Predominance of Left-coiling Globorotalia truncatulinoides (d'Orbigny) between 115,000 and 50,000 yrs BP: A Latest Foraminiferal Biostratigraphic Event in the Western North Pacific.

The planktic foraminiferal species Globorotalia truncatulinoides (d'Orbigny) with left-coiling tests is distributed in limited areas in the modern world oceans, compared to that with right-coiling tests. The left-coiling forms of this species rarely occur in Recent sediments throughout the North Pacific. However, these forms are found to have outnumbered the right-coiling forms between 115, 000 and 50, 000yrs BP, based on the oxygen isotope stratigraphy in three western North Pacific cores. The predominance of the left-coiling forms probably reflects unique characteristics of the Western North Pacific Central Water and its underlying North Pacific Intermediate Water during the time span of about 65, 000 years. We propose the lower and upper limits of this bioevent at respectively 115, 000 and 50, 000yrs BP to be used as the latest biostratigraphic markers in the western North Pacific.