Paleomagnetic analyses were conducted for Holes 897C, 898A, and 900A for this leg (Zhao, this volume). Figure 2 shows the relationship between the Pliocene-Pleistocene nannofossil biostratigraphic results and the paleomagnetic events recognized in each of these three holes. This figure also shows the correlation of the nannofossil biostratigraphic results of Holes 897C, 899A, 898A, and 900A.
The best correlation between the Pliocene-Pleistocene nannofossil biostratigraphic results and paleomagnetic events is seen in Hole 897C. As shown by Figure 2, five normal polarity intervals were identified in Hole 897C (Zhao, this volume). From top to bottom, the first normal polarity interval (0.00 to 108.80 mbsf) lies directly above the LO of R. asanoi (>6.5 µm); thus it is suggested by nannofossils to be the Brunhes chron (0-0.78 Ma). The second normal polarity interval (128.72 to 129.1 mbsf) was located in NN19f; therefore, it should correspond to the Jaramillo subchron (0.91-0.97 Ma). The third normal polarity interval (198.79 to 206.40 mbsf) was found across the NN18/19 boundary (LO of D. brouweri), but mostly within NN19a; it coincides with the Olduvai subchron (1.65-1.88 Ma). The fourth normal polarity interval (234.3 to 234.7 mbsf) is based on two samples. It may possibly represent the Reunion subchron (2.14—2.15 Ma). The fifth normal polarity interval (294.2 to 333.95 mbsf) lies just below the LO of D. surculus, which suggests that its upper boundary represents the Matuyama/Gauss boundary (2.45 Ma).
The nannofossil biostratigraphic results of Hole 898A do not correlate particularly well with the magnetostratigraphy. Three normal polarity intervals are identified in this hole. From top to bottom, the first normal polarity interval (0.0 to 26.81 mbsf) extends from NN21 to the upper part of NN19h; thus, it should belong to the Brunhes chron (0-0.78 Ma).
The second normal polarity interval (101.74 to 106.44 mbsf) was well documented in the U-channel Sample 149-898A-12H-2 (Zhao, this volume). According to the sequence of magnetic polarity zones, this normal polarity interval could represent the Jaramillo subchron (0.99-1.07 Ma). However, nannofossil biostratigraphic data indicate that the sediments between 95.75 and 128.00 mbsf belong to NN19d because of the presence of large Gephyrocapsa (>5.5 µm). Thus, the age of the second normal polarity interval may correspond to the Cobb Mountain subchron (1.1-1.3 Ma), which occurs earlier than the Jaramillo subchron (0.99-1.07 Ma).
Based on the nannofossil biostratigraphy of Hole 898A, the Jaramillo subchron should be located in NN19f (54.69 to 66.00 mbsf), between the FO of R. asanoi (>6.5 µm) and the reoccurrence of Gephyrocapsa spp. (>3 µm) events (Fig. 2). However, no paleomagnetic normal signal was identified in that interval. It is obvious that the paleomagnetic data and nannofossil biostratigraphy do not combine well to identify the Jaramillo subchron. Because the resolution and reliability of the nannofossil biostratigraphy in Hole 898A is high enough to clearly identify NN19f and NN19d, there is no reason to assign the second normal polarity interval to the Jaramillo subchron based on nannofossil biostratigraphy.
If we consider the second normal polarity interval as the Cobb Mountain subchron, a possible explanation as to why we discerned the shorter Cobb Mountain subchron without identifying the longer Jaramillo subchron is that the sediment-accumulation rate within NN19d, which includes the Cobb Mountain subchron, is much higher than within NN19f, which includes the Jaramillo subchron (Fig. 2). Another possible explanation is that the Cobb Mountain subchron was recorded by the high-resolution U-channel sample with a sampling interval of only 0.02 m, which makes it much less likely to be ignored than the Jaramillo subchron, which could have been represented by four or five samples within a sampling interval of 5 to 10m.
The third normal polarity interval (143.85 to 161.82 mbsf) lies immediately below the Pliocene/Pleistocene boundary (i.e., the FO of G. oceanica s.l.). Therefore, this interval corresponds to the Olduvai subchron based on nannofossil biostratigraphy.
Five normal polarity intervals were identified in Hole 900A. From top to bottom, the first normal polarity interval (0.00 to 21.08 mbsf) lies right above the LO of R. asanoi (>6.5 µm) and is thus identified as the Brunhes chron (0-0.78 Ma). The second normal polarity interval (47.00 to 48.15 mbsf) was found around the NN18/NN19 boundary (LO of D. brouweri), which coincides with the Olduvai subchron (1.65-1.88 Ma). According to nannofossil biostratigraphy, the Jaramillo subchron (0.99-1.07 Ma) should be located in NN19f (21.76 to 26.62 mbsf). However, no sample from this interval was analyzed by paleomagnetic techniques, and thus the Jaramillo subchron was not identified. The third and fourth normal polarity intervals (64.55 to 76.9 mbsf and 78.67 to 79.19 mbsf) are found in NN16; these coincide with the Gauss chron. The upper boundary of the third normal polarity interval represents the Matuyama/Gauss boundary at 2.45 Ma. The lower boundary of the fourth normal polarity interval coincides with the Gauss/Gilbert boundary (3.41 Ma). The fifth normal polarity interval (86.18 to 89.39 mbsf) lies in NN14/15, which probably indicates the uppermost subchron (Cochiti subchron) of the Gilbert chron. From the above data, we see a good correlation between the nannofossil and paleomagnetic analytical results.
Figure 3 shows sediment-accumulation rates calculated by assigning each discerned nannofossil event an absolute age based on past studies (Martini and Müller, 1986; Sato and Takayama, 1992; Wei, 1993) as shown below:
FO of Emiliania huxleyi 0.24 Ma
LO of Pseudoemiliania lacunosa 0.39 Ma
LO of Reticulofenestra asanoi 0.83 Ma
Reoccurrence of Gephyrocapsa spp. (>3µm) 0.89 Ma
LO of Gephyrocapsa spp. (>5.5 µm) 1.15 Ma
FO of Gephyrocapsa spp. (>5.5 µm) 1.36 Ma
LO of Calcidiscus macintyrei 1.57 Ma
FO of Gephyrocapsa s.l. (>4 µm) 1.64 Ma
LO of Discoaster brouweri 1.91 Ma
LO of Discoaster surculus 2.45 Ma
LO of Discoaster tamalis 2.63 Ma
LO of Reticulofenestra pseudoumbilicus 3.41 Ma
As shown in Figure 3, the order of the Pliocene-Pleistocene sedimentation rates from highest to lowest at the four Leg 149 sites is Sites 897, 898, 899, and 900. The water depths of these four sites are 5320.0, 5279.0, 5291.0, and 5036.8 m. The general trend of sedimentation rates for Leg 149 indicates that the sedimentation rate increases from the continental margin to the deep sea along with the increasing water depth.