[H-CG23] Earth surface processes related to deposition, erosion and sediment transport

[HCG23-01] Influence of background environmental changes on characteristics of earthquake-induced turbidites, off Omaezaki, Japan

*Ken Ikehara¹, Takashi Ishizawa², Toshiya Kanamatsu³ (1.Institute of Geology and Geoinformation, National Institute of Advanced Industrial Science and Technology, 2.Tohoku University, 3.Japan Agency for Marine-Earth Science and Technology)

Keywords:turbidite, earthquake, sea-level change, sediment source environment

Paleoenvironmental changes including eustatic sea-level changes have been considered as a factor controlling turbidite depositional frequency and depositional processes. Earthquake-induced turbidites have been formed by several processes including submarine landslides and surface sediment remobilization. There are several discussions on influence of environmental changes on triggering submarine landslides. However, our understanding on the influence of background environmental changes on characteristics and frequency of earthquake-induced turbidites is still insufficient. A drill core C9035B was obtained from a small slope basin off Omaezaki. Numerous thin turbidites were observed in the core. No shelf-connected nature of the basin and no clear terrigenous materials in the previously obtained short cores from the same basin suggests the turbidites found in the drill core are earthquake-induced. Radiocarbon dates of planktonic foraminifera from the hemipelagic muds occurred between the turbidites indicate that a ~50 ky continuous earthquake record is preserved in the core. Recurrence intervals of Holocene–last deglacial turbidites, which are relatively thick (>30 cm in thickness) and dominant with turbidite muds, are similar to those in the historical earthquakes along the Nankai Trough. However, those of LGM–MIS 3 turbidites, which are relatively thin (<10 cm) and mostly lack of turbidite muds, show longer periods (~1000 yr). Benthic foraminifera assemblages in turbidite sands also changed from deep and deep–intermediate water species in Holocene and late last deglaciation to intermediate–deep and shallow–intermediate water species before early last deglaciation. Offsets in radiocarbon dates of bulk organic matter between turbidite muds and underlying hemipelagic muds are small in Holocene (<500 yr) but become larger toward LGM–MIS 3 (~1000 yr). Such temporal differences in turbidite characteristics, frequency and sources suggest that change of slope sediments according to paleoenvironmental changes and its related change in response of slope sediments to earthquake shaking is an important factor of recurrence of earthquake-induced turbidite deposition.