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doi:10.2204/iodp.proc.342.105.2014

Biostratigraphy

Coring at Site U1404 recovered a 300 m thick sequence of Pleistocene to middle Eocene clay, with subordinate nannofossil ooze mainly in the Eocene/Oligocene boundary interval. The clay contains varying amounts of biosilica and nannofossils. Nannofossils and planktonic foraminifers are present in the uppermost brown foraminifer sandy clay and nannofossil ooze (Core 342-U1404B-1H; 0–2.46 mbsf) and indicate Quaternary to Pliocene ages (Zones NN21/NN20–NN15). We were unable to provide biostratigraphic ages for the interval between 4.70 and 40.61 mbsf because of the absence of siliceous or calcareous microfossils. Below 40.61 mbsf, first radiolarians and then radiolarians, nannofossils, and occasionally planktonic foraminifers provide biostratigraphic control indicating middle Miocene to middle Eocene-age sediments. Benthic foraminifers are generally rare and moderately to well preserved in lithostratigraphic Units I and II, whereas Subunit IIb contains well-preserved assemblages, including Globobulimina pacifica, indicative of suboxic conditions. Unit III contains abundant and relatively well preserved benthic foraminifers. Benthic foraminifers suggest a lower bathyal to abyssal paleodepth for this site (van Morkhoven et al., 1986).

An integrated calcareous and siliceous microfossil biozonation is shown in Figure F14. Nannofossil datum and zonal determinations agree well with the radiolarian biostratigraphy. An age-depth plot including biostratigraphic and paleomagnetic datums is shown in Figure F15. A summary of calcareous and siliceous microfossil abundances and preservation is given in Figure F16.

Calcareous nannofossils

Calcareous nannofossil biostratigraphy is based on analysis of core catcher samples and additional working section half samples from Holes U1404A and U1404B to refine datum levels, particularly at the Oligocene–Miocene and Eocene–Oligocene transitions. Depth positions and age estimates of biostratigraphic marker events are shown in Table T3. Calcareous nannofossil occurrence data are shown in Table T4. Note that the distribution chart is based on shipboard study only and is therefore biased toward age-diagnostic species.

At Site U1404, preservation of calcareous nannofossils varies from poor to moderate through the Miocene–Oligocene succession, and very poorly preserved dissolved assemblages are present in middle Eocene sediment. Calcareous nannofossil preservation is good in the carbonate-rich interval around the EOT (Fig. F13).

The uppermost interval from Sample 342-U1404B-1H-1, 40 cm, to 1H-CC (0.41–2.46 mbsf) contains abundant nannofossils indicative of Pliocene–Pleistocene Zones NN20–NN15, as indicated by the base of large Gephyrocapsa (Sample 342-U1404B-1H-1, 40 cm; 0.41 mbsf), top of Discoaster tamalis (Sample 1H-1, 135 cm; 1.36 mbsf), and top of Reticulofenestra pseudoumbilicus (Sample 1H-CC; 2.46 mbsf). The sediment is noncalcareous and barren of nannofossils from Sample 342-U1404B-1H-CC to 342-U1404A-6H-CC (2.46–49.36 mbsf).

The interval from Sample 342-U1404A-6H-CC to 14H-1, 43 cm (49.36–115.63 mbsf), contains rare to common calcareous nannofossils ascribed to lower Miocene Zone NN2 because of the common presence of Triquetrorhabdulus carinatus and the absence of Dictyococcites bisectus. The sporadic presence of Discoaster druggii, the base of which defines the base of Zone NN2, is seen from Sample 342-U1404A-10H-5, 115 cm, to 15H-6, 100 cm (87.86–133.03 mbsf), supporting the early Miocene age for this interval.

The Miocene/Oligocene boundary interval is carbonate poor, and nannofossils are generally poorly preserved and of low diversity or absent. Upper Oligocene Zone NP25 is identified by the top and base of Sphenolithus ciperoensis in Samples 342-U1404A-17H-CC (153.72 mbsf) and 21H-3, 93 cm (185.64 mbsf), respectively. A 9.0 m noncalcareous interval follows, lying above a richly nannofossiliferous Oligocene/Eocene boundary succession (from Sample 342-U1404A-21H-CC to 23H-4, 120 cm; 191.71–200.60 mbsf).

The highest lower Oligocene nannofossil assemblage lies immediately below a sharp boundary in Hole U1404A (200.63 mbsf), which may be a sheared contact drilling artifact (see “Paleomagnetism”). Nannofossils appear in a gradual succession of increasing downhole carbonate in Section 342-U1404B-24H-1. The assemblage contains the tops of Coccolithus formosus, Isthmolithus recurvus, and Reticulofenestra umbilicus, indicating the presence of a short hiatus (0.9 m.y.) or condensed section in the overlying barren interval. The top of C. formosus indicates the presence of Zone NP21, the nannofossil zone that spans the Eocene/Oligocene boundary. The intra–Zone NP21, top acme of Clausicoccus subdistichus event occurs in Sample 342-U1404A-23H-5, 60 cm (201.50 mbsf).

The highest upper Eocene datum event, the top of Discoaster saipanensis, occurs in Sample 342-U1404A-24H-3, 30 cm (207.61 mbsf), and marks the base of Zone NP21. The base of I. recurvus in Sample 342-U1404A-24H-6, 96 cm (212.76 mbsf), marks the base of Zone NP19–NP20. The occurrences of single specimens in several lower samples (downhole to Sample 342-U1404A-26H-1, 72 cm; 221.22 mbsf) may represent rare occurrences prior to the base common occurrence, which is the NP19 zonal boundary datum level.

The upper Eocene interval is characterized by carbonate-poor clay containing dissolved nannofossils; therefore, the datum levels reported should be treated with some caution. The interval from Sample 342-U1404A-26H-1, 72 cm, to 33X-CC (221.22–279.56 mbsf) is assigned to Zones NP17–NP18, which cannot be differentiated because of the absence of Chiasmolithus oamaruensis in the upper Eocene at Site U1404. The top of Chiasmolithus grandis, which lies slightly below the base of Zone NP18, occurs in Sample 342-U1404A-25H-CC (220.56 mbsf).

The base of D. bisectus has been consistently found in the upper part of Chron C18r, predating the MECO (Fornaciari et al., 2010). At Site U1404, D. bisectus is observed from Sample 342-U1404A-28H-CC (244.03 mbsf) above an interval barren of calcareous nannofossils (Samples 342-U1404A-29H-1, 64 cm, to 30H-CC; 245.01–259.72 mbsf). This suggests that the MECO may lie within this carbonate-free interval.

The top of Nannotetrina spp. and presence of R. umbilicus in Sample 342-U1404A-34X-1, 56 cm (281.27 mbsf), indicates the presence of upper Zone NP16 in this lowermost nannofossiliferous sample. Calcareous nannofossils are not present from Sample 342-U1404A-34X-7, 60 cm, to 36X-1, 67 cm (289.83–299.80 mbsf).

Radiolarians

Radiolarian biostratigraphy is based on analysis of selected core catcher and working section half samples from Hole U1404A. No samples from Hole U1404B were examined. Radiolarians occur sporadically and are poorly preserved in the undated upper part of Hole U1404A. Radiolarians are abundant and well preserved through the lower Miocene and upper Oligocene but are absent from the Oligocene–Eocene transition. They are abundant and well preserved in the middle Eocene. Depth positions and age estimates of biostratigraphic marker events are shown in Table T5, and the radiolarian distribution is shown in Table T6. Note that the distribution chart is based on shipboard study and is biased toward age-diagnostic species. Some distinctive elements of Miocene and Eocene radiolarian assemblages are illustrated in Figure F17.

The uppermost interval, from Sample 342-U1404A-1H-CC to 4H-CC (4.7–22.36 mbsf), is barren or contains only rare and poorly preserved radiolarians.

Radiolarians are abundant and well preserved from Sample 342-U1404A-5H-CC to 19H-CC (40.59–172.77 mbsf) and are assigned to radiolarian Zones RN2–RP21, which span the Miocene–Oligocene transition. Lower Miocene Zone RN2 is identified by the co-occurrence of Cyrtocapsella tetrapera and Stichocorys delmontensis in Samples 342-U1404A-5H-CC to 11H-CC (40.59–96.24 mbsf), along with the absence of Zone RN3 index species Stichocorys wolffii. The top of Dorcadospyris ateuchus also occurs within this interval (Sample 342-U1404A-8H-CC; 71.30 mbsf), which is consistent with the Zone RP21–RN2 range reported by Sanfilippo et al. (1985). Samples from this zone contain abundant diatoms.

The interval between the base of S. delmontensis in Sample 342-U1404A-11H-CC (96.24 mbsf) and the bases of C. tetrapera and Calocycletta virginis in Sample 342-U1404A-14H-CC (124.87 mbsf) is assigned to Zone RP1. The base of Eucyrtidium diaphanes is recorded in Sample 342-U1404A-13H-CC (115.07 mbsf), although the species is known to range into underlying Zone RP22 at other locations and is a potential marker for the base of the Miocene (Kamikuri et al., 2012). Samples from this zone contain distinctive large orosphaerid radiolarians (Fig. F17). Diatoms are present but less common than in Zone RN2.

Lowermost Miocene–uppermost Oligocene Zone RP22 extends to the base of Lychnocanoma elongata, which is identified in Sample 342-U1404A-16H-CC (144.00 mbsf). Radiolarian biostratigraphy indicates that the Oligocene/Miocene boundary lies between Samples 342-U1404A-14H-CC and 16H-CC (124.87–144.00 mbsf). Upper Oligocene Zone RP21 extends to the base of D. ateuchus in Sample 342-U1404A-19H-CC (172.77 mbsf). The bases of Artophormis gracilis, Didymocyrtis prismatica, and Theocyrtis annosa occur within this zone in Hole U1404A. All three species have well-established first appearance datums in Zone RP20, which was not identified in Hole U1404A because of a barren interval (see below). Diatoms are generally rare in samples from Zones RP21 and RP22 but are abundant in Sample 342-U1404A-18H-CC (163.20 mbsf). Diatoms are rare to very rare in underlying Samples 342-U1404A-19H-CC to 36X-1, 67 cm (172.77–299.80 mbsf).

The interval from Sample 342-U1404A-20H-CC to 27H-CC (181.99–236.48 mbsf) contains the EOT but is barren of radiolarians. Rare radiolarians occur in Sample 342-U1404A-28H-CC, and below this level radiolarians are abundant and well preserved. Radiolarian assemblages from Samples 342-U1404A-29H-1, 52–53 cm, to 36X-1, 67 cm (244.63–299.80 mbsf), are assigned to middle Eocene radiolarian Zones RP16–RP14. Sample 342-U1404A-29H-1, 52–53 cm (244.63 mbsf), is assigned to Zone RP16 based on the presence of the zonal markers Podocyrtis goetheana, Thyrsocyrtis bromia, and Thyrsocyrtis tetracantha. Samples 342-U1404A-29H-CC to 32H-CC (253.05–270.64 mbsf) are correlated with Zone RP15 based on the faunal crossover from Podocyrtis mitra to Podocyrtis chalara in Sample 342-U1404A-32H-CC (270.64 mbsf). The tops of Lophocyrtis biaurita and Phormocyrtis striata striata occur within this zone. The lowermost sample examined (Sample 342-U1404A-36X-1, 67 cm; 299.80 mbsf) is correlated with Zone RP14 based on the presence of the zonal marker P. mitra.

Planktonic foraminifers

Core catchers and additional samples from working half sections were examined from Hole U1404A. Samples were, for the most part, barren of planktonic foraminifers except for intervals in the lower Miocene and Oligocene. Samples from the lower Miocene through Oligocene yielded rare to few planktonic foraminiferal specimens with moderate to very good preservation. Planktonic foraminifers are of limited use in the development of biostratigraphy but provide some zonal assignments near the Oligocene/Miocene boundary. Depth positions and age estimates of identified biostratigraphic marker events are shown in Table T7. The stratigraphic distribution of planktonic foraminifers is shown in Table T8 and Figure F16. The sporadic occurrence of planktonic foraminifers at Site U1404 (4710 mbsl) is interpreted to be the result of calcite dissolution below or close to the CCD.

Sections 342-U1404A-8H-2 through 11H-2 (64.35–92.55 mbsf) contain reasonably diverse and well-preserved lower Miocene assemblages containing the marker species Globoturborotalita angulisutularis, Globigerinoides altiapertura, Paragloborotalia kugleri, and Globoquadrina dehiscens, indicating Zone M2–Subzone M1a. The base of G. dehiscens marks the Subzone M1a/M1b boundary (Sample 342-U1404A-10H-6, 115–117 cm; 89.36 mbsf). The top and base of P. kugleri occurs in Samples 342-U1404A-9H-3, 115–117 cm (75.36 mbsf), and 11H-2, 100–102 cm (92.54 mbsf), respectively.

Samples 342-U1404A-13H-CC to 21H-5, 129–130 cm (115.07–189.00 mbsf), contain sporadic, moderate to well-preserved planktonic foraminifers of late Oligocene age. However, differentiation of biozones in the upper Oligocene was hindered by the absence of key taxa.

Planktonic foraminifers are absent across the EOT and in the upper and middle Eocene from Samples 342-U1404A-21H-CC to 36H-CC (191.71–299.80 mbsf).

Benthic foraminifers

Benthic foraminifers were examined semiquantitatively in core catcher samples from Hole U1404A and from working section half samples of Cores 342-U1404A-1H to 16H. Additional working section half samples taken from Cores 342-U1404A-21H through 36X were examined for preservation and relative abundance of total benthic foraminifers and individual morphogroups. Benthic foraminifers at this site are rare (the “present” category) relative to total sediment particles >150 μm in the lower Miocene to Oligocene and abundant in the uppermost Eocene (the “dominant” category) (Fig. F16). The abundance of benthic foraminifers decreases from the upper Eocene to lower Eocene. Assemblages generally suggest a lower bathyal to abyssal paleodepth for this site (van Morkhoven et al., 1986). Preservation of foraminifer tests is generally moderate to good and very good in Cores 342-U1404A-7H through 16H (Figs. F16, F18). The occurrences of benthic foraminifers at this site are shown in Tables T9 and T10.

Samples 342-U1404A-1H-3, 73–75 cm, and 1H-CC predominantly contain agglutinated benthic foraminifers, such as Bathysiphon sp. and Cyclammina sp., most probably a result of dissolution below the CCD.

Benthic foraminifer occurrences are rare in Cores 342-U1404A-17H through 23H (153.72–204.53 mbsf). Cores 342-U1404A-8H through 13H (71.30–115.07 mbsf) yield slightly more diverse faunas, including Bathysiphon sp., Dentalina sp., G. pacifica, Gyroidinoides sp., and others. Among these species, G. pacifica, indicative of suboxic bottom water condition, is notably abundant (Fig. F18). Although carbonate content is very low, the preservation of G. pacifica and other constituents of the suboxic assemblage is very good. The occurrence of G. pacifica is accompanied by abundant centric diatoms and higher total organic carbon content, which implies high primary production rate at the sea surface. Enhanced organic matter transport to the seafloor may have contributed to the inferred bottom water suboxia.

The upper Eocene to lower Oligocene is characterized by alternation of barren samples and samples with abundant benthic foraminifers. Abundant benthic foraminifers occur in Samples 342-U1404A-23H-6, 51–53 cm, through 24H-1, 55–56 cm (202.92–204.86 mbsf); 24H-4, 55–56 cm, through 24H-CC (209.36–212.79 mbsf); and 26H-1, 50–51 cm, through 26H-2, 50–51 cm (221.01–222.51 mbsf), with barren intervals in between. Calcareous assemblages of these intervals are mainly characterized by Anomalinoides sp., Cibicidoides spp., Dentalina sp., Nuttallides truempyi, Oridorsalis umbonatus, and Stilostomella lepidula (Table T9). Fluctuations of benthic foraminifer abundances correspond to changes in carbonate content of the sediment and may indicate changes of the late Eocene to early Oligocene CCD.

Although lithostratigraphic Unit IV is predominantly barren of benthic foraminifers, Samples 342-U1404A-31H-CC (265.27 mbsf) and 32H-CC (270.64 mbsf) yield Cibicidoides spp. and N. truempyi.

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