Tidal flexure of ice shelves measured by tiltmeter

first_imgA new design of tiltmeter has been used to measure the tidal bending of the Ronne Ice Shelf, Antarctica. By this means the point at which the ice sheet leaves the land and starts floating on the sea has been defined to within 600m. Monitoring this position in future should give an indication of the state of balance of the West Antarctic ice sheet, often considered1 to be unstable even if not actually disintegrating. The hydrostatic level meter is light and simple and can operate unattended while recording continuously for periods of several weeks. Observations at eight different sites on Rutford Ice Stream (Fig. 1) gave tidal records on ice more than 1,700 m thick and at points up to 600 km from the nearest open sea.last_img read more

First Aspidorhynchidae (Pisces: Teleostei) from Antarctica

first_imgA new fossil teleost, belonging to the family Aspidorhynchidae Nicholson & Lydekker, Aspidorhynchus antarcticus sp. nov., was obtained from a block of reworked Upper Jurassic tuffaceous mudstone in the lower (Albian) part of the mid-Cretaceous Whisky Bay Formation of James Ross Island, Antarctic Peninsula. Additional material, assignable to Aspidorhynchus sp., was collected from in situ Upper Jurassic marine rocks (Nordenskjöld Formation) at Longing Gap, northern Antarctic Peninsula. Not only is this the first reported occurrence of the family from Antarctica, it is also the first unequivocal record of Aspidorhynchus outside Europe; prior to this discovery, the genus had only been reported with certainty in marine deposits from the Middle Jurassic to Lower Cretaceous of England, France and Germany. This is the earliest neopterygian fish so far recovered from marine rocks in the Antarctic.last_img read more

Pangea in Permian to Jurassic time

first_imgThe apparent polar wander paths (= APWPs) for the northern and southern continents have been redetermined using revised time-scales and data. When plotted on the conventional Pangea, Pangea A, the paths are distinct but essentially linear and parallel from Late Carboniferous to Late Permian time. Both paths then undergo a sharp change in direction and converge so as to overlap in Early to Middle Jurassic time. If the Permo-Triassic magnetic field was an axial dipole, then paleomagnetic data show that Pangea was mobile during most, and possibly all, of the mid-Triassic to mid-Jurassic interval. Because of the uncertainties in pole ages and positions, and errors in measurements, the new APWPs are consistent with a range of Pangeas. In particular, the data are compatible with two paleomagnetic Pangeas: A2 and a new class of Pangea here named the D Pangeas. These resemble Morel and Irving’s (1981) Pangea B and the C Pangeas of Smith et al. (1981): unlike the A2 Pangeas, the B, C and D Pangeas all require large (> 2000 km) relative motion between the northern and southern continents. Geological data require the transition from an earlier Pangea to Pangea A to take place along a transform zone, here named the Pangea Transform Zone, or PTZ. The PTZ is considered to be technically analogous to the Great Glen Fault of Scotland. Four types of possible PTZ are examined: passing through either the western or eastern end of the Brunswick Magnetic Anomaly of eastern North America, or north and south of Iberia. The geological data suggest that those Pangeas that require large displacements on PTZs that pass east of the Brunswick Magnetic Anomaly are improbable. B and C Pangeas are snapshots giving precise paleomagnetic reconstructions. D Pangeas fit long stretches of the APWPs as well as possible. A2 Pangeas fit the APWPs moderately well but also require the geological displacement to be smaller. The differences in displacement between the D and A2 Pangeas reflect geometrical relationships between Euler poles, PTZ curvature and the APWPs. The general geological evidence suggests that D Pangeas are less probable than A2 Pangeas but that they cannot be rejected. Transition from A2 to A is most likely to have occurred via a sequence of rotations about different poles rather than a single rotation. In detail, the conventional Pangea requires modification. Africa must be broken into at least two fragments along the Benue Trough and its northern continuation. Northwest Africa then fits more tightly into northern South America, which modifies the APWPs and reduces the gap between them. By including the effects of continental extension and using detailed geological data the gap can be reduced to one in which the A63 circles touch or overlap. Non-dipole fields or an expanding Earth need not be invoked to explain the mismatch of the APWPs on the conventional Pangea. The finite rotation required for the older Pangea to Pangea A is very close to Van der Voo and French’s (1974) single rotation for A2 to A but is actually the sum of three smaller rotations that reflect the modification to Pangea A, continental extension and the postulated PTZ displacement. Of the possible PTZs, a PTZ parallel to the West African continental magnetic anomaly with a displacement of about 170 km gives a better fit of this anomaly to the East Coast magnetic anomaly of North America. Further refinements to Pangea A2 will depend on a better knowledge of stretching factors on the passive continental margins of the modified Pangea A and incorporation of rotations due to major faults such as the South Atlas Fault in Morocco.last_img read more

Spatial patterns of diversity in the sea: bryozoan species richness in the North Atlantic

first_img1. We have examined large-scale geographical patterns in species richness for continental shelf bryozoan assemblages in the North Atlantic. Bryozoans are common and often abundant benthic organisms, but they have not previously been examined at this scale of resolution. 2. Assemblage species richness was estimated by sample species richness. This was highest at intermediate depths (10–75 m) at all latitudes where there were sufficient data, but there was no statistically significant variation with depth for the overall data set (all latitudes pooled). Mean assemblage species richness showed no significant variation with latitude, although the highest individual values were generally from lower latitudes. There is thus as yet no convincing evidence for a latitudinal cline in the alpha diversity of North Atlantic bryozoans. 3. Pooling of data into bins of 10 degrees of latitude, or into biogeographic provinces, to estimate regional species richness revealed significant undersampling. Two independent techniques to correct for this undersampling revealed a latitudinal cline in the regional species richness of North Atlantic bryozoans, with a peak around 10–30°N, and a steady decrease in richness north to 80°N. 4. Two measures of beta diversity (Whittaker and Jaccard) revealed relatively high turnover, presumably related to habitat heterogeneity within regional bins or to significant environmental variation across bins. There was a tendency for beta diversity to be higher at lower latitudes, as would be expected from a combination of a latitudinal cline in regional diversity with a mean assemblage species richness invariant with latitude. Null models were used to clarify the expected relationship between the two measures of beta diversity, and these indicated a strong influence of species-abundance structure in the North Atlantic bryozoan data.last_img read more

Contrasting strategies of provisioning and chick growth in two sympatrically breeding albatrosses at Campbell Island, New Zealand

first_imgThe provisioning strategies of two closely related species of albatross breeding sympatrically were studied at Campbell Island, New Zealand. Black-browed Albatrosses (Diomedea melanophrys) had a higher provisioning rate of chicks than Grey-headed Albatrosses (D. chrysostomaa) as a result of a higher feeding frequency. Provisioning and satellite tracking data suggest that Black-browed Albatrosses forage over neritic waters in trips of up to 5 days, in combination with longer trips over oceanic waters. In contrast, it was not possible to separate clearly short and long trips in Grey-headed Albatrosses, but they probably forage mostly over oceanic waters, combined with rafting or feeding near the colony during stays of short duration at sea. No inter-annual differences in foraging trip duration were apparent between years for either species. Chicks were fed larger meals at older ages and when in poorer condition, probably due to a limitation on the rate of assimilation of food. For both species, chick condition after feeding did not influence the duration of foraging trips. Black-browed Albatrosses from Campbell Island feed locally in neritic waters and up to 2,000 km from the colony, in contrast to conspecifics from other sites which feed principally over neritic waters. Grey-headed Albatrosses were largely dependent on oceanic resources as for conspecifics studied elsewhere. This study shows that foraging and provisioning strategies are flexible within species, allowing them to exploit more or less distant resources.last_img read more

Late Quaternary glacial history, flow dynamics and sedimentation along the eastern margin of the Antarctic Peninsula Ice Sheet

first_imgGeological and geophysical data from the NE Antarctic Peninsula continental shelf are used to reconstruct the glacial history, flow-dynamics and sedimentation of the Antarctic Peninsula Ice Sheet (APIS) along its eastern margin during the Late Quaternary. Ice advanced to the shelf edge during the Last Glacial Maximum (LGM) and deposited a stiff till across the shelf. The presence of highly attenuated bedforms indicates that fast-flowing outlets drained the APIS through cross-shelf troughs to the outer shelf after this ice advance. The bedforms are formed in deformation till in response to deforming bed processes. Deglaciation of Robertson Trough and the troughs of Prince Gustav Channel, Larsen-A and Larsen Inlet was continuous (and possibly rapid) on account of the absence of ice-margin recessional features. In contrast, grounding-zone wedges across the shelf of Northern Larsen-A and south of Prince Gustav Channel indicate that ice retreat was gradual and was punctuated by stillstands. The transition from a grounded ice sheet to ice shelf conditions was completed before 11–12 14C ka BP on the shelf south of Prince GustavChannel, and is marked across the shelf by a change from subglacial till to a transitional heterogeneous unit dominated by coarse-grained facies. Transitional sediments record mainly sub-ice shelf rain-out and restricted bottom current and sediment-laden plume activity, as well as localised debris flows. Meltwater-derived facies are largely absent indicating that release of meltwater was not significant beneath polar ice shelves, or during deglaciation of the APIS. In the broader context, the colder, eastern side of the APIS was extensive and was drained mainly through fast-flowing outlets (palaeo-ice streams). Therefore, the eastern APIS would have been an important contributor to sediment and iceberg flux to the Weddell Sea Embayment during the LGM and subsequent deglaciation.last_img read more

A new surface accumulation map for western Dronning Maud Land, Antarctica, from interpolation of point measurements

first_imgAs a result of intensive field activities carried out by several nations over the past 15 years, a set of accumulation measurements for western Dronning Maud Land, Antarctica, was collected, based on firn-core drilling and snow-pit sampling. This new information was supplemented by earlier data taken from the literature, resulting in I I I accumulation values. Using Geographical information Systems software, a first region-wide mean annual snow-accumulation field was derived. in order to define suitable interpolation criteria, the accumulation records were analyzed with respect to their spatial autocorrelation and statistical properties. The resulting accumulation pattern resembles well-known characteristics such as a relatively wet coastal area with a sharp transition to the dry interior, but also reveals complex topographic effects. Furthermore, this work identifies new high-return shallow-drilling sites by uncovering areas of insufficient sampling density.last_img read more

Numerical simulation of the Filchner overflow

first_imgThe plume of Ice Shelf Water (ISW) flowing into the Weddell Sea over the Filchner sill contributes to the formation of Antarctic Bottom Water. The Filchner overflow is simulated using a hydrostatic, primitive equation three-dimensional ocean model with a 0.5-2 Sv ISW influx above the Filchner sill. The best fit to mooring temperature observations is found with influxes of 0.5 and 1 Sv, below a previous estimate of 1.6 +/- 0.5 Sv based on sparse mooring velocities. The plume first moves north over the continental shelf, and then turns west, along slope of the continental shelf break where it breaks up into subplumes and domes, some of which then move downslope. Other subplumes run into the eastern submarine ridge and propagate along the ridge downslope in a chaotic manner. The next, western ridge is crossed by the plume through several paths. Despite a number of discrepancies with observational data, the model reproduces many attributes of the flow. In particular, we argue that the temporal variability shown by the observations can largely be attributed to the unstable structure of the flow, where the temperature fluctuations are determined by the motion of the domes past the moorings. Our sensitivity studies show that while thermobaricity plays a role, its effect is small for the flows considered. Smoothing the ridges out demonstrate that their presence strongly affects the plume shape around the ridges. An increase in the bottom drag or viscosity leads to slowing down, and hence thickening and widening of the plume.last_img read more

Exploring former subglacial Hodgson Lake, Antarctica Paper I: site description, geomorphology and limnology

first_imgAt retreating margins of the Antarctic Ice Sheet, there are a number of locations where former subglacial lakes are emerging from under the ice but remain perennially ice-covered. This paper presents a site description of one of these lakes, Hodgson Lake, situated on southern Alexander Island, west of the Antarctic Peninsula (72° 00.549′ S, 68° 27.708′ W). First, we describe the physical setting of the lake using topographic and geomorphological maps. Second, we determine local ice sheet deglaciation history and the emergence of the lake using cosmogenic isotope dating of glacial erratics cross-referenced to optically stimulated luminescence dating of raised lake shoreline deltas formed during ice recession. Third we describe the physical and chemical limnology including the biological and biogeochemical evidence for life. Results show that the ice mass over Hodgson Lake was at least 295 m thick at 13.5 ka and has progressively thinned through the Holocene with the lake ice cover reaching an altitude of c. 6.5 m above the present lake ice sometime after 4.6 ka. Thick perennial ice cover persists over the lake today and the waters have remained isolated from the atmosphere with a chemical composition consistent with subglacial melting of catchment ice. The lake is ultra-oligotrophic with nutrient concentrations within the ranges of those found in the accreted lake ice of subglacial Lake Vostok. Total organic carbon and dissolved organic carbon are present, but at lower concentrations than typically recorded in continental rain. No organisms and no pigments associated with photosynthetic or bacterial activity were detected in the water column using light microscopy and high performance liquid chromatography. Increases in SO4 and cation concentrations at depth and declines in O2 provide some evidence for sulphide oxidation and very minor bacterial demand upon O2 that result in small, perhaps undetectable changes in the carbon biogeochemistry. However, in general the chemical markers of life are inconclusive and abiotic processes such as the diffusion of pore waters into the lake from its benthic sediments are far more likely to be responsible for the increased concentrations of ions at depth. The next phases of this research will be to carry out a palaeolimnological study of the lake sediments to see what they can reveal about the history of the lake in its subglacial state, and a detailed molecular analysis of the lake water and benthos to determine what forms of life are present. Combined, these studies will test some of the methodologies that will be used to explore deep continental subglacial lakeslast_img read more