Comparative volcanology and petrology of the atlantic island-arcs

first_imgComparisons are made between the Lesser Antilles and the South Sandwich Islands, the recent volcanic island chains at the eastern margins of the Caribbean and Scotia arcs. Although situated in similar geological and structural environments there are differences in the type of volcanic activity which prevails in these two arcs and in the petrography and chemistry of the lavas emitted. There is good evidence that the South Sandwich Islands are in general appreciably younger than the islands of the Lesser Antilles. Basaltic rocks predominate in the South Sandwich Islands whereas andesite is the dominant rock-type of the Lesser Antilles. Many of the lavas of the South Sandwich Islands, including the andesites and dacites are aphyric whereas those of the Lesser Antilles are almost invariably porphyritic. The basalts of the South Sandwich Islands are of tholeiitic type and the series shows more pronounced iron enrichment than does that of the Lesser Antilles. Basalts of the South Sandwich Islands have a lower Fe2O3/FeO ratio, contain lower concentrations of K, Sr and Ba and higher Cr, Co and Ni than the basalts of the West Indies. It is thought that the South Sandwich Islands may represent a volcanic island-arc in the early stages of development and the Lesser Antilles a later stage.last_img read more

Age-related changes in the testicular and antler cycles of reindeer, Rangifer tarandus

first_imgA total of 111 male reindeer of various ages was shot in all months of the year to study the relationship between the seasonal changes in testicular activity and the antler cycle. From the changes in testis weight, seminiferous tubular tissue area and plasma testosterone values and the occurrence of spermatogenesis, it is concluded that calves attain physiological puberty in their first year, during which they also complete an antler cycle. The amplitude of the cyclical change in testis weight and plasma testosterone values increases with age and can be correlated with the earlier onset of events in the spermatogenic and antler cycles of older animals. The duration of the spermatogenic and testosterone cycles of reindeer is short, and is inversely related to the long period spent without antlers. It is suggested that testosterone strongly influences the antler cycle of reindeer males.last_img read more

Drying processes in the Antarctic collembolan Cryptopygus antarcticus (Willem)

first_imgLive and dead specimens of the Antarctic collembolan Cryptopygus antarcticus (Willem) dry, under constant external conditions of temperature (25 or 35°C) and r.h. (0%), like most hygric soil arthropods. The drying curves of both live and dead specimens have three phases: an initial decrease; a constant rate period and a final falling rate period. The latter is exponential. Cryptopygus dries throughout these phases in a purely physical, passive manner. The constant rate period is not indicative of active metabolic or physiological regulation of water loss. The entire drying curve can be explained using drying theories developed for porous, hygroscopic solids. A similar situation pertains in the Antarctic lichen Umbilicaria antarctica Frey et Lamb. Geometrically, the percentage of water remaining at the onset of the falling rate period is related to the hydrated thickness of the collembolan cuticle or the lichen cortices. For Cryptopygus the hydrated thickness is estimated as 14 μm.last_img read more

The fish diet of black-browed albatross Diomedea melanophris and grey-headed albatrossD. Chrysostoma at South Georgia

first_imgThe fish component of the diet of black browed and grey-headed albatrosses at South Georgia was investigated by intercepting 155 meals from adults arriving to feed chicks during February 1986 and 1994. Fish represented 30% and 72% by mass of the diet of black-browed albatrosses and 14% and 60% by mass of the diet of grey-headed albatrosses in 1986 and 1994 respectively. We determined the identity and quantified the contribution (by numbers, size and mass) of fish species mainly by using otoliths (54 representing 9 taxa and 57 representing 17 taxa in black-browed and greyheaded albatross samples respectively). For blackbrowed albatrosses in 1986 the main fish prey wasPatagonotothen guntheri (77% of otoliths, 51% of estimated fish biomass) and a single large specimen ofIcichthys australis (40% estimated biomass), whereas in 1994 Pseudochaenichthys georgianus was the main fish prey (57% of estimated biomass) withMagnisudis prionosa (30%) andChampsocephalus gunnari (12%) also making substantial contributions. Grey-headed albatross samples from 1986 were dominated by southern lampreys (40% by number, 79% of estimated bio mass), lanternfish (32% of numbers, 9% by mass) andPatagonotothen guntheri (11% by mass); in 1994Champsocephalus gunnari (42% by numbers, 24% by mass),Magnisudis prionosa (13% by number, 36% by mass),Muraenolepis microps (90% by number),Pseudochaenichthys georgianus (15% by mass) and lanternfish (18% by number but only 1% by mass) were the main prey. The importance ofPatagonotothen guntheri to both species in 1986 and its absence in 1994 probably reflect albatrosses obtaining it from the commercial fishery, which was active in 1986 but closed in 1994. Otherwise the fish diet of black-browed albatrosses is dominated by krill-feeding fish, characteristic of the waters of the South Georgia shelf. In contrast, the grey-headed albatross diet comprises deeper water mesopelagic species, especially lanternfish, which reflect its affinity for the Antarctic Polar Frontal Zone and associated oceanic upwellings.last_img read more

Magnetopause motions in a Newton-Busemann approach

first_imgThe buffetting of the magnetosphere by the solar wind is the cause of many phenomena in the magnetosphere and the ionosphere, and affects the way in which we observe many other features. Examples of magnetospheric phenomena caused, wholly or in part, by solar wind pressure variations include cavity and field-line resonances (e.g., [1], [2]), travelling convection vortices [3], ring current particle diffusion [4], quasi-periodic emissions [5], and possibly substorm onsets (e.g., [6]; but see [7]) and magnetopause reconnection [8]. Examples of the way in which solar wind pressure variations affect the way we observe and interpret magnetospheric features include the structure of the magnetopause boundary layer [9] and possibly some FTE identifications [10], and similarly the structure of the magnetotail.last_img read more

Miocene changes in bottom current regime recorded in continental rise sediments on the Pacific Margin of the Antarctic Peninsula

first_imgA Fossil Mounded Sedimentary Body (MB) has been identified in the Miocene sedimentary record on the central continental rise west of Adelaide Island, Antarctic Peninsula, using multichannel seismic reflection profiles. The MB has an elongated NE trend away from a group of seamounts, and it developed between two troughs. We interpret it as a patch drift plastered against the NE (lee) side of an obstacle. The MB’s depositional patterns provide the first clear evidence of Early Miocene bottom current activity on the central rise, and they suggest that flow was towards the NE, probably as part of the Antarctic Circumpolar Current. This segment of the rise is, however, presently affected by a SW-flowing branch of Lower Circumpolar Deep Water. The change in bottom water flow took place during the Middle or Late Miocene, and we suggest that it is probably indicative of more widespread palaeoceanographic changes during this period.last_img read more

Sequential reduction of UV-B radiation in the field alters the pigmentation of an Antarctic leafy liverwort

first_imgUV-B radiation (280–315 nm), incident on the leafy liverwort Cephaloziella varians growing at Rothera Point on the Western Antarctic Peninsula, was manipulated in late austral spring 1998 by screens consisting of a novel combination of Plexiglas panes and polyester sheets. The screens transmitted approximately 79, 68, 48, 41 and 31% of daily UV-B radiation dose. Plants from under and outside the screens were sampled seven times over 35 days. Correlative analyses indicated that concentrations of an anthocyanin-like pigment located in leaf tips were positively associated with UV-B transmission at the last two samplings, that concentrations of total chlorophylls were negatively associated with UV-B transmission at the final sampling, and that those of UV-B screening pigments were positively correlated with transmission at three intermediate samplings. Plants exposed to low UV-B transmission levels were visibly greener than those exposed to high transmission levels after approximately 23 days. The closest relationships between the treatment and concentrations of UV-B screening pigments were associated with increased ambient biologically weighted UV-B dose received by plants in the 5.5 h before each sampling, which coincided with the passage of the ozone hole over Rothera Point. As C. varians emerged from melting snow and ice, concentrations of the anthocyanin-like pigment and chlorophyll respectively increased and decreased faster in plants exposed to high transmission levels of UV-B, relative to those exposed to low UV-B transmission levels.last_img read more

Editors’ comment

first_imgIn September 2011, Aberdeen (UK) hosted the World Conferenceon Marine Biodiversity (WCMB). Within this Conference, themultidisciplinary international Science Research Programme (SRP) of the Scientific Committee on Antarctic Research (SCAR) “Evolution and Biodiversity in the Antarctic — The Response of Life to Change (EBA)” was granted a generous platform, which included a full-day Side Meeting on Advances in Evolution and Biodiversity in MarineAntarctic Environments. To mark the importance of this subject, as an outcome of the WCMB we decided to promote an EBA Special Issue of Marine Genomics, focussing on marine biodiversity in the polar regions. The contributions in this issue address the role of environmentalchange, variability and extreme events in the biologicalprocesses of marine organisms, including subjects such asphylogeography, phylogeny, phenotypic plasticity and molecular/physiological adaptations in the vertebrates and invertebrates of both polar oceans.last_img read more

Could Ecosystem Assessment improve the protection of Antarctic ecosystems?

first_imgGuest Editoriallast_img

The effect of meltwater plumes on the melting of a vertical glacier face

first_imgFreshwater produced by the surface melting of ice sheets is commonly discharged into ocean fjords from the bottom of deep fjord-terminating glaciers. The discharge of the freshwater forms upwelling plumes in front of the glacier calving face. We simulate the meltwater plumes emanated into an unstratified environment using a non-hydrostatic ocean model with an unstructured mesh and subgrid-scale mixing calibrated by comparison to established plume theory. The presence of an ice face reduces the entrainment of seawater into the meltwater plumes, so the plumes remain attached to the ice front, in contrast to previous simple models. Ice melting increases with height above the discharge, also in contrast to some simple models, and we speculate that this ‘overcutting’ may contribute to a tendency of icebergs to topple inwards toward the ice face upon calving. The overall melt rate is found to increase with discharge flux only up to a critical value, which depends on the channel size. The melt rate is not a simple function of the subglacial discharge flux, as assumed by many previous studies. For a given discharge flux, the geometry of the plume source also significantly affects the melting, with higher melt rates obtained for a thinner, wider source. In a wider channel, two plumes are emanated near the source and these plumes eventually coalesce. Such merged meltwater plumes ascend faster and increase the maximum melt rate near the center of the channel. The melt rate per unit discharge decreases as the subglacial system becomes more channelised.last_img read more