SUMMARY AND CONCLUSIONS

In this study, we reported the coupled variations recognized in chemical composition and physical properties in the drilled and logged holes at Sites 1150 and 1151. NGR intensity was one useful parameter for reconstruction of paleoceanographic changes. The analytical results showed that (1) the amount of terrigenous minerals including K, Al, and related elements such as clay minerals were the principal sources of the NGR intensity, (2) the NGR intensity was also affected by high porosity related to the amount of diatom-dominant biogenic silica, and (3) NGR signal might be a proxy for the intensification of the East Asian monsoon off Sanriku.

A continuous sedimentary record was constructed by integration of the whole-core MST NGR intensity from Holes 1150A and 1150B with the wireline NGR log from Hole 1150D. Using a stratigraphic age model, the constructed sedimentary sequence was converted to a time series sequence from 1.3 to 9.7 Ma with a short break at ~5 Ma. The results obtained indicate that

  1. The sequence can be divided into eight stages of low (L) and high (H) sedimentation.
  2. The H stages correspond to the low-amplitude intervals of precession index and eccentricity.
  3. Power spectral density of the precession band component in the NGR signal was high during the H stages.
  4. Obliquity band and eccentricity band components always existed through 10 Ma.
  5. Changes of intensities of the periodic components through time suggested that the climate system reorganized during the H stages.
  6. Transition of the dominant periodicities during the H stages may correspond to major shifts of the climate system. The H1, H2/H3, and H4 stages could be correlated to the onset of major Northern Hemisphere glaciation, the initial stage of the East Asian monsoon intensification, and onset of the East Asian monsoon with uplift of the Himalayas and the Tibetan Plateau, respectively.

This is the first attempt at core-log integration of NGR intensity in an attempt to identify the sources of NGR off Sanriku. It is still necessary to determine the detailed paleoceanographic setting for variations in terrigenous input and marine productivity of diatoms. In addition, because of the uncertainty of time control points, future precise work is necessary to prove the prospective interpretations for the NGR variations in time.

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