonian of Northern Scotland

Synopsis The Middle Old Red Sandstone (mid Devonian) of the Orcadian Basin in northern Scotland is composed predominantly of cyclic lacustrine deposits. The cyclicity recorded reflects regular fluctuations in lake levels that have been attributed to climatic variability resulting from regular variations in the Earths orbital parameters (Milankovitch cycles). However, previous estimates and calculations of the periodicities of these events have produced highly variable results, and no consistent approach has emerged. Variability in the estimated thicknesses of the sequences and regular revisions of the Devonian timescale, combined with poorly defined absolute dating of stratigraphic boundaries, have hindered accurate calculations of cycle periodicities. This study reviews the factors involved in the calculation of published periodicities for the lacustrine cycles of the Orcadian Basin and considers the effect of recent revisions to the Devonian timescale and stratigraphic thicknesses. When considered alongside detailed examination of well exposed sections from Easter Ross, Caithness and Orkney, the calculation of cycle periodicities provides consistent results reflecting the dominance of Precessional (19 886 years) and modulating Eccentricity (100 000 years) cycles calculated for the Devonian.

High Molybdenum availability for evolution in a Mesoproterozoic lacustrine environment

Trace metal data for Proterozoic marine euxinic sediments imply that the expansion of nitrogen-fixing cyanobacteria and diversification of eukaryotes were delayed while the availability of bioessential metals such as molybdenum in the ocean was limited. However, there is increasing recognition that the Mesoproterozoic evolution of nitrogen fixation and eukaryotic life may have been promoted in marginal marine and terrestrial environments, including lakes, rather than in the deep ocean. Molybdenum availability is critical to life in lakes, just as it is in the oceans. It is, therefore, important to assess molybdenum availability to the lacustrine environment in the Mesoproterozoic. Here we show that the flux of molybdenum to a Mesoproterozoic lake was 1 to 2 orders of magnitude greater than typical fluxes in the modern and ancient marine environment. Thus, there was no barrier to availability to prevent evolution in the terrestrial environment, in contrast to the nutrient-limited Mesoproterozoic oceans.

Climatic and eustatic controls on the development of a Late Triassic source rock in the Jameson Land Basin, East Greenland

Complex environmental controls have influenced deposition of the Gråklint Beds, a prospective oil- and gas-prone Late Triassic (Mid-Carnian) source rock in the Jameson Land Basin, East Greenland. The identification of a Late Triassic source rock is significant for hydrocarbon exploration in the North Atlantic region. Detailed sedimentological analysis, biostratigraphy and geochemical analysis provide insights into the controls on source rock development and have wider implications for palaeoclimatic trends and palaeogeographical reconstructions of the North Atlantic at this time. The Gråklint Beds were deposited in a predominantly lacustrine setting during a phase of climatic cooling that can be ascribed to the ‘Mid-Carnian Pluvial Event’. This further extends the evidence for the global effect of such climatic perturbations and furthermore highlights the potential for the use of climatic events for global and regional correlation between varying environmental settings. Evidence is also recorded for marine ingression, which resulted in the precipitation of magadiite (NaSi7(OH)3.3H2O) and the brief influx of a marine fauna. This provides the most southerly record of marine influence from the Boreal Ocean at this time and has important implications for palaeoenvironmental reconstructions and correlation in the region.

Palaeoenvironmental significance of lacustrine stromatolite forms from the Middle Old Red Sandstone of the Orcadian Basin

The form of microbialite accumulations is largely the product of environmental processes and microbial activity. Recent work has largely concentrated on the identification and classification of microbialites with little attention being paid to their environmental significance. This study describes the environmental distribution of the varied stromatolite forms recorded from the Middle Old Red Sandstone sequences of the Orcadian Basin. Comparisons are made with Triassic examples from East Greenland and modern microbialite accumulations. The Middle Old Red Sandstone of Northern Scotland was deposited in a predominantly lacustrine setting. Stromatolites are recorded from both steep basin margin coincident settings and lower gradient settings where the lake margin was distant from the basin margin. In the latter case stromatolite development is largely restricted to transgressive lacustrine sequences, during the deposition of which reduced rates of sedimentation resulted from the migration of sediment input points towards the basin margin. Stromatolite sheets, domal mounds, aligned mounds (and associated runnels), sand-cored stromatolite mounds and reefal stromatolite accumulations have been identified representing the transition from more sheltered to more exposed environments. In basin margin coincident settings stromatolite accumulation is restricted to areas of low sedimentation where microbialites coat boulders and pebbles. A model for the palaeoenvironmental distribution of the stromatolite forms described is proposed and is shown to be applicable to similar examples from the Triassic of East Greenland. It is suggested that this model may be more widely applicable to stromatolitic accumulations in similar lacustrine settings through large portions of the Phanerozoic.

A 2.3 million year lacustrine record of orbital forcing from the Devonian of northern Scotland

Detailed sedimentological examination of onshore sections through cyclic lacustrine deposits of the Middle Devonian succession of northern Scotland has been augmented with offshore data to allow the construction of a continuous 2.3 myr record of orbital forcing. These data provide an important record of climatic variance through the Devonian, which, by comparison with other records from this time, can significantly improve our understanding of the Devonian global climatic system. Precessional and eccentricity cycles are shown to be dominant by the direct measurement and extrapolation of depositional rates in well-exposed sections as well as the analysis of ratios between primary and modulating cycles. Spectral analysis has confirmed the presence of regular cycles throughout the offshore data, which provide a far longer record of orbitally forced cyclicity than previously available. The presence of a half precession cycle is also suggested. Similar up-section variations in cycle thicknesses are recorded from both the onshore and offshore succession. These variations and those in cycle symmetry are related to a trend from an underfilled to a balanced-fill basin state. Of further importance in controlling cycle thickness is the period of each cycle during which lacustrine conditions, and therefore lake level controlled accommodation, existed.

Reply to the discussion on ‘A 2.3 Million Year Lacustrine Record of Orbital Forcing from the Devonian of Northern Scotland’, Journal of the Geological Society, London 173, 474–488

Smith & Bailey (2017 a ) have raised concerns over the methodologies we applied for time series analysis in our study of the cyclic Devonian lacustrine deposits of northern Scotland (Andrews et al. 2016). Similar concerns have been raised on a number of occasions for similar studies (Bailey et al. 2009 on Kemp & Coe 2007; Smith & Bailey 2017 c on Howe et al. 2016; Smith et al. …