Major Trough Research Articles

24-hour Pattern in Lag Time of Response by Firemen to Calls for Urgent Medical Aid

The aim of the study was to assess the group 24-h pattern of lag time (LT) in response by regular and volunteer firemen (RFM and VFM) to calls for medical help (CFMH), specifically calls for out-of-hospital cardiac arrest (OHCA). LT, duration in min between a CFMH and departure of service vehicle equipped with a semiautomated defibrillator and generally staffed with four well-trained and ready-to-go FM, represents the integrated duration of several processes, each with separate reaction and decision-making times. The exact time of each CFHM (in min, h, day, month, yr) was recorded electronically, and the exact departure time from the station of the responding FM vehicle was recorded by an on-duty FM. Overall, CFMH made up 53 ± 9% (SEM) of all emergencies calls for aid. To standardize the study methods, the reported findings are based on 568 CFMH specifically regarding OHCA that occurred during the 4-yr study span (January 2005 to December 2008). CFMH exhibited a 24-h pattern with a major peak at 10:00 h (mean ± SEM: n = 9.5 ± 1.6) and major trough at 01:00 h (n = 1.3 ± 0.3; t test, p < .001). From year to year and season to season, a 24-h pattern was detected in the total of CFMH/h with two peaks (∼10:00 and ∼17:00h) and two troughs (∼01:00 and ∼15:00 h) (analysis of variance [ANOVA], p < .01; Cosinor, p < .05 to < .003), with neither season- nor year-related differences (χ2, p > .05). In CFMH/h pooled time series, ANOVA-detected differences between the hourly means (p < .01), and Cosinor analysis validated a 24-h rhythm (p < .002). In raw data, the longest LT, indicative of poorest performance, occurred at 05:00 h (8.8 ± 0.7 min) and the trough of LT, indicative of best performance, at 16:00 h (4.3 ± 0.8 min (t test, p < .02). 24-h patterning in LT was validated both by ANOVA of hourly means (p < .0006) and Cosinor analysis (p < .05), with longest LT ∼05:00 h and shortest LT ∼16.00 h for data of the individual yearly time-series data. The 24-h LT rhythm was also validated in the pooled time series by Cosinor (p < .0001), with the 24-h mean ± SEM = 6 ± 0.17 min and acrophase (peak) of 03:00 h ± 88 min (SD). Curve patterns of CFMH/h and LT/h differed widely. As a group phenomenon, the LT 24-h rhythm mimics the 24-h pattern of performance, as demonstrated by many laboratory and field investigations. The stability of the LT rhythm between years and seasons and its weak relationship with the CFMH 24-h pattern favors the hypothesis of an endogenous component or origin. The nighttime trough of performance is presumably linked to the elevated risk of work accidents in the same population of FM.

The sedimentary legacy of a palaeo-ice stream on the shelf of the southern Bellingshausen Sea: Clues to West Antarctic glacial history during the Late Quaternary

A major trough (“Belgica Trough”) eroded by a palaeo-ice stream crosses the continental shelf of the southern Bellingshausen Sea (West Antarctica) and is associated with a trough mouth fan (“Belgica TMF”) on the adjacent continental slope. Previous marine geophysical and geological studies investigated the bathymetry and geomorphology of Belgica Trough and Belgica TMF, erosional and depositional processes associated with bedform formation, and the temporal and spatial changes in clay mineral provenance of subglacial and glaciomarine sediments. Here, we present multi-proxy data from sediment cores recovered from the shelf and uppermost slope in the southern Bellingshausen Sea and reconstruct the ice-sheet history since the last glacial maximum (LGM) in this poorly studied area of West Antarctica. We combined new data (physical properties, sedimentary structures, geochemical and grain-size data) with published data (shear strength, clay mineral assemblages) to refine a previous facies classification for the sediments. The multi-proxy approach allowed us to distinguish four main facies types and to assign them to the following depositional settings: 1) subglacial, 2) proximal grounding-line, 3) distal sub-ice shelf/sub-sea ice, and 4) seasonal open-marine. In the seasonal open-marine facies we found evidence for episodic current-induced winnowing of near-seabed sediments on the middle to outer shelf and at the uppermost slope during the late Holocene. In addition, we obtained data on excess 210Pb activity at three core sites and 44 AMS 14C dates from the acid-insoluble fraction of organic matter (AIO) and calcareous (micro-) fossils, respectively, at 12 sites. These chronological data enabled us to reconstruct, for the first time, the timing of the last advance and retreat of the West Antarctic Ice Sheet (WAIS) and the Antarctic Peninsula Ice Sheet (APIS) in the southern Bellingshausen Sea. We used the down-core variability in sediment provenance inferred from clay mineral changes to identify the most reliable AIO 14C ages for ice-sheet retreat. The palaeo-ice stream advanced through Belgica Trough after ∼36.0 corrected 14C ka before present (B.P.). It retreated from the outer shelf at ∼25.5 ka B.P., the middle shelf at ∼19.8 ka B.P., the inner shelf in Eltanin Bay at ∼12.3 ka B.P., and the inner shelf in Ronne Entrance at ∼6.3 ka B.P. The retreat of the WAIS and APIS occurred slowly and stepwise, and may still be in progress. This dynamical ice-sheet behaviour has to be taken into account for the interpretation of recent and the prediction of future mass-balance changes in the study area. The glacial history of the southern Bellingshausen Sea is unique when compared to other regions in West Antarctica, but some open questions regarding its chronology need to be addressed by future work.

Another Look at Interannual-to-Interdecadal Variations of the East Asian Winter Monsoon: The Northern and Southern Temperature Modes

Abstract This study investigates the causes of interannual-to-interdecadal variability of the East Asian (EA; 0°–60°N, 100°–140°E) winter monsoon (EAWM) over the past 50 yr (1957–2006). The winter mean surface air temperature variations are dominated by two distinct principal modes that together account for 74% of the total temperature variance. The two modes have notably different circulation structures and sources of variability. The northern mode, characterized by a westward shift of the EA major trough and enhanced surface pressure over central Siberia, represents a cold winter in the northern EA resulting from cold-air intrusion from central Siberia. The southern mode, on the other hand, features a deepening EA trough and increased surface pressure over Mongolia, representing a cold winter south of 40°N resulting from cold-air intrusion from western Mongolia. The cold northern mode is preceded by excessive autumn snow covers over southern Siberia–Mongolia, whereas the cold southern mode is preceded by development of La Niña episodes and reduced snow covers over northeast Siberia. These remarkably different spatiotemporal structures and origins are primarily associated with interannual variations. On the decadal or longer time scale their structures are somewhat similar and are preceded by similar autumn sea surface temperature anomalies over the North Atlantic and tropical Indian Ocean. The two modes found for the EA region also represent the winter temperature variability over the entire Asian continent. Thus, study of the predictability of the two modes may shed light on understanding the predictable dynamics of the Asian winter monsoon.

Unusual Late-Season Cold Surges during the 2005 Asian Winter Monsoon: Roles of Atlantic Blocking and the Central Asian Anticyclone

Abstract The highest frequency of late-winter cold-air outbreaks in East and Southeast Asia over 50 years was recorded in 2005, when three strong successive cold surges occurred in the South China Sea within a span of 30 days from mid-February to mid-March. These events also coincided with the first break of 18 consecutive warm winters over China. The strong pulsation of the surface Siberian Mongolia high (SMH) that triggered these events was found to result from the confluence of several events. To the east, a strong Pacific blocking with three pulses of westward extension intensified the stationary East Asian major trough to create a favorable condition for cold-air outbreaks. To the west, the dominance of the Atlantic blocking and an anomalous deepened trough in the Scandinavian/Barents Sea region provided the source of a succession of Rossby wave activity fluxes for the downstream development. An upper-level central Asian anticyclone that is often associated with a stronger SMH was anomalously strong and provided additional forcing. In terms of the persistence and strength, this central Asian anticyclone was correlated with the Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) only when SMH is weak (warm winters). During strong SMH seasons (cold winters) the correlation vanishes. However, during late winter 2005 the central Asian anticyclone was strengthened by the Atlantic blocking through both the downstream wave activities and a circulation change that affected the Atlantic and west Asian jets. As a result, the period from mid-February to mid-March of 2005 stands out as a record-breaking period in the Asian winter monsoon.

Morphology of the upper continental slope in the Bellingshausen and Amundsen Seas – Implications for sedimentary processes at the shelf edge of West Antarctica

Swath bathymetric and sub-bottom profiler data reveal a variety of submarine landforms such as gullies, slide scars, subtle shelf edge-parallel ridges and elongated depressions, and small debris flows along the continental shelf break and upper slope of West Antarctica. Gullies cutting through debris flow deposits on the Belgica Trough Mouth Fan (TMF) suggest formation after full-glacial deposition on the continental slope. The gullies were most likely eroded by sediment-laden subglacial meltwater flows released from underneath the ice margin grounded at the shelf edge at the onset of deglaciation. Scarcity of subglacial meltwater flow features on the outer shelf suggests that the meltwater reached the shelf edge mainly either through the topmost layer of soft diamict or in the form of dispersed flow beneath the ice, although locally preserved erosional channels indicate that more focused and higher-energy flows also existed. Concentration of gullies on the upper continental slope in front of the marginal areas of the major cross-shelf troughs, as contrasted to their axial parts, is indicative of higher-energy gully-eroding processes there, possibly due to additional subglacial meltwater flow from beneath the slow moving ice lying over the higher banks of the troughs. The shallow and sinuous gully heads observed on the outermost shelf within the Pine Island West Trough may indicate postglacial modification by near-bed currents resulting either from the subglacial meltwater flow from underneath the ice margin positioned at some distance landward from the shelf edge, or from the currents formed by brine rejection during sea ice formation. On the continental slope outside major troughs, slide scars as well as shelf-edge parallel ridges and elongated depressions indicate an unstable and failure-prone uppermost slope, although failures were probably mainly associated with rapid sediment loading during glacial periods. Complex, cauliflower- and amphitheatre-shaped gully heads biting back into the shelf edge suggest upslope retrogressive, multi-stage small-scale sliding as a contributing factor to the formation of gullies in these areas. Small debris fans immediately downslope of the slide scars suggest that small-scale debris flows have been the main downslope sediment transfer processes in the areas of weak or absent subglacial meltwater flow.

A recount of Ross Sea waters

Abstract Oceanographic observations within the Ross Sea have grown dramatically in recent years, both in number and quality. This has prompted a parallel recount of the circulation and structure of all water masses in the southwestern continental margins of the Pacific Ocean. A high-resolution set of horizontal property distributions was combined into a new climatology, which in turn is the basis of a fine volumetric θ − S census of all Ross Sea water masses. Inshore of the shelf break (700 m isobath) the Ross Sea volume (25×10 4 km 3 ) partitioning into layers of neutral density ( γ n ) is: 25% in the top layer ( γ n −3 ) of Antarctic Surface Water (AASW), 22% in the middle layer (28.00 kg m −3 γ n −3 ) of oceanic thermocline water (12%) and Modified Circumpolar Deep Water (MCDW; 10%), and 53% in the bottom layer ( γ n >28.27 kg m −3 ) of Shelf Water (SW, θ θ >−1.85 °C; 22%), precursor of the Antarctic Bottom Water (AABW) offshore. AASW flows westward along the outer shelf and southward along the eastern coast past Cape Colbeck. Oceanic thermocline waters (28.00 kg m −3 γ n −3 ) cross the shelf break west of 170°W and follow southward paths along banks, shoaling progressively to near the sea surface of the central and western inner shelves. Winter buoyancy loss converts AASW and oceanic thermocline water into denser types of MCDW and SW, which move cyclonically toward the sills of major troughs. The census shows a continuous mode of relatively dense MCDW (28.10 kg m −3 γ n −3 ) directly linked to the Circumpolar Deep Water (CDW) offshore, effectively ventilating and freshening the deep ocean at levels below the salinity maximum and above AABW. MCDW outflows from the Ross Sea shelf are traced to the Antarctic Circumpolar Current: near 155°E, into the Australian-Antarctic Basin, along the northern flank of the Southwest Pacific Ridge, and to near 135°W, past the eastern end of the Ross Gyre. Two sources of salty SW are inferred near the coastal polynyas in the Terra Nova Bay and western Ross Ice Shelf areas. The latter contributes to a major outflow of Ice SW that reaches the shelf break along 180°. Vertical mixing of MCDW and SW produce MSW over the shelf, continuing over the sills as new AABW. Export of low salinity AABW ( S

Characterization of the Interactions of Lysozyme with DNA by Surface Plasmon Resonance and Circular Dichroism Spectroscopy

Association with nucleic acid has been recognized as a unique role of lysozyme and may explain why lysozyme was called a killer protein against HIV infection. In the present study, we characterized the interactions of lysozyme and its derived peptides with a biotin-labeled pUC19 plasmid DNA. Real-time detection of the macromolecular interaction was performed using the SPR (surface plasmon resonance) spectroscopy. The SPR sensorgrams were analyzed and the association and dissociation rate constants as well as the dissociation equilibrium constant KD were, thus, estimated. The results reveal that other than the electrostatic interactions between the basic protein and the nucleotide sequences carrying negative charges, the specific DNA-binding motifs at the N- and C-termini of lysozyme were also involved in the interactions. The nonapeptide RAWVAWRNR (aa 107-115 of lysozyme) reported previously to block HIV-1 viral entrance and replication was also able to bind DNA with its KD value comparable to that of histones. The possibilities of ligand-binding-induced conformational changes were investigated using the circular dichroism spectroscopy. The CD spectra (200-320 nm) reveal that the conformational changes indeed occur as the spectra of lysozyme-DNA interactions are much less at the major trough region than the sum of individual spectra. The interaction of lysozyme with DNA molecules may interfere with DNA replication, modulate gene expression, and block bacterial and viral infections. These all suggest that human lysozyme may represent part of the innate immune system with a very broad protective spectrum.

A new bathymetric compilation highlighting extensive paleo–ice sheet drainage on the continental shelf, South Georgia, sub‐Antarctica

A grid derived from a new compilation of marine echo‐sounding data sets has allowed us to visualize and map the geomorphology of the entire continental shelf around South Georgia at an unprecedented level of detail. The grid is the first continuous bathymetric data set covering South Georgia to include multibeam swath bathymetry and represent them at a subkilometer resolution. Large and previously undescribed glacially eroded troughs, linked to South Georgia's modern‐day fjords, radiate from the island, marking the former pathways of large outlet glaciers and ice streams. A tectonic or geological influence is apparent for the major troughs, where glaciers have exploited structural weaknesses on the continental block. Bed forms lining the troughs give some first insights into glacial dynamics within the troughs, suggesting arteries of fast flowing ice occupied these topographic depressions in the past and operated over both bedrock and sedimentary substrates. On the outer shelf and within the troughs, large ridges and banks are also common, interpreted as terminal, lateral, and recessional moraines marking former positions of ice sheets on the shelf and their subsequent reorganization during deglaciation. A small trough mouth fan has developed at the mouth of at least one of the cross‐shelf troughs, demonstrating a focused sediment delivery to the margin. Slides and slide scars are also present on parts of the margin, showing that margin stability, perhaps also related to glaciation, has been an important factor in depositional processes on the continental slope. Implications of the new observations are that ice sheets have been more extensive on South Georgia than any previous studies have reported. Their age may date back to late Miocene times, and evolution of the shelf system has probably involved numerous late Cenozoic glacial episodes. However, relatively fresh seafloor geomorphology coupled with evidence from other maritime‐Antarctic islands (Heard Island and Kerguelen Island) indicating extensive glaciation at the Last Glacial Maximum raises the possibility that the extent of sub‐Antarctic glaciation for the Last Glacial period has, until now, been underestimated.

Crustal collapse in the Andean backarc since 2 Ma: Tromen volcanic plateau, Southern Central Andes (36°40′–37°30′S)

Analysis of seismic lines and gravity data shows the existence of Quaternary extensional depocenters beneath thick covers of < 1 Ma-old volcanic rocks in the Tromen volcanic plateau backarc region (36°40′–37°30′S). Dating and mapping of pre- and post-extensional volcanic units and structure in this area indicate that the main phase of crustal collapse developed during the 1.7–0.7 Ma time interval. However, Late Quaternary reactivations of the extensional structure show that this process is still going on, perhaps with decreasing intensity. Moreover, identification of “mechanical” rift zones and a potentially related main thermal anomaly leads to propose that an east-dipping detachment exists beneath the Late Cretaceous–Late Miocene fold and thrust belt, controlling the crustal collapse at these latitudes. The southernmost Central Andes (35°–37°30′S) have undergone a period of shallow subduction from ~ 13 to ~ 5 Ma, that led to expansion of the arc toward the foreland, generation of ductile–fragile crustal transitions, and subsequent foreland imbrications more than 550 km away from the trench. This framework shifted to a normal Andean subduction type after ~ 5 Ma, and the arc front re-established in the present western position. The consequences of this readjustment were: a) widespread volcanic eruptions of intra-plate melts in the eastern backarc (foreland plateau flows), and b) a major trough formed between the arc front and the foreland plateau basalts (Las Loicas trough). This extensional basin controlled the emplacement of crustal melts as well as primary mantle-derived products, well represented in the Tromen volcanic plateau.

Lunar domes in Mare Undarum: Spectral and morphometric properties, eruption conditions, and mode of emplacement

In this study we examine five lunar domes in Mare Undarum. Four domes termed Condorcet 1–4 are located between the craters Condorcet P and Dubiago, immediately east of Dubiago V and W. The fifth dome, termed Dubiago 3, is located about 35 km further south. The region under study is situated in a major trough concentric to the Crisium basin. The domes Condorcet 1–3 are aligned radially with respect to the Crisium basin. Similar dome configurations aligned radial to major impact basins are known from other lunar mare dome fields. The spectral signature of the domes derived from Clementine UV/VIS imagery reveals that they consist of basaltic lava with a low TiO 2 content below 2 wt% and with a FeO content around 10 wt%. Three examined domes exhibit highland components in their soils, which we attribute to lateral mixing between the material in the mare ponds and the surrounding highland material due to random impacts. All five domes have moderate diameters between 10 and 12 km. Condorcet 1–3 are similar to effusive domes of intermediate flank slope between 1 ∘ and 2 ∘ like those situated in the Hortensius/Milichius/T. Mayer region, while Condorcet 4 has an exceptionally steep flank slope of 2 . 8 ∘ and a large volume. With its low flank slope of 0 . 9 ∘ , the dome Dubiago 3 is morphometrically very similar to a known intrusive dome in the west of Mare Serenitatis. Hence, this structure is possibly of intrusive origin, but with the available data an effusive origin cannot be ruled out. Based on a rheologic model, we infer the physical conditions under which the domes were formed (lava viscosity, effusion rate, magma rise speed) as well as the geometries of the feeder dikes.

The age and origin of the Labyrinth, western Dry Valleys, Antarctica: Evidence for extensive middle Miocene subglacial floods and freshwater discharge to the Southern Ocean

A 50! -km-long network of bedrock channels and scoured terrain occupies the ice-free portion of a major trough that crosses the Transantarctic Mountains in southern Victoria Land. The channels, collectively termed the Labyrinth, emerge from beneath the margin of the East Antarctic Ice Sheet (Wright Upper Glacier) and are incised into a 300-m-thick sill of Ferrar Dolerite at the head of Wright Valley. Upper- and intermediate-elevation erosion surfaces of the Labyrinth exhibit striations and molding characteristic of glacial erosion. Channels and canyons on the lower surface are as much as 600 m wide and 250 m deep, have longitudinal profiles with many reverse gradients, and contain potholes ! 35 m deep at tributary junctions. These characteristics are most consistent with incision from fast-flowing subglacial meltwater; estimated discharge is on the order of 1.6-2.2 10 6 m 3 s # 1 . Our 40 Ar/ 39 Ar analyses of volcanic tephra from the Labyrinth show that the chan- nels are relict, that major channel incision predates 12.4 Ma, and that the last major subglacial flood occurred sometime between 14.4 Ma and 12.4 Ma. The most plausible origin for the Labyrinth is erosion associated with episodic drainage of subglacial lakes in East Antarctica. One compelling possibility is that discharge of large volumes of sub- glacial meltwater to the Southern Ocean, and to the Ross Sea in particular, may have coincided with, and contributed to, oscillations in regional and/or global climate during the middle Miocene.

Palaeoglaciology of the Welsh sector of the British–Irish Ice Sheet

This palaeoglaciological reconstruction, based on the distribution of glacial lineations interpreted from satellite imagery, considers the extent, thickness and dynamics of the Welsh Ice Cap and its interaction with the British–Irish Ice Sheet. Two major ice-flow events were identified. The oldest phase of ice flow (Event I) is characterized by ice flowing from an ice dispersal centre situated over the higher terrain in north–central Wales and by ice thick enough to cover the mountain summits. At this time the interior of the ice cap is inferred to have been dominated by cold-based ice. During the growth of the Welsh Ice Cap it became confluent with the British–Irish Ice Sheet. The Welsh Ice Cap at this time was again sufficiently thick to submerge the highest mountain summits. The youngest detected phase of ice flow (Event II) was marked by an abrupt change in the dynamics of the Welsh Ice Cap. During Event II the Welsh Ice Cap was drained by at least four ice streams, which followed major troughs in northern and eastern Wales.

Birth seasonality in the Old Order Amish.

The Old Order Amish are a healthy and well-nourished natural fertility population, so that the timing of births is not influenced by behaviours to limit family size, undernutrition or disease. The present study examines the monthly distribution of 8160 births occurring between 1920 and 1991 in the Geauga Settlement in north-east Ohio, USA. The monthly distribution of births in the Geauga Settlement is bimodal, with a major peak extending from August to October, a minor peak in February, and a major trough from April to June. This pattern is almost identical to the pattern found in the US in 1943. The monthly distribution of first births appears to be influenced to some extent by a highly significant seasonal pattern of weddings. The pattern of births in the Old Order Amish is consistent with the hypothesis that the spring trough in US births is at least partially caused by a decrease in coital frequency and/or a decrease in fecundability as a result of hot summer temperatures but is not consistent with the hypothesis that the fall peak in US births is primarily due to an increase in coital frequency during the Thanksgiving and Christmas holiday seasons.

Antarctic ice and sediment flux in the Oligocene simulated by a climate–ice sheet–sediment model

A model of deforming sediment is added to a climate–ice sheet model, and applied to the Eocene–Oligocene transition in Antarctic ice volume around 34 Ma. The coupling between the global climate and ice sheet models is asynchronous, with a climate simulation performed once every 10 000 years, and only for the first 40 000 years. These global climate model solutions are re-used to perform runs of 400 000 years in length, with orbital forcing and different levels of CO 2. The sediment model includes bulk transport under ice, generation of sub-ice till, and river transport, and predicts the continental-scale evolution of sediment thickness and coastal discharge. For the first few 10 000’s of years after the onset of substantial ice (triggered by CO 2 falling below ∼3× pre-industrial level), sediment discharge to the ocean is relatively uniform around the coast, derived from nearby pre-existing regolith. After that, most discharge is concentrated into ∼4 sites at the mouths of major trough systems, with orbitally paced pulses on 10 4-yr scales. These sites and the average magnitudes of the fluxes agree generally with those deduced from offshore seismic and core data, although longer (10 my) integrations are needed to model the real sediment evolution.

Relationship between topography and flow in the north polar cap on Mars

AbstractAssuming that the permanent north polar cap of Mars consists of flowing water ice, the relationship between topography and flow is examined along a profile extending from the pole in the 160° E direction This profile is intersected by scarps and troughs that are characteristic of the north polar cap The flow is calculated by a finite-element ice-flow model which includes divergence of the flow, longitudinal stresses and temperature effects. Ice-flow velocities are generally on the order of 0.1–1 mm a–1 but are enhanced at scarps and troughs to cm a–1. Ice flow smooths out the troughs. Troughs affect the flow to the bottom of the cap. Beneath a trough, ice is dragged upward. Longitudinal stresses are able to drag the lowest part of the ice past smaller troughs. At the pole-facing side of major troughs, ice is stagnant or flows slowly poleward. Implications for formation mechanisms of scarps and troughs are discussed.

The response of the East Antarctic ice-sheet to the evolving tectonic configuration of the Transantarctic Mountains

The landscape of the Transantarctic Mountains is the result of the coupled evolution of the West Antarctic rift system and the East Antarctic ice-sheet. Studies of this glacial–tectonic system generally assume that the evolving surface elevation of the Transantarctic Mountains is a key determinant of the changing East Antarctic ice-sheet dynamics between the Miocene and today. Here, we extend previous work [Huybrechts, Ph., 1993. Glaciological modelling of the Late Cenozoic East Antarctic ice-sheet: stability or dynamism? Geografiska Annaler Stockholm, 75A (4) 221–238.] by using numerical models of the ice-sheet and lithosphere to examine the impact of different bedrock surface elevations of the Transantarctic Mountains on ice-sheet dynamics. There are widely different interpretations of the evolution of the Transantarctic Mountains from the available data, so we explore bedrock surface elevations suggested by empirical evidence in recent papers about the sensitivity of the Late Cenozoic ice-sheet. The results show that the surface elevation of individual mountain blocks has only a very local effect on ice-sheet dynamics. The existing mountain blocks of the Transantarctic Mountains, which force inland ice to drain through troughs adjacent to the mountain blocks, were overriden by inland ice when bedrock elevations were 1 km lower. When the troughs through the mountains were less well developed, in the Pliocene or Miocene, inland ice was thicker and ice-surface gradients and ice-velocities across the mountains were higher. This led to more active and erosive outlet glaciers through the mountains and the further development of these troughs. From these results, the key determinant of East Antarctic ice dynamics appears to be the interplay between the development of major troughs through the Transantarctic Mountains and rising mountain elevations. The glacial history of the central Transantarctic Mountain ranges was very different to that of more peripheral mountain ranges, such as the Dry Valleys and Victoria Land. The development of independent ice centres in the latter regions and the overriding of these ice centres by the main ice-sheet is very sensitive to the timing of surface uplift and the particular climate profile of the period. Conversely, the ice-surface profile across the central ranges is similar under widely different climates. The limitations of such a study stem from the necessarily schematic bedrock elevations input to the model and simplifications within the models. At present, insufficiently detailed modelling of the impact of troughs on ice-sheet dynamics means this paper is necessarily speculative. However, this work points to the importance of the outlet troughs on ice-sheet dynamics, rather than simply the rising surface elevations of the Transantarctic Mountains along the rift margin upwarp.

Role of extensional structures on the location of folds and thrusts during tectonic inversion (northern Iberian Chain, Spain)

The Aguilón Subbasin (NE Spain) was originated during the Late Jurassic–Early Cretaceous rifting due to the action of large normal faults, probably inherited from Late Variscan fracturing. WNW–ESE normal faults limit two major troughs filled by continental deposits (Valanginian to Early Barremian). NE–SW faults control the location of subsidiary depocenters within these troughs. These basins were weakly inverted during the Tertiary with folds and thrusts striking E–W to WNW–ESE involving the Mesozoic–Tertiary cover with a maximum estimated shortening of about 12 %. Tertiary compression did not produce the total inversion of the Mesozoic basin but extensional structures are responsible for the location of major Tertiary folds. Shortening of the cover during the Tertiary involved both reactivation of some normal faults and development of folds and thrusts nucleated on basement extensional steps. The inversion style depends mainly on the occurrence and geometry of normal faults limiting the basin. Steep normal faults were not reactivated but acted as buttresses to the cover translation. Around these faults, affecting both basement and cover, folds and thrusts were nucleated due to the stress rise in front of major faults. Within the cover, the buttressing against normal faults consists of folding and faulting implying little shortening without development of cleavage or other evidence of internal deformation. © Elsevier, Paris.

Role of extensional structures on the location of folds and thrusts during tectonic inversion (northern Iberian Chain, Spain)

The Aguilón Subbasin (NE Spain) was originated daring the Late Jurassic-Early Cretaceous rifting due to the action of large normal faults, probably inherited from Late Variscan fracturing. WNW-ESE normal faults limit two major troughs filled by continental deposits (Valanginian to Early Barremian). NE-SW faults control the location of subsidiary depocenters within these troughs. These basins were weakly inverted during the Tertiary with folds and thrusts striking E-W to WNW-ESE involving the Mesozoic-Tertiary cover with a maximum estimated shortening of about 12 %. Tertiary compression did not produce the total inversion of the Mesozoic basin but extensional structures are responsible for the location of major Tertiary folds. Shortening of the cover during the Tertiary involved both reactivation of some normal faults and development of folds and thrusts nucleated on basement extensional steps. The inversion style depends mainly on the occurrence and geometry of normal faults limiting the basin. Steep normal faults were not reactivated but acted as buttresses to the cover translation. Around these faults, affecting both basement and cover, folds and thrusts were nucleated due to the stress rise in front of major faults. Within the cover, the buttressing against normal faults consists of folding and faulting implying little shortening without development of ceavage or other evidence of internal deformation.

Late Pleistocene–Holocene retreat of the West Antarctic Ice-Sheet system in the Ross Sea: Part 1—Geophysical results

In the 1994, 1995, and 1998 austral field seasons, geophysical research efforts in the Ross Sea focused on acquiring a high-resolution database designed to permit (1) reconstruction of the maximum extent and configuration of the ice sheet during the Last Glacial Maximum (i.e., oxygen isotope stage 2); (2) reconstruction of conditions at the base of the ice sheet; and (3) assessment of the relative retreat history of the ice sheet following the Last Glacial Maximum. Five seismic facies are distinguished on the basis of external geometry, relationships among features, bounding surface amplitude, intensity of internal acoustic signature, and geometry of internal reflectors. Seismic facies 1 is a transparent draping unit correlated to diatomaceous mud, interpreted as being deposited under open-marine conditions. Seismic facies 2 and 3 are comprised of units displaying subdued massive internal signatures with smooth lower bounding surfaces and hummocky upper surfaces characterized by glacial lineations. These units are interpreted to be grounding-zone proximal deposits overridden by the expanded ice sheet. Seismic facies 2 may be a deforming-till unit. Seismic facies 4a is characterized by an internally massive to chaotic signature, an erosional, often flat, lower surface, and a hummocky upper surface displaying glacial lineations. This unit is interpreted to be till and is divided into deposits associated with the most recent glacial expansion (4a) and with an earlier (pre–Last Glacial Maximum) expansion (4b). Seismic facies 5 is an acoustically laminated, ponded, and draping deposit interpreted to contain proglacial and sub–ice-shelf materials deposited continuously since the last interglacial period. The associations of these units provide context for the interpretation of the ice-edge maximum position, conditions at the base of the ice sheet, and the relative retreat history of the region. There is compelling evidence for a much-expanded ice sheet in the Ross Sea during the Last Glacial Maximum. In the western Ross Sea, the maximum grounding position is marked by an isolated grounding-zone wedge, and placed in the vicinity of Coulman Island, approximately 150 km from the continental-shelf edge. In the central Ross Sea, the maximum grounding position is close to the continental-shelf break, based primarily on the presence of an extensive 60-m-thick sheet-like deposit with a fluted upper surface. Streaming ice may have occupied bathymetric lows on the continental shelf, as suggested by (1) the configuration of bathymetry; (2) the presence of glacially eroded troughs; (3) the concentration of sediment (interpreted to be deforming till) within the middle to outer shelf reaches of the troughs; and (4) the fluted nature of the upper surface. Absolute rates of streaming ice flow relative to inter-ice-stream areas are not implied. The western Ross Sea continental-shelf deposits record the retreat history of ice derived predominantly from the East Antarctic Ice Sheet and glaciers of the Transantarctic Mountains. Ice flow on the continental shelf is interpreted to have remained fixed in position, restricted by the walls of the troughs. On the inner shelf of the western Ross Sea, the ice flowed over and eroded lithified sedimentary strata. Slower-moving ice occupied flat-topped banks. A single grounding-zone wedge occurs on the western Ross Sea central shelf; no substantial deposits are observed on the inner continental shelf. This lack of grounding-zone features reflects a restricted sediment supply and a relatively steady, rapid retreat of the ice sheet. During retreat, ice flow acted independently in each major trough and on the bank tops. Ice retreated from Victoria Land Basin–Drygalski Trough before it retreated from JOIDES Basin. Ice remained on the bank tops, shedding material into the abandoned troughs. Grounded ice in the central Ross Sea was derived predominantly from an expanded West Antarctic Ice Sheet. The ice sheet remained grounded on the continental-shelf edge after retreat of ice from the western Ross Sea. Expanded ice eroded the inner shelf and deposited sheets of till on the central and outer shelf. Ice-stream drainage shifted laterally, as recorded by rounded, laterally accreting ridges that separate bathymetric troughs. The central Ross Sea is the repository for larger volumes of sediment, derived from ice flow across basins of relatively thick, unlithified sedimentary deposits. Two till sheets mark grounding-zone positions in the central Ross Sea, reflecting the higher sediment supply and a stepped deglaciation. Ice remained grounded on the Pennell Bank to the west; a series of moraines marks the retreat of ice across the bank. Mega-scale glacial lineations and other streamlined, subglacial geomorphic features occur across the continental shelf, primarily within the troughs. The lineations substantiate the maximum reconstruction and support the interpretation of a deforming substrate beneath the outer reaches of the expanded ice sheet. This deforming substrate may have contributed to the onset of deglaciation. Features associated with meltwater are rare or absent, suggesting that basal meltwater played a minor role in retreat of the ice sheet.

THE LAST ICE SHEET IN NORTH-WEST SCOTLAND: RECONSTRUCTION AND IMPLICATIONS

Recent models of the last Scottish ice sheet suggest that nunataks remained above the ice surface in areas peripheral to the main centres of accumulation. This proposition has been investigated on 140 mountains over an area of 10,000 km 2 in NW Scotland. Outside the limits of the later Loch Lomond Readvance in this area there is evidence for a single high-level weathering limit that separates glacially eroded terrain from higher areas of in situ frost debris. This limit occurs at altitudes ranging from 425 to 450 m in the Outer Hebrides to >950 m on the mainland, and is best developed on lithologies that resisted breakdown after ice-sheet downwastage. Interpretation of this weathering limit as a periglacial trimline cut by the last ice sheet at its maximum thickness is supported by: (1) joint-depth and Schmidt hammer measurements that indicate significantly more advanced rock breakdown above the weathering limit; (2) a much greater representation of gibbsite (a pre-Late Devensian weathering product) in the clay fraction of soils above the limit; (3) cosmogenic isotope dating of the exposure ages of rock outcrops above and below the limit; (4) the sharpness of the limit at some sites and its regular decline along former ice flowlines; and (5) shear stress calculations based on the inferred altitude and gradient of the former ice surface. Reconstruction of the ice surface based on trimline evidence indicates that the mainland ice shed lay near or slightly east of the present watershed and descended northwards from >900 m to ca. 550 m at the north coast. Independent dispersion centres fed broad ice streams that occupied major troughs. On Skye an ice dome >800 m deflected the northwestwards movement of mainland ice, but the mountains of Rum were over-ridden by mainland ice up to an altitude of ca. 700 m. The Outer Hebrides supported an independent ice cap that was confluent with mainland ice in the Minches. Extrapolation of the trimline evidence indicates that most reconstructions of ice extent are too conservative, and suggests that low-gradient ice streams extended across the Hebridean Shelf offshore. Wider implications of this research are: (1) that blockfields and other periglacial weathering covers are not all of the same age or significance, depending on the resistance of different lithologies to frost weathering; (2) that the contrasting degree of glacial modification in the Western and Eastern Highlands of Scotland may reflect a former cover of predominantly warm-based ice in the former and predominantly cold-based ice in the latter; and (3) that the approach and techniques developed in this study have potential application for constraining ice-sheet models, not only in areas peripheral to the main centres of ice accumulation in Britain and Ireland, but also in other mountain areas where nunataks protruded through warm-based Late Pleistocene ice masses.