Caught in the act: the 20 December 2001 gravity flow event in Monterey Canyon

A sediment gravity flow descended through the axis of Monterey Canyon on 20 December 2001 at 13:35 Pacific standard time. The timing of this event is documented by a current-meter package which recorded an 11.9-dbar pressure increase in less than 10 min and was found 550 m down-canyon from its deployment site, buried completely within a >70-cm-thick gravity flow deposit. This event is believed to have started in less than 290 m of water because an instrument at this location was also lost at the same time. A 178-cm core collected after the event from the axis of the canyon at 1,297-m water depth contained fresh, greenish, chlorophyll-rich organic material at 32-cm sub-bottom depth, suggesting the event extended to this water depth. The only trigger identified for this mass movement event appears to be moderate sea and surf conditions. Thus, gravity flow events of this magnitude do not require an exceptional triggering event.     

Boussinesq-type modelling using an unstructured finite element technique

A model for solving the two-dimensional enhanced Boussinesq equations is presented. The model equations are discretised in space using an unstructured finite element technique. The standard Galerkin method with mixed interpolation is applied. The time discretisation is performed using an explicit three-step Taylor–Galerkin method. The model is extended to the surf and swash zone by inclusion of wave breaking and a moving boundary at the shoreline. Breaking is treated by an existing surface roller model, but a new procedure for the detection of the roller thickness is devised. The model is verified using four test cases and the results are compared with experimental data and results from an existing finite difference Boussinesq model.     

Second-order drift force response of offshore guyed towers

An iterative frequency domain method of analysis is presented for determining the response behaviour of Guyed Offshore Towers to low-frequency, second-order wave drift forces generated in a random sea environment. For the response analysis, the tower is idealized as a shear beam with a rotational spring at the bottom support. The guylines are replaced by a non-linear spring. The second-order drift force is considered to be proportional to the square of the wave elevation and is simulated using a drift force coefficient and the time history of a slowly varying wave envelope in random sea. The responses due to drift forces are obtained in frequency domain by incorporating the non-linearities produced due to non-linear guy lines. An example problem is solved under different random sea states to compare the response behaviour of the tower obtained by the second-order wave force, the first-order wave force and a combination of the two.     

The formation and evolution of East Australian current warm-core eddies

Data on East Australian Current (EAC) warm-core eddies were obtained over the period 1976–1978 by the Department of Defence and the Commonwealth Scientific and Industrial Research Organization (CSIRO). In that time we have learned that warm eddies form by pinch-off of poleward EAC meanders, can coalesce with the EAC and appear generally similar to Gulf Stream, Kuroshio and other current system eddies. Two eddies were tracked over 1977–1978 with satellite buoys and one (eddy B) was repeatedly studied over eleven months. A deep winter core formed by winter convective cooling and the following summer a new surface mixed layer formed on top of the core. The seasonal changes have been analysed for heat content and changes in dynamic relief. The eddy decayed with a time constant of 650 ± 150 days, due to upwelling below the seasonal thermocline. Surface cooling had little effect on eddy lifetime. The eddy contracted horizontally, possibly after some interaction with the EAC, giving rise to eddy spin-up with increasing age. Surface currents increased after eleven months to 2.0 m s^?1. The dynamic relief during summer was also apparently boosted by contact with the EAC. Eddy B was observed to coalesce with a new meander of the EAC rather than drift away to the south. It is proposed that the formation of these eddies is governed by the westward propagation of the baroclinic Rossby wave known as the Tasman Front. Pinch-off of eddies adjacent to the coast and the variable flow of the EAC may be caused by the baroclinic wave ‘breaking’ on the coast. The eddy formation rate is about two per year and most eddies coalesce with the EAC and do not escape to the south. Eddies coalesce and re-separate, creating many subsurface isothermal layers from old cores south of 34°S.     

Random wave-current interactions in water of varying depth

Refraction of incoherent random gravity waves with currents and bottom topography results in spatial variations in the spectral characteristics of the free surface. Prediction of such variations based on the radiation transfer equation is in a simple analytic form for the case of one dimensional inhomogeneities in currents and topography. This analytic form is examined in terms of two-dimensional wave number- and polar frequency-direction spectra along the associated dynamic and kinematic constraints relevant to wave breaking and reflection. Results are specialized to the simplest case of horizontal shear currents in deep and shallow water with explicit examples to illustrate the relative and combined effects of currents and topography on free surface spectra.     

A detailed magnetic study of the Reykjanes Ridge between 63°00′N and 63°40′N

Immediately southwest of Iceland, the Reykjanes Ridge consists of a series ofen échelon, elongate ridges superposed on an elevated, smooth plateau. We have interpreted a detailed magnetic study of the portion of the Reykjanes Ridge between 63°00N and 63°40N on the Icelandic insular shelf. Because the seafloor is very shallow in our survey area (100–500 m), the surface magnetic survey is equivalent to a high-sensitivity, nearbottom experiment using a deep-towed magnetometer. We have performed two-dimensional inversions of the magnetic data along profiles perpendicular to the volcanic ridges. The inversions, which yield the magnetization distribution responsible for the observed magnetic field, allow us to locate the zones of most recent volcanism and to measure spreading rates accurately. We estimate the average half spreading rate over the last 0.72 m.y. to have been 10 mm/yr within the survey area. The two-dimensional inversions allow us also to measure polarity transition widths, which provide an indirect measure of the width of the zone of crustal accretion. We find a mean transition width on the order of 4.5±1.6 km. The observed range of transition widths (2 to 8.4 km) and their mean value are characteristic of slow-spreading centers, where the locus of crustal accretion may be prone to lateral shifts depending on the availability of magmatic sources. These results suggest that, despite the unique volcanotectonic setting of the Reykjanes Ridge, the scale at which crustal accretion occurs along it may be similar to that at which it occurs along other slow-spreading centers. The polarity transition width measurements suggest a zone of crustal accretion 4–9 km wide. This value is consistent with the observed width of volcanic systems of the Reykjanes Peninsula. The magnetization amplitudes inferred from our inversions are in general agreement with NRM intensity values of dredge samples measured by De Boer (1975) and ourselves. Our thermomagnetic measurements do not support the hypothesis that the low amplitude of magnetic anomalies near Iceland is the result of a high oxidation state of the basalts. We suggest that the observed reduction in magnetic anomaly amplitude toward Iceland may be the result of an increase in the size of pillows and other igneous units.