Previous   |    Next

doi:10.2204/iodp.proc.322.206.2014

Results

With the exception of several barren interval, 72 taxa belonging to 20 genera of planktonic foraminifers were detected at Site C0012 (Table T1). Fossil preservation is generally moderate to poor. In particular, sediment from the lower part of Unit II to the upper part of Unit III has very rare occurrences of planktonic foraminifers with poor preservation and barren intervals (Table T1; Fig. F3). These barren samples contain only thick-walled benthic foraminifers with surfaces disfigured by dissolution. Therefore, the rare occurrences and barren intervals might be the results of dissolution processes beneath the carbonate compensation depth.

A total of 19 biohorizons were recognized in this study (Table T2). During the onboard preliminary observation, five biohorizons were reported from CC samples (see the “Site C0012” chapter [Expedition 322 Scientists, 2010]). Among them, the lowest biohorizon, characterized by the first occurrence of Orbulina universa, should be disregarded because our study detected a younger biohorizon marked by the first occurrence of Fohsella peripheroacuta below this horizon. Other four biohorizons (8, 10, 14, and 17 of Table T2) were redefined after examination of section samples in this study.

The last occurrences of Dentoglobigerina altispira altispira (the base of Zone PL5) and Sphaeroidinellopsis seminulina sensu lato (the base of Zone PL4) should be located above Sample 322-C0012A-2R-4, 0.0–8.0 cm (64.54 m CSF-A). Globorotalia plesiotumida sparsely occur and the last occurrence of this species is estimated to be above Sample 3R-1, 60.0–68.0 cm (70.14 m CSF-A). The last occurrence of Globoturborotalita nepenthes (the base of Zone PL2) is clearly between Samples 3R-1, 60.0–68.0 cm (70.14 m CSF-A), and 4R-1, 50.0–58.0 cm (79.54 m CSF-A). The zonal maker species Globorotalia tumida appears in two samples, with the first occurrence (the base of Zone PL1a) recognized between Samples 4R-1, 50.0–58.0 cm (79.54 m CSF-A), and 5R-4, 50.0–58.0 cm (93.54 m CSF-A). Only one specimens of Hirsutella margaritae was detected in this study from Sample 10R-5, 60.0–68.0 cm (137.14 m CSF-A). However, this species was also observed in Samples 4R-CC, 14.0–19.0 cm (80.79 m CSF-A), and 7R-CC, 10.0–15.0 cm (107.95 m CSF-A) during the onboard study (see the “Site C0012” chapter [Expedition 322 Scientists, 2010]). Therefore, the first occurrence of this species is suggested to be below Sample 10R-5, 60.0–68.0 cm. Globorotalia lenguaensis, the zonal marker of the base of Zone M14, was found in Sample 9R-1, 42.0–50.0 cm. Considering the sporadic occurrence of this species reported during the onboard study (see the “Site C0012” chapter [Expedition 322 Scientists, 2010]), the last occurrence of this species is implied to be above Sample 9R-1, 42.0–50.0 cm (121.46 m CSF-A). Globigerinoides conglobatus was recognized from Samples 4R-CC, 14.0–19.0 cm (80.79 m CSF-A), and 6R-CC, 10.5–15.5 cm (101.845 m CSF-A) during the onboard study (see the “Site C0012” chapter [Expedition 322 Scientists, 2010]). In this study, however, we detected only one individual of this species in Sample 4R-1, 50.0–58.0 cm (79.54 m CSF-A). Thus the first occurrence of this species should be located below Sample 6R-CC, 10.5–15.5 cm (101.845 m CSF-A). The dominant coiling direction of Neogloboquadrina acostaensis switches from sinistral to dextral between Samples 11R-5, 74.0–82.0 cm (146.78 m CSF-A), and 11R-7, 46.0–54.0 cm (149.50 m CSF-A). The first occurrence of G. plesiotumida (the base of Subzone M13b) is placed between Samples 21R-4, 40.0–48.0 cm (239.64 m CSF-A), and 22R-3, 45.0–53.0 cm (247.69 m CSF-A). Globoturborotalita extremus individuals were obtained from three samples, with the first occurrence of this species implied to occur below Sample 16R-4, 70.0–78.0 cm (192.74 m CSF-A). The last occurrence of Globoquadrina dehiscens lies between Samples 24R-4, 52.0–60.0 cm (268.26 m CSF-A), and 25R-1, 53.0–61.0 cm (273.27 m CSF-A). The first occurrence of N. acostaensis (the base of Subzone M13a) may be located below Samples 26R-2, 68.0–76.0 cm (284.42 m CSF-A). However, the precise position of this biohorizon cannot be determined because of the scarce fossil occurrence within it. The last occurrence of P. siakensis (the base of Zone M12) is detected between Samples 25R-1, 53.0–61.0 cm (273.27 m CSF-A), and 25R-5, 68.0–76.0 cm (279.42 m CSF-A). Globoturborotalita decoraperta is characterized by rare and sporadic occurrences and the lowest horizon where it is found is in Sample 26R-2, 68.0–76.0 cm (284.42 m CSF-A). Therefore, the first occurrence of this species could be suggested to be below this sample. The last occurrence of Globigerinoides subquadratus occurs between Samples 27R-3, 38.0–46.0 cm (295.12 m CSF-A), and 28R-3, 65.0–73.0 cm (304.89 m CSF-A). The first occurrence of G. nepenthes (the base of Zone M11) may be found below Samples 31R-5, 74.0–82.0 cm (336.48 m CSF-A). However, the position of this biohorizon may be subject to uncertainty because the species shows only sporadic occurrence in the middle Miocene interval at this site. The last occurrence of Fohsella peripheroronda is detected between Samples 36R-3, 80.0–88.0 cm (379.84 m CSF-A), and 37R-1, 35.0–43.0 cm (385.89 m CSF-A). The lowermost sample at the studied site (Sample 50R-3, 70.0–78.0 cm; 512.74 m CSF-A) contains F. peripheroacuta. Therefore, this sample may be considered younger than the first occurrence of F. peripheroacuta (the base of the Zone M7).

At Site C0012, the first occurrence of N. acostaensis (the base of Subzone M13a) is located below the last occurrence of P. siakensis (the base of Zone M12). Such discrepancy in these two biohorizons has been reported widely in subtropical to temperate regions. For example, the last occurrence of P. siakensis also crosses over the first occurrence of N. acostaensis at Deep Sea Drilling Project Site 563 in the North Atlantic (Miller et al., 1994) and at IODP Site U1338 in the eastern equatorial Pacific (Hayashi et al., 2013). Wade et al. (2011) suggested that the discrepancy in the Atlantic Ocean may be due to diachrony of the extinction of P. siakensis. In the western Pacific, however, our study is the first report of the overlap range of N. acostaensis and P. siakensis. Further study is required to clarify the reason of the discrepancy in this region.

Figure F3 represents the age-depth plot of Site C0012 using biohorizons of calcareous nannofossils (see the “Site C0012” chapter [Expedition 322 Scientists, 2010]) and planktonic foraminifers (this study). The results indicate that foraminiferal biohorizons of this study generally are consistent with the calcareous nannofossil data. In particular, comparison of the two different timetables reveals that biohorizons of the study by Tian et al. (2008) are more concordant with calcareous nannofossil data than those of the study by Wade et al. (2011) (Fig. F3). This result can be explained by the difference in ecological provinces: Tian et al. (2008) constructed their astronomically tuned timescale by using sites in the South China Sea, approximately 2300 km southwest of Site C0012, whereas that of Wade et al. (2011) is mainly based on Atlantic sites (ODP Sites 925 and 926) and eastern equatorial Pacific sites (ODP Legs 111 and 138). Among these biohorizons, the last occurrences of G. dehiscens and F. peripheroronda have been previously reported as diachronous around the Japanese Islands. Motoyama et al. (2004) mentioned that the age of the last occurrence of G. dehiscens was estimated at approximately 8.4–9.6 Ma at ODP Leg 186 in the Sanriku forearc basin in northeastern Japan. The last occurrence of F. peripheroronda was numerically determined as 13.0 Ma by biotite K-Ar ages in the Karasuyama area in the central part of the Honshu Island (Hayashi and Takahashi, 2002). The precise understanding of diachronous biohorizons around Japan requires further studies.

Top of page   |    Previous   |    Next