Katsura Yamada

Cyclic sea-level changes based on fossil ostracode faunas from the Upper Pliocene Sasaoka Formation, Akita Prefecture, northeast Japan

Abstract The paleoenvironments in the Upper Pliocene Sasaoka Formation (2.20–2.05 Ma), Gojome area, Akita Prefecture, northeast Japan, are reconstructed on the basis of the ostracode assemblages and sedimentary facies. The upper part of this formation is divided into five units representing sedimentary facies. Each unit shows coarsening-upward sequences. Q-mode principal component analysis of ostracodes suggests the following: (1) the Upper Pliocene Sasaoka Formation was deposited entirely under cold-water influences; and (2) paleoenvironments of this formation are interpreted as combinations of a coastal area in sublittoral zone, open-water environment in upper bathyal zone, a bay in upper bathyal zone, and a bay in sublittoral zone. Each of four sedimentary units records one cycle of relative sea-level change based on ostracode assemblages, whereas one sedimentary unit (II) is barren. The sea-level changes reflected in the upper two units (IV and V) might have been shorter in duration than those of the lower two units (I and III). Relatively large-amplitude oscillation is recognized in the middle part by presence of lag deposits, absence of the upper part of a cycle in unit below the lag deposit, and no correlation between ostracode assemblages and paleodepth just above the lag deposits. This large shift might have been caused by the global glacial event at 2.08 Ma. The paleodepth between upper bathyal and sublittoral zone shifts to shoreface at the remarkable boundary. The upper half is characterized by a shallowing-upward sequence based on occurrences of Sinocytheridea latiovata.

Centennial-scale East Asian summer monsoon intensity based on δ18O values in ostracode shells and its relationship to land-ocean air temperature gradients over the past 1700 years

Centennial-scale variations in the East Asian summer monsoon (EASM) remain ambiguous. In the present study, the δ18O of shells from the ostracode Bicornucythere bisanensis from Lake Nakaumi, Japan, were examined to identify the bottom salinity changes caused by EASM variations over the past 1700 yr. Four weak EASM periods were identified in Lake Nakaumi: at A.D. 600, 1000, 1350, and 1900. We detected discordance among the centennial-scale EASM variations in Japan and the δ18O of stalagmites (δ18Ostalagmite) in southern China. Furthermore, both total solar irradiance and atmospheric 14C concentration, which were previously considered to be linked with the centennial-scale EASM variations, were asynchronous with the EASM variations. Conversely, we observed significant positive and negative correlations between the EASM from Japan and the δ18Ostalagmite in north-central China, and the relationship was associated with the Asia-Pacific Oscillation (APO) index: centennial-scale EASM periods dominated during the high APO period. These synchronous variations imply that the strength of the EASM variations is tied to temperature gradients between East China and the north-central Pacific Ocean.

The Impact on Bottom Sediments and Ostracods in the Khlong Thom River Mouth Following the 2004 Indian Ocean Tsunami

Abstract. Fossil ostracods are a useful tool for identifying tsunamigenic sediments. However, the behavior of ostracod shells within the bottom tsunami sediments in Recent river mouths and estuaries is poorly understood. In this study, we analyzed bottom sediments and ostracod specimens taken from sites within the Khlong Thom River and sites adjacent to the Malacca Strait along the Malay Peninsula during three intervals—pre-tsunami, four months after the tsunami, and post-tsunami—to determine the impact of the 2004 Indian Ocean tsunami on the bottom sediments in the river mouths and estuaries. The broad distribution of the terrigenous plant material-bearing sediments in the Malacca Strait and the southern part of river mouth areas after the tsunami indicates that the sediments and the suspended materials deposited on bottoms were preserved for four months after the tsunami. However, no plant debris was recorded in the Malacca Strait, the southern part of the river mouth (RM), or junction areas between the river mouth and the estuary in 2008, suggesting that they had dispersed from the bottom during the three years and eight months after the tsunami. Of the bottom sediments taken four months after the tsunami, a few containing no plant debris were recorded in the northern and middle parts of RM, characterized by no ostracods or an abundance of adult and late juvenile instar specimens of Keijella reticulata. Based on these observations, we believe that small materials, such as plant debris and early juvenile instar ostracods, were transported from the bottom after the tsunami by the ordinary current. Previous investigations have captured changes in the abundance and density of meiofauna within a few days of a tsunami; therefore, the existence of some changes in ostracods that were able to recover during the four months may be considered, although there was no change in ostracod biofacies caused by the tsunami in the study area.