Late Quaternary Paleoceanography in the Northwestern Pacific Margin Reconstructed with a Submarine Core Recovered Off Sanriku: An Approach Using Diatom Fossil Assemblage

摘要

A Q-mode principal components analy- sis (PCA) based on downcore variation of diatom fossil assemblage was carried out in order to reveal the late Quaternary paleoceanography of the NW Pacific mar- gin with the qualified submarine core KH 94-3, LM-8, recovered off the Sanriku coast, NE Japan. It is recognized that increase of diatom valve concentration (DVC. valves/g) oc- curred in oxygen isotopic Stage 3 (inter- stadial during the last glacial) and Stage 1 (Holocene). However, diatom assemblages show drastic change between the two peri- ods. As the result of PCA, we delineated four principal components explaining 93. 6 of total variance. Principal compoent 1 is composed mainly of Thalassionema nitzschioides (Grunow) H. et M., which dominates in the coldest Stages 4 and 2 with diatom valves being poorly pre- served and DVC being the lowest. Princi- pal component 2 with some ice-related diatoms predominates in Stage 3, whereas principal component 3, which is cha- racterized by Neodenticula seminae (Si- monsen et Kanaya) Akiba et Yanagisawa and Odontella aurita (Lyngbye) Agardh, displays high values in Stage 1. Here, we suggest that intensified dis- charge of sea ice originated in the Sea of Okhotsk in Stage 3. This interpretation subsequently enables us to explain the high DVC during the period as being due to high productivity at the sea ice edge. We can also infer that the qualitative difference in the Oyashio between the glacial and the interglacial is due to change in the supplying mode of the Okhotsk-originating factor such as sea ~ ice, and consequently that there is