Novel discrete element modelling of Gilbert‐type delta formation in an active tectonic setting—first results

Gilbert deltas are now recognised as an important stratigraphic component of many extensional basins. They are remarkable due to their coarse‐grained nature, large size and steep foresets (up to 30–35°) and may exhibit a variety of slope instability features (faulting, slump scars, avalanching, etc.). They are also often closely related to major, basin‐margin normal faults. There has been considerable research interest in Gilbert deltas, partly due to their economic significance as stratigraphic traps for hydrocarbons but also due to their sensitivity to relative base level changes, giving them an important role in basin analysis. In addition to field studies, numerical modelling has also been used to simulate such deltas, with some success. However, until now, such studies have typically employed continuum numerical techniques where the basic data elements created by simulations are stratigraphic volumes or timelines and the sediments themselves have no internal properties per se and merely represent areas/volumes of introduced coarse‐grained, clastic and sedimentary material. Faulting or folding (if present) are imposed externally and do not develop (naturally) within the modelled delta body itself. Here, I present first results from a novel 2D numerical model which simulates coarse‐grained (Gilbert‐type) deltaic sedimentation in an active extensional tectonic setting undergoing a relative base level rise. Sediment is introduced as packages of discrete elements which are deposited beneath sea level, from the shoreline, upon a pre‐existing basin or delta. These elements are placed carefully and then allowed to settle onto the system. The elements representing the coarse‐grained, deltaic sediments can have an intrinsic coefficient of friction, cohesion or other material properties appropriate to the system being considered. The spatial resolution of the modelling is of the order of 15 m and topsets, foresets, bottomsets, faults, slumps and collapse structures all form naturally in the modelled system. Examples of deltas developing as a result of sediment supply from both the footwall and hanging‐wall of a normal fault, and subject to changes in fault slip rate are presented. Implications of the modelling approach, and its application and utility in basin research, are discussed.     

Intertidal foraminifera in the macro-tidal estuaries of the Gulf of Cambay: Implications for interpreting sea-level change in palaeo-estuaries

The Gulf of Cambay is a macro-tidal estuarine embayment lying in an active monsoon zone characterized by a high annual sediment load. The present study examines the distribution of foraminifera in the Narmada and Tapti estuaries of the Gulf in order to use these as analogues for the study of palaeo-macro-tidal estuarine environments and as a means of recording the extent of sea-level change in estuarine settings. Foraminifera are widely distributed in sand flats, mud flats and marshes at the mouths of the estuaries. The overall foraminiferal assemblage is low diversity, comprising epifaunal and infaunal taxa dominated by Ammonia, Murrayinella, Haynesina, Quinqueloculina, Nonion, Cibicides, Cibicidoides, Elphidium, Trochammina and Miliammina. The triserial planktonic foraminifer Gallitellia vivans, an indicator of stressed and up-welling areas, is also recorded in the intertidal sediments of the estuaries. These foraminifera are carried into estuaries up to 50 km up the river mouths by the strong tidal currents in the bay. Three biofacies are recognized: Trochammina–Miliammina biofacies (high marsh); Murrayinella-Haynesina biofacies (low marsh and mud flats) and Ammonia-Elphidium-Quinqueloculina biofacies (shallow marine).     

Comparing quality of estuarine and nearshore intertidal habitats for Carcinus maenas

Estuarine and nearshore marine areas are vital habitats for several fish and benthic invertebrates. The shore crab Carcinus maenas (Crustacea: Brachyura: Portunidae) inhabits a variety of coastal, estuarine and lagoon habitats. At low tide, habitat structural complexity may be most important for crabs in the intertidal, providing refuge from predation and desiccation. The quality of different vegetated and nonvegetated estuarine and rocky shore habitats in SW Portugal and SW England was evaluated for intertidal C. maenas populations. We estimated population density, size–structure, and potential growth (RNA/DNA ratios) to investigate habitat quality. Vegetated estuarine habitats supported higher crab densities, than nonvegetated estuarine and rocky shore habitats. Investigation of population size–structure revealed that all habitats seem important recruitment and nursery areas although estuarine habitats in SW Portugal appeared to support higher densities of new recruits than equivalent habitats in SW England. Significant variation was found in RNA/DNA ratios among habitats. Ratios were highest in the rocky shore suggesting a high quality habitat where growth potential is high. We speculate that competition from other top-predators (Pachygrapsus spp.) rather than low habitat quality may limit the occurrence of C. maenas in intertidal rocky shore habitats in SW Portugal. In estuarine environments RNA/DNA ratios were significantly higher in the vegetated than in the nonvegetated estuarine habitats in SW Portugal but not in SW England, suggesting geographic differences in the extent to which highly structure habitats represent high quality. Our results challenge the current paradigm that structured habitats are necessarily those of higher quality for C. maenas.     

Particulate organic matter and ballast fluxes measured using time-series and settling velocity sediment traps in the northwestern Mediterranean Sea

Prompted by recent data analyses suggesting that the flux of particulate organic carbon sinking into deep waters is determined by fluxes of mineral ballasts, we undertook a study of the relationships among organic matter (OM), calcium carbonate, opal, lithogenic material, and excess aluminum fluxes as part of the MedFlux project. We measured fluxes of particulate components during Spring and Summer of 2003, and Spring of 2005, using a swimmer-excluding sediment trap design capable of measuring fluxes both in a time-series (TS) mode and in a configuration for obtaining particle settling velocity (SV) profiles. On the basis of these studies, we suggest that distinct OM–ballast associations observed in particles sinking at a depth of 200 m imply that the mechanistic basis of the organic matter–ballast association is set in the upper water column above the Twilight Zone, and that the importance of different ballast types follows the seasonal succession of phytoplankton. As in other studies, carbonate appears to enhance the flux of organic matter over opal. Particles must be at least half organic matter before their settling velocity is affected by ballast concentration. This lack of change in ballast composition with SV in particles with <40% OM content suggests that particle SV reaches a maximum because of the increasing importance of inertial drag. Relative amounts of OM and opal decrease with depth due to decomposition and dissolution; carbonates and lithogenic material contribute about the same amount to total mass, or increase slightly, throughout the water column. The high proportion of excess Al cannot be explained by its incorporation into diatom opal or reverse weathering, so Al is most likely adsorbed to particulate oxides. On shorter time scales, dust appears to increase particle flux through its role in aggregation rather than by nutrient inputs enhancing productivity. We suggest that the role of dust as a catalyst in particle formation may be a central mechanism in flux formation in this region, particularly when zooplankton fecal pellet production is low.     

Settling velocity spectra and the ballast ratio hypothesis

Recently it has been observed that a strong quantitative relationship exists between asymptotic fluxes of particulate organic carbon (POC) to the deep ocean and asymptotic fluxes of “ballast” minerals (opal; calcium carbonate; dust). It has further been suggested that this relationship might provide a mechanistic basis for improved representations of remineralization in ocean carbon models. Since the depth scale of remineralization z* is the ratio k/v of a remineralization rate k and a settling velocity (SV) v, a mechanistic understanding of settling velocities will be crucial in developing such models.Historically, there have been two approaches to estimating the speed with which POC is transported to the deep ocean. First, settling speeds of single particles have been observed directly in both field and laboratory settings; estimates of fecal pellet sinking velocities tend to be higher and more variable than those of aggregates. Second, estimates have been made of the velocity at which temporal patterns in flux propagate between pairs of sediment traps separated in depth (the “benchmark approach”); recent studies have shown these results to be variable and to depend on mineral ballasting. Here we present SV estimates using a relatively new technology: indented rotating sphere (IRS) sediment traps run in settling velocity (SV) mode. In this approach, particles are separated into SV classes during settling to collection cups. In MedFlux, SV data were collected concurrently with time-series (TS) data; the latter were used to construct benchmark estimates for comparison to the SV estimates. From the SV data, the range of modal settling velocities (sinking velocities having the largest time-averaged mass flux densities on a logarithmic scale of SV) in the fast-sinking fraction was estimated to be 287–503 m/d; the average of these modal values is 353 m/d, with standard deviation 76 m/d. In contrast, mean settling velocities of the fast-sinking fraction depend on the range of settling velocity classes included in the estimate. If only SV classes settling at >50 m/d are included, the range of SVs at MedFlux was 214–298 m/d, with average mean value 242 m/d and standard deviation 31 m/d. These mean-velocity results are in excellent agreement with benchmark estimates of signal propagation velocities at Medflux (220±65 m/d); they are also well within the range of other recent benchmark studies. The agreement between the benchmark estimates and mean settling velocity estimates at MedFlux, but not with modal velocities, argues that the benchmark method estimates mean settling velocities.     

Functional composition, but not richness, affected the performance of sessile suspension-feeding assemblages

The efficiency by which communities capture limiting resources may be related to the number of species or functional types competing therein. This is because species use different resources (i.e. complementarity effect) or because species-rich communities include species with extreme functional traits (positive selection effect). We conducted two manipulative studies to separate the effects of functional richness and functional identity on the feeding efficiency (i.e. filtration rate) of suspension-feeding invertebrates growing on vertical surfaces. In addition, one experiment tested whether the density of organisms influences the effect of functional diversity. Monocultures and complete mixtures of functional types were fed with a solution of microalgae of different sizes (6 μm–40 μm). Experiments conducted at two locations, Helgoland and Plymouth, showed that functional identity had far larger effects on filtration rate than richness. Mixtures did not outperform the average monoculture or the best-performing monoculture and this pattern was independent on density. The high efficiency of one of the functional types in consuming most microalgae could have minimised the resource complementarity. The loss or gain of particular species may therefore have a stronger impact on the functioning of epibenthic communities than richness per se.     

Origin and history of ureilitic material in the solar system: The view from asteroid 2008 TC3 and the Almahata Sitta meteorite

Asteroid 2008 TC₃ (approximately 4 m diameter) was tracked and studied in space for approximately 19 h before it impacted Earth's atmosphere, shattering at 44–36 km altitude. The recovered samples (>680 individual rocks) comprise the meteorite Almahata Sitta (AhS). Approximately 50–70% of these are ureilites (ultramafic achondrites). The rest are chondrites, mainly enstatite, ordinary, and Rumuruti types. The goal of this work is to understand how fragments of so many different types of parent bodies became mixed in the same asteroid. Almahata Sitta has been classified as a polymict ureilite with an anomalously high component of foreign clasts. However, we calculate that the mass of fallen material was ≤0.1% of the pre‐atmospheric mass of the asteroid. Based on published data for the reflectance spectrum of the asteroid and laboratory spectra of the samples, we infer that the lost material was mostly ureilitic. Therefore, 2008 TC₃ probably contained only a few percent nonureilitic materials, similar to other polymict ureilites except less well consolidated. From available data for the AhS meteorite fragments, we conclude that 2008 TC₃ samples essentially the same range of types of ureilitic and nonureilitic materials as other polymict ureilites. We therefore suggest that the immediate parent of 2008 TC₃ was the immediate parent of all ureilitic material sampled on Earth. We trace critical stages in the evolution of that material through solar system history. Based on various types of new modeling and re‐evaluation of published data, we propose the following scenario. (1) The ureilite parent body (UPB) accreted 0.5–0.6 Ma after formation of calcium‐aluminum‐rich inclusions (CAI), beyond the ice line (outer asteroid belt). Differentiation began approximately 1 Ma after CAI. (2) The UPB was catastrophically disrupted by a major impact approximately 5 Ma after CAI, with selective subsets of the fragments reassembling into daughter bodies. (3) Either the UPB (before breakup), or one of its daughters (after breakup), migrated to the inner belt due to scattering by massive embryos. (4) One daughter (after forming in or migrating to the inner belt) became the parent of 2008 TC₃. It developed a regolith, mostly ≥3.8 Ga ago. Clasts of enstatite, ordinary, and Rumuruti‐type chondrites were implanted by low‐velocity collisions. (5) Recently, the daughter was disrupted. Fragments were injected or drifted into Earth‐crossing orbits. 2008 TC₃ comes from outer layers of regolith, other polymict ureilites from deeper regolith, and main group ureilites from the interior of this body. In contrast to other models that have been proposed, this model invokes a stochastic history to explain the unique diversity of foreign materials in 2008 TC₃ and other polymict ureilites.     

A hard instrument goes soft: The implications of the Convention on Biological Diversity's current trajectory

The relentless loss of biological diversity, which will have a direct impact on human society and degrade ecosystem buffers against the extremes of climate perturbation, requires a strong global governance response. Of the numerous international legal instruments relating to the protection of nature, the Convention on Biological Diversity (CBD) is the most comprehensive. This paper examines its current emphasis on global biodiversity targets to extend our understanding of its trajectory, and its evolving nature as an instrument of global governance. We review CBD documents, and early examinations of its emergent character, in the context of the distinction between hard and soft law approaches, and combine analysis on the issue of targets from the literature on development, climate change and conservation biology. We emphasise that the CBD, created as a hard law instrument with a framework character, had the clear facility to develop subsidiary hard law instruments in the form of protocols but has not significantly followed this route. We document how its approach - which has been typically ‘soft’, as exemplified by its focus on global biodiversity targets which are not backed up by obligations - suggests it operates de facto as policy rather than an instrument requiring state action. The adoption of global targets has parallels with other initiatives within global governance and may influence international political agendas, but they have failed to provide practical instruments for national implementation. Conditions may now exist for the CBD to develop focussed hard legal instruments in specific areas of its wide remit that support realistic targets.