Takayuki Kuramoto

Seasonal variations of snow chemistry at NEEM, Greenland

Abstarct We conducted a pit study in July 2009 at the NEEM (North Greenland Eemian Ice Drilling) deep ice-coring site in northwest Greenland. To examine the seasonal variations of snow chemistry and characteristics of the drill site, we collected snow/firn samples from the wall of a 2 m deep pit at intervals of 0.03 m and analyzed them for electric conductivity, pH, Cl–, NO3 –, SO4 2–, CH3SO3 – (MSA), Na+, K+, Mg2+, Ca2+ and stable isotopes of water (δ18O and δD). Pronounced seasonal variations in the stable isotopes of water were observed, which indicated that the snow had accumulated regularly during the past 4 years. Concentrations of Na+, Cl– and Mg2+, which largely originate from sea salt, peaked in winter to early spring, while Ca2+, which mainly originates from mineral dust, peaked in late winter to spring, slightly later than Na+, Cl– and Mg2+. Concentrations of NO3 – showed double peaks, one in summer and the other in winter to spring, whereas those of SO4 2– peaked in winter to spring. The winter-to-spring concentrations of NO3 – and SO4 2– seem to have been strongly influenced by anthropogenic inputs. Concentrations of MSA showed double peaks, one in spring and the other in late summer to autumn. Our study confirms that the NEEM deep ice core can be absolutely dated to a certain depth by counting annual layers, using the seasonal variations of stable isotopes of water and those of ions. We calculated the annual surface mass balance for the years 2006–08. The mean annual balance was 176 mm w.e., and the balances for winter-to-summer and summer-to-winter halves of the year were 98 and 78 mm, respectively. Snow deposition during the winter-to-summer half of the year was greater than that during the summer-to-winter half by 10–20mm for all three years covered by this study.

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.

Asynchrony between Antarctic temperature and CO 2 associated with obliquity over the past 720,000 years

The δD temperature proxy in Antarctic ice cores varies in parallel with CO2 through glacial cycles. However, these variables display a puzzling asynchrony. Well-dated records of Southern Ocean temperature will provide crucial information because the Southern Ocean is likely key in regulating CO2 variations. Here, we perform multiple isotopic analyses on an Antarctic ice core and estimate temperature variations at this site and in the oceanic moisture source over the past 720,000 years, which extend the longest records by 300,000 years. Antarctic temperature is affected by large variations in local insolation that are induced by obliquity. At the obliquity periodicity, the Antarctic and ocean temperatures lag annual mean insolation. Further, the magnitude of the phase lag is minimal during low eccentricity periods, suggesting that secular changes in the global carbon cycle and the ocean circulation modulate the phase relationship among temperatures, CO2 and insolation in the obliquity frequency band.The Antarctic temperature record displays a puzzling asynchrony with changes in CO2 through glacial cycles. Here, the authors show that a 720,000-year Antarctic temperature record is affected by variations in obliquity-induced local insolation that are associated with phase modulation of eccentricity cycle.

Greenland records of aerosol source and atmospheric lifetime changes from the Eemian to the Holocene

The Northern Hemisphere experienced dramatic changes during the last glacial, featuring vast ice sheets and abrupt climate events, while high northern latitudes during the last interglacial (Eemian) were warmer than today. Here we use high-resolution aerosol records from the Greenland NEEM ice core to reconstruct the environmental alterations in aerosol source regions accompanying these changes. Separating source and transport effects, we find strongly reduced terrestrial biogenic emissions during glacial times reflecting net loss of vegetated area in North America. Rapid climate changes during the glacial have little effect on terrestrial biogenic aerosol emissions. A strong increase in terrestrial dust emissions during the coldest intervals indicates higher aridity and dust storm activity in East Asian deserts. Glacial sea salt aerosol emissions in the North Atlantic region increase only moderately (50%), likely due to sea ice expansion. Lower aerosol concentrations in Eemian ice compared to the Holocene are mainly due to shortened atmospheric residence time, while emissions changed little.Past climate changes in Greenland ice were accompanied by large aerosol concentration changes. Here, the authors show that by correcting for transport effects, reliable source changes for biogenic aerosol from North America, sea salt aerosol from the North Atlantic, and dust from East Asian deserts can be derived.

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.