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Three ice cores were recovered on or near Mount Logan, Yukon, Canada, at 3017, 4135 and 5340 ma.s.l. in 2002. Prior to ice-core drilling, we collected snow-pit and shallow core samples from Mount Logan in 2001 to study seasonal and spatial variations of snow chemistry. We dug snow pits at six sites between 2420 and 5340 m a.s.l. before the beginning of the melt season, with the exception of a pit at 3180 m a.s.l., where the melt season had just started but had affected only the near-surface stratigraphy. Three of the pits were extended deeper with a shallow core. The snow-pit and core samples were analyzed for ion chemistry and δ18O. A series of depth profiles of ions and δ18O shows spatial variations, though characteristic peaks can usually be traced across all the profiles. Concentrations and deposition fluxes of Na+ and Cl−, which are mainly of sea-salt origin, decrease with altitude. On the other hand, deposition fluxes of NO3−, SO42–, Ca2+ and NH4+ show a weak positive relationship with elevation below the summit plateau. Stable isotopes (δ18O) decrease with altitude, with a distinctive jump between 3200 and 4500 m a.s.l., as was reported previously. Stable isotopes (δ18O), Cl−, CH3SO3− (MSA), Na+ and Ca2+ show clear seasonal variations, which would enable us to date the cores by annual-layer counting.
To better understand how ice fabric evolves in polar ice sheets, we use X-ray diffraction to measure ice crystal orientations. X-ray measurement equipment which can measure the orientation of the c axis and a axis of each crystal in a thin section with high measurement accuracy was developed. In this study, we present a-axes orientation distribution of the deep part of the GRIP (Greenland summit) ice core. At some depths, we find an anisotropic distribution of a-axes orientation. Long-term uniaxial compression tests are also carried out on the GRIP ice core to investigate the ice fabric evolution process. The c-axis orientation distribution develops into a stronger single maximum as the strain increases up to about 20% strain. We find that the a axes of each grain also tend to cluster close to nearly a mutual direction. We discuss the development process of ice fabrics, taking into consideration the distribution of the a-axis orientations. It is suggested that these fabrics may be attributed to a local simple shear deformation in the deep part of an ice sheet.
Laboratory experiments were done to better understand the electrical conduction mechanisms of impure, polycrystalline ice as represented by the 2503 m Dome Fuji (Antarctica) ice core. Also, two electrical measurement techniques for ice cores were compared and their usefulness for determining the acidity of ice cores was studied. We measured the electrical conductivity and complex permittivity of 167 slab-ice samples at frequencies from 20 Hz to 1 MHz. Measurements were performed at –21˚C for all samples, and at –110˚ to –20˚C for several samples, to examine the effects of temperature. We found linear relations between the AC loss factor and the molarity of sulfuric acid, and between the high-frequency-limit conductivity and the AC loss factor. Thus, the acidity levels can be determined from the AC loss factor. In contrast, the electrical conductivity measurement (ECM) current correlated weakly with the other parameters; furthermore, the correlation worsens at larger signal. In several samples containing high acidity, the dielectric properties had distinct changes near –81˚C. We argue that these changes were caused by a change from a liquid-vein-mediated conduction mechanism above the eutectic point of the solute/water/ ice system to a solid-phase conduction mechanism at lower temperatures.
The geophysical metronome (Milankovitch components of the past surface temperature variations) and the isotope–temperature transfer function deduced from the borehole temperature profile at Vostok station, Antarctica, are applied to date the 2500 mdeep ice core from Dome Fuji station, Antarctica, and to reconstruct paleoclimatic conditions at the drilling site on the basis of the local δ18O isotope record. Special attention is paid to consistency of this depth–age relation with the mass-balance reconstruction and predictions of ice-flow modeling. the present-day ice mass-balance rate at Dome Fuji is estimated as 3.2 cm a–1. the ice age at the borehole bottom (590m above the bedrock) is around 335 ± 4.5 kyr and may reach 2000 kyr at about 3000 mdepth.The difference in the ice-sheet surface temperatures between Holocene optimum and Last Glacial Maximum is found to be 17.8˚C at the temporal isotope/temperature slope, about 30% lower than the modern geographical estimates. A good agreement between modeled and measured (preliminary data) borehole temperatures is obtained at the geothermal flux 0.059 Wm–2 and ice-fusion temperature (–2˚C) at the ice–rock interface with minimum (zero) melt rates.
To better understand how ice sheets respond to climate, we designed a new multi-frequency ice-penetrating radar system to investigate subsurface structures of ice sheets. The system is mounted on a single platform and handled by a single operator. Three radio frequencies, 30,60 and 179 MHz, were used. An underlying principle of these multi-frequency observations is that the lower frequencies are more sensitive to electrical conductivity changes, whereas the higher frequencies are more sensitive to dielectric permittivity fluctuations in the ice. The system is composed of three single-frequency pulse radars, a trigger-controller unit and a data-acquisition unit. The trigger controller is the key component of this system. It switches transmitters on at different timings to prevent mixing of signals among the three radars. The timing difference was set as 50 μs, which is equivalent to the two-way travel time for radio waves reflecting from 4250m below the surface. A field test was done along a 2000 km long traverse line in east Dronning Maud Land, Antarctica. The multi-frequency system successfully acquired data that are equivalent in quality to our earlier single-frequency measurements along the same traverse line. The details of the system and preliminary data are described.
The 320 kyr climatic record from the 2503 m Dome Fuji (Antarctica) ice core was analyzed using two electrical methods: AC-ECM and ECM (electrical conductivity measurements). AC-ECM is a method to detect the complex admittance between electrodes dragged on the ice surface with mm-scale resolution and uses 1V and 1 MHz. the ratio of the real to imaginary part of the admittance is the AC loss factor, which responds linearly to the amount of sulfuric acid and hydrogen ions. Both the AC loss factor and the ECM current respond to acid, but the ECM signal tends to saturate at high acidities. Dome Fuji ice was measured to be highly acidic, with background values of 2–7 μM, and had 4500 major peaks with acidities of up to 90 μM. This ice-core evidence and earlier snow-chemistry survey around the dome region indicates that Dome F may have a better connection to the stratosphere than have sites at lower altitude, which allows more stratospheric aerosol and gases to reach the snow surface. Acidity tends to be high in interglacial periods, but correlation between acidity and δ18O is not straightforward. Electrical signals decreased and smoothed out with increasing depth; the diffusion coefficients deduced from this smoothing were 10–102 times greater than in solid ice. the ice core exhibited electromechanical effects and expelling effects from sulfate peaks.
The North Greenland Icecore Project (NorthGRIP) was initiated in 1995 as a joint international programme involving Denmark, Germany, Japan, Belgium, Sweden, Iceland, the U.S.A., France and Switzerland. the main goal was to obtain undisturbed high-resolution information about the Eemian climatic period (115–130 kyr BP). the records from the Greenland Icecore Project (GRIP) and Greenland Ice Sheet Project 2 (GISP2) in central Greenland are different and disturbed down in the ice covering this period. Internal radio-echo sounding layers show that NorthGRIP, placed 325 km north-northwest of GRIP at the Summit of the Greenland ice sheet, is located on a gently sloping ice ridge with very flat bedrock and internal layers found so high that an undisturbed Eemian record is possible. Internal layers much farther above bedrock than their apparent counter parts at GRIP suggest that conditions are favourable for recovery of an undisturbed Eemian record. So far, a 1351 mdeep ice core (NorthGRIP1) and a 3001 mdeep ice core (NorthGRIP 2) have been recovered. the ice thickness is expected to be 3080 m, and the ice temperature at 3001 m is –5.6°C, so we expect basal melting at the bedrock. Most of the Eemian ice will be melted away, leaving only the last part and the transition between the Eem and the Last Glacial Period. At 3001 m the age of the ice is 110 kyr BP and the annual layers are of the order 1 cm.With modern methods the annual layers can be resolved, resulting in detailed information on the decline of the warm Eemian period into the Last Glacial Period.
To determine annual layers for reconstructing the past environment at annual resolution from ice cores, we employed snow-stake data back to 1972, tritium content, solid electrical conductivity measurements (ECM) and stratigraphic properties for the 73m ice core at the H72 site, east Dronning Maud Land, Antarctica. the average annual surface mass balance at H72 is 307 mma–1w.e. during the last 27 years from continuous accumulation data, 317 mma–1 w.e. according to the densification model and 311 mma–1 w.e. according to the average surface mass balance for 167 years based on annual-layer counting. the ECM age is closely coincident with tritium age, and corresponds with the snow-stake record back to AD 1972 from the surface to 15 m depth. the H72 ice core is dated as AD 1831by ECMat 73.16 mdepth.The time series of yearly surface mass balance at H72 shows an almost constant 311 mm a–1 w.e. for the last 167 years. the oxygen-isotope records indicate a significant trend to lower values, with negative gradient of 1.7% (100 years)–1.
Snowpack and ice-core samples were collected from the dome of Austfonna ice cap, Svalbard, in the spring of both 1998 and 1999. The samples were analyzed for anions, cations, pH, liquid electrical conductivity and oxygen isotopes. Concentrations of chemical components in snowpack with a history of melting were much lower than those in unmelted snowpack. There was a clear difference between Mg2+/Na+ ratios previously in melted snowpack (0.03 ± 0.02) and in unmelted snowpack (0.11 ± 0.02). We propose that the Mg2+/Na+ ratio can be used as an indicator of whether or not firn or bubbly ice in the Austfonna ice core has experienced melt percolation. The Mg2+/Na+ ratio indicates that firn or bubbly ice prior to AD 1920 was much less affected by melt percolation than firn or bubbly ice formed after 1920.
Uniaxial compression tests were performed on samples of the Greenland Ice Gore Project (GRIP) deep ice core, both in the field and later in a cold-room laboratory, in order to understand the ice-flow behavior of large ice sheets. Experiments were conducted under conditions of constant strain rate (type A) and constant load (type B). Fifty-four uniaxial-compression test specimens from 1327-2922 m were selected. Each test specimen (25 mm x 25 mm x 90 mm) was prepared with its uniaxial stress axis inclined 45° from the core axis in order to examine the flow behavior of strong single-maximum ice-core samples with basal planes parallel to the horizontal plane of the ice sheet. The ice-flow enhancement factors show a gradual increase with depth down to approximately 2000 m. These results can be interpreted in terms of an increase in the fourth-order Schmid factor. Below 2000 m depth, the flow-enhancement factor increases to about 20-30 with a relatively high variability When the Schmid factor was > 0.46, the enhancement factor obtained was higher than expected from the .-axis concentrations measured. The higher values of flow-enhancement factor were obtained from specimens with a cloudy band structure. It was revealed that cloudy bands affect ice-deformation processes, but the details remain unclear.
The Antarctic ice sheet preserves paleoclimate information in the form of physical and chemical stratigraphy. A deep ice core down to 2503 m depth was drilled at Dome Fuji station, East Antarctica, during the 1993-96 Japanese Antarctic Research Expedition inland operations. Oxygen isotope measurements were conducted on 50 cm long samples selected from the entire core length. A palco-temperature profile was obtained for the past 340 ka by assuming the same conversion factors for the past relation as exist today between isotope ratio and both surface temperature and accumulation rate, in the inland region of Dronning Maud Land. The environmental-index profiles such as major chemical and dust contents coincide quite well withVostok ice-core data in general but not in detail. Detailed analysis of these climatic and environmental signals is in progress.
A 2.2 m deep pit and the top 42.5 m of an ice core recovered at Snøfjellafonna, northwestern Spitsbergen, were continuously analyzed for Na+, Cl−, NO3−, SO42− and pH. Seasonal variations in ionic concentrations seem to have remained in the pit and the core, in spite of the relatively severe summer melting. We dated the core by counting annual peaks of Na+ and made an adjustment with the use of a tritium peak in 1963 as a reference horizon. It turned out that the depth of 42.5 m went back to the early 1930s or late 1920s. The 60–70 year record of snow chemistry showed that the concentrations of both NO3− and SO42− had increased in the 1950s and had decreased in the late 1970s and the 1980s. The increase would be explained in terms of anthropogenic inputs from the industrial areas. The later decrease of the same ions may have been caused by a combination of the reduction of the atmospheric precursors due to pollution controls and the meltwater-associated processes.
The surface mass balance in east Dronning Maud Land has been observed mainlvbv means of the snow-stake method. The surface mass balance generallv decreased with distance from the coast; from more than 250 mm a−1 in the coastal region to less than 50 mm a−1 in the inland region higher than 3500 m in altitude. At Mizuho Station (2230 m a.s.l.). the sublimation was about 50 mm a−1. precipitation was between 140 and 260 mm a−1, and the loss from the surface by theredistribution was estimated to be about 100 mm a−1, which agrees with the surface mass balance estimated as 70 mm a−1 from the grain-growth rate. Around the mountainous area, the balance was small or in some cases negative, where a bare-ice field has developed. In the inland area, 3000– 3200 m a.s.l..the surface mass balance was less than 50 mm a−1, i.e. lower than in the surrounding areas. This low mass-balance area can be explained bv redistribution by the drifting snow. The whole mass input in five drainage basins with a total area of 620 × 103 km2 is 61.2Gton a−1 and the mean surface mass balance is 99 mm a−1.
Two empirical equations for firn densification have been obtained,considering firn porosity as a function of overburden pressure. In the first equation, thereduction ratio of porosity in firn is assumed to be proportional to the increasing ratioof overburden pressure and the mth power of the porosity. The porosity exponent m should be close to -2, so as to have a best-fit with 14 depth-density profiles fromGreenland and Antarctica. In the second equation, the reduction ratio of porosity wasassumed to increase proportionally to the increment of overburden pressure and thenth power of the porosity. The most satisfactory values of the exponent range from -1 to 1. It has been suggested that firn density, determined primarily by overburdenpressure and firn temperature, contribute to a lesser degree.
Undulating topography on the East Antarctic ice sheet was clearly revealed by NOAA AVHRR. The following three patterns of undulating topography were detected by using high-pass filtered images from the visible to thermal infrared channels. In coastal regions (below 2000 m a.s.l.), undulation can be clearly detected by the fluctuation of reflectance in visible channel. It has wavy structure with spacing less than 10 km and alignment at a right angle to the ice-flow lines. In the katabatic zone (from 2000 m a.s.l. to 3000 m a.s.l) well defined fluctuations of albedo stpectrum can be seen with spacing from 10 km to 20 km, aligned at right angles to the ice-flow lines or prevailing katabatic wind direction. Ground-survey data show that the undulating topography is associated with large variations of net accumulation rate. On the inland plateau (above 3000 m a.s.l.), undulation can be clearly seen in the fluctuation of thermal infrared channel in winter. Ground-survey data show that the signal corresponds to the undulating topography. The alignment of the undulation is at a right angle to the ice-flow lines and the spacing is longer than 20 km. The characteristics of these undulations represent the ice-flow dynamics and accumulation anomaly.
Dry and wet air-extraction systems and precise analysis systems of the CO2 and CH4 concentrations for a polar ice core were developed to reconstruct their ancient levels. A dry-extraction system was capable of crushing an ice sample of 1000 g into fine powder within 2 min, and its air-extraction efficiency was found to be 98%. The CO2 and CH4 concentrations of extracted air were determined using gas chromatography with a flame-ionized detector. The overall precision of our measurements, including air extraction, was estimated to be better than ± 1 ppmv for CO2 and + 10 ppbv for CH4. Preliminary analysis of the ice core drilled at Mizuho Station, Antarctica, showed that the CO2 and CH4 concentrations at 3340–3700 year BP were about 280 ppmv and 700ppbv, respectively. The Yamato core drilled at the terminus of the glacial flow near the Yamato Mountains, Antarctica, yielded concentrations of 230–240 ppmv for CO2 and 520–550 ppbv for CH4, suggesting that the core had formed during the glacial period.
A linear relation between total gas content in ice and the elevation of ice formation (i.e. pore close-off) was obtained from seven shallow ice cores in Mizuho Plateau, Antarctica. The derived relation was applied to the vertical profile of total gas content in a 700 m long ice core at Mizuho Station. A general trend of gradual increase in total gas content was observed from 600 to 200 m in depth, from which toward the upper layer it showed a steep increase. After eliminating the effect of down-slope flow of ice around Mizuho Station, it was estimated that the thickness of the ice sheet decreased by about 350 m at maximum during the last 2000 years. This tendency also appears in the δ18O profile of the same ice core.
We have investigated two 30 m cores at two different spots in the most heavy snow-accumulation regions on Mizuho Plateau, East Antarctica. Marked seasonal variations periodically appear in oxygen isotope records of the cores. We analyzed one core with no trace of snow melting and found it had a complete record showing a yearly change of annual net snow accumulations from 1920 through 1980. The analysis shows that a yearly variation of annual net accumulation (N.A.) has some relations with that of the annual maximum value (δmax) of δ18O and that of the annual amplitude (Δδ) of the δ18O-change in an annual snow layer. Power spectral analyses with respect to the variation of N.A., δmax and Δδ also indicate that there is commonly a predominant periodicity of about five years.
Recently, a 700 m long ice core was drilled at Mizuho Station (2230 m a.s.1.), 270 km south-east of Syowa Station and situated in a typical katabatic-slope region.
In order to obtain basic knowledge for dating the core and for interpreting climatic change and depositional environment change along the core, a study of the regional characteristics of the snow-deposition regime on Mizuho plateau has started. Surface-firn cores 10–30 m deep and snow-stake data obtained along the traverse routes on Mizuho plateau since 1970 were analyzed.
The general trend of annual snow accumulation and the regional characteristics of the δ18O profile of snow cover were obtained.
The present paper gives the preliminary results of the analyses on microparticle concentration and electrical conductivity of a 700.56 m ice core from Mizuho Station, Antarctica. Concentration of microparticles coarser than 0.63 μm in diameter increases more than twofold at the 240-440 m depth interval compared with that below 440 m in depth. The higher particle concentration is well associated with higher electrical conductivity and lower δ18O. Periods of high particle concentration are estimated to be 3000-6000 years B.P. A visible volcanic dirt band was found at 500.7 m below the surface. This dirt band may be isochronous with the shallowest ash band of the Byrd Station core, found at 799 m depth. The present study indicates that large-scale environmental changes possibly occurred in the Southern Hemisphere in the middle of the Holocene.