Yuichiro Tanaka

Florisphaera profunda, a possible nannoplankton indicator of late Quaternary changes in sea-water turbidity at the northwestern margin of the Pacific

Three piston cores from the Ryukyu Trench slope, northwestern margin of the Pacific Ocean, contain nearly continuous paleoceanographic records of the last 320, 165 and 105 kyr. Ages of the cores are constrained by plankticδ18O stratigraphy and roughly substantiated by the presence of two calcareous nannofossil events. Spectral analysis of the changes in relative abundance ofFlorisphaera profunda, a nannoplankton species presently living in the lower euphotic zone, reveals a maximum power at ∼0.01 cycles/kyr, corresponding to the ∼ 100 kyr/cycle of orbital eccentricity. Much lower power is observed at the ∼ 23 kyr precessional band. This differs from the equatorial Atlantic Ocean variations, which show dominant power in the ∼ 23 kyr precessional band in the changes ofF. profunda abundances (Molfino and McIntyre, 1990a). Besides, our cores were taken outside the equatorial divergence zone. n nWe also show a rather close relationship betweenF. profunda abundance and sea-water transparency (Secchi depth) in the present northwest Pacific, based on data from 204 sites of surface sediment samples. Because sea-water transparency is primarily affected by bio-productivity change in open ocean, stratigraphic changes ofF. profunda abundances in our cores seem to have been caused by productivity change as well pointed out by Molfino and McIntyre (1990a,b). A mechanism proposed by them for its change, however, is not applied to our cases due to differences in spectral analysis results and in geographic situation. Alternatively, we suggest that geographic change of the northwestern margin of the Pacific during the late Quaternary causedF. profunda abundance change through the sea-water turbidity change which has been affected not only by bio-productivity but also by land-origin particles supply.

Regional and interannual productivity of biogenic components and planktonic foraminiferal fluxes in the northwestern Pacific Basin

Abstract Time-series sediment trap experiments at subtropical (WCT-1) and subarctic (WCT-2) stations in the northwestern Pacific indicate seasonal, latitudinal and depth variations in total particulate, biogenic and foraminiferal fluxes. At the subtropical station, the average total mass flux was 19.4 mg m −2 day −1 in the shallow trap (1060 m) and 21.5–26.1 mg m −2 day −1 in the deep trap (3930 m) during the sampling period. At subarctic station, these values were 91.5–176.9 mg m −2 day −1 in the shallow and 68.6–112.3 mg m −2 day −1 in the deep trap. We recognized 12 and 15 planktonic foraminiferal species at Station WCT-1 and Station WCT-2, respectively. The planktonic foraminiferal flux and species turnover are related to seasonal and interannual changes in source water and water column conditions at both stations. At Station WCT-1, the highest flux was recorded during the summer, with a peak in mid to late June associated with similar flux patterns of the dominant species, Globigerinoides ruber and Globigerinita glutinata . The total flux of foraminiferal tests at the shallow and deep traps is similar in numbers and magnitude. At Station WCT-2, the peaks of total flux of foraminiferal tests at the two trap depths differ in number, and their magnitude in the deep trap is almost half of that in the shallow trap. A distinctive seasonal pattern occurred in the shallow and the deep trap, with a peak in total foraminiferal flux in mid June to mid July. Globigerina quinqueloba , Neogloboquadrina pachyderma and Neogloboquadrina dutertrei dominate the planktonic population throughout the year. Subtropical Station WCT-1 was characterized by low total foraminiferal fluxes and low total mass flux, which is dominated by calcium carbonate and depleted in opal, whereas high foraminiferal fluxes and a high total mass flux dominated by high biogenic opal, and less calcium carbonate and organic matter characterize subarctic Station WCT-2. The foraminiferal carbonate that reaches the seafloor accounts for an average 20–27% and 22–23% of the total calcium carbonate at Station WCT-1 and Station WCT-2, respectively. The primary reason for the difference in flux at both stations thus lies in the different contributions of siliceous and calcareous planktonic assemblages. The seasonal variation in biogenic particulate flux at both stations implies that temporal changes in biological productivity are governed by large-scale seasonal climatic variability and local hydrography.

Coccolith fluxes and species assemblages at the shelf edge and in the Okinawa Trough of the East China Sea

Abstract Coccolith assemblages and flux were investigated in detail by both time-series and short-term sediment trap experiments from the shelf edge to the Okinawa Trough in the East China Sea. Time-series sediment traps were moored at water depths of 600, 800, and 1040 m ( 50 m above the bottom) from March 1993 to February 1994 in the western region of the Okinawa Trough. A short-term sediment trap array was also moored for 2 days in both February and October 1993 at the shelf edge. The seasonal assemblage patterns of coccoliths can be recognized in time-series sediment traps and were reflective of the overlying surface water masses. Emiliania huxleyi dominated the community from April to May, while Florisphaera profunda was dominant from October to December at all three depths in the time-series traps. In the time-series traps, coccolith fluxes increased markedly at 800 and 1040 m , indicating lateral input of coccoliths. These results revealed that biogenic particles are transported by lateral advection in the middle and deep layers, or that a nepheloid layer exists. On the other hand, settling coccolith assemblages and fluxes were clearly different between the western region of the Okinawa Trough and the continental shelf edge. A comparison between traps at the two sites indicated that settling coccolith assemblages and fluxes in the deep water in the western region of the Okinawa Trough are influenced mainly by lateral transportation in the direction of the Kuroshio Current.

Seasonal occurrence of coccoliths in sediment traps from West Caroline Basin, equatorial West Pacific Ocean

Abstract Coccolith fluxes were investigated by sediment trap studies in the West Caroline Basin, which is located in the equatorial western Pacific. The investigation was conducted from June 1991 to March 1992 at two water depths, 1592 and 3902xa0m, as part of the Northwest Pacific Carbon Cycle Study (NOPACCS) program. Two seasonal maxima of coccolith fluxes were observed during September–early October and late December–January. The average coccolith and coccosphere fluxes at the depth of the shallow trap were 1800×106xa0coccolithsxa0m−2xa0day−1 and 1.9×106xa0coccospheresxa0m−2xa0day−1, respectively. The flux of coccoliths followed the same trend as the total flux, and was closely correlated with the flux of organic matter flux. Florisphaera profunda, Gladiolithus flabellatus, Gephyrocapsa oceanica, Umbilicosphaera sibogae var. sibogae, Emiliania huxleyi, and Oolithotus fragilis were the most abundant species together comprising more than 85% of the total flora. Observed seasonal changes of the species composition of coccolith flora, as well as analysis of the R-mode cluster, revealed that during the summer, the assemblage was marked by the dominance of G. oceanica and U. sibogae. However, during the winter, the assemblage was dominated by E. huxleyi and O. fragilis. These assemblage changes were influenced by monsoonal events, which were observed off the New Guinea coast. F. profunda dominated the community in the shallow trap throughout most of the year; peak values of this species were recorded during the winter. The coccosphere assemblage was dominated by G. oceanica at both water depths. In the deep trap, the sedimentation pattern was similar to that observed at the shallow depth. Mean coccolith and coccosphere fluxes at the deep trap were 2000×106xa0coccolithxa0m−2xa0day−1 and 0.08×106xa0coccospheresxa0m−2xa0day−1, respectively. The increase in coccolith flux with water depth suggests a lateral influx. The estimated average daily mass of CaCO3 flux in coccoliths and coccospheres was 16.6xa0mgxa0m−2xa0day−1 at the 1592xa0m trap and 17.9xa0mgxa0m−2xa0day−1 at the 3902xa0m trap, respectively. These calculated values contributed only 23.3% to the total CaCO3 flux at the shallow trap and 27.9% at the deep trap.

Anthropogenic impacts on meiobenthic Ostracoda (Crustacea) in the moderately polluted Kasado Bay, Seto Inland Sea, Japan, over the past 70 years.

Two sediment cores were obtained from Kasado Bay, a moderate-polluted enclosed bay in Japan, to examine anthropogenic impacts on Ostracoda over the past ca. 70 years. We analyzed ostracode abundance and diversity, grain size, and CHN, and used (210)Pb and (137)Cs as the dating method. The present study showed that cross-plot comparisons of ostracode abundance and each environmental factor, based on sediment core data, could be used to identify ostracode species as indicators for anthropogenic influences. Ostracode abundance reflected mainly the changes that had occurred in total organic carbon content in sediments related to eutrophication, but heavy metal concentration did not directly influence several ostracode abundance in the bay. Environmental deterioration because of eutrophication started in the 1960s. The regulations regarding the chemical oxygen demand in waters introduced in the 1980s probably influence ostracode abundance for certain species in this period. Currently, Kasado Bay is not experiencing severe degradation.

An in situ experiment of calcium carbonate dissolution in the central Pacific Ocean

Abstract We developed a new in situ experimental device and conducted a field experiment in the central Pacific Ocean to calculate with a high degree of accuracy a quantitative evaluation of the neutralizing effect and the CaCO 3 dissolution rate of sinking particles for effective CO 2 ocean sequestration. Devices utilizing a continuous flow system to avoid the effects of water stagnation were deployed in two layers, ca. 3800 and 5000xa0m deep, over a period of 23 days to measure the dissolution rate of CaCO 3 particles that consisted of samples of planktonic foraminiferal tests and fragmental calcite and aragonite crystals. Furthermore, sample chambers (moored chambers) were attached to the same mooring system in 26 layers at intervals of 100 or 250xa0m below 1700xa0m deep to obtain the dissolution rate versus depth. Comparison of dissolution rates between experimental device and moored chambers showed the influence of the stagnation in ambient water of particles largely effects on dissolution rates. Foraminiferal tests showed higher dissolution rates than those of crystal fragments because of the larger surface reacting with water. It is concluded that the dissolution rate of foraminiferal tests increases notably with the decreasing degree of saturation with calcite ( Ω c ) value immediately less than ca. 0.8. In case of increasing the degree of undersaturation in mid-deep layer by CO 2 injection, the dissolution rate of CaCO 3 particles accelerate exponentially.

Taxonomy and Stratigraphic Changes of Diatom Resting Spores from DSDP Leg 41, Offshore Northwest African Margin, Based on Nannofossil Biostratigraphy

Abstract. We reinvestigated the standard calcareous nannofossil and diatom biostratigraphy zonations from Deep Sea Drilling Project (DSDP) Leg 41 Holes 366 and 369A in the eastern equatorial Atlantic Ocean in order to determine the geologic ages for the low-latitude diatom zonation. Applying the ages of nannofossil zonations to diatom ones, the ages of several diatom bioevents (first occurrence, FO) which determine the biozones of DSDP Holes 366 and 369A are evaluated as follows; FOs of Baxteriopsis brunii (ca. 38.1 Ma), Coscinodiscus excavatus (ca. 33.6 Ma), Cestodiscus reticulatus (ca. 32.9 Ma), Rocella vigilans (ca. 29.8 Ma), Rossiella symmetrica (ca. 29.5 Ma) and Bogorovia veniamini (ca. 28.3 Ma). These ages may be applicable not only for these holes but also for biostratigraphic studies of other holes which contain Paleogene diatoms. Moreover, this paper describes the taxonomy and stratigraphic ranges of marine diatom Chaetoceros resting spores from DSDP Holes 366 and 369A, including two new morpho-species (Vallodiscus truncatuhis Suto sp. nov. and Xanthioisthmus fortii Suto sp. nov.) and its allied species with synonymy lists, light microscopic observations and several key references for each taxon. As the results of counting of each diatom taxon, it was clear that the sedimentation rate and Chaetoceros resting spore abundances and diversities changed abruptly in the earliest Oligocene (ca. 32 Ma). During the separation of the South American and African continents from the Late Cretaceous to earliest Oligocene, a passage arose that acted for the transfer of Antarctic Bottom Water and modulated the bottom current velocities. The southward bottom water and the passage influenced the sedimentation rate and upwelling of nutrient-rich deeper waters stimulated marine productivity and ecology of Chaetoceros across the late Eocene to early Oligocene.

Transportation of foraminifers in the East China Sea. Sediment trap results from the shelf edge to the slope.

東シナ海の陸棚縁辺部から大陸斜面域において, 1995年10月27日から11月4日の間に簡易型および時系列型のセジメント・トラップ実験によって有孔虫フラックスの観察を行った.2種類のセジメント・トラップにより採集された試料中に, 保存良好の浮遊性有孔虫以外に保存不良の浮遊性有孔虫と底生有孔虫が認められた.このことから, 海底堆積物からの有孔虫殻の巻き上がりが示唆される.簡易型トラップと海底表層堆積物の底生有孔虫の種組成の比較から, 陸棚縁辺部のトラップには主に陸棚縁辺部から大陸斜面最上部の表層堆積物に含まれていた有孔虫が, 大陸斜面域のトラップには主に大陸斜面域から沖縄トラフ域の有孔虫が捕集されていることが明らかとなった.また, 時系列式トラップの観測結果から, 大量の有孔虫殻が, 内部潮汐流の変動をきっかけとして巻き上がり, その後平行輸送され, セジメント・トラップに到達したと推定される.