[B-PT04] 地球生命史

[BPT04-P04] 津波堆積物中の現世生痕から読み取る生痕化石の古生態学的情報

*清家 弘治^1,2 (1.産業技術総合研究所・地質調査総合センター、2.東京大学大学院新領域創成科学研究科自然環境学専攻)

キーワード：底生生物、生痕化石、古生態

The 2011 Great East Japan Earthquake and its accompanying tsunami severely affected shallow marine ecosystems along the northeastern Japanese coasts. Soft-bottom ecosystems such as tidal flats and inner bay bottoms are the most heavily disturbed by the tsunami currents as they are greatly altered by rapid erosion and deposition. Investigation on modern traces in the tsunami deposits provides useful information on the nature of their fossil counterparts (trace fossils) because there are no pre-existing burrows in the sediments. Reestablishment of some burrowing invertebrate populations began during 18 months from the tsunami disturbance (Seike et al., 2013, 2016). This situation provides a rare opportunity to determine an accurate depth for bioturbation in natural marine environments (Seike et al., 2017). The 2011 tsunami deposits after 4 years from the disaster event revealed that the nature of bioturbation of the heart urchin Echinocardium cordatum such as burrowing depth, substrate-softening effect by the bioturbation (Seike et al., 2018). In addition, we revealed that the size of the echinoid burrow represents population density of the burrow producer (Seike et al., 2020). The result indicates that fossilized echinoid burrows will prove useful for palaeoecological reconstruction, i.e., size of trace fossils comparable to the echinoid burrow, such as the trace fossils Scolicia and Bichordites, would be indicators of population density of the trace producer in an ancient marine ecosystem.

References

Seike, K., Shirai, K., & Kogure, Y. (2013). Disturbance of shallow marine soft-bottom environments and megabenthos assemblages by a huge tsunami induced by the 2011 M9. 0 Tohoku-Oki earthquake. PLoS One, 8(6), e65417.
Seike, K., Kitahashi, T., & Noguchi, T. (2016). Sedimentary features of Onagawa Bay, northeastern Japan after the 2011 off the Pacific coast of Tohoku Earthquake: sediment mixing by recolonized benthic animals decreases the preservation potential of tsunami deposits. Journal of Oceanography, 72(1), 141-149.
Seike, K., Shirai, K., & Murakami-Sugihara, N. (2017). Using tsunami deposits to determine the maximum depth of benthic burrowing. Plos One, 12(8), e0182753.
Seike, K., Sassa, S., Shirai, K., & Kubota, K. (2018). Lasting impact of a tsunami event on sediment-organism interactions in the ocean. Journal of Geophysical Research: Oceans, 123(2), 1376-1392.
Seike, K., Shirai, K., Kubota, K., Ota, Y., & Sassa, S. (2020). Does trace fossil size correspond with burrower population density? An example from the modern counterpart of the trace fossil Bichordites. Palaeogeography, Palaeoclimatology, Palaeoecology, 557, 109946.