Hideki Narita

High-resolution record of Northern Hemisphere climate extending into the last interglacial period

Two deep ice cores from central Greenland, drilled in the 1990s, have played a key role in climate reconstructions of the Northern Hemisphere, but the oldest sections of the cores were disturbed in chronology owing to ice folding near the bedrock. Here we present an undisturbed climate record from a North Greenland ice core, which extends back to 123,000 years before the present, within the last interglacial period. The oxygen isotopes in the ice imply that climate was stable during the last interglacial period, with temperatures 5 °C warmer than today. We find unexpectedly large temperature differences between our new record from northern Greenland and the undisturbed sections of the cores from central Greenland, suggesting that the extent of ice in the Northern Hemisphere modulated the latitudinal temperature gradients in Greenland. This record shows a slow decline in temperatures that marked the initiation of the last glacial period. Our record reveals a hitherto unrecognized warm period initiated by an abrupt climate warming about 115,000 years ago, before glacial conditions were fully developed. This event does not appear to have an immediate Antarctic counterpart, suggesting that the climate see-saw between the hemispheres (which dominated the last glacial period) was not operating at this time.Two deep ice cores from central Greenland, drilled in the 1990s, have played a key role in climate reconstructions of the Northern Hemisphere, but the oldest sections of the cores were disturbed in chronology owing to ice folding near the bedrock. Here we present an undisturbed climate record from a North Greenland ice core, which extends back to 123,000 years before the present, within the last interglacial period. The oxygen isotopes in the ice imply that climate was stable during the last interglacial period, with temperatures 5 °C warmer than today. We find unexpectedly large temperature differences between our new record from northern Greenland and the undisturbed sections of the cores from central Greenland, suggesting that the extent of ice in the Northern Hemisphere modulated the latitudinal temperature gradients in Greenland. This record shows a slow decline in temperatures that marked the initiation of the last glacial period. Our record reveals a hitherto unrecognized warm period initiated by an abrupt climate warming about 115,000 years ago, before glacial conditions were fully developed. This event does not appear to have an immediate Antarctic counterpart, suggesting that the climate see-saw between the hemispheres (which dominated the last glacial period) was not operating at this time.

A continuous 770-year record of volcanic activity from east Antarctica

A 100-m ice core from site G 15 (accumulation rate 0.1 m water yr−1, mean annual temperature −38°C) on the Mizuho plateau, Dronning Maud Land, East Antarctica, has been analysed using the dielectric profiling (DEP) technique. The capacitance and conductance of the core were measured at ac frequencies (20 Hz-300 kHz). The high-frequency conductivity profile shows variations that are primarily related to the strong acids derived from volcanic activity. The Tambora (1815) eruption can be identified with the aid of an approximate chronology based on the firn densification rate, other historic eruptions can then be recognised. Beyond about 300-years historical observations are very few, however if a constant overall accumulation rate is assumed, a well-known eruption of 1259 A.D. can be found near the bottom of the core. Other peaks in the conductivity profile can then be assigned dates accurate to within a few years. Using the conductivity profile it is possible to estimate the relative acid deposition fluxes produced by the main eruptions with reasonable accuracy. The estimated acid deposisition fluxes realtive to the Tambora (1815) eruption, of Agung (1963) is 27%, Krakatoa (1883), 25%, the signal of 1601, 28%, and that of 1259, 53%.

Acid ions at triple junction of Antarctic ice observed by Raman scattering

We measured the Raman spectra of the junctions where three crystal grains met (triple-junctions) in polycrystalline ice from two Antarctic sites (Nansen ice and South Yamato ice) in order to obtain direct evidence that sulfuric and nitric acids are present as liquid at the triple-junctions. We found that Raman spectra of Nansen ice have a peak (1050 cm−1) of HSO4− and NO3− in sulfuric and nitric acid solutions when the measured temperatures of the ice are −8 ∼ −35°C. Thus, sulfuric and nitric acids dissociate to HSO4− and NO3− at the triple-junctions of Nansen ice. Raman spectra of South Yamato ice have a peak (980 cm−1) of SO42− in sulfuric acid solution when the measured temperatures of the ice are −8 ∼ −20°C. Thus, sulfuric acid dissociates to SO42− at the triple-junctions of South Yamato ice. The results showed that aqueous solutions of the acids exist at Antarctic ice-sheet temperatures.

Shear cell experiments of snow and ice friction

Snow and ice friction was investigated with a shear cell in which two surfaces of annular snow and/or ice samples were in contact and sheared by rotation. The temperature ranged from 0 to −25 °C, normal stress from 205 to 1292 Pa, and velocity from 0.9 to 25.3 m/s. The total friction coefficients measured by friction of two hard sintered snow plates ranged roughly from 0.2 to 0.8, which were separated into velocity‐independent dry friction and linearly dependent viscous friction; only at velocities larger than 15 m/s did friction increase parabolically, evidence of turbulent friction. The dry friction coefficient (μD) was found to consist of Coulomb friction and adhesion, both of which were strongly dependent on temperature and hardness of snow; μD decreased linearly with lowering temperature from 0.47 at 0 °C to 0.22 at −25 °C. Viscosity was found to be dependent on snow type (grain characteristics) but independent of temperature; the average kinematic viscosity was roughly 5×10−5 m2/s for snow of densit...

The Internal Structure of Powder-Snow Avalanches

The internal structure of powder-snow avalanches was investigated at Kurobe Canyon in the Shiai-dani area of Japan in 1988. Internal velocity was derived for avalanches of this kind by frequency analysis of impact-force data, and was found to undergo a remarkable change with time. The shear of the avalanche flow was estimated to range from I to 7 S1. The front region of the avalanche wind was observed to precede the front of the avalanche by a distance of 17.3 m. The maximum wind velocity was comparable with the internal velocity of the front region of the avalanche.

MEASUREMENTS OF FRICTION COEFFICIENTS OF SNOW BLOCKS

Snow blocks were slid down natural snow slopes and filmed with a video camera. Friction coefficients were calculated from time-distance curves and the equation of motion. Dry-friction coefficients ranged from 0.57 to 0.84, and could be separated into Coulomb friction and a friction component proportional to the contact area of the blocks (adhesion). These values are greater than the values usually used in avalanche dynamics, but are consistent with previous coefficients obtained for snow blocks sliding over snow. When uniform ploughing occurred and a shear layer developed along the track the apparent friction coefficien ts increased with velocity, and could be modelled by considering the kinematic viscosity of the snow. The values of kinematic viscosity ranged from 10-3 to 10-4 m2/s and agreed well with those values obtained by other researchers.

Characteristics of air bubbles and hydrates in the Dome Fuji ice core, Antarctica

Air bubbles trapped near the surface of an ice sheet are transformed into air hydrates below a certain depth Their volume and number varies partly with environment and climate. Air bubbles and hydrates at 120-2200 m depth in the Dome Fuji (Dome F) ice core were examined with a microscope. This depth range covers the Holocene/Last Glacial/Last Interglacial/Previous Glacial periods. No air bubbles were seen below about 1100 m depth, and air hydrates began to appear from about 600 m. The observed number of air bubbles and hydrates was similar to that found in the Vostok ice core. For the ice covering the Last Glacial Maximum period, however the hydrate concentration in the Dome F core is about half that of the Vostok core. Reference to snow metamorphism and packing does not explain this finding.

Linear and non-linear relations between the high-frequency-limit conductivity, AC-ECM signals and ECM signals of Dome F Antarctic ice core from a laboratory experiment

Abstract 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.

A shallow ice core re-drilled on the Dunde Ice Cap, western China: recent changes in the Asian high mountains

A 51?m deep ice core was re-drilled on the Dunde Ice Cap of western China in 2002, 15 years after the previous ice core drilling in 1987. Dating by seasonal variations in ?18O and particle concentration showed that this 51?m deep ice core covered approximately the last 150 years. The stratigraphy and density showed that more than 90% of the ice core was refrozen ice layers, which comprised less than 5% of the annual accumulation in the older core. This indicates that the ice cap had experienced a more intense melting since 1987, possibly due to climate warming in this region. Mean net accumulation since the last drilling (2002?1987) was 176?mm?a?1, which was considerably smaller than that obtained from the 1987 core (390?mm?a?1, 1987?1963), indicating a significant decrease of net accumulation on the ice cap in the more recent period. The ?18O record showed an increasing trend in the late 19th century and the highest in the 1950s, which is consistent with the previous core findings. However, there has been no significant increase in ?18O during the last two decades, in contrast to the warming trends suggested by the melt features and other climate records. This discrepancy may be due to the modification of ?18O records by melt water runoff, percolation, and refreezing on the ice cap. Results strongly suggest recent significant mass loss of glaciers in the Asian high mountains and serious shortage of water supply for local people in this arid region in the near future.

Electrical measurements on the 2503 m Dome F Antarctic ice core

Abstract 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.