Shiro Hasegawa

Stratigraphic relationships between the last occurrence of Neogloboquadrina inglei and marine isotope stages in the northwest Pacific, D/V Chikyu Expedition 902, Hole C9001C

The stratigraphic relationships between the last occurrence (LO) of the planktic foraminiferan Neogloboquadrina inglei in the middle Pleistocene and established marine isotope stages (MIS) was investigated using a 365-m-long sediment core from a continental slope in the northwest Pacific near the Shimokita Peninsula, Japan. Two tephra layers (Shikotsu-Daiichi and Aso-4 tephras) and two nannofossil datum planes (first occurrence of Emiliania huxleyi and LO of Pseudoemiliania lacunosa) were used as age- control points, and the oxygen isotope stratigraphy of Hole C9001C was established by correlating the oxygen isotope values of the benthic foraminiferan Uvigerina akitaensis with the standard oxygen isotope curve LR04. Hole C9001C provides the first continuous, high-sedimentation-rate (20-90 cm/kyr) record from MIS 18 to present in the northwestern Pacific near Japan. The stratigraphic position of the LO of N. inglei is in late MIS 16 or near the MIS 16/15 boundary.

Destruction of radiolarian shells during sample drying and its effect on apparent faunal composition

Radiolarian concentration and faunal composition are compared between pairs of dried and wet sub-samples from 24 samples in sediment cores from the Japan Sea, in order to evaluate the alteration of radiolarian assemblages during the sample-drying process. Radiolarian shell count is reduced by nearly half after oven drying at 50? C as well as at ambient room temperature. In contrast, the shell count is unchanged by freeze drying. The reduction in radiolarian test count is probably caused by contraction and consolidation of the sediment. The reduction in shell count is species-specific and results from the capacity of the shell to resist crushing owing to sedi- ment contraction. Faunal composition definitely is altered by the drying process, especially in assemblages dominated by fragile species. The original faunal composition can be reconstructed from the observed faunal composition in dried material, based on the shell-strength of major species in a sample.

A new index to quantify diatom dissolution levels based on a ratio of Neodenticula seminae frustule components

Abstract Distortion of diatom assemblages by differential dissolution has long hindered their utilization for paleoceanographic research. We propose a quantitative index defined as the ratio of number of complete valves/number of complete copulae (V/C ratio) to evaluate dissolution levels of Neodenticula seminae. We observe that an almost equal number of valves and copulae of this species occur in nearly undissolved assemblages from the water column. In spite of this, copulae are dominant in surface sediments, indicating differential dissolution between these frustule components. To confirm this, we carried out an experiment and found the copulae to be more dissolution tolerant than the valves. Based on these results, valve number in any assemblage prior to the start of dissolution is predicted with an expression using; 1) the number of both components after dissolution and their ratio; 2) the ratio of both components before dissolution starts (1.0) and 3) the ratio of dissolution tolerance between both c...

Paleoceanographic implications of benthic foraminiferal biostratigraphy. Examples from the Neogene in Hokkaido.

Biostratigraphic correlation using benthic foraminifera is discussed in a paleoceanographic point of view. Based on the compiled data on the geographic distribution of shallow-water benthic foraminifera during the Mid-Neogene climatic optimum (MNCO), the coastal seas of the Northwest Pacific at around 16Ma are divided into four paleobiogeographic provinces; subtropical, warm temperate, mild temperate, and cool temperate.A similar pattern of thermal gradient is recognized in the transition period between the MNCO and the following cooler stage (early Middle Miocene), though each province shifted southward compared with those in the MNCO. This biogeographic trend is a useful guideline to test the validity of inter-regional correlation.