氷期-間氷期スケールおよびMillennialスケールの気候変動の研究 : 同位体地球化学的・地球物理学的手法によるアプローチ

<p>Determining the timing and magnitude of the past global environmental changes are the key to understand the Earth's climate systems. Combined geochemical and geophysical analyses for geological samples that contained sea-level changes, ocean environment, and cryospheric environmental changes have been conducted. In this paper, I present some results we obtained using multi-proxy approaches to understand the last ice age climate history. Sea-level changes, at locations far away from former ice sheets (so called far-field sites), are relatively immune from isostatic effects: expressed as ice volume equivalent sea-level (ESL), they relate to variations in ice volume. The tectonically stable North Western Australian continental shelf, and the tectonically uplifting Huon Peninsula, Papua New Guinea (HP) were examined to construct ESL for the past 50,000 years. Sedimentological, micropaleontological and radiocarbon (¹⁴C) analyses of gravity-core samples, from the North Western Australian shelf, show details of the ESL during the LGM and timing of the termination of LGM, which appears to have been in two stages. During the LGM, maximum ESL occurred at a depth of ca. - 135 m and relatively constant. The end of LGM, at 19,000 years ago, marks the start of major melting. These results are in excellent agreement with previously reported observation from Barbados corals. After the LGM, the melting history of the global ice sheets was reconstructed using reported far-field sealevel observations combined with the Glacio-hydro-isostatic modeling. Good coherence was found and we reached the conclusions as (i) The maximum global ice volume (ie. LGM) was marked between ca. 30,000 years ago to 19,000 years ago. (ii) The sea-level rise after the LGM was paced as the rate of ca. 1.5 cm/yr between 16 cal ka (× 1000 calendar years ago) to 12.5 cal ka and again from 11.5 cal ka and 9 cal ka. During the Younger Dryas, little change in volume of global ice was observed, (iii) The initial melting rate of the ice after the LGM was originally high in 500 years followed by about 2,500 years of a comparatively slow melting period, (iv) The beginning of the LGM was characterized as rapid sea-level fall of about 50 m occurring in a few thousand years. For the pre-LGM climate change, uranium series and radiocarbon ages were measured in corals from the uplifted coral terraces of HP, to provide a detailed history of climate change. Rapid sea-level changes (10-15 m) were found and they appear to be synchronous with Heinrich events and concentrations of ice-rafted debris found in North and South Atlantic deep sea cores. Both U-series nuclides as well as ¹⁴C measurements revealed the time sequence of events: An initial sea-level high is followed by a large increase in atmospheric radiocarbon as the sea-level subside. Over 3,000 years to 4,000 years the atmospheric radiocarbon drops to below present ambient levels. This cycle bears a close resemblance to ice-calving episodes of Dansgaard-Oeschger and Bond cycles and the slow-down or complete interruption of the North Atlantic thermohaline circulation. The increases in the atmospheric radiocarbon levels are attributed to the cessation of the North Atlantic circulation. The measured initial ²³⁴U/²³⁸U ratio of last glacial period corals from HP, when compared with previously published data for the Holocene and Last Interglacial, appears to be about 1% lower. This suggests the necessity of ²³⁴U cycle model in the surface ocean. The equatorial ocean plays an important role to drow the detail picture of the past climate changes. Sediment cores recovered from Eastern Equatorial Pacific (EEP) were used for δ¹⁸O, micropaleontological, ¹⁴C and sedimentological analyses to reconstruct the paleo SST in the EEP region.</p><p>(View PDF for the rest of the abstract.)</p>