Coats Land Research Articles

Subglacial geology in Coats Land, East Antarctica, revealed by airborne magnetics and radar sounding

During the austral summer of 2001/02 five thousand line kilometres of airborne radio echo sounding and aeromagnetic data were collected in the region of three tributaries of Slessor Glacier, East Antarctica, which drains into the Filchner Ice Shelf. Basal topography and roughness estimates were obtained from the echo sounding data, which have been combined, here, with the subglacial geological units inferred from an analysis of the aeromagnetic data. This analysis included Euler and Werner deconvolution to determine the location and depth of sills, dykes and faults. In addition, we high-pass filtered the magnetic anomaly profiles to obtain information about the near surface subglacial geology. This revealed a prominent area of low magnetic susceptibility lying in a deep topographic basin underneath the northern tributary of Slessor Glacier. Forward and inverse magnetic models indicate that this area is likely underlain by a 3 km thick sedimentary basin. Ice flow modelling suggests that this sedimentary basin influences the ice dynamics in the tributary. These results have important implications for understanding the dynamics of this and other East Antarctic flow features that may be underlain by a weak sedimentary bed.

Structure and Dynamics of Katabatic Flow Jumps: Idealised Simulations

For the first time, results from a high-resolution numerical simulation (with horizontal grid spacing of 35m) were used to reveal the detailed structure near an atmospheric katabatic jump over an idealized slope. The simulation represents flow over the slopes of Coats Land, Antarctica for austral winter conditions. The katabatic jump is characterised by an updraft with vertical velocities of order 1ms−1 and serves as a possible forcing mechanism for the gravity waves frequently observed over the ice shelves around the Antarctic. Results also indicate that strong turbulence is generally confined within a mixing zone near the top of the katabatic layer upstream of the jump and extends downstream through the top of the strong updraft associated with the jump. Detailed analyses of momentum and heat budgets across the katabatic jump indicate that, upstream of the jump, turbulent mixing is important in decelerating the upper part of the katabatic layer, while within the jump the upslope pressure gradient force associated with the pool of cold air plays a role in decelerating the flow near the surface. The heat budget near the jump reveals a simple two-term balance: the turbulent heat flux divergence is balanced by the advection. A comparison of model results with available theories indicates that mixing between layers of different potential temperature structure indeed plays some role in the development of katabatic flow jumps, especially for strong jumps. Theories used to study katabatic jumps should include this mixing process, of which the amount depends on the intensity of the jump. A conceptual model of a katabatic jump, including the main dynamical processes, is constructed from these detailed analyses.

An Autonomous Doppler Sodar Wind Profiling System

Abstract An autonomous Doppler sodar wind profiling system has been designed, built, tested, and then deployed for 2 years at a remote site in Coats Land, Antarctica. The system is designed around a commercially available phased-array sodar (a Scintec flat-array sodar, FAS64) and powered from five modular power system units. Each power unit comprises two batteries, two photovoltaic solar panels, and two vertical axis wind generators, plus charging control and isolation circuitry. The sodar’s main processing unit is located at the antenna, but is controlled from a manned research station 50 km distant, in real time, by a line-of-sight UHF radio link. Data from an integral automatic weather station (AWS) are also transmitted over the radio link, allowing meteorologically informed decisions on whether or not to operate the Doppler sodar. Over the 2-yr experiment dozens of sounding episodes, lasting from a few hours to a few days, were obtained. Successful soundings were obtained in temperatures down to −33°C, and wind speeds up to 12 m s−1. In general, the wind data quality was good, but the range was disappointing, probably as a result of the strongly stable atmospheric conditions that were experienced. The wind profiling system that is described has been used to obtain the first remote wintertime observations of katabatic winds over the Antarctic continent.

Pb isotope record over one century in snow from Victoria Land, Antarctica

Pb and Ba concentrations and Pb isotopic compositions are reported for firn core and snow pit samples from Victoria Land, Antarctica, dating from 1872 AD to 1994 AD. From variations in Pb/Ba ratios and Pb isotopic compositions, two periods of major Pb enhancements were identified, from 1891 to 1908 AD and from 1948 to 1994 AD. The earlier pollution event is attributed to Pb emissions from non-ferrous metal production and coal combustion in the Southern Hemisphere and is in excellent agreement with coincident pollution inputs reported in firn/ice cores from two other regions of Antarctica, at Coats Land and Law Dome. Using Pb isotopic systematics, it was calculated that ∼50% of Pb deposited in Victoria Land in 1897 originated from anthropogenic emission sources. The more recent period of Pb enhancements, from 1948 to 1994 AD, corresponds to the introduction and widespread use of gasoline alkyl Pb additives in automobiles in the Southern Hemisphere, with anthropogenic Pb inputs averaging 60% of total Pb but with large uncertainty. Intra- and inter-annual variations in Pb concentrations and isotopic compositions were evaluated in snow pits samples corresponding to the period 1991–1994. Substantial variations in Pb/Ba and 206Pb/ 207Pb ratios were detected but the absence of a regular seasonal pattern for these parameters suggests that the transport and deposition of aerosols to the Antarctic ice sheet are complex and vary from year to year.

Numerical simulations of katabatic jumps in coats land, Antartica

A non-hydrostatic numerical model, the Regional Atmospheric Modeling System (RAMS), has been used to investigate the development of katabatic jumps in Coats Land, Antarctica. In the control run with a 5 m s � 1 downslope directed initial wind, a katabatic jump develops near the foot of the idealized slope. The jump is manifested as a rapid deceleration of the downslope flow and a change from supercritical to subcritical flow, in a hydraulic sense, i.e., the Froude number (Fr) of the flow changes from Fr > 1t oFr < 1. Results from sensitivity experiments show that an increase in the upstream flow rate strengthens the jump, while an increase in the downstream inversion-layer depth results in a retreat of the jump. Hydraulic theory and Bernoulli's theorem have been used to explain the surface pressure change across the jump. It is found that hydraulic theory always underestimates the surface pressure change, while Bernoulli's theorem provides a satisfactory estimation. An analysis of the downslope momen- tum balance for the katabatic jump indicates that the important forces are those related to the pressure gradient, advection and, to a lesser extent, the turbulent momentum divergence. The development of katabatic jumps can be divided into two phases. In phase I, the pressure gradient force is nearly balanced by advection, while in phase II, the pressure gradient force is coun- terbalanced by turbulent momentum divergence. The upslope pressure gradient force associated with a pool of cold air over the ice shelf facilitates the formation of the katabatic jump.

The dynamics of idealized katabatic flow over a moderate slope and ice shelf

AbstractA non‐hydrostatic numerical weather prediction model has been employed to simulate idealized katabatic flows over a moderate slope and adjoining ice shelf. The topography of Coats Land and the adjoining Brunt Ice Shelf, Antarctica, has been used; this is typical of much of the Antarctic coastline. The Regional Atmospheric Modeling System Version 4.3 has been adapted for simulations over compacted snow, most notably through changes to the multi‐layer soil model. The simulations are initialized using clear‐sky conditions and at rest. On the slope, a shallow katabatic flow develops, the winds becoming approximately steady on the slope by ∼12 h. The peak downslope winds are about 7 m s−1 at 30 m above the snow surface. The katabatic flow depth ranges from 50 to 100 m down the slope. Over the ice shelf the katabatic flow peters out, while a pool of cold air develops, primarily through sensible‐heat loss into the surface and partially balancing the net radiative‐heat loss to space. Near‐surface and sounding data from the model simulations compare well with archetypal and typical katabatic flow observations, especially after some tuning of the model's turbulence parametrization.An analysis of the downslope flow dynamics shows the buoyancy force is generally balanced by the inertial force, except towards the foot of the slope where it is balanced by upslope forces related to gradients in the potential‐temperature deficit and katabatic‐layer height, caused by the pool of cold air over the ice shelf. Over time, the cooling of the ice shelf boundary layer leads to an apparent retreat of the katabatic flow from the ice shelf and some way up the slope. The dynamical analysis explains the surface climatology observed, such that the persistent katabatic winds of Coats Land rarely reach the Brunt Ice Shelf. The simulated katabatic flow moves from ‘shooting’ to ‘tranquil’ towards the foot of the slope. This transition acts to trigger a train of internal gravity waves which propagate energy upwards away from the katabatic flow jump. Previous studies have also found shooting to tranquil katabatic flow transitions, so to generalize these findings would suggest that internal gravity‐wave generation is ubiquitous around much of coastal Antarctica and Greenland. Copyright © 2004 Royal Meteorological Society.

Himalayan-type indenter-escape tectonics model for the southern part of the late Neoproterozoic–early Paleozoic East African– Antarctic orogen

The East African–Antarctic orogen is one of the largest orogenic belts on the planet. It resulted from the collision of various parts of proto–East and West Gondwana during late Neoproterozoic–early Paleozoic time (between 650 and 500 Ma). We propose that the southern part of this Himalayan-type orogen can be interpreted in terms of a lateral-escape tectonic model. Modern Gondwana reconstructions show that the southern part of the East African– Antarctic orogen can best be reassembled when a number of microplates (the Falkland, Ellsworth-Haag, and Filchner blocks) are positioned between southern Africa and East Antarctica. This microplate assemblage is unusual. The microplates probably represent shear-zone–bounded blocks, produced by tectonic translation during lateral escape, similar to those currently evolving in Southeast Asia. One of the escape-related shear zones is exposed as the 20-km-wide Heimefront transpression zone in western Dronning Maud Land. Coats Land, a crustal block within the orogen, probably represents a block of older crust that was not subjected to tectonometamorphic reworking ca. 500 Ma by lateral tectonic escape. The southern part of the orogen is also typified by very large volumes of late-tectonic A2-type granitoids, intruded ca. 530–490 Ma, probably as a consequence of delamination of the orogenic root and the subsequent influx of hot asthenospheric mantle during tectonic escape. Erosional unroofing of the orogen is documented by the remnants of originally massive areas covered by Cambrian– Ordovician molasse-type sedimentary rocks throughout Africa, Arabia, and Antarctica, testifying to the past extent and size of this largest of orogens.

The role of ice thickness and bed properties on the dynamics of the enhanced-flow tributaries of Bailey Ice Stream and Slessor Glacier, East Antarctica

AbstractAirborne radio-echo sounding investigations in the upper reaches of Bailey Ice Stream and Slessor Glacier, Coats Land, East Antarctica, have shown that enhanced-flow tributaries are associated with well-defined areas of relatively thicker ice, and are separated from each other by areas of relatively thinner ice. A numerical modelling study has revealed that while internal ice deformation might account for all the observed flow in inter-tributary areas and the majority in the Slessor tributaries, a significant proportion of the flow of Bailey tributary is attributable to basal motion. Further, investigations of depth-corrected basal reflection power indicate that the bed underlying both Bailey and Slessor enhanced-flow tributaries is significantly smoother than in the slower-moving inter-tributary areas. It is thus proposed that enhanced motion within Bailey tributary (and also perhaps Slessor) may be facilitated by a reduction in basal roughness, caused by the accumulation of water and/or sediments within subglacial valleys, or by the erosion and smoothing of bed obstacles.

Direct determination of mercury at the sub-picogram per gram level in polar snow and ice by ICP-SFMS

An analytical method for the direct determination of mercury (Hg) in polar snow and ice cores and surface snow based on inductively coupled plasma sector field mass spectrometry (ICP-SFMS) has been developed. Various Hg isotopes, such as 199Hg, 200Hg, 201Hg and 202Hg, appear to be free of polyatomic interferences in such matrices and allow the measurements to be made in low resolution mode, leading to high sensitivity. Ultra-low concentration Hg standards (from 1.5 to 20 pg g−1) were used for the calibration of the Thermo Finnigan MAT Element2, and a detection limit as low as 0.18 pg g−1 was achieved using 202Hg. Ultra-clean procedures used from field sampling to final laboratory analysis show no significant blank contributions and appear suitable for the reliable determination of Hg at ultra-low concentrations. Precision of the Hg measurements was estimated to be 15% in terms of relative standard deviation on five replicates and accuracy was checked with an analytical reference material (102% recovery). Hg concentrations in surface snow samples from the Northern Hemisphere collected in the Canadian Arctic and in Svalbard (Norway) show high variability (1.2–32.0 pg g−1). In Antarctica, Hg was determined in different ice core sections from Dome C, spanning the last 18000 years BP (range from 0.7 to 3.2 pg g−1), and in snow samples from Coats Land covering the last 150 years (range from 0.2 to 16.1 pg g−1).

Basal topography and ice flow in the Bailey/Slessor region of East Antarctica

An airborne radio‐echo sounding campaign carried out in the upper reaches of Bailey Ice Stream and Slessor Glacier, in Coats Land, East Antarctica, has revealed that tributaries of enhanced flow lie within well‐defined basal troughs and are separated from each other by bed highs. These new data indicate significant differences in ice thickness compared with those estimated in the Bedmap database. A numerical modeling study has revealed that driving stresses are high enough to account for flow by ice deformation alone in intertributary areas. Most flow in the enhanced flow tributaries of Slessor Glacier may also be explained by ice deformation alone. However, although ice deformation is also significant in the Bailey Ice Stream tributary, a large amount of basal motion is also required to fully explain flow velocities here. It is proposed that the trough in which Bailey tributary lies is sufficiently deep that marine sediments may have accumulated here in preglacial times. Along with water produced by geothermal heating, frictional heating, and through a reduction in the pressure melting point, basal motion may therefore be facilitated by the presence of a deformable, saturated till layer at the bed.

New age constraints for Grenville-age metamorphism in western central Dronning Maud Land (East Antarctica), and implications for the palaeogeography of Kalahari in Rodinia

New SHRIMP zircon data from Gjelsvikfjella and Muhlig–Hofmann–Gebirge (East Antarctica) indicate that the metamorphic basement is composed of Grenville-age rocks that are most likely part of the north-eastern continuation of the Namaqua–Natal–Maud Belt. Crystallisation ages of meta-igneous rocks range between ca. 1,150 to 1,100 Ma, with little inheritance recorded. Metamorphic zircon overgrowth during high-grade metamorphism is dated between ca. 1,090 to 1,050 Ma. Both, the crystallisation ages and the metamorphic overprint are similar to U–Pb data from a number of areas along a ca. 2,000-km stretch from Natal in South Africa to central Dronning Maud Land. The basement underwent in part strong high-grade reworking during the collision of East and West Gondwana at ca. 550 Ma. The timing of Grenville-age metamorphism has important implications for the position of Kalahari in Rodinia. It also questions that Coats Land is part of the Maud Belt because the undeformed volcanic rocks of Coats Land are older than the main metamorphism within the Maud Belt and, therefore, must rest on older basement. This interpretation explains why the pole of Coats Land at ca. 1,110 Ma differs from the Kalahari poles by 30°, i.e. Coats Land had not yet amalgamated to Kalahari. On the other hand, the palaeopoles from Coats Land and Laurentia at 1,110 Ma are identical within error. Thus, Coats Land could have been part of Laurentia prior to the final amalgamation of Rodinia, the Namaqua–Natal–Maud Belt could have been a part of the Grenville Belt and the entire Kalahari Craton could indeed have opposed Laurentia on its eastern side.

A marginal marine ichnofauna from the Blaiklock Glacier Group (?Lower Ordovician) of the Shackleton Range, Antarctica

ABSTRACTA diverse arthropod-dominated ichnofauna, associated with a poorly preserved crustacean fauna and soft-bodied ?medusoid impressions, is described from the Blaiklock Glacier Group of the north-western Shackleton Range (Coats Land), Antarctica. The ichnofauna consists of Asaphoidichnus, Beaconites, Didymaulichnus, Diplichnites, Gordia, ?Laevicyclus, Merostomichnites, Monomorphichnus, Palaeophycus, Planolites, Rusophycus, Selenichnites, and Taphrhelminthoides (ichnogen nov.). Three new ichnotaxa are recognised: Taphrhelminthoides antarcticus n. ichnogen. et ichnosp. is a bilobate trail, composed of two parallel flat lobes, separated by a median ridge with a characteristic figure-of-eight pattern. Merostomichnites gracilis n. ichnosp. is characterised by its proportions (external:internal width ratio &gt;3) and series of 10 to 12, thin, linear tracks. Selenichnites antarcticus n. ichnosp. is characterised by small elongate horseshoe-shaped marks, the medial portion showing three to five transverse scratch-marks.The palaeoenvironment is interpreted as extremely shallow marine water, possibly a tide-dominated estuary, based on sedimentological evidence and the composition of the ichnofauna. Radiometric and palaeomagnetic data indicate that this assemblage is Lower Ordovician in age, representing the first autochthonous Ordovician fossiliferous succession to be described from Antarctica. The succession shows several sedimentological and palaeontological similarities with the basal units of the Ordovician Table Mountain Group in South Africa, supporting palaeogeographic models placing the Palaeozoic Blaiklock basin close to the Ordovician Table Mountain basin.

One hundred fifty–year record of lead isotopes in Antarctic snow from Coats Land

A record of the concentrations of Pb and Ba and the isotopic composition of Pb has been established for a remote, low accumulation site in the Atlantic sector of Antarctica (Coats Land) by means of thermal ionization mass spectrometry. The snow samples cover the period ∼1840 to 1990. They were taken from the walls of a pit to a depth of 7.8 m and as a core to 16 m; ultraclean procedures were used. Detailed laboratory subsampling provided both long-term (secular scale) and short-term (intra-annual) Pb, Ba, and Pb isotope variations. The results show that there have been significant variations in Pb concentrations (range, 0.1 to 9.3 pg/g) and isotopic composition (range, 1.096 to 1.208 for 206Pb/ 207Pb ratio) since the 1840s. The data show evidence of pollution for this metal in Antarctica as early as the 1880s. Several Pb maxima were observed: the first at the beginning of the 20th century and the last in the 1970s to 1980s, with a clear decrease during recent years. Although the last maximum is clearly linked to the rise and fall in the use of leaded gasoline in the Southern Hemisphere, especially in South America, the reason for the first remains uncertain. The pattern of changing isotopic composition of Pb reveals the changing origin and character of the anthropogenic inputs to Antarctica. An interesting feature in this pattern is the relatively large contribution of unradiogenic Pb in the ∼1890s, possibly originating from Australia. Another interesting feature is the pronounced intra-annual variation in the isotopic composition of Pb, which illustrates the complexity of the changing inputs of Pb to Antarctica.

Short-term variations in the occurrence of heavy metals in Antarctic snow from Coats Land since the 1920s

Short-term variations in heavy metals concentrations in Antarctic snow have been investigated by analysing 13 metals (Al, V, Cr, Mn, Co, Cu, Zn, Ag, Cd, Ba, Pb, Bi and U) in a series of ultraclean samples collected from two snow pits in Coats Land in the Atlantic sector of Antarctica. The samples covered a ∼70 years time period from the 1920s to 1990. They were analysed by inductively coupled plasma sector field mass spectrometry and graphite furnace atomic absorption spectrometry. The results conclusively show that there is a pronounced short-term (intra- and inter-annual) variability of heavy metal in Antarctic snow, with the highest concentrations being up to 100 times higher than the lowest ones for metal such as Cr, Mn, Pb and U. Contributions from the different possible natural and anthropogenic sources are found to be extremely variable. This is especially due to the large variability in the transport patterns of heavy metals from Southern America, Southern Africa and Australia to the Antarctic continent. Inputs from several volcanic events were identified in our samples from non-seasalt sulfate concentrations profiles. The only ones that gave clear signals for heavy metals were eruptions at Deception Island the proximity of which allowed for tropospheric transport to Coats Land.

The surface climatology of an ordinary katabatic wind regime in Coats Land, Antarctica

The surface climatology of Coats Land, Antarctica, is described through observations from automatic weather stations, from Halley station, from upper air soundings and from satellite remote sensing. Coats Land consists of the Brunt Ice Shelf and the adjoining continent to the south. The topography of this region is typical of much of the Antarctic coastal fringes: a modest slope (5% at most) and relative uniformity across the slope. A basic climatology broken into site and season is presented. In winter, and to an extent in the equinoctial seasons, the region clearly divides into two dynamical regimes. Over theice shelf winds are usually from the east or occasionally from the west, whereas over the continental slopes winds are from the east to south quadrant. Over the ice shelf the surface layer is about 10 K colder, in terms of potential temperature, than on the continent, and is also more stable than on the steeper parts of the slope. Motivated by case studies, three criteria are developed to select a subset of the data that are katabatic in the sense that the flow is believed to be primarily due to a downslope buoyancy forcing. On the continental slope, the criteria pick out a coherent subset of the data that aretightly clustered in wind speed and wind direction. Typical katabatic winds are from 10° to the east ofthe fall line and 7.5 m s^-1 at the steepest part of the slope (5.1 m s^-1 higher up). They are rarely morethan 15 m s^-1 in this region; hence their description as ordinary, in contrast with those extraordinary katabatic regimes that have been the focus of previous studies. The katabatic flow remains close to adiabatic as it moves down the slope, and is relatively dry near the slope foot. We estimate the flow tobe primarily katabatic at most 40—50% of the time, although it may appear to be katabatic, from windspeed and wind direction characteristics, some 60—70% of the time. There is no coherent katabatic-flow signature on the ice shelf.

Changes in heavy metals in Antarctic snow from Coats Land since the mid-19th to the late-20th century

V, Cr, Mn, Cu, Zn, Co, Ag, Cd, Ba, Pb, Bi and U have been measured in a series of dated snow samples, covering the period from 1834 to 1990, collected at remote, low accumulation sites in Coats Land, Antarctica. They were determined by ultrasensitive inductively coupled sector field mass spectrometry in ultraclean conditions. Concentrations are found to be extremely low, down to 3×10 −15 g/g, for most metals, then confirming the high purity of Antarctic snow. The results show contrasting time trends for the different metals. For Mn, Co, Ba, and possibly V and Cd, no clear time trends are observed. For Cr, Cu, Zn, Ag, Pb, Bi and U, on the other hand, pronounced enhancements are observed during the recent decades. They are attributed to emissions of heavy metals to the atmosphere from human activities in Southern America, Southern Africa and Australia, especially non-ferrous metal mining and smelting in Chile, Peru, Zaire, Zambia and Australia. It shows that atmospheric pollution for heavy metals in the remote Antarctic continent is not limited to Pb and Cu, as previously thought, but also affects several other metals. It is a further indication that atmospheric pollution for heavy metals is really global.

The southern Weddell Sea: combined contourite-turbidite sedimentation at the southeastern margin of the Weddell Gyre

Abstract Sedimentary processes in the southeastern Weddell Sea are influenced by glacial-interglacial ice-shelf dynamics and the cyclonic circulation of the Weddell Gyre, which affects all water masses down to the sea floor. Significantly increased sedimentation rates occur during glacial stages, when ice sheets advance to the shelf edge and trigger gravitational sediment transport to the deep sea. Downslope transport on the Crary Fan and off Dronning Maud and Coats Land is channelized into three huge channel systems, which originate on the eastern, the central and the western Crary Fan. They gradually turn from a northerly direction eastward until they follow a course parallel to the continental slope. All channels show strongly asymmetric cross sections with well-developed levees on their northwestern sides, forming wedge-shaped sediment bodies. They level off very gently. Levees on the southeastern sides are small, if present at all. This characteristic morphology likely results from the process of combined turbidite-contourite deposition. Strong thermohaline currents of the Weddell Gyre entrain particles from turbidity-current suspensions, which flow down the channels, and carry them westward out of the channel where they settle on a surface gently dipping away from the channel. These sediments are intercalated with overbank deposits of high-energy and high-volume turbidity currents, which preferentially flood the left of the channels (looking downchannel) as a result of Coriolis force. In the distal setting of the easternmost channel-levee complex, where thermohaline currents are directed northeastward as a result of a recirculation of water masses from the Enderby Basin, the setting and the internal structures of a wedge-shaped sediment body indicate a contourite drift rather than a channel levee. Dating of the sediments reveals that the levees in their present form started to develop with a late Miocene cooling event, which caused an expansion of the East Antarctic Ice Sheet and an invigoration of thermohaline current activity.

The surface climatology of an ordinary katabatic wind regime in Coats Land, Antarctica

The surface climatology of Coats Land, Antarctica, is described through observations from automaticweather stations, from Halley station, from upper air soundings and from satellite remote sensing.Coats Land consists of the Brunt Ice Shelf and the adjoining continent to the south. The topography ofthis region is typical of much of the Antarctic coastal fringes: a modest slope (5% at most) and relativeuniformity across the slope. A basic climatology broken into site and season is presented. In winter, andto an extent in the equinoctial seasons, the region clearly divides into two dynamical regimes. Over theice shelf winds are usually from the east or occasionally from the west, whereas over the continentalslopes winds are from the east to south quadrant. Over the ice shelf the surface layer is about 10 Kcolder, in terms of potential temperature, than on the continent, and is also more stable than on thesteeper parts of the slope. Motivated by case studies, three criteria are developed to select a subset ofthe data that are katabatic in the sense that the flow is believed to be primarily due to a downslopebuoyancy forcing. On the continental slope, the criteria pick out a coherent subset of the data that aretightly clustered in wind speed and wind direction. Typical katabatic winds are from 10° to the east ofthe fall line and 7.5 m s-1 at the steepest part of the slope (5.1 m s-1 higher up). They are rarely morethan 15 m s-1 in this region; hence their description as ordinary, in contrast with those extraordinarykatabatic regimes that have been the focus of previous studies. The katabatic flow remains close toadiabatic as it moves down the slope, and is relatively dry near the slope foot. We estimate the flow tobe primarily katabatic at most 40—50% of the time, although it may appear to be katabatic, from windspeed and wind direction characteristics, some 60—70% of the time. There is no coherent katabatic-flowsignature on the ice shelf.

Ultrasensitive determination of heavy metals at the sub-picogram per gram level in ultraclean Antarctic snow samples by inductively coupled plasma sector field mass spectrometry

Assessing changes in heavy metals concentrations in Antarctic snow dated from the last century is of high interest to determine to which extent the most remote regions of our planet are contaminated for these metals, and to have a better understanding into their long-range transport from the different natural and anthropogenic source areas to the Antarctic continent. Such investigations are unfortunately very difficult because the concentrations to be measured are exceedingly low, down to the sub-picogram per gram level. They require a strict control of contamination problems from field sampling to laboratory analysis, and the use of ultrasensitive analytical techniques. We present here important advances in the analytical protocols for obtaining reliable data on the occurrence of heavy metals in Antarctic snow. Utmost precautions were taken to obtain a series of large size ultraclean snow blocks from the wall of a 8.3 m clean hand-dug pit at a remote site in Coats Land, Antarctica. These blocks were then sub-sampled inside a laminar flow clean bench in a cold room, using ultraclean protocols, to provide high-resolution heavy metal times series. V, Cr, Mn, Cu, Ag, Ba, Pb, Bi and U were then determined directly, without any pre-concentration step, by the ultrasensitive inductively coupled plasma sector field mass spectrometry (ICP-SFMS) technique in clean room conditions. Calibration of the instrument was performed using ultralow concentrations standards, and extreme precautions were taken to ensure the cleanliness of the instrument and its introduction system. The results show that it is possible to accurately measure a variety of heavy metals in Antarctic snow, down to the sub-picogram per gram level, using this approach. Examples of the data obtained for the Coats Land site are finally presented. For U, the observed concentrations range from 0.004 to 0.21 pg/g; they are the first data ever obtained for this metal for Antarctic snow and ice.

Sapphirine-Kornerupine Granulites from the Precambrian of Southern Madagascar: Implications for the Evolution of the Deep Crust in East Gondwana

Dalziel, I.W.D. (1991) Pacific margins of Laurentia and East AntarcticaAustralia as a conjugate rift pair: evidence and implications for an Eocambrian supercontinent. Geology, v. 19, pp. 598-601. Gose, W.A., Helper, M.A., Conelly, J.N., Hutson, EE. and Dalziel, I.W.D. (1987) Palaeomagnetic data and U-Pb isotopic age determinations from Coats Land, Antarctica: implications for late Proterozic plate reconstructions. J. Geophys. Res., v. 102, pp. 7887-7902. Groenewald, PB., Moyes,A.B., Grantham, G.H. andKrynaw, J.R. (1995) East Antarctic crustal evolution: geological constraints and modelling in western Dronning Maud Land, Antarctica. Precamb. Res., v. 75,