Toshimitsu Sakurai

State dependence of climatic instability over the past 720,000 years from Antarctic ice cores and climate modeling

Global cooling in intermediate glacial climate with northern ice sheets preconditions climatic instability with bipolar seesaw. Climatic variabilities on millennial and longer time scales with a bipolar seesaw pattern have been documented in paleoclimatic records, but their frequencies, relationships with mean climatic state, and mechanisms remain unclear. Understanding the processes and sensitivities that underlie these changes will underpin better understanding of the climate system and projections of its future change. We investigate the long-term characteristics of climatic variability using a new ice-core record from Dome Fuji, East Antarctica, combined with an existing long record from the Dome C ice core. Antarctic warming events over the past 720,000 years are most frequent when the Antarctic temperature is slightly below average on orbital time scales, equivalent to an intermediate climate during glacial periods, whereas interglacial and fully glaciated climates are unfavourable for a millennial-scale bipolar seesaw. Numerical experiments using a fully coupled atmosphere-ocean general circulation model with freshwater hosing in the northern North Atlantic showed that climate becomes most unstable in intermediate glacial conditions associated with large changes in sea ice and the Atlantic Meridional Overturning Circulation. Model sensitivity experiments suggest that the prerequisite for the most frequent climate instability with bipolar seesaw pattern during the late Pleistocene era is associated with reduced atmospheric CO2 concentration via global cooling and sea ice formation in the North Atlantic, in addition to extended Northern Hemisphere ice sheets.

Direct observation of salts as micro-inclusions in the Greenland GRIP ice core

We provide the first direct evidence that a number of water-soluble compounds, in particular calcium sulfate (CaSO 4 .2H 2 O) and calcium carbonate (CaCO 3 ), are present as solid, micron-sized inclusions within the Greenland GRIP ice core. The compounds are detected by two independent methods: micro-Raman spectroscopy of a solid ice sample, and energy-dispersive X-ray spectroscopy of individual inclusions remaining after sublimation. CaSO 4 .2H 2 O is found in abundance throughout the Holocene and the last glacial period, while CaCO 3 exists mainly in the glacial period ice. We also present size and spatial distributions of the micro-inclusions. These results suggest that water-soluble aerosols in the GRIP ice core are dependable proxies for past atmospheric conditions.

The chemical forms of water-soluble microparticles preserved in the Antarctic ice sheet during Termination I

This study clarifies changes in the chemical forms of microparticles during Termination I, the period of drastic climate change between the Last Glacial Maximum (LGM) and the Holocene. We determine the chemical forms of individual water-soluble microparticles through micro-Raman spectroscopy and compare the relative frequencies of different types with the ion concentrations in melted ice. Micro-Raman spectroscopy shows that Na2SO4� 10H2O and MgSO4� 11H2O are abundant in Holocene ice, while CaSO4� 2H2O and other salts are abundant in LGM ice. Further, the number of CaSO4� 2H2O particles is strongly correlated with the concentration of Ca 2+ during Termination I. Taken together, the evidence strongly suggests that most of the Ca 2+ exists as CaSO4� 2H2O. The different compositions of microparticles from the Holocene and LGM can be explained by ion balance arguments.

Magnesium methanesulfonate salt found in the Dome Fuji (Antarctica) ice core

Using micro-Raman spectroscopy, we identified the chemical forms of methanesulfonate salt particles in reference samples of the Dome Fuji (Antarctica) ice core. We found only (CH3SO3)2MgnH2O among methanesulfonate salts, and this salt particle is most prevalent in the Last Glacial Maximum (LGM) ice. We suggest that during the LGM, (CH3SO3)2MgnH2O may have formed in the atmosphere through the chemical reaction of CH3SO3H with sea salts, but probably not in the firn and ice due to the neutralization of acid in LGM ice of inland Antarctica.

Meridianiite detected in ice

Inclusions affect the behavior of ice, and their characteristics help us understand the formation history of the ice. Recently, a low-temperature magnesium sulfate salt was discovered. This paper describes this naturally occurring MgSO4 � 11H2O mineral, meridianiite, derived from salt inclusions in sea ice of Lake Saroma, Japan and in Antarctic continental core ice. Its occurrence is confirmed by using micro-Raman spectroscopy to compare Raman spectra of synthetic MgSO4 � 11H2O with those of the inclusions.

A Technique for Measuring Microparticles in Polar Ice Using Micro-Raman Spectroscopy

We describe in detail our method of measuring the chemical forms of microparticles in polar ice samples through micro-Raman spectroscopy. The method is intended for solid ice samples, an important point because melting the ice can result in dissociation, contamination, and chemical reactions prior to or during a measurement. We demonstrate the technique of measuring the chemical forms of these microparticles and show that the reference spectra of those salts expected to be common in polar ice are unambiguously detected. From our measurements, Raman intensity of sulfate salts is relatively higher than insoluble dust due to the specific Raman scattering cross-section of chemical forms of microparticles in ice.

Chemical compositions of sulfate and chloride salts over the last termination reconstructed from the Dome Fuji ice core, inland Antarctica

The flux and chemical composition of aerosols impact the climate. Antarctic ice cores preserve the record of past atmospheric aerosols, providing useful information about past atmospheric environments. However, few studies have directly measured the chemical composition of aerosol particles preserved in ice cores. Here we present the chemical compositions of sulfate and chloride salts from aerosol particles in the Dome Fuji ice core. The analysis method involves ice sublimation, and the period covers the last termination, 25.0–11.0 thousand years before present (kyr B.P.), with a 350 year resolution. The major components of the soluble particles are CaSO4, Na2SO4, and NaCl. The dominant sulfate salt changes at 16.8 kyr B.P. from CaSO4, a glacial type, to Na2SO4, an interglacial type. The sulfate salt flux (CaSO4 plus Na2SO4) inversely correlates with δ18O in Dome Fuji over millennial timescales. This correlation is consistent with the idea that sulfate salt aerosols contributed to the last deglacial warming of inland Antarctica by reducing the aerosol indirect effect. Between 16.3 and 11.0 kyr B.P., the presence of NaCl suggests that winter atmospheric aerosols are preserved. A high NaCl/Na2SO4 fraction between 12.3 and 11.0 kyr B.P. indicates that the contribution from the transport of winter atmospheric aerosols increased during this period.

Amplification characteristics of a cryogenic Yb 3+ :YAG total-reflection active-mirror laser

We have studied the amplification characteristics of a cryogenically cooled Yb³⁺:YAG total-reflection active-mirror (TRAM) ceramic laser including wavefront distortion, birefringence loss, small signal gain (SSG), and temperature rise for developing high-performance master oscillator power amplifier (MOPA) systems. A 0.6 mm thick Yb³⁺:YAG ceramic sample was used, and maximum pump intensity ~10  kW/cm² was reached. The transmitted wavefront was measured by using a Shack-Hartmann wavefront sensor, and we evaluated the thermal lens focal length and Strehl ratio for different pump conditions. We have also observed a butterfly-like leakage profile of thermally induced birefringence loss at the maximum pump intensity. From SSG measurements, we obtained moderate laser gain of G=3 for one bounce with a near aberration-free wavefront. Gain calculations, which included also temperature dependence of the emission cross section and reabsorption of Yb³⁺:YAG, were in good agreement with the experiments. These experimental results will be useful as benchmark data for numerical simulations of temperature distribution in TRAM and for designing multikilowatt-class high-performance MOPA systems.

Ernstburkeite, Mg(CH3SO3)2·12H2O, a new mineral from Antarctica

The new mineral (IMA 2010–059) ernstburkeite, Mg(CH 3 SO 3 ) 2 · 12H 2 O, occurs as solid inclusions, typically with a grain size up to 5 μm, in an ice core from the Dome Fuji station, East Antarctica. Due to the small crystal size most physical and optical properties cannot be established on natural material. Optically, the mineral is uniaxial (+), ω 1.402(1), ɛ 1.408(1) (589 nm), and nonpleochroic. Ernstburkeite is trigonal, space group R -3 (no. 148), a = 9.27150(8) A, c = 21.1298(4) A, V = 1572.99(4) A 3 , Z= 3. Strongest X-ray powder-diffraction lines (relative intensities in parentheses) are: 7.04(42), 6.39(39), 4.64(100), 4.41(44), 3.87(89), 3.75(31), 3.74(35). The chemistry of the mineral in the ice core was confirmed by Raman microspectroscopy. The name is for Ernst A J. Burke, Belgian-born mineralogist at the Vrije Universiteit Amsterdam, Netherlands (1966–2005), chairman of the IMA Working Group on Inclusions in Minerals (1994–1998) and chairman of the IMA Commission on New Minerals, Nomenclature and Classification (2003–2008).

Physicochemical properties of bottom ice from Dome Fuji, inland East Antarctica

The deepest ice in inland Antarctica is expected to preserve the oldest ice records and to potentially contain microorganisms. However, little is known about the physicochemical conditions in the deepest part of ice sheets. This study investigates the physicochemical properties of the bottom section (3000–3035 m) of the Dome Fuji inland ice core, which is located immediately above unfrozen bedrock. The ubiquitous presence of air hydrates and the water isotope composition of ice comparable to the upper main ice core show that the bottom ice is meteoric. However, ion concentrations exhibit abnormal drops at the greatest depths (approximately below 3033 m). In the same depth range, microscopic investigations reveal that considerable relocation of air hydrates and micro-inclusions (water-soluble impurities) occurs, suggesting that the observed reduction in ion concentration results from the segregation of inclusions to ice grain boundaries and the subsequent discharge of chemicals through liquid-water veins. Principal component analysis of ion data supports the meteoric-ice hypothesis, suggesting that the bottom ice had similar original chemistry through all depths. Statistical analyses of chemical data suggest that the water-soluble impurities attached to hydrates or dust (water-insoluble), the ice-soluble chemical species (such as chlorine), and solid particles are less affected by this chemical displacement phenomenon. It is also noteworthy that, in the bottom ice, impurity chemicals, which are limiting nutrients for ice-dwelling microorganisms, are concentrated largely to ice–hydrate interfaces, where oxygen, another vital matter for aerobic microorganisms, is also enriched.