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doi:10.2204/iodp.proc.303306.210.2009

Results

A total of 11 uppermost Pliocene to Quaternary nannofossil datums are detected in the Site U1308 section (Table T1; Figs. F2, F3). Based on the datum numbers of Sato and Takayama (1992), Datum 2, the first occurrence of Emiliania huxleyi, which defines the Zone NN20/NN21 boundary of Martini's zonation, is found at 16.88–16.93 mcd. Datum 3, the last occurrence of Pseudoemiliania lacunosa, which correlates to the Zone NN19/NN20 boundary, is detected between 28.14 and 28.19 mcd. Because Helicosphaera inversa occurs rarely throughout the section, Datums 1 and 4 were not recognized here. Datums 5 and 7, the last and first occurrences of Reticulofenestra asanoi, are respectively found at 55.47–55.52 and 80.28–80.33 mcd. Datums 6 and 8, the first occurrence of Gephyrocapsa parallela and last occurrence of the large form Gephyrocapsa spp., are situated at 66.40–66.45 and 84.56–84.86 mcd, respectively. Datums 9 (last occurrence of Helicosphaera sellii; 84.86–90.16 mcd), 10 (first occurrence of large Gephyrocapsa spp.; 100.59–103.52 mcd), 11 (first occurrence of Gephyrocapsa oceanica; 123.07–131.27 mcd), 12 (first occurrence of Gephyrocapsa caribbeanica; 131.27–132.74 mcd), and 13 (last occurrence of Discoaster brouweri; 114.51–153.29 mcd) are situated in the uppermost Pliocene to lower Pleistocene interval.

Based on the stratigraphic position of these datums at Site U1308, we also correlate the datums to magnetostratigraphy, which was studied by Channell et al. (2006) (Fig. F2). Both Datums 2 and 3 are traceable to the Brunhes Chron. Datum 5 is situated between the Brunhes Chron and Jaramillo Subchron of the Matuyama Chron, and Datum 6 is traceable to the top of the Jaramillo Subchron. Datum 7 is found between the Jaramillo and Cobb Mountain Subchrons, and Datum 8 is situated within the Cobb Mountain Subchron of the Matuyama Chron (Fig. F2). Datums 9–12 are present between the Cobb Mountain and the Olduvai Subchrons in the Matuyama Chron. Datum 13 is detected just below the Olduvai Subchron (Fig. F2).

We also compared the nannofossil datum events with oxygen isotope stratigraphy (Fig. F2). Datums 2 and 3 are respectively situated in marine isotope Stage (MIS) 8 and 12. Both datums are situated just before the highest peak of the respective MIS. Datums 5 and 6 are respectively found in MIS 21 and 27, and Datums 7, 8, 9, and 10 are found in MIS 34, 35, 38, and 44, respectively. Ages of nannofossil datums and the relationship between nannofossil biostratigraphy, magnetostratigraphy, and oxygen isotope stratigraphy are summarized in Figure F3 and Table T1.

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