Bioerosion by chemosynthetic biological communities on Holocene submarine slide scars

Geomorphic, stratigraphic, and faunal observations of submarine slide scars that occur along the flanks of Monterey Canyon in 2.0–2.5 km water depths were made to identify the processes that continue to alter the surface of a submarine landslide scar after the initial slope failure. Deep-sea chemosynthetic biological communities and small caves are common on the sediment-free surfaces of the slide scars, especially along the headwall. The chemosynthetic organisms observed on slide scars in Monterey Canyon undergo a faunal succession based in part on their ability to maintain their access to the redox boundaries in the sediment on which they depend on as an energy source. By burrowing into the seafloor, these organisms are able to follow the retreating redox boundaries as geochemical re-equilibration occurs on the sole of the slide. As these organisms dig into the seafloor on the footwall, they often generate small caves and weaken the remaining seafloor. While chemosynthetic biological communities are typically used as indicators of fluid flow, these communities may be supported by methane and hydrogen sulfide that are diffusing out of the fresh seafloor exposed at the sole of the slide by the slope failure event. If so, these chemosynthetic biological communities may simply mark sites of recent seafloor exhumation, and are not reliable fluid seepage indicators.     

Application of wavelet transforms to the decoupling of free-roll decay data

This study presents the results of the decoupling of a free-roll decay test by wavelet transforms. A free-roll decay test was performed to determine the coefficients of damping terms in equations of motion. No motion of the model was restricted during the experiment. A slight yaw motion was found while the model was in the free-roll decay motion. It was necessary to extract a pure roll motion from the experimental data. Wavelet transforms were applied to the signals to extract the pure roll motion. The results were compared to those found through the Fourier transform. Discrete wavelet transforms were able to efficiently decouple the test signals, while the continuous wavelet transform and the Fourier transform could not.     

Copper, lead and cadmium in coastal waters of the western North Sea

Cadmium, lead and copper have been determined in both dissolved and particulate phases for 130 surface-water samples from the coastal zone of Scotland and England in the western North Sea. The concentrations reported are, in general, lower than those published previously for the same area.The distribution of total metal concentration between the dissolved and particulate phases varies for the three elements. The lead distribution is dominated by its associations with particulate material while cadmium shows the reverse behaviour, existing almost totally in the dissolved phase. Copper is intermediate in behaviour, with the dissolved phase generally being dominant.An inverse relationship between salinity and dissolved copper concentration suggests a freshwater origin for this element. It is suggested that sedimentary and/or anthropogenic sources affect dissolved copper concentrations in coastal waters. The dissolved lead distribution reveals plumes of contaminated coastal water. The lead distribution is, however, dominated by associations with particulate material and sediments which produce a sharp concentration gradient close to the coast. The Humber estuary is identified as a significant source of cadmium to the North Sea.     

Problems with biogenic silica measurement in marginal seas

Surface sediment samples from the Bohai, Yellow Sea, and the Pacific were used to assess biogenic silica (BSi) content and to study uncertainties in BSi measurements. The contents of BSi in the Bohai and Yellow Sea are all less than 1%. The dissolution of BSi in sediments from the Bohai and Yellow Sea is very important to maintain high levels of silicate in the water column. The non-biogenic silica from clay minerals has an obvious effect on BSi of sediment samples in the Bohai and Yellow Sea with low BSi and high clay minerals. The solid to solution ratio was found to have a great influence on BSi measurement, which can induce uncertainties up to 75%. The effect of loss by sorption and centrifugation is negligible. Interlaboratory comparison of techniques for BSi measurement by the wet alkaline extraction technique of Mortlock and Froelich [Deep-Sea Res. 36 (1989) 1415-1426] with clay correction was suggested to give no significant differences. However, differences in sediment compositions and reagent to sample ratio may limit the application of the wet alkaline extraction method.     

Seismic evidence of small-scale lacustrine drifts in Lake Baikal (Russia)

High resolution, single-channel seismic sparker profiles across the Akademichesky Ridge, an intra-basin structural high in Lake Baikal (Russia), reveal the presence of small sediment mounds and intervening moats in the upper part of the sedimentary cover. Such features interrupt the generally uniform and even acoustic facies and are not consistent with the hemipelagic sedimentation, which is expected on such an isolated high and which would produce a uniform sediment drape over bottom irregularities. The influence of turbidity currents is excluded since the ridge is an isolated high elevated more than 600-1000 m above adjacent basins. The mounded seismic facies, including migrating sediment waves and non-depositional/erosional incisions, strongly suggest that sediment accumulation was controlled by bottom-current activity. We interpret the mounds as small-scale (< few tens of km² in area) lacustrine drifts. Four basic types of geometry are identified: 1) slope-plastered patch sheets; 2) patch drifts; 3) confined drifts; 4) fault-controlled drifts. The general asymmetry in the sedimentary cover of the ridge, showing thicker deposits on the NW flank, and the common location of patch drifts on the northeast side of small basement knolls indicate that deposition took preferentially place at the lee sides of obstacles in a current flowing northward or sub-parallel to the main contours. Deep-water circulation in the ridge area is not known in detail, but there are indications that relatively cold saline water masses are presently flowing out of the Central Basin and plunging into the deep parts of the North Basin across the ridge, a process that appears to be driven mainly by small differences in salinity. We infer that the process responsible for the observed bottom-current-controlled sedimentary features has to be sought in these large-scale water-mass movements and their past equivalents. The age of the onset of the bottom-current-controlled sedimentation, based on an average sedimentation rate of 4.0 cm/ky, is roughly estimated to be as least as old as 3.5 Ma, which is generally regarded as the age of the onset of the last major tectonic pulse of rift basin development in the Baikal region.     

The Equatorial Atlantic Mid-Ocean Channel: An Ultra High-Resolution Image of Its Burial History Based on TOPAS Profiles

Multibeam bathymetric and ultra high-resolution seismic data reveal that the distal course of the Equatorial Atlantic Mid-Ocean Channel (EAMOC) extends further east and south than was previously known, and is controlled by the presence of morphologic highs related to the Fernando de Noronha Fracture Zone. Distal course of the EAMOC is buried by sediments, and does not have bathymetric expression on the seafloor. The channel fill consists of three seismic sequences, suggesting that the recent geological evolution of the channel is composed of successive phases of decreasing sedimentary activity that finally resulted in its complete burial. Tectonic and volcanic activity related to the Fernando de Noronha Fracture Zone and Ridge, together with the effect of strong pulses of the Antarctic bottom water current during the upper Pliocene are suggested to have contributed to the progressive burial and the final abandonment of the EAMOC.     

Fishers' responses to management measures and their socio-economic effects

This session presented a wide range of papers on cooperativeresearch, design of alternative gears and fishing strategies,and new analyses to support management. The contributed papersreflected a real desire to explore different approaches andstrategies to fishery management. It is apparent that fishery policy has had a mixed record ofsuccess and failure at best. In most regions of the world, manystocks are still being overfished or have been severely overfishedin the past. Although rebuilding programmes for overfished stockshave been developed, nationally as well as internationally,only a few have been successful, many others are works in progressand, in too many cases, rebuilding     

Isotopic and Elemental Variations of Carbon and Nitrogen in a Mangrove Estuary

Variations in elemental and isotopic ratios of suspended particulate matter (SPM) were investigated in the Guayas River Estuary Ecosystem (GREE) that empties into the Gulf of Guayaquil, Ecuador. Detritus in the system was identified on the basis of extremely high carbon:chlorophyll aratios (>1000). This material had mean δ¹³C of −26·4±0·3, δ¹⁵N of +4·8±0·2, and (C:N)_atomicof 14·1±0·9. The isotopic data were comparable to measurements reported for fresh and degrading mangrove leaves, whereas the elemental ratio was comparatively enriched in nitrogen. Isotope measurements of SPM throughout the GREE were more similar to values for riverine material and detritus compared with that for the coastal end-member. Values indicative ofin situproduced algae, sewage and shrimp pond effluent were only found at selected sites. Bacterial bioassays, which were used to document potential sources of dissolved organic matter in the GREE, were isotopically similar to SPM. This correspondence coupled with the relatively low (C:N)_aof SPM could be explained by bacterial immobolization of nitrogen onto detritus. Finally, tidal variations of (C:N)_aand δ¹³C at a brackish mangrove site were similar in magnitude to spatial variations encountered throughout the GREE. Based on these results, the authors caution that care must be taken when samples are taken for food-web studies in these systems.     

A detailed geochemical and rock magnetic study of dredged basalt from the Sheba Ridge,Gulf of Aden

We report the results of petrological, geochemical and rock magnetic studies of basalt dredged from the eastern end of the west Sheba Ridge during cruise 11/1979 of R. R. S. Shackleton to the Western Gulf of Aden. The ridge is part of the Red Sea-Gulf of Aden spreading axis and the basalts are olivine tholeiites. The abundances of some elements are characteristic of normal MORB (mid-ocean ridge basalt) but other elemental abundances suggest affinites with transitional-type MORB.The observed magnetic properties are interpreted in terms of the composition, concentration and microstructure of the magnetic mineral fraction by recourse to the available data on synthetic analogues. The analysis has been carried out in greater detail than has been attempted in previous magneto-petrological studies. It appears that submarine weathering of the magnetic minerals (maghemitization) brings about not only the expected change in composition but also a fall in concentration of the magnetic fraction. This could result from the removal-of-iron oxidation mechanism operating in the submarine environment. It is also found that the fall in remanence with increasing degree of maghemitization is not explicable in terms of the change in composition and concentration of the magnetic minerals. A further influence—probably microstructural change—significantly reduces the remanence intensity.     

Magnetic anomalies in the canary basin and the Mesozoic evolution of the central North Atlantic

The data from a recent magnetic compilation by Verhoefet al. (1991) off west Africa were used in combination with data in the western Atlantic to review the Mesozoic plate kinematic evolution of the central North Atlantic. The magnetic profile data were analyzed to identify the M-series sea floor spreading anomalies on the African plate. Oceanic fracture zones were identified from magnetic anomalies and seismic and gravity measurements. The identified sea floor spreading anomalies on the African plate were combined with those on the North American plate to calculate reconstruction poles for this part of the central Atlantic. The total separation poles derived in this paper describe a smooth curve, suggesting that the motion of the pole through time was continuous. Although the new sea floor spreading history differs only slightly from the one presented by Klitgord and Schouten (1986), it predicts smoother flowlines. On the other hand, the sea floor spreading history as depicted by the flowlines for the eastern central Atlantic deviates substantially from that of Sundvik and Larson (1988). A revised spreading history is also presented for the Cretaceous Magnetic Quiet Zone, where large changes in spreading direction occurred, that can not be resolved when fitting magnetic isochrons only, but which are evident from fracture zone traces and directions of sea floor spreading topography.Deceased 11 November 1991