Sedimentary environments and facies of the subtropical mgeni estuary,Southeast Africa

The Mgeni Estuary is situated on the subtropical, mainly microtidal Natal coast. Modern sedimentary environments in the estuary comprise two groups. Barrier-associated environments include inlet channel, inlet beachface, tidal delta, washover fans, transverse intertidal bars and aeolian dunes. Estuarine environments include subtidal channels, interidal bars, back-barrier lagoon, tidal creek, tidal creek side-attached bars, creek mouth bar, mangrove fringe and supratidal mudflats. Each sedimentary facies is described in terms of grain-size, sedimentary structures, and sedimentary processes. The distinctive flora and fauna play an important role in facies recognition. Vertical sequences produced by infilling of the estuary and subsequent coastal erosion are discussed. The facies are considered sufficiently distinct to warrant recognition in the geological record.     

Phytoplankton community in the Western Arctic in July–August 2003

The phytoplankton community was studied in Bering Strait and over the shelf, continental slope, and deep-water zones of the Chukchi and Beaufort seas in the middle of the vegetative season (July–August 2003). Its structure was analyzed in relation to ice conditions and the seasonal patterns of water warming, stratification, and nutrient concentrations. The overall ranges of variation in phytoplankton abundance and biomass were estimated at 2.0 × 10² to 6.0 × 10⁶ cells/l and 0.1 to 444.1 mg C/m³. The bulk of phytoplankton cells concentrated in the seasonal picnocline, at depths of 10–25 m. The highest values of cell density and biomass were recorded in regions influenced by the inflow of Bering Sea waters or characterized by intense hydrodynamics, such as the Bering Strait, Barrow Canyon, and the outer shelf and slope of the Chukchi Sea. In the middle of the vegetative season, the phytoplankton in the study region of the Western Arctic proved to comprise three successional (seasonal) assemblages, namely, the early spring, late spring, and summer assemblages. Their spatial distribution was dependent mainly on local features of hydrological and nutrient regimes rather than on general latitudinal trends of seasonal succession characteristic of arctic ecosystems.     

Three-dimensional numerical modeling of cohesive sediment transport and wind wave impact in a shallow oxbow lake

It was observed that in some closed inland lakes sediment transport was dominated by wind-induced currents, and the sediment resuspension was primarily driven by wind-induced waves. This paper presents the development and application of a three-dimensional numerical model for simulating cohesive sediment transport in water bodies where wind-induced currents and waves are important. In the model, the bottom shear stresses induced by currents and waves were calculated, and the processes of resuspension (erosion), deposition, settling, etc. were considered. This model was first verified by a simple test case consisting of the movement of a non-conservative tracer in a prismatic channel with uniform flow, and the model output agreed well with the analytical solution. Then it was applied to Deep Hollow Lake, a small oxbow lake in Mississippi. Simulated sediment concentrations were compared with available field observations, with generally good agreement. The transport and resuspension processes of cohesive sediment due to wind-induced current and wave in Deep Hollow Lake were also discussed.     

The interpretation of the generalised derivative operator

The Generalised Derivative Operator is an image‐processing tool for the enhancement of potential field data. It produces an amplitude‐balanced image of the derivative of a potential field in any direction in three‐dimensional space. This paper shows how, by using the correct inclination angle ?, the Generalised Derivative Operator can be used to produce images where its maxima/minima lie directly over dipping contacts and thin dykes with arbitrary magnetisation vectors. The dip of contacts and dykes can be found by varying ? until a symmetrical result is obtained (in the absence of unknown remanent magnetisation). Furthermore, the width of the peak of the Generalised Derivative Operator can then be used to determine the depth of the contact or dyke.     

Applying the tilt‐depth and contact‐depth methods to the magnetic anomalies of thin dykes

A new method for the calculation of the depth, location, and dip of thin dykes from pole‐reduced magnetic data is introduced. The depth can be obtained by measuring the distance between chosen values of a tilt angle that is based upon the ratio of the magnetic field and its Hilbert transform over the dyke. Alternatively, it can be obtained from the horizontal derivative of the ratio of the Hilbert transform of the field to the field itself, over the dyke. The latter method also allows the dip of the dyke to be estimated from the gradient of the depth estimates.     

Improved structural interpretation of legacy 3D seismic data from Karee platinum mine (South Africa) through the application of novel seismic attributes

Seismic detection of faults, dykes, potholes and iron-rich ultramafic pegmatitic bodies is of great importance to the platinum mining industry, as these structures affect safety and efficiency. The application of conventional seismic attributes (such as instantaneous amplitude, phase and frequency) in the hard-rock environment is more challenging than in soft-rock settings because the geology is often complex, reflections disrupted and the seismic energy strongly scattered. We have developed new seismic attributes that sharpen seismic reflections, enabling additional structural information to be extracted from hard-rock seismic data. The symmetry attribute is based on the invariance of an object with respect to transformations such as rotation and reflection; it is independent of the trace reflection amplitude, and hence a better indicator of the lateral continuity of thin and weak reflections. The reflection-continuity detector attribute is based on the Hilbert transform; it enhances the visibility of the peaks and troughs of the seismic traces, and hence the continuity of weak reflections. We demonstrate the effectiveness of these new seismic attributes by applying them to a legacy 3D seismic data set from the Bushveld Complex in South Africa. These seismic attributes show good detection of deep-seated thin (∼1.5 m thick) platinum ore bodies and their associated complex geological structures (faults, dykes, potholes and iron-rich ultramafic pegmatites). They provide a fast, cost-effective and efficient interpretation tool that, when coupled with horizon-based seismic attributes, can reveal structures not seen in conventional interpretations.     

Modeling cave cross-section evolution including sediment transport and paragenesis

Deposits within caves are often used to interpret past landscape evolution and climate conditions. However, cave passage shapes also preserve information about past conditions. Despite the usefulness of passage shape, no previous models simulate cave cross-section evolution in a realistic manner. Here we develop a model for evolving cave passage cross-sections using a shear stress estimation algorithm and a shear stress erosion rule. Our model qualitatively duplicates observed cave passage shapes so long as erosion rates vary with shear stress, as in the case of transport limited dissolution or mechanical erosion. This result provides further evidence that erosion rates within caves are not typically limited by surface reaction rates, even though current speleogenesis models predict surface-rate limitation under most turbulent flow conditions. By adding sediment transport and alluviation to the model we successfully simulate paragenetic channels. Simulations duplicate the hypothesized dynamics of paragenesis, whereby: 1) the cross-section of a phreatic passage grows until shear stress is sufficiently reduced that alluviation occurs, 2) the floor of the passage becomes armored and erosion continues on the ceiling and walls, 3) negative feedback produces an equilibrium cross-sectional area such that shear stress is sufficient to transport incoming sediment. We derive an approximate scaling relationship that indicates that equilibrium paragenetic channel width scales with the square root of discharge, and weakly with the inverse of sediment supply. Simulations confirm this relationship and show that erosion mechanism, sediment size, and roughness are secondary controls. The inverse scaling of width with sediment supply in paragenetic channels contrasts with surface bedrock channels, which respond to larger sediment supplies by widening. Our model provides a first step in simulating cave cross-section evolution and points to the need for a better understanding of the dominant erosion mechanisms in soluble bedrock channels. © 2020 John Wiley & Sons, Ltd.     

The Aguas Zarcas (CM2) meteorite: New insights into early solar system organic chemistry

To date, the CM2 class of carbonaceous chondrites has provided the most detailed view of organic synthesis in the early solar system. Organic‐rich chondrites actually observed falling to Earth (“Falls”), for example, the Murchison meteorite in 1969, are even more rare. The April 23, 2019 fall of the Aguas Zarcas meteorite is therefore the most significant CM2 fall since Murchison. Samples collected immediately following the fall provide the rare opportunity to analyze its bulk mineralogy and organic inventory relatively free of terrestrial contamination. According to the Meteoritical Bulletin, Aguas Zarcas (“AZ” or “Zarcas”) is dominated by serpentine, similar to other CM2 chondrites. Likewise, our initial analyses of AZ were meant to give a broad view of its soluble organic inventory relative to other carbonaceous chondrites. We observe that while it is rich in hydrocarbons, carboxylic acids, dicarboxylic acids, sugar alcohols, and sugar acids, some of these classes may be of lesser abundance than in the more well known carbonaceous chondrites such as Murchison. Compared generally with other CM2 meteorites, the most significant finding is the absence, or relatively low levels, of three otherwise common constituents: ammonia, amino, acids, and amines. Overall, this meteorite adds to the building database of prebiotic compounds available to the ancient Earth.     

Practice versus policy-led coastal defence management

Throughout northwest European coastal countries risks associated with coastal erosion are significant but spatially and temporally variable. The level of this risk is largely dependent on the extent of development within the coastal zones and a variety of approaches have been adopted for its management. The decision-making process for responding to erosion risk depends to a large extent on national policy. Coastal protection policy in northwest European countries varies in terms of the level of centralisation and formality of arrangements. In this paper the practical outworking of the informal practice-based system of Ireland, where there is no national policy framework, is compared with the policy-led system of England and Wales where formal national guidelines exist. Using case studies, the strengths and weaknesses of both the bottom-up and top-down approaches are assessed. The findings reveal strengths and weaknesses in both existing types of approach.     

Mafic Plinian volcanism and ignimbrite emplacement at Tofua volcano,Tonga

Tofua Island is the largest emergent mafic volcano within the Tofua arc, Tonga, southwest Pacific. The volcano is dominated by a distinctive caldera averaging 4 km in diameter, containing a freshwater lake in the south and east. The latest paroxysmal (VEI 5–6) explosive volcanism includes two phases of activity, each emplacing a high-grade ignimbrite. The products are basaltic andesites with between 52 wt.% and 57 wt.% SiO₂. The first and largest eruption caused the inward collapse of a stratovolcano and produced the ‘Tofua’ ignimbrite and a sub-circular caldera located slightly northwest of the island’s centre. This ignimbrite was deposited in a radial fashion over the entire island, with associated Plinian fall deposits up to 0.5 m thick on islands >40 km away. Common sub-rounded and frequently cauliform scoria bombs throughout the ignimbrite attest to a small degree of marginal magma–water interaction. The common intense welding of the coarse-grained eruptive products, however, suggests that the majority of the erupted magma was hot, water-undersaturated and supplied at high rates with moderately low fragmentation efficiency and low levels of interaction with external water. We propose that the development of a water-saturated dacite body at shallow (<6 km) depth resulted in failure of the chamber roof to cause sudden evacuation of material, producing a Plinian eruption column. Following a brief period of quiescence, large-scale faulting in the southeast of the island produced a second explosive phase believed to result from recharge of a chemically distinct magma depleted in incompatible elements. This similar, but smaller eruption, emplaced the ‘Hokula’ Ignimbrite sheet in the northeast of the island. A maximum total volume of 8 km³ of juvenile material was erupted by these events. The main eruption column is estimated to have reached a height of ∼12 km, and to have produced a major atmospheric injection of gas, and tephra recorded in the widespread series of fall deposits found on coral islands 40–80 km to the east (in the direction of regional upper-tropospheric winds). Radiocarbon dating of charcoal below the Tofua ignimbrite and organic material below the related fall units imply this eruption sequence occurred post 1,000 years BP. We estimate an eruption magnitude of 2.24 × 10¹³ kg, sulphur release of 12 Tg and tentatively assign this eruption to the AD 1030 volcanic sulphate spike recorded in Antarctic ice sheet records.