A PULSED PLASMA JET ACOUSTIC SOURCE FOR PROFILING THE OCEAN FLOOR1

A plasma gun source has been successfully used to obtain sub-bottom profiles. The profiles show better penetration than with a 3-5 kHz source and more resolution than with an air gun. The plasma gun source is compact, self-contained, and requires no complex auxiliary equipment. The device was deployed from large research vessels and a small boat. The plasma gun produces sufficient acoustic energy and with its characteristically short pulse length and broad bandwidth it is an attractive sub-bottom profiler, especially in shallow fresh or salt water.     

Zones of Progressive Regional Burial Metamorphism in Part of the Tasman Geosyncline, Eastern Australia

A succession of metamorphic zones can be recognized along theMolong Geanticline, an ancient rise within the Tasman Geosynclineof Eastern Australia. In order of increasing grade, the zonesin the volcanogenic rocks are: (1) an albite-quartz zone, containingneither prehnite, pumpellyite, actinolite, nor biotite; (2)a prehnite zone; (3) a prehnite-pumpellyite zone; (4) an actinolitezone; and (5) a biotite zone. Zeolite associations are not developed.Zones 4 and 5 lie within the greenschist facies. The rocks alongthis geanticline are generally only slightly deformed, lackingcleavage. The grade of this metamorphism within the Molong Geanticlineincreases partly in accordance with increasing stratigraphicdepth and partly increases eastwards towards the adjacent HillEnd Trough. Within the Trough, rocks show metamorphic assemblagesakin to those of Zones 4 and 5 as defined within the MolongGeanticline, but deformation is more extreme and a regionalcleavage is characteristic. In the region about the Hill EndTrough and the adjacent geanticlines there is an apparent variationfrom essentially static burial metamorphism of the geanticlinesthrough transitional types to tectonic dynamothermal metamorphismof the trough. Both metamorphisms are envisaged as having arisenduring the same general period of metamorphism, but deformationof the geanticlines was much less than deformation in the interveningHill End Trough.     

Spatial and temporal patterns of iceberg rafting (IRD) along the East Greenland margin,ca. 68°N,over the last 14 cal.ka

Twelve 1–2 m, 10-cm-diameter gravity cores collected in 1988 and 1991, from the continental shelf and fjords of East Greenland near Kangerlussuaq Fjord/Trough (ca. 68°N, 32°W), have distinct changes in lithofacies and in the quantity of iceberg rafted (IRD) sediments. These changes are readily observed in X-radiographs of the split cores. We quantify the IRD contribution through grain-size analyses and counting the number of clasts >2 mm from the X-radiographs. Chronological control is provided by acclerated mass spectroscopy ¹⁴C dates on foraminifera. During deglaciation, after 14 cal.ka there was one interval of IRD accumulation ca. 12–13 cal.ka, followed by a brief return to IRD conditions centred at 9 cal.ka. Thereafter, a prominent feature of most cores on the shelf is an increase in IRD accumulation that started ca. 5–6 cal.ka, and which has increased toward the present. Indicators of iceberg rafting, such as the net sand flux and numbers of clasts >2 mm ka⁻¹, follow a power law distribution when graphed against distance from the present East Greenland coast, a measure of the position of the glacier margins. The form of the relationship indicates that there is a dramatic decrease in the supply of sediment from the fjords to the shelf. These relationships are used to estimate changes in the location of the ice margin during the late Quaternary based on a site on the East Greenland slope, Denmark Strait, and to discuss factors that can negate such a simple transfer function. © 1997 by John Wiley & Sons Ltd.     

THE INFLUENCE OF PERMEABILITY ON COMPRESSIONAL WAVE VELOCITY IN MARINE SEDIMENTS*

To investigate the effect of permeability on the propagation of seismo-acoustic waves through marine sediments, a theoretical model based on Biot's equations is established which relates the compressional wave velocity measured at a fixed frequency to computed velocities at zero and infinite frequencies in terms of sediment porosity and permeability. The model is examined experimentally in a standard soil mechanics consolidation test (itself dependent, among other things, on sediment porosity and permeability) which has been modified to include measurements of compressional wave velocity at 1 MHz and shear-wave velocity at 5 kHz. This test allows the elastic modulus of the sediment frame to be assessed under different load conditions simultaneous with the velocity determinations. From a number of tests on different samples, five samples are chosen to typify the range of sediment sizes. The results show that the difference between the measured velocity at 1 MHz and the model-derived velocity at zero frequency increases with increasing particle size (from clays to fine sand), with decreasing porosity, and with increasing permeability. For sediments coarser than fine sand the simple model breaks down, possibly because of the dominance of scattering/diffraction effects at the high frequency of the experiment. Within this limitation the model seems satisfactory to offer a capability of predicting the permeability of a sea floor sediment to an order of magnitude by the in situ measurement of seismic velocities over a wide range of frequencies; the prediction process requires a good in situ determination of sediment porosity such as that offered by electrical formation factor measurements.     

The role of the microbial loop in Antarctic pelagic ecosystems

The Paradigm pelagic food web organization in Antarctic waters is undergoing fundamental revision evidence that large fractions of material and energy flow through the microbial food web. because of the unique Antarctic ecosystem conditions, the microbial food web performs some roles that are fundamentally different from those in oligotrophic temperate and tropical waters: 1) during winter, bacterial production, at the expense of slow-turnover DOM (dissolved organic matter) from the previous summer, could be a significant factor in the survival of over wintering animal populations; 2) microbial regeneration of ammonium in nitrate-replete Antarctic waters may spare the reductants necessary for nitrate assimilation and thus enhance primary productivity of deep-mixed light-limited phytoplankton; and 3) the small diatoms and phytoflagellates which dominate the Antarctic pelagic primary production are apparently directly digestible by the metazoan herbivores, whereas cyanobacteria which dominate the primary productivity in lower latitude oligotrophic waters are not digestible by the metazoan herbivores. These roles performed by the microbial loop may, in part, explain why Antarctic waters, in contrast to the lower latitude oligotrophic waters, have high levels of tertiary productivity despite low primary productivity.