ALBERT

Description: Force chain buckling, leading to unjamming and shear banding, is examined quantitatively via a discrete element analysis of a two-dimensional, densely-packed, cohesionless granular assembly subject to quasistatic, boundary-driven biaxial compression. A range of properties associated with the confined buckling of force chains has been established, including: degree of buckling, buckling modes, spatial and strain evolution distributions, and relative contributions to non-affine deformation, dilatation and decrease in macroscopic shear strength and potential energy. Consecutive cycles of unjamming–jamming events, akin to slip–stick events arising in other granular systems, characterize the strain-softening regime and the shear band evolution. Peaks in the dissipation rate, kinetic energy and local non-affine strain are strongly correlated: the largest peaks coincide with each unjamming event that is evident in the concurrent drops in the macroscopic shear stress and potential energy. Unjamming nucleates from the buckling of a few force chains within a small region inside the band. A specific mode of force chain buckling, prevalent in and confined to the shear band, leads to above-average levels of local non-affine strain and release of potential energy during unjamming. Ongoing studies of this and other buckling modes from a structural stability standpoint serve as the basis for the formulation of internal variables and associated evolution laws, central to the development of thermomicromechanical constitutive theory for granular materials.

Description: A simple unified critical state constitutive model for bonded geomaterials is presented in this paper. The model, which is called CASM-n, is an enhancement of an existing unified critical state model for reconstituted geomaterials (CASM). Characteristic behaviours of a bonded geomaterial such as the pre-yield higher strength and stiffness and the cohesive–frictional shearing mode in the post-yield region are taken into account and included in the reference model. The salient feature of the proposed model is the incorporation of cohesive component into the stress–dilatancy relationship. Consideration of the contribution of cohesion to plastic flow allows the modelling of delayed dilatancy and softening–contraction behaviour, which are two interesting phenomena observed in bonded geomaterials.

Description: Tropical cyclones (TC) under different climate conditions in the Northern Hemisphere have been investigated with the Max Planck Institute (MPI) coupled (ECHAM5/MPI-OM) and atmosphere (ECHAM5) climate models. The intensity and size of the TC depend crucially on resolution with higher wind speed and smaller scales at the higher resolutions. The typical size of the TC is reduced by a factor of 2.3 from T63 to T319 using the distance of the maximum wind speed from the centre of the storm as a measure. The full three-dimensional structure of the storms becomes increasingly more realistic as the resolution is increased. For the T63 resolution, three ensemble runs are explored for the period 1860 until 2100 using the IPCC SRES scenario A1B and evaluated for three 30 yr periods at the end of the 19th, 20th and 21st century, respectively. While there is no significant change between the 19th and the 20th century, there is a considerable reduction in the number of the TC by some 20% in the 21st century, but no change in the number of the more intense storms. Reduction in the number of storms occurs in all regions. A single additional experiment at T213 resolution was run for the two latter 30-yr periods. The T213 is an atmospheric only experiment using the transient sea surface temperatures (SST) of the T63 resolution experiment. Also in this case, there is a reduction by some 10% in the number of simulated TC in the 21st century compared to the 20th century but a marked increase in the number of intense storms. The number of storms with maximum wind speeds greater than 50 m s−1 increases by a third. Most of the intensification takes place in the Eastern Pacific and in the Atlantic where also the number of storms more or less stays the same. We identify two competing processes effecting TC in a warmer climate. First, the increase in the static stability and the reduced vertical circulation is suggested to contribute to the reduction in the number of storms. Second, the increase in temperature and water vapour provide more energy for the storms so that when favourable conditions occur, the higher SST and higher specific humidity will contribute to more intense storms. As the maximum intensity depends crucially on resolution, this will require higher resolution to have its full effect. The distribution of storms between different regions does not, at first approximation, depend on the temperature itself but on the distribution of the SST anomalies and their influence on the atmospheric circulation. Two additional transient experiments at T319 resolution where run for 20 yr at the end of the 20th and 21st century, respectively, using the same conditions as in the T213 experiments. The results are consistent with the T213 study. The total number of TC were similar to the T213 experiment but were generally more intense. The change from the 20th to the 21st century was also similar with fewer TC in total but with more intense cyclones.

Description: The measurements of dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) in seawater are key in global change and coastal eutrophication studies. Nowadays, the high-temperature combustion (HTC) technique is a widely used method for DOC and TDN analysis. However, uncertainties exist about the operation of the catalyst in the conversion process of DOC and TDN in the HTC method. In this study, five different 'catalyst' materials were tested for their blanks, calibration slopes, and conversion efficiency of DOC and TDN using the Shimadzu TOC 5000A total organic carbon analyser coupled to a Sievers NCD 255 nitrogen chemiluminescence detector. The materials included four metallic catalysts (Shimadzu and Johnson 0.5 Pt-alumina, 13 Cu(II)O-alumina, 0.5 Pd-alumina) and quartz beads. The results indicated that DOC blank signals for the HTC approach using metallic catalysts with an alumina support are higher compared with quartz beads, as a result of the amphoteric nature of the alumina. However, the slopes of the standard calibration graphs were lowest for DOC and TDN determinations on the quartz beads. The DOC recoveries for the metallic catalysts were close to 100 for all compounds tested, with the exception of ammonium pyrrolidine dithiocarbamate. Using quartz beads, poor recoveries were obtained for a range of organic compounds, including the commonly used calibration compounds potassium hydrogen phthalate and glycine. The TDN recoveries for all compounds were typically >90, with the exception of NaNO2. Furthermore, analysis using the CuO-alumina and Pd-alumina catalysts and quartz beads showed low recoveries for NH4Cl. This study showed that catalyst performance should be verified on a regular basis using model compounds and blank checks made during every run, and that the Shimadzu 0.5 Pt-alumina material was an efficient catalyst for DOC and TDN analyses using the coupled total organic carbon-nitrogen chemiluminescence detector (TOC-NCD) analyser. © 2007 Taylor & Francis.

Description: Data from two cross-shelf sediment sampling cruises were used to explain reasons for the sediment biogeochemical variability in respect of carbon, nitrogen and sulphur, and how the cycling of these elements governs the biogeochemistry of the overlying water through their control of the redox conditions. The spatial extent of this benthic–pelagic flux link is limited to the innershelf mud belt system on the Namibian shelf. The inshore mud belt is the primary deposition area of the carbon and nitrogen new production export flux. The offshore organic-rich zones are thought to be relict particulate organic matter originating from the inshore mud belt rather than from an overlying pelagic source. These data were used to set up a multi-layer sediment model that was used through sensitivity analyses to elucidate the input characteristics that result in the most significant feedbacks on hypoxia in the overlying water. The analyses showed that, although the new production flux is a requirement to drive an oxygen demand in the sediments, the onset and persistence of anoxia may depend critically on a low-oxygen boundary condition threshold. This is thought to be a key differentiating factor between systems that, despite comparable carbon export fluxes, are characterised by a persistent hypoxia/anoxia signal and those that are characterised by episodic hypoxia events. It was concluded that sediment oxygen demand and methane and 'sulphide' emissions from the central Benguela sediments are responses to external hypoxia boundary conditions rather than the local drivers of oxygen variability.

Description: The functional response of Sepia officinalis early stages, preying on mysids of the species Mesopodopsis slabberi was investigated. The effects of five prey densities (12.5, 25, 37.5, 50 and 125 mysids l−1) and two hatchling ages (1-day-old and 7-day-old) on consumption rate and the frequency of non-feeding animals were tested. Older animals were approximately 50% heavier than newly hatched ones. Hatchlings were individually assayed under 0.25 W m−2 natural light, 37.8 psu in salinity and 19°C. The effect of prey density on consumption rate was highly significant and no effect of age was detected within the age range tested. Maximal values recorded for consumption rate were about 0.45 mysids h−1. The frequency of non-feeding individuals was strongly reduced at saturating prey densities. The functional response curve showed an interval of prey densities for which density-dependent prey mortality is probable.

Description: We report data on the stomach contents of the long‐finned pilot whale, Globicephala melas, recovered from a group of whales stranded on Ruakaka Beach, northeastern New Zealand, in November 2006. In nine whales for which identifiable stomach contents were recovered (three that stranded on 10 November and six that stranded on 11 November) prey remains comprised exclusively cephalopod beaks attributed to five squid species. The stomachs of a further two whales contained unidentifiable upper beaks only, while the stomachs of five whales were completely empty. No whale appeared to have been satiated immediately before stranding, given that the maximum biomass of prey recently consumed by any one whale was calculated to be 〈5 kg. All squids ingested represented oceanic species, found from 50 to 1000 m but more common towards the deeper end of this range. These data both complement and contrast with the only other dietary information available for this species in New Zealand waters, reported from stomach contents of whales stranded on Farewell Spit, South Island in December 2005.

Description: Stomach contents of the long‐finned pilot whale, Globicephala melas, are reported for the first time from New Zealand waters. Analyses based on two male and three female whales (2.5–5.3 m in length) that stranded on Farewell Spit, Golden Bay, South Island in December 2005 revealed a diet comprised exclusively of cephalopods (2-33 lower cephalopod beaks per stomach). Two genera of cephalopod from two orders; arrow squid, Nototodarus spp. (Teuthoidea: Ommastrephidae), and common octopus, Pinnoctopus cordiformis (Octopoda: Octopodidae) were represented. A further five pilot whale stomachs were examined and found to be empty.