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doi:10.2204/iodp.proc.311.211.2009

Results

Sites U1325 (2195 m water depth), U1326 (1828 m water depth), U1327 (1305 m water depth), and U1329 (946 m water depth) were cored along a southwest–northeast transect on the continental slope, and Site U1328 (1268 m water depth) was drilled south of Site U1327. The shipboard diatom analyses of these five sites revealed that all sites contained diatom remains, though their abundances vary drastically, ranging from barren to >10,000 diatom valves per slide. Species compositions of diatoms also vary rather drastically (see Figs. F1, F2, and F3 in the "Expedition 311 summary" chapter). Diatom assemblages are mostly composed of shallow-marine and nonmarine species (Pls. P1, P2, P3). Nonmarine diatoms recognized in the Expedition 311 sediments are dominated by planktonics. Diatom biostratigraphic zonation showed that all sediments recovered from Sites U1325–U1328 are Pleistocene and those from Site U1329 are Pleistocene to upper Miocene. The Pleistocene zonations are represented by the Neodenticula seminae (NPD12; 0–0.3 Ma), Proboscia curvirostris (NPD11; 0.3–1.0 Ma), and Actinocyclus oculatus (NPD10; 1.0–2.0 Ma) Zones. The upper Miocene zonations are represented by the Rouxia californica (NPD7A; 6.4–7.6 Ma) and Thalassionema schraderi (NPD6B; 7.6–8.5 Ma) Zones (Pls. P1, P4).

Sites U1325 and U1326, situated near the southwest end of the transect, show similar occurrences of diatoms (Fig. F1). At Site U1325 diatoms are rare to common in lithostratigraphic Units I (see the "Site U1325" chapter) and II, where Zones NPD12 and NPD11 are recognized. Unit III yields very rare diatoms, and no diatom zonal species were detected. At both sites, abundance peaks of diatoms are recognized near the core tops. At Site U1326, diatoms are rare to common in lithostratigraphic Unit I and the upper part of Unit III, where Zones NPD12 and NPD11 are recognized. The two zones are separated by a barren interval of Unit II. The lower part of Unit III contains very rare to no diatoms.

Sites U1327 and U1328, both located midslope of the accretionary prism, show similar patterns of diatom occurrences (Fig. F2). Abundances of overall diatoms and resting spores of Chaetoceros spp. fluctuate, with common to very abundant occurrences in the middle part of the sections (lithostratigraphic Unit II) at both sites. The three Pleistocene diatom zones, NPD12, NPD11, and NPD10, are recognized at these sites. The zonal boundaries of the former two zones and the latter two zones are indicated by 0.3 Ma and 1.0 Ma (Yanagisawa and Akiba, 1998), respectively. Zone NPD10 ranges from 1.0 to 2.0 Ma (Yanagisawa and Akiba, 1998), but this zone at both sites contains P. curvirostris in basal samples, which restricts the age of the sites to <1.6 Ma.

The most characteristic feature recognized at these sites is large and frequent fluctuations in the ratio of marine versus nonmarine diatoms, which varies between 0% and 100%. Marine diatom assemblages are dominated by shallow-marine species, and their abundances are mostly concordant with Chaetoceros resting spores. Nonmarine diatoms assemblages are dominated by such planktonic genera as Aulacoseira and Cyclotella, which constitute >90% of nonmarine assemblages (Table T1; Pls. P1, P2). This suggests that sediments supplied from terrestrial areas were lacustrine, not river deposits. Fluctuation patterns at the two sites appear very similar to each other. At both sites, 7–8 cycles of peaks and troughs can be recognized, and one cycle ranges from 30 to 40 m thick. The more closely spaced detailed diatom analysis of Site U1328 carried out in this study (Fig. F3; Table T1) revealed that the fluctuation of marine versus nonmarine diatoms is represented by very high frequencies several times higher than those detected by the shipboard analysis. This fluctuation pattern was compared to one of the global oxygen isotope curves (de Garidel-Thoron et al., 2005), which are generally thought to reflect the global glacial sea level changes in the Pleistocene (Fig. F4). The comparison shows that our diatom data are comparable to the oxygen isotopic curve at least in the order of cycle frequency in both data sets, especially in the interval younger than 1 Ma, although not all the peaks and troughs between the two curves are correlative, probably because most of the diatoms are displaced from landward areas as is explained next. This may imply that the fluctuation of marine versus nonmarine diatoms at Site U1328 reflects the global glacial sea level changes during the Pleistocene. Nonmarine diatoms might be easily transported to the site during the low sea level glacial stages and not during highstands of interglacial stages.

The dominant and fluctuating occurrences of shallow-marine and nonmarine diatoms at Site U1328 might indicate that the paleobathymetry of this site became significantly shallower. To test this possibility, foraminiferal assemblages from Holes U1328B and U1328C were examined (Table T2). The result shows that benthic foraminifers in the site are characterized by predominance of calcareous forms, composed mostly of such deep-sea dwellers as Bolivina spissa, Epistominella pacifica, Islandiella norcrossi, and Uvigerina curticosta with unexpectedly very few admixtures of shallow-marine species in the uppermost part of the site, indicating that the paleobathymetry of the site has been almost constantly upper middle bathyal (1500–2000 m), namely within a depth range of the present water depth at Site U1328, and became slightly shallower near the top of the site. This result is consistent with that of Zellers (1995) for Leg 146 Hole 889A, which is located between Sites U1327 and U1328. Occurrence patterns of diatoms and foraminifers look rather different from each other, suggesting that the high-productivity areas for diatoms and foraminifers were different. This difference may well be reflected by their dominance in different habitats; diatom assemblages dominated in shallow-marine and nonmarine habitats and foraminiferal assemblages were deep-sea dwellers. In conclusion, the results of diatom and foraminiferal analyses suggest that the Pleistocene turbidite sediments originated mainly from the shelf and inland water areas and were displaced to the deepwater site. We conclude this because the diatom assemblages are mainly composed of shallow-marine and nonmarine species and the benthic foraminiferal assemblages comprise those of upper middle bathyal depths. The displacement of shallow-marine and nonmarine diatoms and sediments to the present deep site might be closely related to the frequently occurring earthquakes associated with the subsidence of the Juan de Fuca plate under the North American plate.

At Site U1329, located most landward of the transect and near the edge of continental slope and shelf, diatoms are rare in lithostratigraphic Units I and III and common to very abundant in Unit II (Fig. F4). Diatom biostratigraphy reveals that Units I and II are Pleistocene, represented by Zones NPD12, NPD11, and NPD10, in descending order. The upper and middle part of Unit III is upper Miocene, represented by Zones NPD7A and NPD6B. The basal part of the unit contains no age-diagnostic diatoms, and its geologic age is unknown at present. The Pleistocene section of this site is dominated by marine diatoms, and no obvious marine/nonmarine fluctuation is recognized. The diatom zonal assignment suggests that the sediments representing ~5 m.y. are missing by either a hiatus or a fault at the boundary of Units II and III (Fig. F5).

Using the diatom biostratigraphic events recognized at the five sites (Table T3), sedimentation rate curves for them are constructed (Fig. F6). The figure demonstrates that the sedimentation rates of the Pleistocene section of these sites increase concordantly with their distance from the shelf edge.

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