Mid-Quaternary Paleoceanographic Trend in Near-shore Waters of the Northwest Pacific : A Case Study Based on an Offshore Well

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An exploratory oil and gas well was drilled on the continental shelf in water 38 m deep at lat. 36°9′33.8″N., long. 140°43′49.3″E., 13 km off the coast of Kashima, Ibaraki Prefecture, Kanto District, Japan. This well sank to the total subbottom depth of 2100 m penetrated sediments ranging back to Late Cretaceous in age. The upper 500 m of well sequence which comprises largely Quaternary sediments was subjected to paleoenvironmental analyses by jointly analyzing microfossils (calcareous nannoplankton and benthic and planktonic foraminifera), mineral content, and oxygen and carbon isotope stratigraphies. Three dated horizons, 0.128, 0.27 and 0.46 Ma, were established by recognizing one marked excursion to negative values in the oxygen isotope record and two calcareous nannoplankton datum levels in the well sequence. A possible hiatus at about 465 m depth terminates this mid-Quaternary sequence which is underlain by late Pliocene strata of about 1.9 Ma and older. Mid-Quaternary paleobathymetric trends were reconstructed for the well site area mainly on the basis of Q-mode cluster analysis of benthic foraminiferal faunas. Deposition of the mid-Quaternary sequence began in the upper bathyal zone and a steady shallowing followed through outer shelf environments to the present-day inner neritic zone. Other paleo-depth indicators such as abundance variations of nannoplankton Florisphaera profuuda and planktonic versus total foraminifera ratios, and variations of ^<13>C isotope record are all supportive of this benthic foraminifera-based depth-trend. Paleotemperature fluctuations were estimated by analyzing oxygen isotope compositions of benthic and planktonic foraminifera and by calculating with the aid of paleoecological transfer functions quantitative estimates of past winter temperatures. The paleotemperature fluctuations estimated for the well site area by these two methods agree well for the lower part of the well sequence, but begin to diverge from the mid-sequence and upwards, probably reflecting increasing effects of local water masses to the oxygen isotope variation. In the Quaternary isotope record of Kashima SK-1 well, no definite interval correlatable with the even-numbered isotope stages was observed. In view of the fact that distinct hiatuses occur in the onshore Quaternary record of Kanto District located to the west of the well site, the lack of even-numbered isotope stages is interpreted to signify periods of greatly reduced sedimentation or possibly non-deposition in a broad coastal region encompassing also the continental shelf around the well site. Mid-Quaternary paleoceanography around the well site prior to about 0.35 Ma was under the general influence of cold Oyashio Current with its strength varying greatly from time to time, whereas the latter time period responding to the diminishing Oyashio Current saw a seesaw game of two warm currents, Kuroshio and Tsugaru, expanding and retreating alternatively over a broad region of the sea off Northeast Honshu, Japan.Mid-Quaternarypaleoenvironmentnear-shore watersmicrofossilsmineral contentoxygen isotopecarbon isotopetransfer functionsIbaraki Prefecture
東北大学の論文
1987-03-31

Mid-Quaternary Paleoceanographic Trend in Near-shore Waters of the Northwest Pacific : A Case Study Based on an Offshore Well

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