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doi:10.2204/iodp.proc.317.202.2013

Results

Detailed diatom assemblage data are given by species and sample number in Figure F2 and Table T1. These revealed a low to high diversity, although abundances in the majority of samples containing diatoms were low with moderate to poor preservation. Several robust diatom species with thickly silicified valves (e.g., Paralia sulcata, Thalassionema nitzschioides, and Chaetoceros resting spores) were better preserved than delicate forms with weakly silicified valves. The number of diatom valves preserved in sediments depends on diatom productivity, preservation, and/or dissolution of the valves and dilution with terrigenous and/or organic materials (Koizumi et al., 2004). Although the reasons for low diatom occurrences in cores from Hole U1352B are not clear, poor preservation might result from dissolution by pore water in coarse-grained sediments (see the “Site U1352” chapter [Expedition 317 Scientists, 2011c]).

Although the diatom occurrences are not continuous and their abundances are generally few, some slides contain common (1–10 valves/field of view [FOV]) to few (≥1 valve/10 FOV and <1 valve/FOV) abundances. Therefore, 111 slides with better preserved and more abundant diatoms (≥50 valves, highlighted in Table T1) were selected to provide paleoceanographical data (Fig. F3). After procedures to concentrate siliceous microfossils, materials including diatoms, poorly preserved silicoflagellates, radiolarians, and several types of pollen with very low abundances were also recognized in some slides (indicated by a “+” in Table T1).

In general, Expedition 317 age assignments relied on nannofossils for the Holocene to middle Pliocene and planktonic foraminifers for the early Pliocene to late Miocene. Both fossil groups were integral to biostratigraphic control for the middle Miocene (Fig. F2). Cody et al. (2008) achieved a remarkable advance in Neogene diatom biostratigraphy of the Antarctic by integrating diatom biostratigraphy, magnetostratigraphy, and tephrostratigraphy from 32 Neogene sections around the Southern Ocean and the Antarctic continental margin. The locations are concentrated around the Atlantic sector of the Southern Ocean except for those from the Dry Valley Drilling Project (DVDP), Cenozoic investigations in the western Ross Sea (CIROS), and DSDP Leg 28. Nevertheless, we have tried to use their model in order to see whether it can be applied in the case of Expedition 317 sites influenced by the Subantarctic Front in the Pacific sector.

In this study, we mainly used the age model provided by the shipboard calcareous nannoplankton biostratigraphy (see the “Methods” chapter [Expedition 317 Scientists, 2011b]) because most occurrences of diatom markers are sporadic except for some discussed below. Compared to the nannofossil datums, the ages of the first appearance of Fragilariopsis rhombica in Sample 317-U1352B-59X-1W, 99–100 cm (505.29 mbsf) (Total Range Model [TRM] = 1.69–1.92 Ma; Average Range Model [ARM] = 1.37–1.45 Ma in Cody et al., 2008), and the last appearance of Hemidiscus karstenii in Sample 317-U1352B-12H-4W, 130–131 cm (108.96 mbsf) (TRM = 0.28–0.33 Ma; ARM = 0.35–0.43 Ma), are consistent. Several first and last occurrences (FOs and LOs, respectively) of other biostratigraphic diatom markers indicated by Cody et al. (2008) were also recognized in this study (Fig. F2), but these taxa appeared sporadically above/below these levels and their reliability is uncertain.

Biostratigraphically useful diatom resting spores have been found in several studies (e.g., Suto, 2006b, 2006c, 2007). The LO of Liradiscus var. castaneus recognized in Sample 317-U1352B-25H-3W, 25–26 cm (220.95 mbsf), from the middle Pleistocene (~0.5 Ma) corresponds to that reported from the middle Pleistocene Zone NPD 11 of Yanagisawa and Akiba (1998) at DSDP Site 436, northwestern Pacific. Therefore, this species might be a potentially useful marker for improving the resolution of diatom biostratigraphy in the North and South Pacific Oceans.

Diatom assemblages from Hole U1352B contain several ecotype diatoms that typically live in oceanic (60 species), littoral to neritic (32 species), and freshwater (7 genus-level taxa) environments, including some warm- and cold-water species (Fig. F3). Moreover, 23 fossil resting spore morphospecies of the marine genus Chaetoceros, which is indicative of high productivity in nearshore upwelling regions and coastal areas, are also preserved (Fig. F2).

Oceanic diatoms live in the open ocean, away from coastal influences, and are transported by wind and currents. Neritic to littoral diatoms in shallower waters are also affected by coastal processes and reduced salinity resulting from freshwater drainage from land. The oscillations of these floras are somewhat heterogeneous. The large fluctuations of oceanic and neritic to littoral species indicate that there were several transgressions and regressions at the site (Fig. F3). Of those, stronger regressions at ~1600, 1140, and 600 ka coincide with slight peaks in land-derived freshwater diatoms at these ages (Fig. F3), although freshwater diatoms do not commonly coincide with continental detritus throughout the core. These signals of regression, however, generally coincide with the results from planktonic foraminiferal abundances (see the “Site U1352” chapter [Expedition 317 Scientists, 2010c]). On the other hand, freshwater diatoms also peak along with increases of oceanic species that indicate transgressions at ~1300, 800, and 350 ka (Fig. F3).

During warm periods indicated by abundant occurrences of warm-water species, oceanic species are also abundant. This synchronous fluctuation indicates transgressions took place during warm periods. On the other hand, some peaks in cold-water species are simultaneous with those of warm-water taxa. For the most part, warm-water species increase during the warm marine isotope stages (MIS) of Lisiecki and Raymo (2005; i.e., MIS 39, 37, 35, 17, 11, and 9), but a peak in warm-water species is present during the cold period MIS 56 (Fig. F3).

The marine diatom genus Chaetoceros is a major contributor to primary production in nearshore upwelling regions and coastal areas (Rines and Hargraves, 1988), and its resting spores are usually taken as a measure of diatom productivity and an indicator of nutrient-rich conditions (Sancetta, 1982). Therefore, abundant occurrences of resting spores from 1700 to 1100 ka indicate that eutrophication increased in the coastal regions after upwelling strengthened. Furthermore, resting spores are believed to increase because of an unstable and sporadic supply of nutrients from upwelling rather than seasonal upwelling conditions (Suto, 2006b). Abrupt increases of resting spores, therefore, from 1250 to 1100 ka, during the transitional period from 41,000 to 100,000 k.y. MIS cycles, indicate that the upwelling system became unstable. In the future, we will reconstruct the past environments of marine deposition under the influence of coastal upwelling and river input by comparing the results in this paper to other paleoenvironmental analyses.

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