Paleoenvironmental Change of the Kuroshio Region since the Last Glacial Age
1992 Volume 31 Issue 5 Pages 329-339
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1992 Volume 31 Issue 5 Pages 329-339
The δ18O of four planktonic and two benthic foraminiferal species, and the benthic foraminiferal assemblages were analyzed from a piston core KT-89-18, P-4, from off the coast of Shikoku, southwestern Japan. The comparison of these data with published δ18O values from three cores of the Northwest Pacific off Japan and with the standardized deep sea benthic δ18O values clearly indicates paleoenvironmental change in the Kuroshio region during the past 34, 000 years.
The δ18O measurements of individual foraminiferal specimens collected from sediment traps in the Japan Trench suggest that the average depths of test secretion of Globigerinoides sacculifer, Globorotalia truncatulinoides, and Globorotalia inflata are about 100m, 350m, and 400m, respectively.
Between 30, 000 and 13, 000yrs BP, the benthic foraminiferal δ18O values of KT-89-18, P-4, are relatively lighter than the standard δ18O values. This means that sea water with lighter δ18O values had been supplied to this basin. Benthic foraminiferal assemblages indicate that the deep water was low in dissolved oxygen during this period. Sea water with light δ18O values is inferred to have been supplied by the northern Oyashio Area, based upon comparison with planktonic foraminiferal δ18O curves of four Northwest Pacific cores.
During the last glacial, most of the Pacific coast of southern Japan was occupied by a cool, mixed water mass, which is considered to be the same as the modern surface water in the Perturbed Area off Sanriku, northeastern Japan, between the Kuroshio and Oyashio Area. Additionally, the so-called Oyashio Undercurrent, which is an intermediate water mass formed in the Oyashio Area, was also supplied to the Shikoku Basin area between 30, 000 and 13, 000yrs BP.
The Kuroshio Front, which moved southward during glacial times, is thought to have moved northward again, appearing off Shikoku at about 14, 000yrs BP, passing off Enshu-nada at 13, 000 yrs BP, and reaching the latitude of the Boso Peninsula by 10, 000yrs BP.
