Biogeochemical characterization of particulate organic matter from a coastal hydrothermal vent zone in the Aegean Sea

Three particle interceptor traps were moored from June to September 1996 along the SE coast of Milos Island (Aegean Sea), in an area known for its extensive seabed geothermal activity. The settling particles collected differed between sites not only in quantity but also in their geochemical composition. In the area directly influenced by warm water vents, the vertical flux of particulate material was almost one order of magnitude higher than that observed at the reference sites 6.5 km away from the vents, with a higher contribution of biogenic material in the former. The neutral lipid fractions exhibited a significant zooplankton biomarker signature (e.g. cholesterol and wax esters), followed by those of microalgal and terrigenous remains. The biogeochemical cycle of sterols included the formation of steroidal ketones, thiols and stanols, the latter two particularly associated with the venting activity. Moreover, the aliphatic and aromatic hydrocarbons in the particles from the venting area reflected a locally enhanced maturity of the organic matter. Although part of the material collected in the traps may have been exported in the vent waters, the higher fluxes observed in the vicinity of the hydrothermal plumes are mainly the result of an assemblage of zooplankton which, in turn, may contribute significantly to the sedimentation, through fast sinking faecal pellets, of the petrogenic hydrocarbons possibly generated in the vents.     

Non-linear predictions ofAp by activity class and numerical value

Two neural network algorithms are applied to the short-term,1 to 3 days, prediction of theAp geomagnetic index. A multi-layer, back-propagation (MBP) network is used to implement a self-prediction filter forAp and this provides a forecast of the numerical value of the index. A probabilistic neural network (PNN) is used to estimate the probability distribution of theAp index, in six activity classes, and to provide a forecast of the single most likely activity class for each day. BothAp and an index of solar activity, based on the daily reports issued by the Space Environment Services Centre (Boulder), are input to the probabilistic net. It is found that the numerical forecasts of the MBP filter are most accurate at low, non-storm, levels of activity. This non-linear method provides quantitatively better estimates of activity than are produced by an existing linear prediction filter, particularly with increasing forward forecasting lag. At high levels of the solar activity index the PNN is found to anticipate storm classAp with around 60% accuracy in 1992 and 1993. Some details of the algorithms and implementation issues are described. It is concluded that interplanetary field and solar wind data will be significant components of any of the possible future developments which are discussed.     

The 26 December 2004 Sumatra Tsunami: Analysis of Tide Gauge Data from the World Ocean Part 1. Indian Ocean and South Africa

The M_w = 9.3 megathrust earthquake of December 26, 2004 off the northwest coast of Sumatra in the Indian Ocean generated a catastrophic tsunami that was recorded by a large number of tide gauges throughout the World Ocean. Part 1 of our study of this event examines tide gauge measurements from the Indian Ocean region, at sites located from a few hundred to several thousand kilometers from the source area. Statistical characteristics of the tsunami waves, including wave height, duration, and arrival time, are determined, along with spectral properties of the tsunami records.     

Numerical Modelling of Tsunamis Generated by Hypothetical Landslides in the Strait of Georgia,British Columbia

--A modified and corrected version of the viscous slide model of Jiang and LeBlond (1994) is used to assess the tsunami risk associated with hypothetical underwater slope failures in two coastal areas of British Columbia having potentially unstable sediment deposits: (a) Malaspina Strait, separating the mainland coast and Texada Island in the central Strait of Georgia; and (b) Roberts Bank on the foreslope of the Fraser River Delta in the southern Strait of Georgia. The intent of this study is to demonstrate the capability of the model for tsunami risk assessment and to improve upon previous studies of tsunami risk in the region based on reasonable submarine landslide scenarios. The potential risk from tsunamis associated with slide failures has been examined, but the likelihood of failure events themselves was not considered. For the Malaspina Strait scenarios, simulated tsunamis are generated by failure of a lobe of perched sediment situ ated on the slope of eastern Texada Island. Failure as a flow slide of the estimated 1,250,000 m³ of sediment generates wave troughs reaching ц.9 m and trough-to-crest heights of 6 to 8 m along the coast of Texada Island. At Cape Cockburn, on the opposite side of the strait, wave heights of 1.5 to 2.0 m are produced. For Roberts Bank, simulated waves are examined for two separate failure scenarios. The larger slide (Case 1) involves the failure of a sediment lobe with lateral dimensions of 7 2 3 km² and volume of 0.75 km³ while the smaller slide (Case 2) fails a sediment lobe with dimensions of 4 2 2.6 km² and volume of 0.23 km³. Computations were made both for high (+3 m) and low (х m) tide conditions. For both failure volumes, maximum wave amplitudes (up to 18 m for Case 1 and 8 m for Case 2) occur on the coasts of Mayne and Galiano Islands, opposite the source area. Wave amplitudes are much smaller (1 to 4 m) on the mainl and coast because of the reflection of the initial waves from Roberts Bank. Additional numerical experiments were conducted for both regions to estimate the sensitivity of the computed tsunami wave heights to input parameters, such as slide viscosity, bulk density, and slide position.     

Mobility of authigenic rhenium, silver, and selenium during postdepositional oxidation in marine sediments

Sedimentary records of redox-sensitive trace elements hold significant potential as indicators of paleoceanographic environmental conditions. Records of Re can reveal the intensity of past reducing conditions in sediments at the time of deposition, whereas records of Ag may record the magnitude of past diatom fluxes to the seafloor. Confidence in paleoenvironmental reconstruction from records of either metal, however, requires it to have experienced negligible redistribution since deposition. This study examines diagenetic rearrangements of Re and Ag that occur in response to exposure to bottom-water O₂ in environments of low sedimentation rate, including Madeira Abyssal Plain turbidites and eastern Mediterranean basin sapropels. Authigenic Re was remobilized quantitatively by oxidation but poorly retained by the underlying sediments. All records are consistent with previous work demonstrating that only a limited reimmobilization of Re occurs preferentially in C_org-rich, reducing sediments. Silver was also mobilized quantitatively by oxidation, but it was subsequently immobilized more efficiently in all cases as sharp peaks immediately into anoxic conditions below active oxidation fronts, and these peaks remain immobile in anoxic conditions during long-term burial. Comparison of Ag, S, and Se records from various cores suggests that Ag is likely to have been immobilized as a selenide, a mechanism previously proposed for Hg in similar situations (Mercone et al., 1999). Coexisting narrow peaks of Ag and Hg with Se offer a means of assessing whether oxidative burndown has ever occurred at the top of C_org- and sulfide-rich sedimentary units. Although these results suggest that caution must be used when inferring paleoenvironmental information from records of Ag and Re in cores with low sediment accumulation rates (<5 cm ka⁻¹), they should not affect the promise that authigenic Ag and Re records hold for paleoenvironmental reconstruction in sediments with higher accumulation rates and where anoxic conditions have been maintained continuously.     

Minor axis rotation and the intrinsic shape of the shell elliptical NGC 3923

We present kinematic observations of NGC 3923, one of the best-known examples of an elliptical galaxy with shells. Although NGC 3923 has no rotation on its major axis, it has minor axis rotation with an amplitude of some 20 km s⁻¹ out to 25 arcsec radius. This is possibily the result of a kinematically decoupled core, but we present arguments against this hypothesis, and if the core were formed by a merger, this must have been a different merger from that which gave rise to the shells. The minor axis rotation suggests that NGC 3923 is prolate or triaxial, at least in the inner region. A prolate geometry for the whole galaxy would favour the merger model over the interaction model for the shell formation mechanism. There is other evidence, however, that the shape of the underlying galaxy changes with radius, in which case it is possible that the minor axis rotation could be confined to the galaxy core. The kinematics could reflect a triaxial mass distribution, in which the long and short axes are aligned with the photometric major and minor axes, and the axial ratios change such that the galaxy is near-prolate in the inner regions and near-oblate at larger radii. Measuring the rotation curve to about double this radius along the minor axis, as well as offset from the nucleus along the major axis at the radius of the innermost shells, would enable the true extent of the minor axis rotation to be determined.     

The impact of discharges from seafood processing on southeastern estuaries

A study to determine the impact of seafood packing and processing effluents discharged to southeastern estuarine waters was conducted in July and August of 1979. The environmental impact of current seafood processing wastes on Georgia’s estuaries appears to be minimal when compared with the natural organic load. One large estuary demonstrated a high residual capacity to receive processing effluents without significant change. The BOD load from shrimp thawing, peeling, sorting, and cleaning operations at a large seafood processing plant was shown to be equivalent to the organic material generated by a 302 m² plot (57 ft×57 ft) of salt marsh. NH₄?N levels were greater than, but the same order of magnitude as, natural runoff from marsh land.     

The growth response of an estuarine diatom (Melosira nummuloides [dillw.] Ag.) to UV-B (290–320 nm) radiation

An increase in the transmission of solar radiation in the UV-B region (specifically, 290–320 nm) is expected to occur as a result of anthropogenic degradation of stratospheric ozone. The potential impact of increased levels of UV-B radiation upon the biosphere is of ecological concern. In a previous study a community of estuarine organisms received a daily exposure to a simulated solar spectrum enhanced in UV-B radiation for a period of six weeks. A dominant species of diatom growing at the surface of these communities was isolated and identified to beMelosira nummuloides. Short chains of this diatom were irradiated for a four-hour period on each of three consecutive days. Fluorescent sunlamps filtered by a 290 nm cut-off filter (0.13–0.50 mm cellulose acetate) or a 315 nm cut-off filter (0.13mm Mylar ‘S’) provided a range of fluence which closely approximated natural fluence levels. A least squares regression analysis of the number of cell divisions on the biologically weighted fluence indicated a significant depression in the growth of this species by radiation in the 290–320 nm waveband.