Tokiyuki Sato

The chronostratigraphic significance of seismic reflections along the Bahamas Transect

Continuous cores drilled during the Bahamas Drilling Project (BDP) and the Ocean Drilling Program (ODP) Leg 166 along a transect from the top of Great Bahama Bank to the basin in the Straits of Florida provide a unique data set to test the assumption in seismic stratigraphy that seismic reflections are time lines and, thus, have a chronostratigraphic significance. Seismic reflections that are identified as seismic sequence boundaries (SSBs) were dated by means of biostratigraphy in the five ODP sites and by a combination of biostratigraphy, magnetostratigraphy and Sr isotope stratigraphy in the two BDP sites. The seismic reflection horizons are carried across a variety of facies belts from shallow-water carbonates over slope carbonates to drift deposits in the Straits of Florida. Within this system 17 SSBs were identified and dated. Despite the fact that the seismic reflections cross several facies belts, their ages remain remarkably constant. The average offset in all sites is 0.38 Myr. In no cases do the seismic reflections cut across time lines. The age differences are the combined result of the biostratigraphic sampling frequency, the spacing of marker species that required extrapolation of ages, and the resolution of the seismic data. In addition, uncertainties of age determination in the proximal sites where age-diagnostic fauna are rare add to the age differences between sites. Therefore, it can be concluded that the seismic reflections, which mark the SSBs along the Bahamas Transect, are time lines and can be used as stratigraphic markers. This finding implies that depositional surfaces are preferentially imaged by reflected seismic waves and that an impedance contrast exists across these surfaces. Facies successions across the sequence boundaries indicate that the sequence boundaries coincide with the change of deposition from times of high to low sea level. In the carbonate setting of Great Bahama Bank, sea-level changes produce changes in sediment composition, sedimentation rate and diagenesis from the platform top to the basin. The combination of these factors generates differences in sonic velocity and, thus, in impedance that cause the seismic reflection. The impedance contrasts decrease from the proximal to the distal sites, which is reflected in the seismic data by a decrease of the seismic amplitude in the basinal area. = 2002 Elsevier Science B.V. All rights reserved.

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.

Timing of Shell Size Increase and Decrease of the Planktic Foraminifer Neogloboquadrina pachyderma (Sinistral) During the Pleistocene, IODP Exp. 303 Site U1304, the North Atlantic Ocean

We made shell size measurements of the planktic foraminifer Neogloboquadrina pachyderma (sinistral) using samples from IODP Site U1304, the North Atlantic. We found that mean and maximum shell sizes began to increase around 1.1 Ma, and that several episodic changes in test size occurred during 0.6 - 0.35 Ma. Test size reached a maximum during the late Quaternary. Based on correlation with previous investigations, we have confirmed that these changes in fo- raminiferal shell size occurred on an inter-ocean scale.

Temporal changes in biotic and abiotic composition of shallow-water carbonates on submerged seamounts in the northwestern Pacific Ocean and their controlling factors

ABSTRACT The lithology of Cretaceous to Pleistocene shallow-water carbonates, which were collected from 29 sites on 24 submerged seamounts in the northwestern Pacific Ocean using the Deep-sea Boring Machine System, are described. The shallow-water carbonate deposits examined in the present study can be roughly divided into three types based on their composition: Cretaceous, Eocene (to lowest Oligocene?), and Oligocene to Pleistocene. The Cretaceous type is characterized by an abundance of molluscs (including rudists), smaller foraminifers, microencrusters, non-skeletal grains (e.g., peloids, cortoids, and intraclasts), and microbial sediments. Most components have been micritized and possess thick micrite envelopes. The Eocene type is characterized by the dominance of larger foraminifers, Halimeda spp., nongeniculate and geniculate coralline algae, bryozoans, and dasycladacean algae. Scleractinian corals are very minor components. The Oligocene to Pleistocene type is similar in composition to the Eocene type, but it differs from the latter by the abundant occurrence of scleractinian corals and nongeniculate coralline algae. Corals, nongeniculate coralline algae, and Halimeda spp., which precipitate carbonates within closed to semi-closed spaces in and around their bodies (intra-tissue), are major components of the Eocene and Oligocene to Pleistocene types. In contrast, the Cretaceous-type sediments contain relatively more carbonates of extra-tissue origin (i.e. carbonates deposited in relatively open spaces around the bodies of organisms, such as rudists, as well as microbialite and ooids) than the Eocene and Oligocene to Pleistocene types. The changes in the major constituents of the carbonate factory depend on local environments, such as nutrient availability, as well as a global factor: seawater chemistry in the surface waters. Temporal variations in the abundance of the shallow-water carbonates on the examined seamounts suggest that carbonate accumulation was not necessarily controlled by climatic conditions; instead, it was related to the volcanism and tectonics that served as the foundations for reef/carbonate-platform formation.

Millennial-Scale Interaction between Ice Sheets and Ocean Circulation during Marine Isotope Stage 100

Waxing/waning of the ice sheets and the associated change in thermohaline circulation have played an important role in global climate change since major continental ice sheets appeared in the northern hemisphere about 2.75 million years ago. In the earliest glacial stages, however, establishment of the linkage between ice sheet development and ocean circulation remain largely unclear. Here we show new high-resolution records of marine isotope stage 100 recovered from deep-sea sediments on the Gardar Drift, in the subpolar North Atlantic. Results of a wide range of analyses clearly reveal the influence of millennial-scale variability in iceberg discharge on ocean surface condition and bottom current variability in the subpolar North Atlantic during marine isotope stage 100. We identified eight events of ice-rafted debris, which occurred mostly with decreases in sea surface temperature and in current components indicating North Atlantic Deep Water. These decreases are interpreted by weakened deep water formation linked to iceberg discharge, similarly to observations from the last glacial period. Dolomite fraction of the ice-rafted events in early MIS 100 like the last glacial Heinrich events suggests massive collapse of the Laurentide ice sheet in North America. At the same time, our early glacial data suggest differences from the last glacial period: absence of 1470-year periodicity in the interactions between ice sheets and ocean, and northerly shift of the ice-rafted debris belt. Our high-resolution data largely improve the picture of ice-sheet/ocean interactions on millennial time scales in the early glacial period after major Northern Hemisphere glaciation.

Carbonate sediments of the Bahama Platform

ODP(国際深海掘削計画)の第166次航海は, バハマトランセクトの炭酸塩堆積シークエンスというテーマで, 海水準変動とバハマ炭酸塩プラットフォームの形成史の関係を解明するために, 1996年2月～4月にかけて行われた(Eberli,Swart et al.,1997).1997年10月に第2回ポストクルーズミーティングがバハマのグレートエクスーマ (Great Exuma) 島のジョージタウンで開催され, それぞれの試料の分析結果を持ち寄って討論がなされた. 特に, 本航海で得られた, 更新統と中新統の“サイクル”に多くの研究者が重点をおいて, 試料の解析を行っていた. また, シークエンス境界についても, 航海後, 計算し直した年代値が提案された. このポストクルーズミーティングの後, 主席研究員の2人であるマイアミ大学Gregor P. Eberli教授, Peter K Swart 教授らによって準備された巡検がエクスーマ諸島の一つであるリーストッキング (Lee Stocking) 島で行われ, バハマプラットフォームの現世堆積物を目の当たりにすることができた. ここでは, 潮下帯(subtidal)で発達したストロマトライトや, ウウイドの瀬(ooid shoals)など,その一部を紹介する.