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The quality of the ice core from Siple Dome, West Antarctica, varied widely, with significant fracturing below 400 m. Bubbly ice persisted to the ice–rock interface at 1004 m and constituted the brittle zone. The core has undergone minimal relaxation and has remained brittle and prone to fracturing more than 5 years after it was drilled. This behavior is attributed to unrelieved stresses from Kamb and Bindschadler Ice Streams (former Ice Streams C and D) flanking the dome. Melt layers were identified sporadically throughout the core, as were inclined layers tilted at angles that occasionally exceeded 10°. Structurally, the ice was characterized by extensive recrystallization including grain-size changes from 0.074 cm2 at 59 m to >50 cm2 at 992 m, and major transitions in c-axis fabrics. Unusual fabrics included vertical c-axis clusters superimposed on vertical girdles that may reflect vertical compression acting in conjunction with horizontal tension. The sudden appearance of a shear-type fabric at 700–800 m appears closely linked to the occurrence of abundant tephra particles embedded in the ice. The occurrence of dispersed sediment in the bottom 2 m is attributed to freeze-on of basal meltwater.
Approximately 300 volcanic ash and dust layers were observed in the Siple Dome (Antarctica) ice core. Most of this tephra, deposited between 700 and 800 m depth, consisted primarily of glass shards with varying amounts of crystalline material and groundmass fragments. The pattern of distribution of tephra fallout closely replicates that found in the Byrd ice core, indicative of contemporaneous deposition at both locations. Peak fallout occurred approximately 19 500 years ago, based on methane tie points in the Siple Dome and Greenland Ice Sheet Project 2 (GISP2) ice cores. Mount Berlin was identified as a potential source of tephra, although other volcanoes in West and East Antarctica appear to have contributed ash and dust. Ice between 697 and 730 m, in which fine-grained tephra is concentrated, has undergone enhanced thinning compared to ice with a similar concentration of tephra deposited contemporaneously between 1300 and 1540 m at Byrd. It is speculated that this thinning has occurred in response to dynamic interaction between ice at Siple Dome and the two ice streams flanking it. A dramatic change to a shear fabric appears to be directly related to the higher concentration of volcanic particles in the ice between 700 and 800 m.
Shallow ice cores were obtained from widely distributed sites across the West Antarctic ice sheet, as part of the United States portion of the International Trans-Antarctic Scientific Expedition (US ITASE) program. The US ITASE cores have been dated by annual-layer counting, primarily through the identification of summer peaks in non-sea-salt sulfate (nssSO42–) concentration. Absolute dating accuracy of better than 2 years and relative dating accuracy better than 1 year is demonstrated by the identification of multiple volcanic marker horizons in each of the cores, Tambora, Indonesia (1815), being the most prominent. Independent validation is provided by the tracing of isochronal layers from site to site using high-frequency ice-penetrating radar observations, and by the timing of mid-winter warming events in stable-isotope ratios, which demonstrate significantly better than 1 year accuracy in the last 20 years. Dating precision to ±1 month is demonstrated by the occurrence of summer nitrate peaks and stable-isotope ratios in phase with nssSO42–, and winter-time sea-salt peaks out of phase, with phase variation of <1 month. Dating precision and accuracy are uniform with depth, for at least the last 100 years.
The Holocene portion of the Siple Dome (Antarctica) ice core was dated by interpreting the electrical, visual and chemical properties of the core. The data were interpreted manually and with a computer algorithm. The algorithm interpretation was adjusted to be consistent with atmospheric methane stratigraphic ties to the GISP2 (Greenland Ice Sheet Project 2) ice core, 10Be stratigraphic ties to the dendrochronology 14 C record and the dated volcanic stratigraphy. The algorithm interpretation is more consistent and better quantified than the tedious and subjective manual interpretation.
Results of analyses of snow annual accumulation variability, density and crystal growth measurements in firn and ice cores recovered from the upper layers of the West Antarctic ice sheet during the US component of the International Trans-Antarctic Scientific Expedition (ITASE) are presented. Annual-layer structure was analyzed on the basis of the visible stratigraphy and electrical conductivity measurement record in each core. Annual accumulation varied appreciably between core sites and within cores at individual sites where undulating surface topography appears to be exerting a significant impact on the magnitude of snow deposition. All density profiles except one exhibited densification that was normal with respect to snow annual accumulation and 10 m firn temperatures. Snow annual accumulation was determined stratigraphically, and 10m firn temperatures were either measured in the holes drilled for cores or inferred using elevation changes relative to Byrd Station, the 10m temperature at Byrd Station and an assumed lapse rate. Measurements at the one exceptional location indicated that the firn had undergone extremely rapid densification to ice, with the transition to ice occurring at 35–36m depth. Furthermore, thin-section measurements of grain-size show that the growth of crystals accelerated below the firn–ice transition. The behavior at this one site is attributed to localized deformation in the upper layers of firn and ice. Enhanced crystal growth was also observed at another site. At all other locations where grain-sizes were measured, the rates of crystal growth were in accord with age–temperature relationships observed by other researchers in Antarctica and Greenland. Profiles illustrating pore–crystal structure changes with increasing depth of burial are also presented.
Successful core-drilling to bedrock of both the Greenland and Antarctic ice sheets offers unique opportunities for examining processes acting at the bed. At Byrd Station, Antarctica, penetration of the bed was accompanied by upwelling of glacial meltwater into the drillhole. The nature and disposition of sediment in the 4.83 m thick debris-rich basal ice, together with stable-isotope and gas analyses of the enclosing ice, confirm that incorporation of the debris occurred simultaneously with periodic “freeze-on” of basal meltwater. Currently, the presence of substantial meltwater at the ice/rock interface likely precludes any erosive activity at the bed. At GISP2, Greenland, basal silly ice, 13.1 m thick, is currently frozen to the bed at −9° C. Limited studies of the silty ice at GISP 2, together with results of more comprehensive investigations obtained by GRIP researchers on basal ice at a companion site at Summit indicate that the sediment-bearing basal ice likely formed in the absence of an ice sheet and was therefore unrelated to direct interaction of the present ice sheet with its bed. The fact that the basal ice at Summit is frozen to the bottom also precludes any likelihood of erosive activity at the bed.
A survey of ice-front changes since 1910–13 shows that the Koettlitz Ice Tongue, located along the western shore of McMurdo Sound, Antarctica, has undergone significant retreat during the past 80 years. The ice front in 1910–13 was located 5 km in front of the Dailey Islands. Today, only two of the six Dailey Islands remain connected to the Koettlitz Ice Tongue. The most recent break-out of ice is believed to have occurred in 1979 or 1980, resulting in an estimated loss of 80 km2 of ice. Based on the current position of the ice front, it is estimated that a minimum of 300 km of ice has called off the Koettlitz Ice Tongue during the 80 year period that has elapsed since the ice front was first mapped in 1910–13.
The Transantarctic Mountains of East Antarctica provide a new milieu for retrieval of ice-core records. We report here on the initial findings from the first of these records, the Dominion Range ice-core record. Sites such as the Dominion Range are valuable for the recovery of records detailing climate change, volcanic activity, and changes in the chemistry of the atmosphere. The unique geographic location of this site and a relatively low accumulation rate combine to provide a relatively long record of change for this potentially sensitive climatic region. As such, information concerning the site and general core characteristics are presented, including ice surface, ice thickness, bore-hole temperature, mean annual net accumulation, crystal size, crystal fabric, oxygen-isotope composition, and examples of ice chemistry and isotopic composition of trapped gases.
During the austral summers of 1976–77 and 1978–79, several ice cores were taken from the McMurdo Ice Shelf brine zone to investigate its thermal, physical, and chemical properties. This brine zone consists of a series of superimposed brine layers (waves) that originate at the seaward edge of the ice shelf and migrate at various rates, depending on their age and position in the ice shelf. The brine in these layers becomes increasingly concentrated as the waves migrate inland through the permeable ice-shelf firn. Chemical analyses of brine samples from the youngest (uppermost) brine wave show that, except for the advancing front, it contains sea salts in normal sea-water proportions. Further inland, deeper and older brine layers, though highly saline (S > 200°/00), are severely depleted in SO42-, with the SO42-/Na+ ratio being an order of magnitude less than that of normal sea-water. Consideration of the solubility of alternative salts, together with analyses of Na+, K+, Ca2+, Mg2+, SO42-, and Cl- concentrations, shows that the sulfate depletion is probably due to selective precipitation of mirabilite, Na2SO4·10H2O. The location of the inland boundary of brine penetration is closely related to the depth at which the brine encounters the firn/ice transition. However, a small but measureable migration of brine is still occurring in otherwise impermeable ice; this is attributed to eutectic dissolution of the ice by concentrated brine as it moves into deeper and warmer parts of the McMurdo Ice Shelf.
We summarize here data on in situ nitrate ion concentrations in snow pits and firn cores over the last ∼3 250 a. Nitrate fluctuations show seasonal, 11 and 22 a periodicities, and long-term changes both at South Pole station and Vostok. High nitrate levels conform to winter darkness and solar activity peaks. Long-term lows and highs conform to solar activity minima and maxima. The data available support the hypothesis that nitrate is fixed in the upper atmosphere by some solar-mediated phenomenon causing a periodicity in East Antarctica snow. Background levels and non-periodic spikes in nitrate come from other sources.
Large, simply supported beams of temperate lake ice generally yield significantly higher f1exural strengths than the same beams tested in the cantilever mode. Data support the view that a significant stress concentration may exist at the fixed corners of the cantilever beams. Maximum effects are experienced with beams of cold, brittle ice substantially free of structural imperfections; the stress concentration factor may exceed 2.0 in this kind of ice. In ice that has undergone extensive thermal degradation the stress concentration effect may be eliminated entirely. Simply supported beams generally test stronger when the top surface is placed in tension. This behavior is attributed to differences in ice type; the fine-grained, crack-free top layer of snow-ice usually reacting more strongly in tension than the coarse-grained bottom lake ice which is prone to cracking.
The size of firn crystals as a function of age has been investigated in thin sections to a depth of 49 m at the South Pole. Grain cross-sections increased in size from 0.24 mm2 at 0.1 m depth to 0.63 mm2 at 10 m. Crystals, as distinct from grains, increased in size from 0.18 to 0.43 mm2 over the same interval, implying that grains are generally composed of just one or two crystals rather than several as is frequently contended. The mean crystal cross-section increased linearly with the age of the firn at a rate of 0.0006 mm2 year−1; in 388 year old firn at 49 m the crystal size measured 0.63 mm2. Analysis of crystal-growth data from other locations in Antarctica and Greenland also revealed a strong linear relationship between the mean cross-sectional arcas (D2) of crystals (in mm2) and their ages in years (t), i.e. . The fact that the temperature dependence of the crystal growth rate K can be expressed very satisfactorily in an equation of the form K = K0 exp (E/RT) confirms predictions that crystal growth in firn is essentially analogous to grain growth in metallic and ceramic sinters. An extrapolation of available data indicates that crystal growth rates in dry firn could be expected to vary by two orders of magnitude (0.0003 to 0.03 mm2 year−1) over the temperature range −60° to −15°C. A method of utilizing crystal growth-mean annual temperature data to determine accumulation rates in snow is demonstrated.
Application of the gas law to fourth-place density measurements of ice samples from two deep drill holes at “Byrd” station and “Little America V”, Antarctica, shows that virtually all density increase beyond the pore close-off density (0.830 g cm−3) can be attributed to compression of the entrapped bubbles of air. Data from “Byrd” station also indicate that the lag between overburden pressure and bubble pressure, initially 4–5 kg cm−2 at pore close-off, diminishes to less than 1.0 kg cm−2 at about 200 m depth. By substituting the overburden pressure for the bubble pressure in the pressure-density relationship based on the gas law, one can determine ice densities below 200 m more accurately than they can be measured per se on cores, because of the relaxation that occurs in samples recovered from high confining pressures. This relaxation, resulting in a progressive increase in the bulk volume of the ice with time, is generally attributed to decompression of the entrapped air bubbles following removal of the ice from high confining pressures. However. calculations of the stress in ice due to bubble pressure, together with measurements of bubble sizes in cores from various depths at “Byrd” station, both tend to indicate that there has been negligible decompression of the inclosed bubbles. It is suggested that most of this relaxation may be due to the formation of micro-cracks in the ice. Anomalous bubble pressure–density relations at “Little America V” tend to confirm abundant petrographic evidence of the existence of considerable deformation in the upper part of the Ross Ice Shelf.
Studies of crystal–bubble relations at “Byrd” station revealed that the concentration of bubbles in ice remains remarkably constant at approximately 220 bubbles/cm3. Bubbles and crystals were found to be present in approximately equal numbers at pore close-off at 64 m depth, at which level the average bubble diameter was 0·95 mm, decreasing to 0.49 mm at 116 m and to 0·33 mm at 279 m. Despite a ten-fold increase in the size of crystals between 64 and 279 m, the bubbles showed no tendency to migrate to grain boundaries during recrystallization of the ice. The observation that most of the bubbles had assumed substantially spherical shapes by 120 m depth points to essentially hydrostatic conditions in the upper layers of the ice sheet at “Byrd” station.
Art examination of bullet crystals in precipitation at the South Pole indicates that combinations of bullets originate as primary growth structures and that individual bullets are formed as a result of the disintegration of these primary growth forms rather than by independent crystallization of pyramidally terminated columns.
Recent measurements of snow accumulation on undulating surfaces around “Byrd station”, Antarctica indicate that the undulations are tending to be filled in. These results are discussed in the light of current knowledge of the origin and migration of such features.
The seasonal distribution of snow at the South Pole and its relationship to stratigraphy was investigated to pits dug beside a number of four-year-old accumulation stakes. Results show that conventional stratigraphic methods yield thoroughly reliable values of accumulation rates. Even hiatuses in accumulation can be identified from the intensity of sublimation of layers of depth hoar in the stratigraphic section. Such hiatuses are due almost invariably to the prolonged absence of accumulation rather than to widespread scouring of pre-existing layers of snow. The bulk of the year’s accumulation is deposited as dunes during winter. The majority of dunes are subsequently transformed into linear sastrugi by wind with the result that the amplitude of surface relief observed at the end of winter frequently exceeds the average thickness of snow accumulated annually. Such gross relief does not persist to the end of summer, however. Instead the dunes and sastrugi arc gradually worn down by a process of sublimation-deflation. It is this leveling of the surface relief in summer and the resultant redistribution of snow more uniformly over the surface that are believed to be the significant factors in the formation of the systematic stratigraphy observed in pits at the South Pole.
Temperature, inclination, and closure have been measured in a 309 m. deep drill hole at Byrd Station, Antarctica. The results of five series of measurements taken yearly since February 1958 show that temperatures below 70 m. have remained constant since December 1958, that the closure rate has accelerated, and that the hole has undergone negligible inclination from the vertical. Anomalous temperatures in the upper levels of the drill hole are attributed to the steel casing that was permanently emplaced to a depth of 36 m. during drilling in 1957–58. A positive temperature gradient was observed in the casing, but negative gradients exist below the casing and a constant gradient profile is developed below 170 m. Both ice motion and climatic changes at Byrd Station are thought to have contributed to the formation of the observed negative temperature gradients. Insignificant bending of the drill hole would imply negligible differential motion in the upper 300 m. of the 2,400 m. thick ice sheet at Byrd Station. The rate of hole closure has accelerated throughout the 4 yr. period of observations, except at the bottom of the drill hole, where the most recent measurements (February 1962) show that some constraint is now developing. Deformation rates throughout the drill hole are not proportional to some constant power of the stress; instead the value of the power has been found to increase with both increasing stress and time of application of stress. This behavior is attributed to some process of continuous deformational recrystallization of ice in the walls of the drill hole. A recoring of the deformed drill hole to investigate such effects is advocated.
The age hardening of artificially and naturally compacted snow has been investigated at the South Pole. Results show that the age-hardening process is greatly retarded at low temperatures. Artificially compacted samples of density 0.55 g./cm.3 attained a compressive strength of less than 3.0 kg./cm.2 after one year’s ageing at −49° C. Exposure to solar radiation accelerated the age hardening. Irradiated samples attained a strength of 6.0 kg./cm.2 after 100 hr., increasing to a virtual maximum of 8.0 kg./cm.2 at the end of 600 hr. Compressive strengths increased with decrease in snow-particle size and with increasing angularity of the particles. Below 3 m. the strength of naturally compacted snow was found to increase rapidly with increase in density. Naturally compacted snow of density 0.55 g./cm.3 possessed considerably greater strength than any of the age-hardened samples of artificially compacted snow of the same density. Thin-section studies show that age hardening can be correlated with the formation and growth of intergranular bonds, and that bond growth falls off rapidly with decreasing temperature. In view of the low strengths found in both naturally compacted snows near the surface and in artificially compacted snow at the South Pole, “cut-and-cover” under-snow camp construction may not prove too practical at the South Pole.
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