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doi:10.2204/iodp.proc.329.203.2015

Results

All samples from Hole U1371D contained sufficient diatoms per slide (>400, usually nearly 1000 valves) and Chaetoceros resting spores (>100, usually nearly 400 valves), except for Sample 329-U1371D-12H-2W, 45–46 cm (104.35 m CSF-A), with very rare diatoms, and barren samples between Samples 12H-3W, 45–46 cm (105.85 m CSF-A), and 14H-4W, 35–36 cm (126.25 m CSF-A). The preservation and abundance of fossil diatoms and resting spores are good and common to abundant throughout all microslides except for some rare and barren samples mentioned above (Table T1). Diatom assemblages are mostly composed of useful biostratigraphic markers with continuous and abundant occurrences (Fig. F2; Tables T1, T2).

Core materials from Expedition 329 Hole U1371D investigated in this study correspond to several diatom bioevents (i.e., FAD and LAD) defining the diatom zones of Harwood and Maruyama (1992) (Fig. F3):

• Thalassiosira lentiginosa Zone (0.0–0.54 Ma, recalculated ages in midpoint of ARM by Cody et al. [2008]),

• Actinocyclus ingens Zone (0.54–1.24 Ma),

• Fragilariopsis kerguelensis/Thalassiosira kolbei/Thalassiosira vulnifica Zone (1.24–2.48 Ma),

• Thalassiosira insigna–T. vulnifica/Fragilariopsis interfrigidaria Zone (2.48–4.11 Ma),

• Fragilariopsis barronii Zone (4.11–4.40 Ma),

• Thalassiosira inura Zone (4.40–4.74 Ma),

• Shionodiscus oestrupii/Fragilariopsis reinholdii b Subzone (4.74–6.00 Ma),

• F. reinholdii a/A. ingens var. ovalis Zone (6.00–8.67 Ma), and

• Thalassiosira torokina Zone (8.67–? Ma).

However, the bioevents that define the bottoms of the F. kerguelensis Zone, the T. kolbei Zone, the T. insigna–T. vulnifica Zone, the S. oestrupii Zone, and Subzone a of the F. reinholdii Zone were not clarified because of the absence of index species (Fig. F2). Here, the magnetostratigraphic chron and subchron datums are presumed according to shipboard research results (Expedition 329 Scientists, 2011). We selected biostratigraphic diatom markers that did not contradict the magnetostratigraphic datums (Fig. F3). Several datums of other age indicators conflicted with chron and subchron datums, and these indicators were not used in this study because they might include reworked and/or contaminated diatoms. Moreover, some diatom species also were not used because of their sporadic and rare occurrences (Fig. F2).

The LAD of A. ingens Rattray (0.5–0.57 Ma) is recognized between Samples 329-U1371D-1H-3W, 66–67 cm, and 1H-4W, 112–113 cm (4.64 ± 0.98 m CSF-A). The interval between the top of Hole U1371D and the LAD of A. ingens is assigned to the T. lentiginosa Zone of Harwood and Maruyama (1992).

The LAD of F. barronii (Gersonde) Gersonde et Bárcena (1.19–1.29 Ma) is recognized between Samples 3H-3W, 112–113 cm, and 3H-4W, 112–113 cm (21.77 ± 0.75 m CSF-A). The interval between the LAD of F. barronii and the LAD of A. ingens is assigned to the A. ingens Zone of Harwood and Maruyama (1992).

In the interval between the LAD of F. barronii and the LAD of T. insigna (Jousé) Harwood et Maruyama, the F. kerguelensis Zone and the T. kolbei Zone are defined by Harwood and Maruyama (1992), but the boundaries of these zones, which are defined by the LAD of T. kolbei (Jousé) Gersonde (1.98 Ma) and the LAD of T. vulnifica (Gombos) Fenner (2.17 Ma), are not recognized because of the absence and sporadic occurrences of T. kolbei and T. vulnifica.

The LAD of T. insigna (2.45–2.5 Ma) is recognized between Samples 5H-2W, 112–113 cm, and 5H-3W, 112–113 cm (38.87 ± 0.75 m CSF-A), and assigned to the bottom of the T. vulnifica Zone of Harwood and Maruyama (1992). The LAD of Shionodiscus tetraoestrupii var. reimeri (Mahood et Barron) Alverson et al. (1.31–1.34 Ma), the LAD of Proboscia barboi (Brun) Jordan et Priddle (1.6–1.73 Ma), the FAD of Fragilariopsis obliquecostata (Van Heurck) Heiden et Kolbe (1.66–1.73 Ma), the FAD of Shionodiscus gracilis (Karsten) Alverson et al. (1.87 Ma), and the FAD of F. kerguelensis (2.18–2.21 Ma) are also recognized in the intervals consisting of the F. kerguelensis Zone, the T. kolbei Zone, and the T. vulnifica Zone (Fig. F2; Table T2).

The FAD of T. vulnifica (3.12–3.18 Ma), which is assigned to the bottom of the T. insigna–T. vulnifica Zone, is not defined in this study because of sporadic occurrences.

The FAD of F. interfrigidaria (McCollum) Gersonde et Bárcena (3.93–4.19 Ma), which defines the bottom of the F. interfrigidaria Zone of Harwood and Maruyama (1992), is recognized between Samples 7H-4W, 15–16 cm, and 7H-5W, 15–16 cm (60.30 ± 0.75 m CSF-A). The LAD of T. inura Gersonde (2.53–2.55 Ma), the FAD of Actinocyclus actinochilus (Ehrenberg) Simonsen (2.72–2.81 Ma), and the LAD of Fragilariopsis praeinterfrigidaria (3.45–3.49 Ma) are also recognized in the interval consisting of the T. insigna–T. vulnifica Zone and the F. interfrigidaria Zone (Fig. F2; Table T2).

The FAD of F. barronii (4.28–4.52 Ma), which defines the bottom of the F. barronii Zone of Harwood and Maruyama (1992), is recognized between Samples 7H-6W, 30–31 cm, and 8H-1W, 82–83 cm (63.96 ± 1.26 m CSF-A).

The FAD of T. inura (4.71–4.77 Ma), which defines the bottom of the T. inura Zone of Harwood and Maruyama (1992), is recognized between Samples 8H-4W, 82–83 cm, and 8H-5W, 82–83 cm (70.47 ± 0.75 m CSF-A). The FAD of Rouxia diploneides Schrader (4.61–4.7 Ma) is also recognized between Samples 8H-1W, 82–83 cm, and 8H-2W, 82–83 cm (65.97 ± 0.75 m CSF-A).

The bottom of the S. oestrupii Zone of Harwood and Maruyama (1992), defined by the FAD of S. oestrupii (4.8–4.95 Ma), was not determined.

The LAD of Hemidiscus triangularus (Jousé) Harwood et Maruyama (5.14–6.86 Ma), which defines the bottom of Subzone b of the F. reinholdii Zone, is observed between Samples 9H-4W, 92–93 cm, and 9H-5W, 58–59 cm (79.90 ± 0.58 m CSF-A).

The FAD of Thalassiosira miocenica Schrader (6.25–8.33 Ma), which defines the bottom of Subzone a of the F. reinholdii Zone, is not recognized in this study.

The FAD of A. ingens var. ovalis (8.6–8.74 Ma) defines the boundary between the A. ingens var. ovalis Zone and the T. torokina Zone; however, the FAD of A. ingens var. ovalis and the FAD of Hemidiscus karstenii Jousé in Jousé et al. (9.68–10.24 Ma) may be uncertain because lower samples lack continuous occurrences of those diatoms (Fig. F2; Table T2).

These biostratigraphic results match well with those of Cortese and Alvarez Zarikian (2015) based on radiolarian biostratigraphy from 30 meters below seafloor (mbsf) to the top of core and at ~100 mbsf, although the radiolarian and diatom biostratigraphic ages from 70 to 60 mbsf have more differences. The reason for these differences might be that reworked siliceous fossils are included in these cores.

Moreover, the abundant occurrence of Chaetoceros resting spores, which are a major contributor to primary production in nearshore upwelling regions and coastal areas (Rines and Hargraves, 1988) and are usually taken as a measure of diatom productivity and an indicator of nutrient-rich conditions (Sancetta, 1982), may indicate that eutrophication increased in the coastal regions after upwelling strengthened (Suto, 2006b). Also, the peak at ~2.5 Ma might coincide with the Pacific Chaetoceros Explosion Event-2 (Suto et al., 2012), which is characterized by relatively higher occurrences at ~2.5 Ma in the North Pacific region.

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