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doi:10.2204/iodp.proc.340.110.2013

Paleontology and biostratigraphy

Core catcher samples at Site U1400 contain calcareous nannofossils and planktonic and benthic foraminifers of varying abundances. The consistency of the sediment within the core catcher samples varied between loose and highly indurated, particularly at the base of Hole U1400C. Calcareous nannofossil and planktonic foraminiferal data both suggest late Pleistocene ages (Fig. F3) for the majority of the samples. However, near or at the base of each hole, species indicative of older material are present (see Fig. F4 for a summary). Major to minor reworking is evident throughout Site U1400. Because of both the reworking and the presence of substantial mass-flow deposits, a calculated sedimentation rate would be grossly misleading.

Calcareous nannofossils

Samples from Site U1400 are mainly composed of late Pleistocene sediment, changing to late Pliocene sediment at the base of Hole U1400C. Only five core catcher samples from Hole U1400A were analyzed for nannofossils because of the coarse nature of the material collected. For Holes U1400B and U1400C, the sediment collected was generally finer grained. A total of 20 core catcher samples were analyzed for Hole U1400B, and 41 core catcher samples were analyzed for Hole U1400C. In Holes U1400A and U1400B, the uppermost core catcher samples (340-U1400A-1H-CC and 2H-CC and 340-U1400B-2H-CC) contained common Gephyrocapsa sp., but >90% of the assemblage consisted of small 2–3 µm forms presumed to be Emiliania huxleyi, which would indicate an age younger than 84 ka within Zone CN15 (Okada and Bukry, 1980). Reworked discoasters and Sphenolithus abies/neoabies species were observed in Sample 340-U1400A-2H-CC. Specimens within the genus Discoaster are heavily overgrown and could not be identified to species level. However, Sample 340-U1400A-7H-CC contains a variety of reworked discoaster species such as Discoaster surculus, Discoaster brouweri, and Reticulofenestra umbilicus, along with late Pleistocene species. In general, background sedimentation in Hole U1400A was assigned to Zone CN15 (Okada and Bukry, 1980).

Core catcher samples (340-U1400B-2H-CC and 28H-CC) from Hole U1400B contain abundant E. huxleyi, Gephyrocapsa parallela, Gephyrocapsa oceanica, Ceratolithus cristatus, and small Gephyrocapsa species (<2.5 µm). These samples were assigned to Zone CN15 (Okada and Bukry, 1980). Pseudoemiliania lacunosa was common in both Holes U1400A and U1400B; however, they were observed to be reworked. Hole U1400C is similar to Hole U1400B in terms of biostratigraphic nannofossil zonation. Because of the high occurrence (abundant to few) of E. huxleyi, Gephyrocapsa sp., C. cristatus, Ceratolithus telesmus, and Ceratolithus simplex, Samples 340-U1400C-2H-CC to 36X-CC were assigned to Zone CN15. At the bottom of Hole U1400C, many samples did not provide reliable age markers because of poor preservation. Gephyrocapsa sp. and E. huxleyi are largely absent in Samples 340-U1400C-39X-CC, 40X-CC, 41X-CC, 43X-CC, and 44X-CC. However, Samples 340-U1400C-45X-CC to 49X-CC yielded a characteristic upper Pliocene assemblage of D. brouweri, D. surculus, Calcidiscus macintyrei, and Discoaster asymmetricus. These samples correlate with Zone CN12 (Kameo and Bralower, 2000).

Planktonic foraminifers

All core catcher samples from Holes U1400A, U1400B, and U1400C (9, 28, and 49 samples, respectively) were analyzed for planktonic foraminiferal content. Planktonic foraminifers are present in most of the analyzed samples but are absent in Samples 340-U1400A-6H-CC, 340-U1400B-4H-CC and 9H-CC, and 340-U1400C-9HCC, 24X-CC, and 30X-CC, possibly because of the high volume of volcanic material in these samples. In samples with abundant specimens, the assemblage of planktonic foraminifers is diverse but dominated by Globigerinoides ruber (white and pink), Globigerinoides sacculifer, and Neogloboquadrina dutertrei (dextral). Other abundant species include Pulleniatina obliquiloculata and Sphaeroidinellopsis dehiscens. In all holes, the population changes toward the base, reflecting a likely Pliocene assemblage. This change is most obvious in Hole U1400C; in Sample 340-U1400C-36X-CC, P. obliquiloculata is reduced in number, and absent in Sample 38X-CC, whereas Globorotalia miocenica and Globorotalia exilis become more abundant. All these changes reflect an upper Pliocene rather than a lower Pleistocene foraminiferal assemblage. All species present are indicative of warm subtropical waters.

Several datum species were found in all three holes. The most frequently encountered datum species was Globigerinella calida (bottom occurrence at 0.22 Ma), which was found in low numbers throughout most of each hole. A further late Pleistocene marker, Globorotalia flexuosa (0.07–0.40 Ma), was only found in three samples (340-U1400B-22H-CC and 340-U1400C-19H-CC and 20H-CC). The co-occurrence of Globorotalia tosaensis (top occurrence [T] at 0.61 Ma), which does not have an overlapping range with either G. calida or G. flexuosa, indicates reworking through a large portion of the late Pleistocene sediments. At the base of Hole U1400A (Sample 340-U1400A-7H-CC), Hole U1400B (Sample 340-U1400B-28H-CC), and Hole U1400C (Samples 340-U1400C-33X-CC to 49X-CC), G. miocenica (T 2.39 Ma) is abundant and, without any conflict from a younger species, most likely reflects the true age of sediments. This suggests a large gap of >2 m.y. Other late Pliocene markers present include Globigerinoides extremus (T 1.99 Ma) and G. exilis (T 2.10 Ma), further corroborating the PL5 zonation. The early Pliocene marker species Globorotalia cibaoensis (T 4.60 Ma) was also found in low numbers throughout the late Pliocene sediments in Hole U1400C, though it appears to be reworked.

Figure F4 summarizes the occurrences of important planktonic foraminifer species and highlights the extent to which reworking was apparent within the three holes. It should be noted that in Hole U1400C, the bottom contact of the reworked G. tosaensis is not well defined, as both preservation and abundance deteriorate for ~40 m below its lowest occurrence and no specimens are found within those samples. This may help to explain the seemingly impossible relationship of G. tosaenesis and G. flexuosa between Holes U1400B and U1400C.

Benthic foraminifers

A total of 31 genera and 26 species were identified at Site U1400 in the >150 µm size fraction. Benthic foraminifers examined in Holes U1400A, U1400B, and U1400C have very low abundances. Preservation varies from poor to very good. Ten species are present in Hole U1400A, fourteen in Hole U1400B, and sixteen in Hole U1400C. Overall, the abundance of specimens per species is very low (1–5 per sample) in most samples. Rotaliids have a very low diversity mainly represented by Amphistegina, Cibicides wuellerstorfi, Melonis sp., and Osangularia sp. Miliolids are present; Pyrgo murrhina and Quinqueloculina granulocostata are the more dominant species in all holes. Agglutinated foraminifers are extremely rare, with only Sigmoilopsis sp. present, and only in seven samples. Benthic foraminiferal abundance is low at Site U1400, ranging between 1 and 16 foraminifers/g of sediment with the exception of Sample 340-U1400B-7H-CC, which has an abundance of 33 foraminifers/g of sediment.

Pleurostomella alternans, Siphonodosaria cooperensis, Siphonodosaria sargrinensis, and Siphonodosaria pomuligera are present in very low abundances (1 specimen per sample) throughout Site U1400. Samples 340-U1400B-8H-CC, 14H-CC, 18H-CC, and 20H-CC contain relative higher abundances of S. cooperensis (≤10 individuals per sample). The top for this foraminiferal group is 0.58 Ma (Hayward et al., 2006).

The very low abundance of indicative species like C. wuellerstorfi, P. murrhina, and Osangularia sp. and the absence of other abyssal key taxa make it difficult to provide a reliable paleodepth estimate.

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