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doi:10.2204/iodp.proc.311.106.2006

Biostratigraphy

Holes U1328B and U1328C contain a ~300 m thick Quaternary sedimentary sequence. The biostratigraphy determined for Site U1328 was based on an examination of all core catcher samples from Holes U1328B and U1328C (Table T2).

Diatoms

Diatoms are mostly very rare and poorly preserved in the upper stratigraphic section from the shallowest sample to ~128 mbsf and mostly rare to abundant but poorly to moderately preserved in the deeper section cored from ~134 to ~301 mbsf. Diatom assemblages are dominated either by marine species Neodenticula seminae, Stephanopyxis spp., Thalassionema nitzschioides, and Thalassiosira spp. or nonmarine diatoms Aulacoseira granulata s.l. and Cyclotella spp. The abundance of the dominant species varies significantly among samples. Abundant resting spores are frequently associated with marine diatom-dominant assemblages but are limited to the deeper section.

Marine- and nonmarine-dominant assemblages alternate throughout the cored section, suggesting terrigenous input by turbidites. Several distinct patterns in upward-increasing nonmarine diatoms are recognized at this site, especially in the lower part. These patterns might reflect sedimentary cycles, but detailed postcruise diatom analyses of more closely spaced samples are needed to ascertain if this hypothesis is true or an artifact caused by the coarsely spaced shipboard sampling.

The interval between the shallowest sample and Sample 311-U1328C-6H-CC (4.75–103.20 mbsf) contains Neodenticula seminae but lacks Proboscia curvirostris and was assigned to North Pacific Diatom (NPD) Zone 12 (N. seminae Zone; 0–0.3 Ma). The last occurrence (LO) of P. curvirostris was recognized in Sample 311-U1328C-7X-CC (110.07 mbsf), marking the NPD Zone 12/11 boundary (P. curvirostris Zone). The occurrence of Actinocyclus oculatus in this hole is very sporadic, and it is recognized only in Samples 311-U1328C-14X-CC (174.74 mbsf) and 25X-CC (281.99 mbsf). Its shallower occurrence is tentatively interpreted as reworked because it is separated from its deeper occurrence by as many as 10 samples that lack A. oculatus. The deeper occurrence was designated as the LO of A. oculatus in this hole, although detailed postcruise diatom analyses of more closely spaced samples are needed to confirm this interpretation. Based on the shipboard interpretation, the following zonation is made. The interval between the LO of P. curvirostris and 311-U1328C-24X-CC (110.07–272.18 mbsf) contains P. curvirostris but lacks A. oculatus, except for the reworked occurrence, and is assigned to NPD Zone 11 (P. curvirostris Zone; 0.3–1.0 Ma). The interval between the LO of A. oculatus and Sample 311-U1328C-27X-CC (281.99–301.97 mbsf) contains P. curvirostris and A. oculatus but lacks Neodenticula koizumii and is assigned to NPD Zone 10 (A. oculatus Zone; 1.0–2.0 Ma). Sample 311-U1328C-27X-CC (301.97 mbsf), the lowest in this hole, contains P. curvirostris. Therefore, the base of NPD Zone 10 in this hole, which is not the base of the entire NPD Zone 10, is younger than 1.6 Ma, the first occurrence of P. curvirostris (see Table T2).