Motoyoshi Oda

Postglacial environmental change of the Pacific Ocean off the coasts of central Japan

Abstract Downcore changes in microfossil assemblages and oxygen isotope ratios in three piston cores recovered from the Northwestern Pacific, off central Japan, show that the subtropical Kuroshio front was located to the south of C-4 core site (Lat. 33° N) during the last glacial. The front then advanced northward, passing over the C-4 site and the C-6 site (34.6° N) at about 13 ka and 10 ka, respectively, and reached the C-1 core site (36° N) at about 7 ka. After 5.5 ka it retreated to the area between the C-1 and C-6 sites. A brief but significant cold event, the readvance of the cold Oyashio Current, is recognized between 11 and 10 ka in the two northern cores, but the current did not reach the southern C-4 site. A contemporaneous cold event is known in the North Atlantic, and the cooling was probably a global phenomenon likely to be associated with lowering of sea level. Contamination of isotopically light water is apparent between 14 and 11 ka in the marked change in isotopic composition of benthic foraminifers. Oxygen isotope ratios of planktonic foraminifers show that prior to the advance of the Kuroshio front, the surface water at these core sites was isotopically lighter than the Kuroshio water at that time.

Transgressive and highstand systems tracts and post-glacial transgression, the East China Sea

Abstract The Late Pleistocene–Holocene depositional sequence on the shelf in the East China Sea (ECS) is interpreted on the basis of the analyses of four sediment cores and high-resolution seismic reflection sub-bottom profiler records along a NE–SW across-shelf transect. Sedimentary strata deposited above a lowermost planar erosional surface that was formed during sea-level fall of the last glacial are divided into two units along a NE–SW across-shelf transect (Changjiang–Okinawa). The Lower Unit is characterized by seaward-dipping tangential clinoforms with a thickness of 30–40 m at mid-shelf depths and less than 30 m beneath the outer shelf. Prograding clinoforms are more obvious in mid-shelf environments. The Upper Unit is characterized by an upper transparent layer that is formed into ridge-and-swale topography. The boundary between the Upper and Lower Units is sharp and erosional. Surficial sediments taken by cores from the central ECS shelf are also divided into two facies: a sandy facies consisting of sand or sandy gravel with moderately abundant molluscan shell fragments, and a muddy facies comprising mud intercalated with thin sand layers. The sandy facies is widely distributed on the middle to outer shelf seafloor and has a measured thickness of 30 cm, up to several metres. Radiocarbon ages of molluscan shells in this sediment are less than approximately 10 ka BP except for ages from basal shell lag deposits, which are >10–40 ka BP. The muddy facies underlies the sandy facies and has radiocarbon ages of 2.7–2.9 ka BP (previous work) and 20–38 ka BP (this study) and TL ages of 27–50 ka BP (previous work). The boundary between these lithologic units is an erosional sharp contact. The sandy and muddy facies are correlated with the Upper and Lower Seismic Units, respectively. The Lower Unit is characterized by prograding clinoforms and is interpreted to represent deltaic or nearshore tidal ridge sediments of the paleo-Changjiang River that were deposited during the last glacial lowstand of sea-level as a lowstand systems tract. The underlying erosional surface is interpreted to be a sequence boundary formed as a regressive marine erosional surface during the fall of sea-level. The erosional boundary between the Lower and Upper Units is interpreted as a transgressive surface of erosion formed during the subsequent rise of sea-level. The Upper Unit has a modern sand ridge topography and is interpreted to represent offshore shelf sand ridges of the transgressive to highstand systems tract.

Change in the thickness of the warm Tsushima Current at the initiation of its flow into the Sea of Japan

Abstract The initiation of flow of the warm Tsushima Current created significant changes in Quaternary paleoceanography and ecosystems in the continental shelf of the Japanese islands in the Sea of Japan. In order to understand the behavior of the current at the initiation of its flow, we reconstruct the change in the relative thickness of the current during the initiation of flow based on combining planktonic and shallow-water molluscan fossil analyses of the Early Pleistocene Omma Formation in central Japan. The result indicates that warm-water planktonic foraminifera and cold-water molluscs coexisted during about 2300–2400 years just after the initiation of the intrusion of the Tsushima Current. Migration of a warm-water molluscan fauna to the shelf occurred shortly after the local extinction of cold-water molluscs. This means that the rate of increase in thickness of the Tsushima Current during the initiation of flow was faster than that of the sea-level rise. Using deep-sea core records and modern biogeographic data, the rates of the former and latter are calculated to be 17.2±8.9 m per 1000 years and 1.9±1.1 m per 1000 years, respectively. Because bottom water change might lag behind surface water change during a rapid climatic change in all shallow water, combining planktonic and benthic fossil analyses is necessary to reconstruct shallow paleoceanography.