Thesis or Dissertation Chemical compositions of past soluble aerosols reconstructed from Greenland and Antarctic ice cores

大藪, 幾美

Aerosols play an important role in the global climate balance, and therefore they could be important in climate change. Chemical compositions of aerosols are one of the important information for understanding the role of aerosols. Therefore, it is required to clear spatial and temporal distributions of chemical compositions of aerosols. Polar ice cores preserve past atmospheric aerosols, which is a useful proxy for understanding the interaction between climate changes and atmospheric aerosols. One useful technique for reconstructing past soluble aerosols from ice cores is the determination of dissolved ion species. For instance, Na+ and Ca2+ are the major cations of both Greenland and Antarctic ice cores. The Na+ originates from sea-salt (NaCl), and the Ca2+ originates from terrestrial materials (CaCO3 and CaSO4). The NaCl and CaCO3 react with sulfuric acid (H2SO4), and change to Na2SO4 and CaSO4. It has been clearly known that the contribution of terrestrial aerosols is higher in the glacial period, whereas that of sea-salt aerosols is higher in the interglacial period. However, since salts and acids melt into ions, chemical compositions of soluble aerosols in the ice cores have not been cleared. To clarify the temporal variations in the chemical compositions of past soluble aerosols, and the interaction between past soluble aerosols (sulfate-salt aerosols) and temperature changes, this study investigated chemical compositions of soluble particles preserved in the Greenland and Antarctic ice cores by focusing on the last termination, the most recent climate transition. The ice core samples are selected from the sections from the last termination (the Last Glacial Maximum (LGM) to Holocene) of the Dome Fuji and Dome C (inland Antarctica) ice cores, and the sections of the last interglacial period to Holocene of the Greenland NEEM ice core. Using the ice-sublimation method, soluble salt particles were extracted without melting. Chemical components of extracted particles were analysed by scanning electron microscope and energy dispersive spectroscopy, and micro-Raman spectroscopy. The major components of soluble salts particles in the Antarctic ice cores (Dome Fuji and Dome C) are CaSO4, Na2SO4 and NaCl. The CaSO4 and NaCl fractions were high in the first half of the last termination, whereas the Na2SO4 fraction is high in the latter half of the last termination. The major components of soluble salts particles in the NEEM ice core are CaCO3, CaSO4, NaCl and Na2SO4. The fractions of CaCO3, CaSO4 and NaCl were high in LGM, whereas those of NaCl and Na2SO4 were high in Holocene. The changes in the salt compositions in Antarctica are mainly controlled by concentration of terrestrial material (Ca2+). In the first half of the last termination, most of the terrestrial material (CaCO3) reacted with H2SO4 but some of sea-salt (NaCl) did not react with H2SO4 due to high Ca2+ concentration. As a result, the CaSO4 and NaCl fractions were high in this period. In the latter half of he last termination, reaction of NaCl with H2SO4 enhanced due to decreased in the Ca2+ concentration. As a result, Na2SO4 fraction was high in the latter half of the last termination. Using fractions of salt compositions derived from the sublimation method and ion concentrations, the CaCO3, CaSO4, NaCl, Na2SO4 and sulfate-salt fluxes of the Dome Fuji, Dome C and NEEM ice cores were estimated. The fluxes of CaCO3, CaSO4 and NaCl of the three ice cores in the LGM are significantly higher than in the Holocene, whereas Na2SO4 fluxes of the three ice cores are not changed from the LGM to the Holocene. The sulfate-salt flux in the NEEM ice core is relatively high in Greenland cold climate stages, whereas the flux is relatively low in Greenland warm climate stages. In Antarctica, the sulfate-salt flux decreases in Antarctic warming phases, whereas the flux stops decreasing in Antarctic cooling phases. The changes in sulfate-salt fluxes of three ice cores have significant inverse correlations with temperature changes in each three sites, respectively. The production of sulfate-salt was regulated not by SO42- which mainly originates from oceanic phytoplankton, but by physical process of oceanic and atmospheric systems. Sulfate-salt aerosols are a key component of cloud condensation nuclei in the atmosphere, which lead to increased solar scattering that cools Earth’s climate. The reduction in the sulfate-salt flux may have contributed to the last deglacial warming both in inland Antarctica and Greenland. The above achievements are new knowledge for the interpretation of ice core records and for the clarification of the role of the soluble aerosols for climate changes, which have not been obtained by traditional ice core studies of soluble aerosols (ion concentrations).
Hokkaido University(北海道大学). 博士(環境科学)

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