The applicability of fossil foraminifera as a paleo-tsunami indicator: evidence from the 2011 Tohoku and A.D. 869 Jogan tsunami events
Abstract
The 11 March 2011 megathrust earthquake (Mw 9.0) off the coast of Japan generated a tsunami that reached the Sendai coastal plain with wave heights of ~10 to 12 m above sea level. In May 2011 a series of trench sections (n = 13) and surface samples (n = 17) were collected along a transect on the northern perimeter of the Sendai airport (38.145°N, 140.933°E) in an effort to document the sedimentological characteristics of the Tohoku deposit. In this study we (1) examine the taphonomic signature of recent and fossil foraminifera from the Tohoku tsunami deposit, and (2) apply this technique to detect older events in the paleo-record through comparison with sediments deposited by the Jogan tsunami of A.D. 869 (Mw 8.3). The trench sections containing the Tohoku tsunami deposit are characterized by a poorly-sorted, dark-brown, medium basal sand, overlain by a well-sorted, light-brown, medium sand and a mud cap. The deposit, with sheet-like geometry that is 5 to 30 cm thick, extends ~5 km inland and contains marine sand and a distinct paucity of recent foraminifera. The recent foraminifera that were present showed evidence of prolonged subaerial exposure (e.g. pitting, corrosion, fragmentation) and likely originated from coastal dune and beach sediments that were breached by the tsunami. In contrast, high abundances of robust sediment-filled fossil foraminifera were found within the overwash sands and were ascribed a marine origin since they were absent in units outside of the tsunami deposit. Along a surface transect, greatest abundances were found in offshore samples. Trends associated with test size (e.g. decreasing concentration of large test sizes with distance inland) are in agreement with particle-size data. Preliminary analysis shows the presence of fossil foraminifera within sediments emplaced by the Jogan tsunami of A.D. 869, indicating that a taphonomic based approach is effective in identifying paleo-tsunamis at Sendai where there is a paucity of recent foraminifera, and could prove useful at other locations.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFMOS33A1641P
- Keywords:
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- 3022 MARINE GEOLOGY AND GEOPHYSICS / Marine sediments: processes and transport;
- 3030 MARINE GEOLOGY AND GEOPHYSICS / Micropaleontology;
- 4304 NATURAL HAZARDS / Oceanic