Pedestal craters on Ganymede

The high-resolution Voyager images of Ganymede show a class of fresh craters 6–89 km in diameter which is distinguished by an ejecta blanket similar to those seen for some types of Martian craters. One hundred and eighty-five were identified and studied for trends with respect to latitude, longitude, and terrain type. No correlation of the ratio of ejecta diameter to crater diameter was found as a function of latitude or longitude, and there is only a suggestion of a trend in this ratio with respect to major terrain types. Central peak frequency is greatest for the smaller crater diameters. Central pit occurrence dominates central peak occurrence at crater diameters ?35 km. We conclude that the ejecta morphology probably results from impact into an icy target. The question of whether atmospheric ejecta-particle drag contributes to ejecta blanket morphologies on planets with an atmospheric cannot be resolved entirely from the Voyager images. The image resolution is insufficient to show diagnostic flow features on the ejecta, if they exist, or to detect evidence of any other ejecta deposits which would lie beyond the pedestal, predicted by some researchers to exist only on bodies with an atmosphere.     

Quantifying turbulent mixing and oxygen fluxes in a Mediterranean-type,microtidal estuary

Experiments were carried out on an intermittent estuary during its closed (summer) and open (winter) states to identify the physical processes responsible for vertical mixing across the halocline, and to quantify vertical fluxes of oxygen and salt between water layers. During the blocked phase a two-layer structure was observed, with a brackish surface layer overlying old seawater. Within a deep basin the wind-driven turbulent mixing was consistent with the measured surface-layer turbulent dissipation, but the dissipation in the bottom layer appeared to be driven by internal seiching. In the shallow regions of the estuary vertical fluxes of dissolved oxygen were indicative of oxygen demand by respiration and remineralization of organic material in bottom water and sediments. During the estuary's open phase a three-layer structure was observed, having a fresh, river-derived surface layer, a middle layer of new seawater, and a bottom layer of old seawater. In the shallower regions surface-layer turbulent diffusion was consistent with the strong, gusty winds experienced at the time. The dissolved oxygen of the incoming seawater decreased to very low values by the time it reached the upstream deep basin as a result of the low cross-pycnocline oxygen flux being unable to compensate for the oxygen utilization. At least 50 % of the cross-pycnocline salt fluxes in the shallow reaches of the open estuary are suggested to be driven by Holmboe instabilities. Responsible Editor: Hans Burchard     

Geotechnical characterisation of stratocone crater wall sequences, White Island Volcano, New Zealand

Geotechnical characterisation is undertaken for 3 broad units comprising the bulk of the stratigraphy identified on White Island Volcano, Bay of Plenty, New Zealand, an active island stratovolcano. Field and laboratory measurements were used to describe rock mass characteristics for jointed lava flow units, and ring shear tests were undertaken to derive residual strength parameters for joint infilling materials within the lavas. Rock Mass Rating (RMR) and Geological Strength Index (GSI) values were calculated and converted to Mohr-Coulomb strength parameters using the Hoek-Brown criterion. Backanalysis of known landslide scarps was used to derive strength parameters for brecciated rock masses and hydrothermally altered rock masses. Andesite lava flows have high intact strength (σ_ci = 184 ± 50 MN m^− 2; γ = 24.7 ± 0.3 kN m^− 3) and typically 3 wide, infilled joint sets, one parallel to flow direction and two steeply inclined, with spacings of 0.3-1.7 m. Joints are rough, with estimated friction angles for clean joints of ?_j = 42-47°. Joint infill materials are clayey silts derived from weathering of wall rocks and primary volcanic sources; they have low plastic (54%) and liquid (84%) limits and residual strength values of c_r = 0 kN m^− 2 and ?_r = 23.9 ± 3.1°. RMR values range from 70 to 73, giving calculated strength parameters of c′ = 1161-3391 kN m^− 2 and ?′ = 50.5-62.3°. Backanalysis suggests brecciated rock masses have c′ = 0 kN m^− 2 and ?′ = 35.4°, whereas GSI observations in the field suggest higher cohesion (c′ = 306-719 kN m^− 2) and a range of friction angles bracketing the backanalysed result (?′ = 30.6-41.7°). Hydrothermally altered rock masses have c′ = 369 kN m^− 2 and ?′ = 14.9°, indicating considerable loss of strength, especially frictional resistance, compared with the fresh lava units. Values measured at outcrop scale in this study are in keeping with other published values for similar volcanic edifices; backanalysed data suggest weaker rock mass properties than those determined at outcrop. This is interpreted as a scale issue, whereby rock mass characteristics of a large rock mass (crater wall scale) are weaker than those of small outcrops, due in part to the overestimation of friction angle from measurements on small exposures.     

The analysis of nitrate in highly saline waters

The analysis of nitrate in seawater and hypersaline waters should take account of a significant “salt effect”. Procedures developed for fresh water and marine waters must be applied cautiously to highly saline waters. The most widely used standard method for the determination of nitrate-nitrogen (NO₃-N) in fresh and marine waters involves the quantitative reduction of nitrate by the Cd column technique followed by colorimetric procedures. In our study, three approaches to estimate NO₃-N in highly saline waters were examined. The first involved dilution. This approach overcame the salt effect but dilution limited the detection of low concentrations of nitrate in highly saline waters. The second involved the use of standard nitrate solutions in saline water. This method is not recommended because of nitrate impurities in AR grade salts. The third- and preferred approach-involved the use of standard additions. “Spikes” of a known volume of NO₃-N standard solution were added to natural saline waters. Nitrate values estimated by the stadard addition method were used to calculate an equation for salt error correction at different salinities applicable to waters with the same relative ionic composition as seawater. This could then be used to correct nitrate determinations in highly saline waters where standards made in distilled water were used for calibration. Many previously published data for NO₃-N in saline water used methods of analysis which do not take account of salt error and are therefore probably in error.     

Stable isotope analysis of some representative fish and invertebrates of the Newfoundland and Labrador continental shelf food web

We examined stable carbon and nitrogen isotopic signatures of 17 fish and 16 invertebrate taxa common to the Newfoundland and Labrador (NL) continental shelf food web. Particular sampling emphasis was placed on Atlantic cod (Gadus morhua) and related prey species (e.g. shrimp, Pandalus borealis, and capelin, Mallotus villosus). We found highly significant (p < 0.0001) differences between near-shore (bays) and offshore (shelf edge) δ¹⁵N signatures for cod, ‘other fish’ (pooled) and invertebrates (pooled). In contrast, there were only minor differences in δ¹³C signatures of ‘other fish’ (p < 0.05) and no difference for cod and invertebrates among the two habitats. We sampled at two times of the year (January and June) and found no systematic effect of season on both δ¹³C and δ¹⁵N in cod, ‘other fish’ and invertebrates. We calculated isotopic fractionation factors for cod from the entire shelf (mixed diet) and for cod with diets composed mainly of capelin or shrimp. These values ranged between 2.2‰ and 3.9‰ for δ¹⁵N and −0.4‰ and 0.8‰ for δ¹³C and, for δ¹⁵N, may reflect diet-related differences in bioenergetic status. We discuss potential mechanisms for near-shore versus offshore enrichment of δ¹⁵N signatures, and demonstrate the implications of this spatial variation on δ¹⁵N-derived trophic position estimates.     

The DMS-cloud albedo feedback effect: Greatly underestimated?

There are a number of ways by which the biosphere may counter any impetus for global warming that might be produced by the rising CO₂ content of earth's atmosphere. Evidence for one of these phenomena, the DMS-cloud feedback effect, is discussed in light of recent claims that it is not of sufficient strength to be of much importance.     

Terracette morphology and soil properties: A note on a canonical correlation study

Morphological and soil properties of terracettes have been examined using canonical correlation methods. Highly significant statistical relationships have been found linking some members of the soil property set to the morphological properties of the terracettes studied.