Response to comments by on “Evidence for the biotic origin of seabed pockmarks on the Australian continental shelf”

Nicholas et al. (2016) restate the results and conclusions of their own past work and that of colleagues on the North West Shelf. Although they accept that fish may be involved in ‘pockmark’ maintenance, they do not accept that fish can initiate such features. The fluid flow theory with which they have been working therefore remains intact. However, the restatement also provides no new information and no cause to revise the conclusions of Mueller (2015). More field data is required.     

Synthetic seismic illumination of small-scale growth faults,paralic deposits and low-angle clinoforms: A case study of the Triassic successions on Edgeøya,NW Barents Shelf

Fault activity and sandstone-body geometries and spread, all bear significant weight to understanding the potential hydrocarbon systems on the NW Barents Shelf. Synthetic seismic modelling of onshore sedimentary successions provides insight into the seismic resolution and expression of various sedimentological features on Edgeøya: (i) sandy growth basins in pro-delta mudstones (ii) paralic sedimentary deposits and (iii) low-angle tidally influenced progradational successions.Synthetic modelling suggests that the lithological contrast associated with sand-infilled growth basins in pro-deltaic shales, even with offsets as small as 50–75 m, will create distinct geometries in seismic data. Modelling suggests that while optimal 90° illumination, low frequency bands and more typical sedimentary velocities around 2000 m/s will generate very clear discernible growth faults, the angular lithological contrast should generate discernible features even with the high velocities and typical 20–30 Hz frequency band of seismic sections on the NW Barents Shelf. Comparing to actual seismic data in which multiples, noise and in places overburden are influential, it is possible to identify growth-fault geometries more confidently, and to link them to larger planar fault activity.Modelling other features identified in the paralic sedimentary system it is apparent that many of the massive channel or lenticular shaped sandstone bodies should be identifiable in the actual seismic, although their expression is less distinct and more easily misinterpreted than that of growth faults. It is apparent that features such as igneous intrusions, unless imaged in a “perfect” survey, can be difficult to properly identify, particularly near-vertical connections. The velocity contrast creates strong impedance along horizontal sections, but heavily fractured igneous intrusions with lower velocities could easily be assumed to be isolated sandstone bodies. While the modelling appreciates the overall wedging nature of the successions, the simplified lithological observations onshore cannot predict the probable erosion/condensation contrast associated with low-angle clinoforms which are visible in the seismic data, and hence not reproduce them easily in the models.     

Recognizing cryptic environmental changes by using paleoecology and taphonomy of Pleistocene bivalve assemblages in the Oga Peninsula,northern Japan

Multivariate analyses applied to Pleistocene bivalve assemblages from the Oga Peninsula (northern Japan) discriminate three distinct assemblages. The assemblages and their taphonomy were used to recognize environmental settings and changes. The Astarte–Cyclocardia–Glycymeris assemblage indicates shelf environment (below the storm wave base) where gravels and shells are transported from shallower settings. Supply of the exotic coarse sediment probably enabled epifaunal bivalves to inhabit the sea floor. The Glycymeris assemblage is characterized by dominance of G. yessoensis and represents current-swept shoreface environment (above the storm wave base). The Moerella assemblage is characterized by bivalves inhabiting bay to open-marine conditions and diverse deposit-feeders, indicating a moderately land-locked environment, such as an open bay or a bay mouth. Fine-grained substrata rich in organic matters in the bay were probably suitable for the deposit-feeders. Ordination also shows the assemblages along two environmental gradients, a bathymetrical one and the other related to open-marine and bay conditions. The environmental changes are explained mainly by glacio-eustatic sea-level changes and alternation of coastal geomorphology caused by local crustal movements. This study also suggests that fossil assemblages can be a powerful tool to reconstruct environments and depositional dynamics even in intensely bioturbated sedimentary facies.     

A 3-D coastal tidal rectification process: observations, theory and experiments

In homogeneous rotating fluid, when there is an oscillating forcing in the interior fluid with a period long enough for an Ekman layer to develop, there is an interaction between the oscillatory Ekman layer and the vertical wall, since the latter imposes an alternating adjustment flow confined near the wall. As a result, this coastal rectification process leads to a Lagrangian transport along the coast. The Ekman number, the Rossby number and the temporal Rossby number of the forcing flow are the governing parameters of that mechanism which can be described by a simplified analytical model taking into account both the vertical time-dependent structure of the current and the presence of the wall. The model shows that the residual (rectified) current flowing with the coast to its right results from the strong nonlinear interaction between along- and cross-shore tidal currents leading to asymmetrical momentum exchanges between the Ekman bottom layer and the coastal boundary layer. The model provides simple scaling laws for the maximum intensity and width of the residual current. The latter is significantly larger than the friction (Stokes) lateral boundary layer of the forcing flow. A comprehensive set of experiments is performed in the 13 m diameter rotating tank by oscillating an 8 m×2 m horizontal plate and vertical wall in a homogeneous fluid at rest in solid-body rotation and measuring the two horizontal components of the current at several locations and depths above the central part of the plate. The predicted and experimentally measured maximum intensity and width of the residual current are in very good agreement, within the range of validity of the model, i.e. when the Ekman number is sufficiently small. However experiments also show that the residual current still occurs when the Ekman layer thickness is of the same order as the fluid depth, but it is then confined to a narrower band along the vertical wall. The flow structure found experimentally is also correctly described by a numerical model developed by Zhang et al. (1994). Current measurements in the Eastern part of the English Channel near the French coast reveal a significant coastal residual current flowing Northward and the coastal rectification process described here may account for part of it.     

Sources, sinks and resuspension of suspended particulate matter in the eastern English Channel

Seasonal observations on the nature and concentration of suspended particulate matter (SPM) are presented for a cross-section of the English Channel, between the Isle of Wight (UK) and Cotentin peninsula (France) i.e. the western boundary of the eastern English Channel. The highest concentrations of suspended material are found adjacent to the English coastline, whereas the offshore waters are associated with low concentrations. Seasonal variations in the concentration and nature of suspended material are identified, with highest concentrations in winter. At this time, the suspended particles are characterised generally by peaked grain size spectra and an enrichment in coarse silt particles; in summer, the distributions are generally flat. The diatom communities found within the suspended matter indicate that material resuspended in the coastal zone and the estuarine environments is transported offshore. SPM fluxes (based upon the observed SPM concentrations and the output from a 2-D hydrodynamic model) from the western Channel through the Wight–Cotentin Section, ranged between 2 and 71×10⁶ t a⁻¹ with a mean of around 20×10⁶ t a⁻¹ over the period of the observations (1994–1995). These fluxes are comparable to the order of magnitude and mean value reported as output through the Dover Strait. Therefore, it is possible that the eastern English Channel may be characterised as an area of fine-grained sediment ‘bypass'. This interpretation is corroborated by: (a) the absence of fine-grained sediment deposits over the area; and (b) correlation between the potential resuspension time of the fine particles and the seabed sediment distribution.     

Fluxes and seasonal changes in composition of organic matter in the English Channel

Dissolved and particulate organic matter (DOM and POM) have been investigated along a transect between Cherbourg and the Isle of Wight. In addition, the relative contribution of different sources of POM have been assessed by the use of lipid biomarkers (e.g. fatty acids). Seawater samples were collected at two depths (subsurface and above the bottom) at five stations located on the transect during five cruises (from September 1994 to July 1995). Particulate organic carbon (POC) and dissolved organic carbon (DOC) concentrations vary between 30–530 μg l⁻¹ and 0.5–2.7 mg l⁻¹, respectively, for all the cruises. Fluxes of POM and DOM have been estimated at 0.6×10¹² g yr⁻¹ and 6.5×10¹² g yr⁻¹ of carbon, respectively. General fluxes of water and therefore of DOC and POC are oriented eastward. However, around the Isle of Wight a westward oriented flux exists due to a gyre located in the area. The major DOC and POC fluxes occur in the central part of the Channel where the water column is deepest. Seasonal variations of different sources of POM (algal, bacterial and terrigenous) have been examined for the five cruises. The fresh algal organic fraction is relatively important in September in coastal waters with a predominance of diatom species on the English side, whereas it has a low or undetectable contribution during winter months. The bacterial fraction generally varies in concert with the algal component. It is low during the winter period and more important in bloom or post-bloom conditions, as for example in May. Terrestrial organic matter is restricted to coastal areas in September, and is present at low levels in May and July. Nevertheless, in November and February, terrigenous inputs have been clearly identified for the whole transect even in central waters.     

Design of hail measurement networks*

Abstract

Hail and rain data collected in the National Hail Research Experiment's 1976 dense precipitation network have proved useful in defining the requirements of hail measuring networks. It is shown, at least for the hailstorm of 22 June 1976, that the primary maxima and minima of the spatial distribution of hail mass are revealed by a hailpad spacing of about 4 km, and that increasing detail obtains with smaller spacings until with spacings of 0.4 to 0.8 km finer scale features with dimensions of 1–3 km become defined. Monte‐Carlo and conventional statistical analysis show that the confidence limits on the errer in estimating the true hail mass for a storm increase approximately linearly with the mean spacing of hailpads. For the hailfall of 22 June 1976, there is 90% confidence that the true hail mass is estimated within ± 10% for a hailpad spacing of 1.7 km. Estimates of hail kinetic energy and number of hailstones of this accuracy require that hailpads be approximately 10–20% closer or farther apart, respectively. There is no simple numerical relation between the densities of hailpad and wedge raingauge networks covering the same area such that, if satisfied, the networks would then provide estimates of hail mass and total precipitation of the same accuracy for any storm. There is considerable daily variation in the size of hailswaths and in the spatial distribution of hail mass within them, pointing to the need for a climatological study of these aspects of hailfall to assess properly the requirements that a hail network must meet in a given region.     

The North Devon Basin: a Devonian passive margin shelf succession

The North Devon Basin, situated in a more proximal passive margin regime than the rift basins to the south, is not constrained but its succession is thought to represent in large part the sediments debouched from a northerly hinterland. Rather than that immediate source being South Wales an original location of the basin well to the south-east and west of the Ardennes massif is considered probable, with its present position being attained by Carboniferous displacement along the Bristol Channel-Bray Fault. The basin's thick (6000 m) succession comprises terrestrial and marine deposits that form two major sedimentary cycles, which are apparently closely linked to rift basin formation to the south. The GCR sites span a relatively straightforward shelf succession that extends from the late Early Devonian to the Carboniferous. The sedimentology, palaeontology, and depositional environments of terrestrial and marine facies lithostratigraphical units are detailed, some sites providing the macrofossil assemblages important in the identification and definition by Sedgwick and Murchison of the Devonian System.     

Mechanisms controlling the intra-annual mesoscale variability of SST and SPM in the southern North Sea

Thermal and optical remote sensing data were used to investigate the spatial and temporal distribution of sea surface temperature (SST) and of suspended particulate matter (SPM) in the southern North Sea. Monthly SST composites showed pronounced seasonal warming of the southern North Sea and delineated the English coastal and continental coastal waters. The East-Anglia Plume is the dominant feature of the English coastal waters in the winter and autumn SPM composites, and the Rhine region of freshwater influence (ROFI), including the Flemish Banks, is the dominant feature of the continental waters. These mesoscale spatial structures are also influenced by the evolution of fronts, such as the seasonal front separating well-mixed water in the southern Bight, from the seasonally stratified central North Sea waters. A harmonic analysis of the SST and SPM images showed pronounced seasonal variability, as well as spring-neap variations in the level of tidal mixing in the East Anglia Plume, the Rhine ROFI and central North Sea. The harmonic analysis indicates the important role played by the local meteorology and tides in governing the SST and near-surface SPM concentrations in the southern North Sea. In the summer, thermal stratification affects the visibility of SPM to satellite sensors in the waters to the north of the Flamborough and Frisian Fronts. Haline stratification plays an important role in the visibility of SPM in the Rhine ROFI throughout the year. When stratified, both regions typically exhibit low surface SPM values. A numerical model study, together with the harmonic analysis, highlights the importance of tides and waves in controlling the stratification in the southern North Sea and hence the visibility of SPM.     

Late quaternary chronology of the riverine plain of southeastern Australia

Studies of the Murray River have revealed that its ancestral river system is the result of three separate phases of river activity, each consisting of a degradational and aggradational sub‐phase. To illustrate the relations, certain sections of the rivers were mapped in detail revealing characteristic terrace levels for the various phases. The regional pattern shows that immediately upstream from the Cadell Fault, and for a distance of 100 miles (160 km.) downstream, tectonically induced topographical changes have separated the individual ancestral rivers representative of each phase. Elsewhere, the three rivers are superimposed and follow the same general course. This pattern of separation by diversion also applies to the present rivers so that, in the diverted sector of the river system, old floodplains occur which were deserted by the present river. Initial water logging and salting problems were restricted to these deserted floodplains where they occur in the Irrigation Districts. A relative chronology of Quaternary events has been drawn up.