Spatial variability in beach biogeomorphology in a tropical archipelago

Beaches of tropical island coasts exhibit high levels of diversity in composition and form in comparison with their continental counterparts. To investigate the nature and origin of this diversity, individual beach morphology and sedimentology was investigated in the British Virgin Islands (BVI), a Caribbean archipelago of > 60 high volcanic and low reef islands. The islands exhibit a diversity of orientations (some facing the Atlantic and some the Caribbean), elevation and gradient, rock type and wave energy. An examination of 100 beaches in the archipelago revealed a first-order division into sand (70 beaches) and coral rubble (30 beaches). These beaches occur in seven planform types (determined by the antecedent geological framework) and are further subdivided according to shoreface type (seagrass, sandy shoreface, or reef). Mainland-attached headland-embayment beaches are the most common form of sand beach while coral rubble beaches usually occur as barriers that enclose salt ponds and wetlands. Among sand beaches, carbonate content is greatest on Atlantic-facing beaches, and coral rubble beaches are more common on Caribbean-facing beaches. Grain size characteristics on sandy beaches are highly variable and range from fine to very coarse sands while coral rubble beaches range up to boulder-sized clasts. The local source material is a primary determinant of sediment composition. The local factors such as the underlying geology, source and availability of sediments are the primary determinants of beach form, composition and texture in the BVI. Oceanographic and climatic conditions such as the prevailing easterly trade winds and waves which seasonally range in direction from east-northeast to southeast as well as beach orientation to Atlantic- or Caribbean-facing waves also contribute to the variability, but in a secondary role. © 2019 John Wiley & Sons, Ltd.     

Morphodynamic interactions of continental shelf,beach and dunes: the Cabopino dune system in southern Mediterranean Spain

The complex morphodynamic interactions between nearshore, shoreface and dune systems are usually simplified by studying these zones and their associated processes in isolation. However, the established relationships between each of them suggests that an integrated approach is required to examine the genesis, evolution and adaptation of the entire morphodynamic system. The Cabopino dune system in the southern Spanish Mediterranean Sea provides a clear example of a linked morphodynamic system in which a relatively large dune system has been generated and grown through the supply of sediments from an adjacent littoral supply environment. Here, we present a conceptual model of how the nearshore has provided suitable conditions for beach and dune development. We purport that synchronization of sediment activation in the marine and aeolian sections of the system have played a major role in this microtidal setting in which temporal aspects are not only tied to storm action, but to large sedimentary features moving alongshore. © 2019 John Wiley & Sons, Ltd.     

The deglaciation of the western sector of the Irish Ice Sheet from the inner continental shelf to its terrestrial margin

This paper provides a new deglacial chronology for retreat of the Irish Ice Sheet from the continental shelf of western Ireland to the adjoining coastline, a region where the timing and drivers of ice recession have never been fully constrained. Previous work suggests maximum ice-sheet extent on the outer western continental shelf occurred at ~26–24 cal. ka BP with the initial retreat of the ice marked by the production of grounding-zone wedges between 23–21.1 cal. ka BP. However, the timing and rate of ice-sheet retreat from the inner continental shelf to the present coast are largely unknown. This paper reports 31 new terrestrial cosmogenic nuclide (TCN) ages from erratics and ice-moulded bedrock and three new optically stimulated luminescence (OSL) ages on deglacial outwash. The TCN data constrain deglaciation of the near coast (Aran Islands) to ~19.5–18.5 ka. This infers ice retreated rapidly from the mid-shelf after 21 ka, but the combined effects of bathymetric shallowing and pinning acted to stabilize the ice at the Aran Islands. However, marginal stability was short-lived, with multiple coastal sites along the Connemara/Galway coasts demonstrating ice recession under terrestrial conditions by 18.2–17. ka. This pattern of retreat continued as ice retreated eastward through inner Galway Bay by 16.5 ka. South of Galway, the Kilkee–Kilrush Moraine Complex and Scattery Island moraines point to late stage re-advances of the ice sheet into southern County Clare ~14.1–13.3 ka, but the large errors associated with the OSL ages make correlation with other regional re-advances difficult. It seems more likely that these moraines are the product of regional ice lobes adjusting to internal ice-sheet dynamics during deglaciation in the time window 17–16 ka.     

Radial Permeability Measurements for Shale Using Variable Pressure Gradients

Shale gas is becoming an important component of the global energy supply, with permeability a critical controlling factor for long-term gas production. Obvious deviation may exist between helium permeability determined using small pressure gradient(SPG) methods and methane permeability obtained under actual field production with variable pressure gradients(VPG). In order to more accurately evaluate the matrix permeability of shale, a VPG method using real gas(rather than He) is established to render permeability measurements that are more representative of reservoir conditions and hence response. Dynamic methane production experiments were performed to measure permeability using the annular space in the shale cores. For each production stage, boundary pressure is maintained at a constant and the gas production with time is measured on the basis of volume change history in the measuring pump. A mathematical model explicitly accommodating gas desorption uses pseudo-pressure and pseudo-time to accommodate the effects of variations in pressuredependent PVT parameters. Analytical and semi-analytical solutions to the model are obtained and discussed. These provide a convenient approach to estimate radial permeability in the core by nonlinear fitting to match the semi-analytical solution with the recorded gas production data. Results indicate that the radial permeability of the shale determined using methane is in the range of 1×10-6– 1×10-5 mD and decreases with a decrease in average pore pressure. This is contrary to the observed change in permeability estimated using helium. Bedding geometry has a significant influence on shale permeability with permeability in parallel bedding orientation larger than that in perpendicular bedding orientation. The superiority of the VPG method is confirmed by comparing permeability test results obtained from both VPG and SPG methods. Although several assumptions are used, the results obtained from the VPG method with reservoir gas are much closer to reality and may be directly used for actual gas production evaluation and prediction, through accommodating realistic pressure dependent impacts.     

A peridotitic komatiite from the Dalradian of Shetland

Komatiite lavas occur in the Dunrossness Spilitic Group in southeast Shetland in the Cunningsburgh area. This metavolcanic group is the easternmost member of the East Mainland Succession of Shetland and probably the youngest. It overlies a thick succession of deep-water metasediments and metavolcanic rocks probably forming part of an extensional basin and has been correlated with a part of the upper Dalradian of Scotland. Komatiites are not common in rock sequences as young as this. Their eruption is thought to require tectonic and thermal conditions which have been unusual since Archean times. Such conditions could arise when the crust beneath an extensional basin splits to form a constructive margin.     

Correlating specific conductivity with total hardness in gypsum karst waters

Modern digital conductivity meters are readily portable, robust, cheap, and give precisely reproducible values of specific electrical conductivity (SpC, in µS cm^?1). Here we investigate the accuracy of their estimates of the amounts of gypsum dissolved in waters collected in gypsum karst terrains, expressed as total hardness (TH) in mg L^?1 of CaSO₄·2H₂O (GYP). Total dissolved solid concentrations (TDS) are also considered. Curves obtained with the program PHREEQC, for the dissolution of pure gypsum in water at 25 C, are compared with 574 comprehensive water chemical analyses selected from gypsum karst studies in Europe and the Americas. Principal common and foreign ions encountered are the BNC group (bicarbonates, nitrates, chlorides). It is found that GYP = 1·12^·SpC + 62 where BNC < 33% (Cl^? < 5%), with one standard error <5% for waters with SpC > 2400 µS cm^?1; GYP = 0·74^·SpC + 777 where BNC < 33% (5% ≤ Cl^? < 15%), with one standard error <10% for waters with SpC > 3100 µS cm^?1; GYP = 0·97^·SpC ? 209 where BNC < 33% and Cl^? ≥ 15%, with one standard error <10% for samples with SpC > 4300 µS cm^?1. There are similar results for the more complex waters found in gypsum karsts where much carbonate rock or salt is also present, to the limit of BNC < 50% for what may reasonably be defined as ‘gypsum waters’. Values of R² for linear correlations of different subsets of the water samples range from 0·69 to 0·96, the majority being >0·8. Copyright © 2006 John Wiley & Sons, Ltd.     

Biodiversity reduction on contaminated sediments in the sea: contamination or sediment mobility?

Where contaminants in sediments correlate with biodiversity reductions, the contamination may not be the cause. Other physical factors may be responsible, and should be investigated so that any remedial action is directed properly. Two examples are given where unusual sediment mobility caused biodiversity reductions in sediments with elevated trace contaminants.