Form, function and feedbacks in a tidally dominated ooid shoal, Bahamas

Although the general aspects of oolitic depositional systems are well documented, seascape‐scale (≈10³–10⁶ m²) patterns of oolitic shoals and the details of processes acting on them are not well understood or quantified. To begin to fill this basic gap in understanding, this paper describes the morphology and hydrodynamics of Lily Bank, a Modern tidally dominated Bahamian ooid shoal. In this study, integrating remote sensing imagery with quantitative, geo‐located bathymetrical, hydrological and granulometric data in a Geographic Information System documents geomorphic and sedimentological patterns and facilitates interpreting these patterns in the context of the processes operating in this system. The results of these analyses reveal that parabolic bars up to several kilometres in wavelength and several metres in height form a common morphologic motif, although there is considerable variation on that general theme. The seascape‐scale configuration of bars and superimposed sedimentary structures is closely linked to spatial patterns of tidal movements, and includes the presence of mutually evasive flood and ebb channels. Sedimentologically, bars are neither homogenous nor random bodies; instead, granulometric parameters such as sorting and percentage mud vary systematically, as shaped by hydro‐geomorphic controls. The best sorted, coarsest ooids are on bar crests, whereas the finest grains are found in the lower energy, deeper interior and flanking regions. In short, results clearly document hydrodynamic‐bathymetrical influences on these ooid shoals and their granulometry, linkages akin to siliciclastic analogues. Sedimentological, hydrodynamic and geomorphic observations are consistent with a conceptual model for the formation of parabolic bars in which initial irregularities in non‐parabolic bars are enhanced through their effect of focusing flow. Constricted flow leads to higher flow velocities, tidal flow velocity asymmetries, differential net sediment transport and growth of bathymetrical highs. This bathymetrical divergence creates separate paths for flood‐ and ebb‐tides, facilitating emergence of better‐developed parabolic forms. The resultant parabolic geometries and component bedforms appear to be either in dynamic equilibrium with both ebb‐ and flood‐tide flows, or evolving toward that state. In exploring patterns and processes within carbonate shoals, this study illustrates some of the first documented insights on quantitative details of morphology and dynamics and in the links between geomorphic framework and grain‐size and sorting in an oolitic carbonate system. Assuming a continuity of processes between ancient and modern, the insights from this shoal provide information on possible facies geometries and on the characteristics of grains and depositional porosity of analogous facies within ancient ooid shoals.     

Basal Waitemata Group lithofacies: rapid subsidence in an early Miocene interarc basin, New Zealand

The abrupt transition from coastal and shallow shelf sediments to bathyal sediments provides a record of rapid subsidence and deepening of the early Miocene Waitemata basin. Basal shallow marine strata (Kawau Subgroup) accumulated upon a highly dissected surface that overlies deformed Mesozoic metagreywacke. The early Miocene coast was characterized by an embayed and cliffed shoreline with numerous sea stacks and islands. Kawau Subgroup lithofacies, which include pocket beach, shallow shelf and base-of-cliff talus deposits, reflect rapidly changing coastline configuration and water depths as the rugged bedrock surface was buried. The response to continued rapid subsidence and transgression in Waitemata basin was a decrease in the supply of coarse clastic sediment. Beach gravels were locally displaced to greater water depths by avalanching down steep bedrock slopes. The first bathyal turbidite facies, which abruptly overlie the shallow-water Kawau Subgroup, include locally derived sediment gravity flows commonly ponded by remnant bedrock submarine highs. When this local supply of sediment had been exhausted, coarse sediment starvation ensued and bathyal muds accumulated. With the resumption of sediment supply and gradual burial of submarine bedrock relief, submarine fans coalesced and increased in lateral extent. Subsidence of the Waitemata basin to bathyal depths is thought to have occurred in less than a million years. From the above hypothesis, a general model of sedimentation is proposed.     

QUARTZ WEATHERING IN FREEZE–THAW CYCLES: EXPERIMENT AND APPLICATION TO THE EL'GYGYTGYN CRATER LAKE RECORD FOR TRACING SIBERIAN PERMAFROST HISTORY

The object of this study is to test the assumption that cryogenic weathering (here understood as in‐situ disintegration of rock under cold‐climate conditions including ice as a weathering agent) preferentially breaks up quartz grains. We apply the results of laboratory tests to a Quaternary sediment record. The combination of silt production, relative quartz enrichment in the silt fraction, and quartz grain micromorphology is traced in a multi‐100‐kyr lake sediment archive as indicator data for cryogenic weathering. Constant cryogenic weathering conditions are inferred for at least the last 220 000 years from a lake sediment core of El'gygytgyn Crater, northeast Russia. This is the longest continuous terrestrial archive currently known for the continental Arctic. Quartz enrichment in the fines evolves from seasonal freeze–thaw weathering as demonstrated in laboratory testing where over 100 freeze and thaw cycles crack quartz grains preferentially over feldspar. Microscopic grain features demonstrate that freeze–thaw cycling probably disrupts quartz grains along mineral impurities such as bubble trails, gas–liquid inclusions, or mineralogical sub‐grain boundaries. Single‐grain micromorphology (e.g. angular outlines, sharp edges, microcracks, brittle surfaces) illustrates how quartz becomes fragmented due to cryogenic cracking of the grains. The single‐grain features stemming from the weathering dynamics are preserved even after a grain is transported off site (i.e. in mobile slope material, in seasonal river run‐off, into a lake basin) and may serve as first‐order proxy data for permafrost conditions in Quaternary records.     

Large-scale current-induced erosion and deposition in the path of the 1929 Grand Banks turbidity current

New observations concerning the degree of current-induced erosion and deposition in the path of the 1929 Grand Banks turbidity current are presented. Most of the observations are available from Eastern Valley, Laurentian Fan. Seabeam and SeaMARC I data reveal widespread current erosion along the valley over a distance of 200 km from the shelfbreak. Erosional valley-floor channels are preferentially developed adjacent to the valley margins and the flanks of intravalley highs. Asymmetric transverse bedforms (herein termed gravel waves) are moulded in a deflationary pebble and cobble lag that overlies the eroded valley floor. In contrast, at the distal limit of Eastern Valley, thick deposits of massive granule gravel indicate deposition beneath a decelerating turbidity current. Symmetrical transverse bedforms (herein termed macrodunes) are developed within these granule gravel sediments. The spatial distribution of both bedforms and the areas of erosive excavation suggest that the turbidity current in 1929 was accelerating over the first 100 km from the shelfbreak and was eroding and entraining sediment from the valley floor over a distance of at least 200 km. With the loss of lateral constraint at the distal limit of Eastern Valley the turbidity current spread laterally and started depositing sediment as it decelerated. Current-induced erosion of the valley floor represented a potential source of between 50 and 100 km3 of sediment for incorporation into the resulting turbidite.     

HUMAN SPACEFLIGHT: ACHIEVEMENTS, BENEFITS AND FUTURE OPPORTUNITIES FROM A EUROPEAN PERSPECTIVE

The past achievements, benefits and future opportunities of human spaceflight are discussed from a European perspective. Earlier work performed on the Skylab, Salyut, Shuttle, Spacelab, and Mir orbital facilities is reviewed, together with the prospects for new research on the International Space Station (ISS). Major scientific benefits are expected from the ISS in the areas of life science research (including human physiology and medicine) physical sciences, and fundamental physics.     

SUB-DEBRIS MELT RATES ON SOUTHERN INYLCHEK GLACIER, CENTRAL TIAN SHAN

Melt rates of glacier surfaces are strongly influenced by the existence of a debris cover. Dependent on thickness and other physical parameters, the debris layer can enhance or reduce ablation as compared to bare ice conditions. Supraglacial moraines appear very frequently on Central Asian glaciers, greatly affecting water yield from these high mountain regions. In summer 2005, a network of 22 ablation stakes was drilled into locations with varying debris thicknesses on southern Inylchek Glacier in the central Tian Shan. Mean ablation rates varied from 2.8 to 6.7 cm/day, strongly correlated with moraine thickness. Parallel observation of air temperature allowed the application of a simple degree‐day approach and the calculation of ablation rates. Efforts to improve calculations of melt rates by incorporating relative air humidity to account for latent heat fluxes failed. This proves that air temperature is already a very good melt indicator. Ice albedo measurements show that reflectivity might be controlled by the occurrence of evaporation or condensation, but this topic needs further investigation.