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Paleoenvironmental Science

  • Kaoru Kashima, Associate Professor
  • Yusuke Okazaki, Associate Professor
The Earth's climate changes at various timescales beyond instrumental observation. In order to understand long-term climate and environmental changes, we employ paleo-proxies preserved in sediments to reconstruct past environmental changes. To obtain sediment samples, we go to ponds, lakes, wetlands and ocean. Our main research targets are: (1) decadal to centennial-scale anthropogenic impact on local environments; (2) millennial to orbital-scale global environmental dynamics without anthropogenic disturbance.
Figure 1
Figure 1. Benthic oxygen isotope ratio (=ice volume) and pCO2 changes for the past 800 kyrs (Monnin et al., 2001; Lisiecki and Raymo, 2005; Siegenthaler et al., 2005; Lüthi et al., 2008)

Earth's climate for the past 800 kyrs is characterized by glacial-interglacial cycle with 100 kyr cyclicity co-varied with atmospheric CO2 concentration. Present CO2 level (~400 ppm) is far beyond the glacial-interglacial range (~170 to 280 ppm). Understanding the mechanisms for the past pCO2 change is a key issue to clarify the Earth's climate system. In particular, deep ocean is a giant carbon reservoir and thus, plays a critical role in the global carbon cycle at millennial to orbital scale.

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Figure 2. Sediment cores obtained from the NW Pacific (left) and the Lake Kevo in Finland. They records time-series environmental changes. Several tephras are interbedded in the Pacific core samples and the lake sediments are annually laminated.
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Figure 3. Microscopic images of marine sediments from northern North Pacific (left, diatom ooze) and equatorial Pacific (right, carbonate ooze).

Marine and lake sediments record environmental changes in the past (paleo archives). Microfossils with biogenic opal (diatoms and radiolarians )and calcium carbonate (foraminifera and coccoliths) are preserved in sediments. Microfossil assemblages and their geochemical parameters provide clues to reconstruct paleo-environmental changes (paleo proxies).

Figure 4

Topic 1. Lowering water level at the Dead Sea, Jordan. Large hydrological changes of the lake have been recorded in the laminated sediment of the Dead Sea.

Figure 5 Figure 6
SEM images of diatom species living in and around sea-ice in the Okhotsk Sea.

Topic 2. Multi-proxy records in sediment core from the Okhotsk Sea since the last glacial period. Combined geochemical (right figure) and microfossil records have revealed pronounced deglacial events with high productivity by diatoms and sea-ice retreat (Okazaki et al., 2014).

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Topic 3. We try to develop novel paleo-proxy development to reconstruct past environmental change. Right figure is an example, CT cross-section image of planktic foraminiferal shell. Color indicates density of CaCO3 shell, which reduces by ocean acidification. This method allows us to estimate amount of carbonate dissolution of foraminiferal shell quantitatively (Iwasaki et al., 2015).

Iwasaki, S., et al., Observation of the dissolution process of Globigerina bulloides shells (planktic foraminifera) by X-ray micro-computed tomography, Paleoceanography 30, 317–331, 2015.
Okazaki, Y., et al. Glacial to deglacial ventilation and productivity changes in the southern Okhotsk Sea, Palaeo3, 395, 53–66, 2014.

Full publication list: http://paleobio.geo.kyushu-u.ac.jp/paper.html