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Core YK07-12 PC03B obtained from the central Okhotsk plain(52°36'N, 150°08'E ; water depth: 1049 m) was analyzed every 5 cm, focusing on two radiolarian biostratigraphic markers in the late Pleistocen...e: Lychnocanoma nipponica sakaii and Amphimelissa setosa. The last occurrence(LO) datum of A. setosa at 67 ka conformed with those of the LO of the previously published studies. However, the LO of L. nipponica sakaii at 25 ka was off set and significantly younger than those published in the previous studies in the North Pacific and the marginal seas. We introduce herein the conceptual use of the last common occurrence(LCO) of L. nipponica sakaii as the LCO conforms with the LO of this taxon published elsewhere and hence more reliable, especially out side of the northern part of the central Okhotsk plain. The LCO of L. nipponica sakaii is 46 ka, 21 kyrs older than the LO, but is approximately the same as the LOs published elsewhere. We postulate the LCO as a practical datum which can be compared with the data from out side of the studied region. As alternative explanations, the obtained significantly younger LO than the LOs of other studies from the out side of the study region may stem from the following two reasons. Because that the present study is specifically focused on the two biostratigraphic marker radiolarian taxa with details our microscopic counts are significantly greater than those performed in the previous studies. This may have caused the documentation of the LO with the rare microscopic counts on multiple microslides, which could have been missed had we counted only one microslide. It is also possible that the effect of bioturbation due to upward transport in the slow sedimentation rate regime caused the appearance of the rare specimens of L. nipponica sakaii well above the LCO.続きを見る
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