Seismic image of undercrusted serpentinite beneath a rifted margin

During the continental rifting the upper mantle was unroofed, and the mantle rocks were transformed into serpentinite at the ocean-continent transition of the west Galicia margin (Spain). The serpentinite layer, several km thick, extends probably eastwards, beneath the highly thinned continental crust of the margin.
The serpentinite layer was recently imaged by seismic reflection. It is discontinuously and deeply layered. As serpentinized peridotite can have densities and seismic velocities comparable to those of the lower continental crust, we suggest that undercrusting by serpentinite can play a part in building the lower seismic crust in highly stretched continental rifted areas.     

Products of an effusive subglacial rhyolite eruption: Bláhnúkur, Torfajökull, Iceland

We present field observations from Bláhnúkur, a small volume (<0.1 km³) subglacial rhyolite edifice at the Torfajökull central volcano, south-central Iceland. Bláhnúkur was probably emplaced during the last glacial period (ca. 115–11 ka). The characteristics of the deposits suggest that they were formed by an effusive eruption in an exclusively subglacial environment, beneath a glacier >400 m thick. Lithofacies associations attest to complex patterns of volcano-ice interaction. Erosive channels at the base of the subglacial sequence are filled by both eruption-derived material and subglacial till, which show evidence for deposition by flowing meltwater. This suggests that meltwater was able to drain away from the vent area during the eruption. Much of the subglacial volcanic deposits consist of conical-to-irregularly shaped lava lobes typically 5–10 m long, set in poorly sorted breccias with an ash-grade matrix. A gradational lavabreccia contact at the base of lava lobes represents a fossilised fragmentation interface, driven by magma-water interaction as the lava flowed over poorly consolidated, waterlogged debris. Sets of columnar joints on the upper surfaces of lobes are interpreted as ice-contact features. The morphology of the lobes suggests that they chilled within conically shaped subglacial cavities 2–5 m high. Avalanche deposits mantling the flanks of Bláhnúkur appear to have been generated by the collapse of lava lobes and surrounding breccia. A variety of deposit characteristics suggests that this occurred both prior to and after quenching of the lava lobes. Collapse events may have occurred when the supporting ice walls were melted back from around the cooling lava lobes and breccias. Much larger lava flows were emplaced in the latter stages of the eruption. Columnar joint patterns suggest that these flowed and chilled within subglacial cavities 20 m high and 100–200 m in length. There is little evidence for magma-water interaction at lava flow margins which suggests that these larger cavities were drained of meltwater. As rhyolite magma rose to the base of the glacier, the nature of the subglacial cavity system played an important role in governing the style of eruption and the volcanic facies generated. We present evidence that the cavity system evolved during the eruption, reflecting variations in both melting rate and edifice growth that are best explained by a fluctuating eruption rate.     

Breaking recent global temperature records

Global surface temperature was a record in 1988. What is the probability that this record will be surpassed in the next few years? Answers are provided given a variety of simple statistical models for temperature. The answers illustrate how record breaking is influenced by alternative model specifications. Estimates for the probability of a record are shown to range widely. If annual temperature is independent and identically distributed then a new record is unlikely. But probabilities increase rapidly if there is a trend or autocorrelation. Estimates of the probability of a record using data on global temperature suggest that another record in the next few years would not be a rare event.     

Electric potential gradient changes during explosive activity at Sakurajima volcano,Japan

We report electric potential gradient measurements carried out at Sakurajima volcano in Japan during: (1) explosions which generated ash plumes, (2) steam explosions which produced plumes of condensing gases, and (3) periods of ashfall and plume-induced acid rainfall. Sequential positive and negative deviations occurred during explosions which generated ash plumes. However, no deflections from background were found during steam explosions. During periods of ashfall negative electric potential gradients were observed, while positive potential gradients occurred during fallout of plume-induced acid rain from the same eruption. These results suggest that a dipole arrangement of charge develops within plumes such that positive charges dominate in the volcanic gas-rich top and negative charges in the following ash-rich part of the plume. The charge polarity may be reversed for other volcanoes (Hatakeyama and Uchikawa 1952). We suggest that charge is generated by fracto-emission (Donaldson et al. 1988) processes probably during magma fragmentation within the vent, rather than by frictional effects within the plume.     

Couplings of watersheds and coastal waters: Sources and consequences of nutrient enrichment in Waquoit Bay,Massachusetts

Human activities on coastal watersheds provide the major sources of nutrients entering shallow coastal ecosystems. Nutrient loadings from watersheds are the most widespread factor that alters structure and function of receiving aquatic ecosystems. To investigate this coupling of land to marine systems, we are studying a series of subwatersheds of Waquoit Bay that differ in degree of urbanization and hence are exposed to widely different nutrient loading rates. The subwatersheds differ in the number of septic tanks and the relative acreage of forests. In the area of our study, groundwater is the major mechanism that transports nutrients to coastal waters. Although there is some attenuation of nutrient concentrations within the aquifer or at the sediment-water interface, in urbanized areas there are significant increases in the nutrient content of groundwater arriving at the shore’s edge. The groundwater seeps or flows through the sediment-water boundary, and sufficient groundwater-borne nutrients (nitrogen in particular) traverse the sediment-water boundary to cause significant changes in the aquatic ecosystem. These loading-dependent alterations include increased nutrients in water, greater primary production by phytoplankton, and increased macroaglal biomass and growth (mediated by a suite of physiological responses to abundance of nutrients). The increased macroalgal biomass dominates the bay ecosystem through second- or third-order effects such as alterations of nutrient status of water columns and increasing frequency of anoxic events. The increases in seaweeds have decreased the areas covered by eelgrass habitats. The change in habitat type, plus the increased frequency of anoxic events, change the composition of the benthic fauna. The data make evident the importance of bottom-up control in shallow coastal food webs. The coupling of land to sea by groundwater-borne nutrient transport is mediated by a complex series of steps; the cascade of processes make it unlikely to find a one-to-one relation between land use and conditions in the aquatic ecosystem. Study of the process and synthesis by appropriate models may provide a way to deal with the complexities of the coupling.     

Age et croissance de la truite lacustre (Salmo trutta lacustris L.) du Léman

The age and growth of adult Lake Geneva trout (Salmo trutta lacustris L.) caught from 1964 to 1974 for mark-and-recapture experiments was studied by scale analysis. The median lengths of 3 years and older fish were fitted to Von Bertalanffy growth curves. Their growth rates are higher than those of the fish younger than 3 years. It is conculded that this increase of growth rate concides with beginning of life in the lake, after the downstream migration of the juvenile trout which probably takes place in the second and third summer. No general increase of growth rates between cohorts could be detected from the beginning to the end of the period studied, in spite of a serious increase of eutrophication of Lake Geneva during the same time.