Yoshiaki Matsushima

Holocene sea-level change and hydro-isostasy along the west coast of Kyushu, Japan

Abstract Mid- to late-Holocene sea-level variations have been obtained at sites along the west coast of Kyushu, Japan. Sea-level observations for this relatively stable area are very important for evaluating the crustal tilting associated with the hydro-isostatic adjustment due to the last deglaciation, and for examining the cause of underwater Jomon sites (submerged archeological sites during the Jomon period of mid-Holocene) typically observed in this region. For these purposes, we conducted systematic boring samplings at sites from Goto Island to Tamana along the latitude of about 33°N. Then we performed diatom assemblage and pyrite-sulfur analyses of these sediment samples and 14 C datings for intertidal shells in order to get the sea-level variations for these sites. Observations at Tsumizu faced to Oomura Bay with tidal range of 0.9 m show a sea-level high-stand of 1 m at about 5500 yr B.P. (years before present) followed by smoothly falling to the present level. Evidences for sea-level oscillation of greater than 50 cm have not been found here. Relative sea-levels at about 5000 yr B.P. are, however, −2 m at Goto Island (125 km west of Tsumizu) and 2 m at Tamana (50 km east of Tsumizu). Thus observations at sites along the traverse from Goto Island to Tamana are indicative of crustal tilting of about ∼3–4 m for the distance of 175 km, which seems to be consistent with the prediction caused by the hydro-isostatic adjustment.

HOLOCENE FRINGING REEFS AND SEA-LEVEL CHANGE IN MANGAIA ISLAND, SOUTHERN COOK ISLANDS*

Abstract Holocene relative sea levels in oceanic islands that are situated sufficiently far from glaciated regions provide basic information for the study of the melting histories of the continental ice sheets and of the rheological structure of the Earth. We have studied Mangaia Island, South Cooks, as one such oceanic island located in the middle Pacific Ocean. In addition to the usual geological and geomorphological observations of geomorphic features associated with former shorelines, we have used a portable drilling sampler for shallow borings of coral reefs. Geological and geomorphological studies of the coast of Mangaia Island have revealed that there was a higher sea level than the present in the mid-Holocene. The heights and ages of emerged microatolls on the emerged bench indicate that the sea reached a maximum level of +1.7 m around 4000-3400 yr B.P., and then emergence is considered to have occurred between 3400 and 2900 yr B.P. At the same time, the reef crest formed in the period from 5000 to 3400 yr B.P. emerged above the sea and suffered erosion. As a result of this change in sea level, upward reef growth from the fore reef slope began to form a new reef crest by 2000 yr B.P. to seaward of the old reef crest. The reef margin of Mangaia grew outward intermittently with seaward jump of the reef front, in accordance with a fall of sea level, in the late Holocene. The presence of fossil reef crests or eroded algal ridges on modern reef flats is a rather common feature in the South Pacific. The geomorphic development of such features on reef flats can be explained by the effect of the late Holocene fall in sea level.

Abyssal molluscan colony of Calyptogena in the Pliocene strata of the Miura Peninsula, Central Japan

Fossil Calyptogena are found as colonial occurrence in Pliocene abyssal sediments cropping out in the Miura Peninsula, Central Japan. The sediments containing Calyptogena deposited on a talus during drastic rearrangement of the sedimentary basins, caused by a collision process around a trench-trench-trench type triple junction. The population density of Calyptogena shells is 90–130 individuals/m2 which is comparable to the living colony of Calyptogena. The oxygen and carbon isotopes of the fossil shells indicate that the shells lived at abyssal depth and under the influence of seepage of connate water with a high concentration of organic carbon.

Evidence for a Holocene high sea-level stand, Vanua Levu, Fiji

Abstract Two distinct elevations of emerged notches can be recognized on the southeastern coast of Vanua Levu: the higher notch is +2.22 to +3.30 m above low-tide level and the lower +1.52 to +1.65 m above low-tide level. Many emerged Holocene corals encrust higher notch floors and benches or occur as microatolls and from these radiocarbon ages ranging from 6000 to 3400 yr B.P. were obtained. The higher notches are significantly higher than present high-tide level, even allowing for relative tectonic uplift. This is supported by elevations of emerged microatolls and their 14 C ages. Thus, mean sea level of the mid-Holocene was higher than that at present.

Secular changes in the oxygen isotope ratios of mollusc shells during the Holocene of Central Japan

In order to estimate changes in the relative water temperature during the Holocene, oxygen isotopes ratios were measured in mollusc shells collected from archaeological sites and natural shell beds in Central Japan. Analyses were made on the clam Meretrix lamarckii which lives on the open coast at depths of about 5–10 m. Serial samples were analyzed for each shell to obtain isotopic profiles. Daily growth-lines were examined to determine the dates or season. A living specimen was analyzed to compare with the results of the Holocene specimens. The oxygen isotope fluctuation of the specimen shows a clear correspondence with the growth rate pattern, indicating that the fluctuation reflects the annual variation of temperature. Twenty specimens of various ages ranging from 9000 to 1500 yr B.P. were analyzed. The isotopic values indicate that the water temperature was lowest at 9000 yr B.P., highest at 7000 yr B.P., and again became low during 4500-4000 yr B.P. Then it slightly increased after 4000 yr B.P. with an indistinct low temperature episode at about 2000 yr B.P. The change accords well with the climatic and sea-level changes known in central Japan.

Holocene sedimentary history of some coastal plains in Hokkaido, Japan V. Sedimentary history of Kushu Lake and Akkeshi

Diatom assemblages of sediments obtained from Kushu Lake and Akkeshi were analyzed in order to clarify the local Holocene sedimentary history. The results revealed the following: 1) Sometime before about 9000 yr B.P., Kushu Lake was originally a freshwater environment. 2) The first marine diatom zone (MD1 Zone) was deposited on the bottom of the paleo-Kushu Bay between 8500 and 6000 yr B.P. 3) This site changed to become the bottom of the paleo-Kushu Lagoon around 6000 yr B.P. owing to the formation of a bay-mouth bar across the paleo-Kushu Bay and the first Holocene regression resulted in a freshwater lake at about 4500 yr B.P. 4) At this time, the Akkeshi site changed from a sublittoral shore in a drowned valley to a peat moor. 5) The second Holocene transgression at about 3000 yr B.P. was detected at the Akkeshi site. 6) At Kushu Lake site the occurrence ofMastogloia elliptica between 7000 and 5000 yr B.P. suggested the influx of the Tsushima warm current into the Japan Sea.

Holocene Ostracods from the Borehole Core at Oppama Park, Yokosuka City, Kanagawa Prefecture, Central Japan: Paleoenvironmental Analysis and the Discovery of a Fossil Ostracod with Three-Dimensionally Preserved Soft Parts

Abstract. Forty-seven sediment samples were taken from a borehole core at Oppama Park, Yokosuka City, Kanagawa Prefecture, central Japan. Fifty-eight ostracod species, representing 32 genera, were identified in the 32 samples. Three ostracod biofacies (I–III) were identified using Q-mode cluster analysis. On the basis of ostracod biofacies, the following paleoenvironmental changes were recognized in the study area: land mudflat (no ostracods) → muddy bottom environment of inner bay (biofacies III) → sandy bottom environment of bay near rocky shore (biofacies II) → shallow mudflat (biofacies I). Modern analog technique (MAT) showed that the paleoenvironment of the study area was a warm (maximum bottom temperature in summer around 30°C; minimum bottom temperature in winter around 7°C) and shallow bay, similar to that around Honshu Island today. An ostracod specimen, Parakrithella pseudadonta, with exceptionally well preserved soft parts was discovered in one of the horizons, at a core depth of 20.4–20.5 m. In this horizon, an event deposit, namely, one indicating an increase in paleobathymetery, was identified from ostracod assemblages, which were identified as tsunami deposits on the basis of previous studies of neighboring areas. Similar soft part-preserved arthropod fossils have been discovered from the Upper Cambrian (500 Ma) of Sweden. Many fecal pellets have also been found in the Orsten limestone. Such “cesspools” were exceptionally phosphatized during early diagenesis owing to the high local phosphorus levels produced by the accumulated fecal pellets. The “cesspool preservation hypothesis” provides an explanation for this kind of exceptional fossilization, found in the marine sediment record from the Late Cambrian onward.