Sarah J. Bass

The pervasive role of biological cohesion in bedform development

Sediment fluxes in aquatic environments are crucially dependent on bedform dynamics. However, sediment-flux predictions rely almost completely on clean-sand studies, despite most environments being composed of mixtures of non-cohesive sands, physically cohesive muds and biologically cohesive extracellular polymeric substances (EPS) generated by microorganisms. EPS associated with surficial biofilms are known to stabilize sediment and increase erosion thresholds. Here we present experimental data showing that the pervasive distribution of low levels of EPS throughout the sediment, rather than the high surficial levels of EPS in biofilms, is the key control on bedform dynamics. The development time for bedforms increases by up to two orders of magnitude for extremely small quantities of pervasively distributed EPS. This effect is far stronger than for physical cohesion, because EPS inhibit sand grains from moving independently. The results highlight that present bedform predictors are overly simplistic, and the associated sediment transport processes require re-assessment for the influence of EPS.

Sticky stuff: Redefining bedform prediction in modern and ancient environments

The dimensions and dynamics of subaqueous bedforms are well known for cohesionless sediments. However, the effect of physical cohesion imparted by cohesive clay within mixed sand-mud substrates has not been examined, despite its recognized influence on sediment stability. Here we present a series of controlled laboratory experiments to establish the influence of substrate clay content on subaqueous bedform dynamics within mixtures of sand and clay exposed to unidirectional flow. The results show that bedform dimensions and steepness decrease linearly with clay content, and comparison with existing predictors of bedform dimensions, established within cohesionless sediments, reveals significant over-prediction of bedform size for all but the lowermost clay contents examined. The profound effect substrate clay content has on bedform dimensions has a number of important implications for interpretation in a range of modern and ancient environments, including reduced roughness and bedform heights in estuarine systems and the often cited lack of large dune cross-sets in turbidites. The results therefore offer a step change in our understanding of bedform formation and dynamics in these, and many other, sedimentary environments.

The role of biophysical cohesion on subaqueous bed form size

Abstract Biologically active, fine‐grained sediment forms abundant sedimentary deposits on Earths surface, and mixed mud‐sand dominates many coasts, deltas, and estuaries. Our predictions of sediment transport and bed roughness in these environments presently rely on empirically based bed form predictors that are based exclusively on biologically inactive cohesionless silt, sand, and gravel. This approach underpins many paleoenvironmental reconstructions of sedimentary successions, which rely on analysis of cross‐stratification and bounding surfaces produced by migrating bed forms. Here we present controlled laboratory experiments that identify and quantify the influence of physical and biological cohesion on equilibrium bed form morphology. The results show the profound influence of biological cohesion on bed form size and identify how cohesive bonding mechanisms in different sediment mixtures govern the relationships. The findings highlight that existing bed form predictors require reformulation for combined biophysical cohesive effects in order to improve morphodynamic model predictions and to enhance the interpretations of these environments in the geological record.

Ambient noise in the natural surf zone: wave-breaking frequencies

Ambient noise in the surf zone, in the frequency range 120 Hz to 5 kHz, was recorded using a broad-band hydrophone, located approximately 1 m above bottom and 1-2 m below the mean sea surface. The predominant source of this noise is breaking waves. Analysis of simultaneous land-based video observations of the sea surface in the region of the hydrophone, along with wave height data, reveals quantitative correlation between wave-breaking events and the hydrophone signal. In energetic surf, locally breaking waves appear as discrete events in the ambient noise spectra. Distant breaking events do not appear to be detected, as distinct events above the ambient background noise, by the hydrophone. The noise events associated with local breakers are characterized by an asymmetry in the time envelope: low frequencies (less than 500 Hz) are observed leading the breaking crest, followed by a broader range of frequencies peaking in intensity with the passage of the wave crest above the hydrophone, and then decreasing abruptly at all frequencies. Low frequencies are generally not observed trailing the breaking wave. The detection by the hydrophone of breaking waves in the immediate vicinity implies that ambient noise in heavy surf provides a means of studying breaking-wave statistics in the surf zone in situ: in particular, the frequency of occurrence of local breaking.

Magnetic behaviour of some Mn.III2.VI4 compounds and their alloys

Measurement of the magnetic susceptibility χ as a function of temperature were made on polycrystalline samples from the alloy systems Cd1−zMnzGa2Se4, Zn1−zMnzGa2Se4 and Cd1−zMnzIn2Te4 which had been subjected to various heat treatments. The 1/χ versus T curves indicated that for the ZnSe alloys, for all values of z, samples slowly cooled to room temperature were antiferromagnetic showing ideal Curie-Weiss behaviour, but for samples quenched from 700°C the behaviour was a mixture of antiferromagnetic and spin-glass. For the CdSe alloys, samples from the I42 m range (0.6 < z < 1.0) showed very similar behaviour, but in the I4 range (0 < z < 0.6) even the very slowly cooled samples showed a mixture of antiferromagnetic and spin-glass behaviour. For the CdTe alloys, all samples, however heat-treated, showed spin-glass form. Values of the Curie-Weiss constant Ф were determined from all of the 1/χ versus T curves, and by comparison with the T(z) phase diagrams for the different alloy systems, the values of Ф were correlated with the ordering of the Mn atoms on the cation lattice. It is shown that the experimental values of Ф can give a very convenient way of determining the type of ordering and the degree of order in such alloys.

Magnetic susceptibility and exchange interaction parameters for some Mn.III2.VI4 compounds

Abstract Measurements of magnetic susceptibility χ as a function of temperature were made on polycrystalline samples of the compounds MnGa 2 Se 4 , MnIn 2 Te 4 , MnGa 2 Te 4 and MnIn 2 Se 4 . From the 1 / χ versu that MnGa 2 Se 4 was antiferromagnetic but the other three compounds showed spin-glass behaviour. This is consistent with the known crystal structures, since in MnGa 2 Se 4 the Mn atoms are ordered while in MnIn 2 Te 4 they are disordered on the cation sublattice. It is concluded that similar disorder occurs in MnGa 2 Te 4 and MnIn 2 Se 4 . Values of the critical temperature T N and the Curie-Weiss θ were determined for each compound. In the cases of MnGa 2 Se 4 and MnIn 2 Te 4 , analysis was carried out in terms of a simple mean-field theory and using the virtual transition model of Geertsma et al. for exchange interaction, and values of exchange interaction parameters determined from the measured T N and θ data.

Preliminary findings of a study of the upper reaches of the Tamar Estuary, UK, throughout a complete tidal cycle: Part II: In-situ floc spectra observations

A series of field experiments funded by the Natural Environmental Research Council were conducted in the upper reaches of the Tamar estuary (UK), which placed the measurements within the tidal trajectory of the turbidity maximum. The aim of the study was to examine how the distribution of floc characteristics evolved with respect to changes in the turbulent shear stress, suspended concentration and biological constituents, throughout a complete tidal cycle. The main objective of the experiment was to measure simultaneous floc properties (which included: floc size, shape, settling velocity, effective density, porosity and floc dry mass) using the optical INSSEV instrument and associated hydrodynamic components, in-situ , throughout a complete tidal cycle. This paper reports the preliminary findings of the measurements made on the 15 th April 2003, during a spring tide. During the ebb a concentrated benthic suspension layer formed in close proximity to the bed producing a peak concentration of 4.2 g/1 and a maximum shear stress of about 1.5 N/m 2 . The more dynamic flood produced a shear stress which exceeded the peak ebb stress by 0.15 N/m 2. This in turn meant that the suspended matter was more evenly mixed throughout the entire water column on the flood. Local salinity values ranged from 14 at high water, down to completely flesh at low water. A total of 24 INSSEV floc samples were collected on the ebb flow and a further 34 floc populations were obtained on the flood. A combination of a shear stress of 0.38 N/m 2 and a concentration of 4.2 g/l, produced the optimum flocculation conditions which was signified by a bi-modal floc distribution. The microflocs represented 25% of the population, but only 9% of the dry floc mass. These microflocs were generally dense, slow settling aggregates, with typical effective density values of 80–1550 kg/m 3 and settling velocities ranging from 0.03–1.1 mm/s. The macroflocs, which constituted the second mode, contained 91% of the floc mass, one third of which were flocs over 400 μm in spherical-equivalent diameter. These macroflocs had individual settling velocities ranging between 2.2 to 7 mm/s. This translated into the macrofloc fraction constituting 98% of the total mass settling flux.

Ambient noise and wave breaking on a sandy beach: energy dissipation estimates

ABSTRACT Bass, S.J., 2013. Characterising the ambient noise wave breaking environment on a sandy beach. The interest in wave-generated noise is driven by the relationship with energy dissipation. Studies of deep-water waves show that the sound intensity due to wave-breaking correlates with the energy dissipation rate and it has been demonstrated that passive acoustic methods are a viable means of monitoring local wave breaking. Ambient noise measurements in the surf zone of a sandy beach are analysed for one four-hour measurement period over high water and the results are used to identify individual locally breaking waves from which dissipation rates are estimated. Estimates of wave energy dissipation agree broadly with those estimated using a weighted Rayleigh distribution to predict the breaking wave probability distribution. However, predicted dissipation rates displayed greater variability and an overall lower net dissipation over the measurement period when compared with dissipation estimated using the measured breaking wave fraction.

Preliminary findings of a study of the upper reaches of the Tamar Estuary, UK, throughout a complete tidal cycle: Part I: Linking hydrodynamic and sediment cycles

Preliminary results are presented from an experiment in the turbidity maximum region of the Tamar Estuary, UK, to acquire detailed particle information of the suspension in relation to the main physical and biochemical driving processes throughout several complete tidal cycles. Results presented here focus on the relationship between the evolving suspension characteristics and physical mechanisms through one tidal cycle. The experiment took place over 5 days during April 2003. Insitu floc properties (including floc settling velocity, size, shape and effective density) were measured using the video-based INSSEV system. Profiles of full water column velocity, salinity, temperature and suspension concentrations were supplemented with detailed near bed measurements of turbulence and sediment concentrations. Measurements were made during several days approaching spring tides and the results exhibit a strong contrast in flood and ebb hydrodynamics which is reflected in the observed vertical distribution of sediment and particle characteristics. The flood tides were short and well-mixed with high values of turbulent shear stress causing rapid erosion and resuspension at the onset of the turbidity maximum. Suspension concentrations were relatively evenly distributed vertically reaching concentrations of 1.1 kg/m 3 at the INSSEV height of 0.5 m. In contrast ebb tides exhibited tidal straining and development of a concentrated benthic suspension layer with a lutocline in the region of the INSSEV height. Concentrations at this height reached 1.4 kg/m 3 while the INSSEV sampling height was just below the lutocline. Particles within the turbidity maximum on the ebb tide were typically much larger than the flood, with up to 30% greater than 400 μm just inside the lutocline. Turbulent shear stress measurements during the ebb exhibited a significant reduction in the region of the lutocline due to sediment stratification.

Sand and mud flux estimates using acoustic and optical backscatter sensors: measurements seaward of the Wash, southern North Sea

Abstract Optical and acoustic backscatter sensors, more sensitive to fine and sandy sediment respectively, were used to measure the mud and sand components of a mixed suspension at a site seaward of the Wash embayment, in the southern North Sea. Data were acquired from a free-standing instrument frame during a five-week deployment in 12 m water depth about 6 km offshore. Suspended mud at this site was characterized by tidal advection of fine sediment along the coast resulting in semi-diurnal peaks in concentration near slack water. Suspended sand concentrations correlated well with tidal current speeds indicating local resuspension behaviour. Predicted sand flux direction followed the residual current while mud fluxes at the site were different in direction to both the residual current and sand flux. Residual fluxes may be biased by cumulative errors resulting from instrument calibration and inferred vertical concentration profiles. These factors are assessed in relation to both predicted flux magnitudes and directions.