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Remarks on stratigraphic position of datums

Takayama and Sato (1987) identified 11 Quaternary nannofossil datums at Deep Sea Drilling Project Sites 606, 607, 608, 609, 610, and 611, located between 37°N and 53°N in the North Atlantic Ocean. They described that 9 of 11 Quaternary nannofossil datums were traceable to all sites in the North Atlantic Ocean and that the stratigraphic positional relationship of each nannofossil datum to magnetostratigraphy were invariable. Site U1308 is a reoccupation of Site 609.

Site U1308 is suitable for high-resolution study to clarify the relationship between magnetostratigraphy, oxygen isotope stratigraphy, and calcareous nannofossil biostratigraphy in detail (Figs. F2, F3). The relationship between magnetostratigraphy, oxygen isotope stratigraphy, and calcareous nannofossil biostratigraphy has been previously discussed based on study of deep-sea sediments (Thierstein et al., 1977; Wei, 1993; Raffi et al., 1993; Raffi, 2002). Raffi (2002) and Raffi et al. (2006) summarized the biochronology of nannofossil events and discussed the synchroneity of nannofossil datum events.

Although our results are almost the same as those by Raffi et al. (2006), some datums such as Datums 5 and 8 are biochronostratigraphically different from those described by Raffi et al. (2006).

Datum 8: last occurrence of Gephyrocapsa spp. (large)

Gephyrocapsa spp. (large) in this study is correlated to large Gephyrocapsa spp. by Raffi (2002) and Raffi et al. (2006) and Gephyrocapsa spp. A-B by Wei (1993). Raffi (2002) correlated the position of Datum 8 to MIS 37–38 below the Cobb Mountain Subchron. Our results, however, indicate that Datum 8 is found within the Cobb Mountain Subchron at Site U1308 and is correlated to MIS 35. This correlates well with results from Sites 609, 610, and 611 (Takayama and Sato, 1987) and from Quaternary sequences in Japan (Sato and Takayama, 1992).

Datum 5: last occurrence of Reticulofenestra asanoi

Although the stratigraphic position of Datum 5, last occurrence of R. asanoi at Site U1308, is correlated to the lowest peak of MIS 21 in this study, its position by Wei (1993) and Raffi (2002) is traced to MIS 22. Raffi (2002) summarized the biochronological problems introduced by the influence of regional environmental conditions on diachronous placement of biological datum events. However, Datum 5 was interpreted as being abrupt and consistently isochronous and is associated with the mid-Pleistocene revolution (Raffi, 2002). Our results indicate that the presence of a small difference in chronostratigraphic position of this datum may reflect the influence of regional environmental conditions.

Datums 2 and 3: first occurrence of Emiliania huxleyi and last occurrence of Pseudoemiliania lacunosa

Datums 2 and 3 in this study occur in MIS 8 and 12, respectively. These results are the same as those of Thierstein et al. (1977), Wei (1993), and Raffi (2002). Furthermore, our high-resolution study also clarifies the critical stratigraphic positions of both datums, which are situated just before the highest peaks of each MIS (Fig. F2). Datum 2 is at 16.88–16.93 mcd, 0.82 m below the highest peak of MIS 8. Datum 3 is also traceable to 28.14–28.19 mcd, 0.41 m below the highest peak of MIS 12. These differences between MIS peak and datum level are calculated as 12.6 k.y. (Datum 2) and 16.2 k.y. (Datum 3) (Fig. F4).