Katsumi Takayasu

Geochemical record of the Holocene Jomon transgression and human activity in coastal lagoon sediments of the San'in district, SW Japan

Abstract Coastal lagoon muds were analyzed to evaluate changes in sedimentary environments by the Jomon transgression from the lower to middle Holocene age and human activities. Core samples from Lake Shinji, Southwest Japan were utilized, which cover the entire Holocene Nakaumi Formation (ca. 23-m thick), and comprise the Lower, Middle, Upper and Uppermost members. Total sulfur (TS) contents and Fe 2 O 3 /Al 2 O 3 ratios increase from the boundary of Middle and Upper members, which is 1 m below the Kikai-Akahoya (K-Ah) volcanic ash of 6300 years BP. This change coincides to the pollen flora zone boundary of the Pinus – Abies and the Cyclobalanopsis – Castanopsis , suggesting transition to a warming climate. Chemical index of alteration (CIA) values [(ratio of Al to Al+Ca+Na+K)×100] and Rb/K ratios also show gradual increase from the middle part of the Upper member, due to its derivation from highly weathered source material probably formed under warming and rainy condition. Al 2 O 3 /TiO 2 and SiO 2 /TiO 2 show little variation from the Lower to the Upper members, probably related to consistent feldspar composition in the source rocks, and homogeneously mixed clays. In the Uppermost member (from 1500 years), sharp increases in Ti/Zr and decreases in both Nb/Y and Zr/Y occurred, suggesting heavy mineral fractionation. This change was caused by iron sand processing called Kannanagashi and charcoal-making in the most mountainous regions. Soil erosion by these processes brought changes in mud composition, shown by the enrichment in Al 2 O 3 , and depletion in Zr and Cr. Human activities thus had severe impacts on sedimentary environments compared with the natural changes since the Jomon transgression.

FORAMINIFERA IN AN ORGANIC-RICH, BRACKISH-WATER LAGOON, LAKE SAROMA, HOKKAIDO, JAPAN

We investigated the distribution of benthic foraminifera in Lake Saroma, a large lagoon on the coast of the Sea of Okhotsk in Hokkaido, Japan, to establish the characteristics of the foraminiferal fauna in these cool, organic-rich, brackish-water environments. Three types of faunas were identified: a Lagoon-bottom fauna dominated by Haynesina sp. A, a Shallow-water fauna dominated by Trochammina cf. japonica , and a River-mouth fauna dominated by Elphidium excavatum . The distribution of species is controlled by the dissolved oxygen content of bottom water in the summer, and a combination of organic enrichment and high mud content in the substrate that is specific to areas off river mouths. Haynesina sp. A opportunistically adapts to organic-rich and oxygen-poor conditions in deeper basins in the lagoon during the summer, whereas Trochammina cf. japonica mostly occurs in shallower areas where summer oxygen levels are high enough to sustain reproduction. Elphidium excavatum probably adapts to organic-rich sediments specific to areas near river mouths.

Correlation of Virgulinella fragilis Grindell & Collen (benthic foraminiferid) with near-anoxia in Aso-kai Lagoon, central Japan

The distribution of Virgulinella fragilis and the hydro-environment of Aso-kai Lagoon, central Japan, were studied to clarify the foraminifer’s adaptation to low-oxygen conditions. The hypolimnion of the lagoon is oxygen-poor during much of the year. Two faunas (A and B) are recognized, based on cluster analysis. Cluster A fauna consists of species common in brackish lagoons, such as the genera Trochammina and Ammonia, and occurs in seasonally oxygenated waters. Virgulinella fragilis, the predominant species of Cluster B fauna, dominates the central part of the lagoon. This species can tolerate more severe oxygen deficiencies than the typical brackish foraminifers (e.g. Trochammina spp.) and can adapt to long periods of oxygen-poor conditions in coastal lagoon environments, as well as in pelagic to hemi-pelagic settings. In order to survive in the near-anoxia of Aso-kai Lagoon, V. fragilis may have adapted to environments in which little reactive iron is available in the sediment, leaving pore-water and bottom-water sulphide available for symbionts, or may utilize sulphur denitrification processes.

Seasonal variation in nutrients and chlorophyll a in the stratified brackish Lake Nakaumi, Japan

The major inorganic nitrogenous compounds in aquatic environments are nitrate, ammonium and sometimes nitrite. Previously we studied the uptake of ammonium and nitrate by natural phytoplankton (SEIKE et al. 1986a), denitrification (SEIKE et al. 1986b, l986c ), nitrification (SEIKE et al. 1986c, 1997), the behavior of nitrate (SEIKE et al. 1990) and nitrous oxide (SENGA et al. 2001, 2002) in the brackish Lake Nakaumi, Japan. The present study was undertaken to explain seasonal variation in inorganic nitrogen compounds in the water column of Lake Nakaumi, on the basis of existing knowledge of nitrogen cycling in the lake. Data on nitrogenous nutrients such as nitrate, nitrite and ammonium in the lake were collected monthly during 1990-1999. Chlorophyll a was analyzed from data collected at monthly intervals from 1990 to 2002. A further aim of this paper was to evaluate the importance o f coupled nitrifi-

Preliminary Faunistic Survey of Ophiuroids in the Westernmost Part of the Sea of Japan

1999年11月及び2000年6月に日本海最西部の水深43-133mの海域において6科11属13種のクモヒトデが採集された。この中で5種、Ophiophrixus acanthius、Ophiocentrus koehleri、Amphiura (A.) teptobrachia、Amphiophiura pomphophora、Ophiomusium scatrareが日本海初記録であった。日本海全域からこれまでに採集されたクモヒトデ類の動物地理についても論じた。