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doi:10.2204/iodp.proc.340.106.2013

Paleontology and biostratigraphy

Core catcher samples at Site U1396 contain calcareous nannofossils and planktonic and benthic foraminifers of varying abundances and at varying levels of preservation. Preservation deteriorates with depth; however, it remains adequate for confident biostratigraphic age assignment. Both nannofossil and planktonic foraminiferal biostratigraphic data and magnetostratigraphic dates for Site U1396 indicate ages ranging from late Pleistocene to early Pliocene (Fig. F3). Sedimentation rate appears to increase with depth (see “Paleomagnetism” for further discussion). Sponge spicules and ostracods are scarce and generally moderately preserved. Pteropods and heteropods were not found in any core catcher samples.

Calcareous nannofossils

Nannofossil preservation was good to moderate throughout the 13 core catcher samples analyzed from Hole U1396A (of 15 collected), one sample from Hole U1396B, and 13 samples from Hole U1396C (of 15 collected). The remaining core catcher samples were not analyzed because of the coarse nature of the material collected.

Sample 340-U1396A-1H-CC contains abundant Gephyrocapsa caribbeanica, common Helicosphaera hyalina, and small forms (2.5–3 µm) that may be Emiliania huxleyi. The presence of E. huxleyi was further substantiated by the absence of Pseudoemiliania lacunosa. Thus, this sample lies within the E. huxleyi Zone (Zone CN15; Okada and Bukry, 1980). The age of this sample is younger than 0.25 Ma (Kameo and Bralower, 2000).

Samples 340-U1396A-2H-CC to 4H-CC yielded a variety of small Gephyrocapsa species, P. lacunosa, and Crenalithus doronicoides. The latter species defines Zone CN13 (Okada and Bukry, 1980). G. caribbeanica is abundant throughout these three core catcher samples; therefore, these samples are within Subzone CN13b (Okada and Bukry, 1980). Moreover, Calcidiscus macintyrei is common in Sample 340-U1396A-4H-CC. Gartner (1977) has used this species to divide the Quaternary. Herein, Gartner’s methodology has been followed using the revised zonation for this sample. Sample 340-U1396A-4H-CC lies within Subzone CN13b (Okada and Bukry, 1980) or the C. macintyrei Zone (Gartner, 1977). Overall preservation is good to moderate.

Sample 340-U1396A-5H-CC yielded a characteristic Pliocene assemblage with abundant Discoaster brouweri and common C. macintyrei. The D. brouweri are extremely affected by overgrowth; however, identification was possible. Otherwise, the sample contained abundant specimens with good preservation. C. macintyrei and D. brouweri are indicative of an upper Pliocene assemblage, placing this sample within Subzone CN12d. Sample 340-U1396A-6H-CC contains an abundant and well-preserved Pliocene assemblage with Discoaster surculus, D. brouweri, C. macintyrei, and Hayaster perplexus. Generally, Discoaster species were well preserved in comparison to Sample 340-U1396A-5H-CC. Because of the absence of Discoaster tamalis, Sample 340-U1396A-6H-CC was placed within Subzone CN12b (Okada and Bukry, 1980). Samples 340-U1396A-7H-CC to 10H-CC yielded a characteristic lower Pliocene assemblage with a variety of Discoaster species. D. pentaradiatus, D. surculus, D. brouweri, Discoaster asymmetricus, and D. tamalis are common. Discoaster variabilis and Discoaster challengeri are generally less abundant (few to none). The preservation, especially of Discoaster specimens, in this sample is good except for Ceratolithus rugosus, which is heavily overgrown, destroying the general morphology. Because neither Sphenolithus abies/neoabies nor Reticulofenestra pseudoumbilica were detected, this sample was assigned to Subzone CN12a.

Samples 340-U1396A-11H-CC to 15H-CC contain S. abies/neoabies and R. pseudoumbilica, which indicate lower Pliocene Subzone CN11a.

In Hole U1396B, zones, subzones, and ages similar to those in Hole U1396A were identified. The top core catcher samples are placed in the Quaternary, whereas the bottom samples (340-U1396B-6H-CC to 15H-CC) show trends similar to those in Hole U1396A, indicating an age of late to early Pliocene. Hole U1396B recovered only one core catcher sample (340-U1396B-2H-CC), which yielded an upper Pleistocene assemblage with G. caribbeanica, G. oceanica, and H. hyalina. Sample 340-U1396B-2H-CC was assigned to Subzone CN14a (Okada and Bukry, 1980).

Planktonic foraminifers

All 31 core catcher samples from Site U1396 were analyzed for foraminiferal content. Only two samples, 340-U1396C-1H-CC and 2H-CC, did not suggest a reliable age because they lacked marker taxa. Three samples, 340-U1396A-12H-CC and 13H-CC and 340-U1396C-13H-CC, had extremely low foraminiferal content because of an influx of volcanic material. Samples 340-U1396A-13H-CC and 340-U1396C-13H-CC can still, however, be dated to the Sphaeroidinellopsis seminulina Zone (PL3), as they contain S. seminulina. Species abundances in the Pleistocene generally conform to the previous sites (U1394 and U1395), with Globigerinoides ruber and Globigerinoides sacculifer being the two most dominant species. Throughout the Pliocene these abundances shift, with a distinct increase in the keeled genus Globorotalia (specifically: Globorotalia exilis, Globorotalia pertenuis, Globorotalia menardii, and Globorotalia miocenica) becoming more common.

Every planktonic foraminiferal biozone and subzone from the Quaternary to the lower Pliocene was recorded (Zones PT1b, PT1a, PL6, PL5, PL4, PL3, PL2, and PL1). Several differences exist between the two main holes, U1396A and U1396C. Hole U1396A does not appear to capture the 30,000 y long Zone PL4 (Dentoglobigerina altispira Highest Occurrence Zone [HOZ]), although it was identified in Hole U1396C. Postcruise analysis of Hole U1396A may still identify this zone, likely within Core 340-U1396A-10H. Additionally, Zone PL1 (Globorotalia tumida/Globoturborotalia nepenthes Concurrent Range Zone (CRZ) [Atlantic]) was identified at the base of Hole U1396A (Sample 340-U1396A-15H-CC) but not in Hole U1396C. Again, further analysis of Core 340-U1396C-15H may reveal the presence of G. nepenthes, which would indicate Zone PL1. Core catcher Sample 340-U1396C-3H-CC contains both Globorotalia tosaensis and Globorotalia flexuosa, which have conflicting and nonconcurrent ranges. The missing 21,000 y interval between when G. flexuosa goes extinct and G. tosaensis originates appears to have been the victim of time averaging, either through bioturbation, drilling, or some other mechanism. The primary marker between Subzone PT1a and Zone PL6 (Globigerinoides fistulosus) was not recorded at the expected depth, above the base of Globorotalia truncatulinoides. G. fistulosus was recorded in only three samples (340-U1396A-3H-CC and 340-U1396C-5H-CC and 6H-CC). Zone PL6 was recognized, instead, on the base of G. truncatulinoides, the first secondary datum within Zone PL6. Many different datums were recorded: G. flexuosa (0.07–0.40 Ma), G. tosaensis (top occurrence [T] at 0.61 Ma), G. exilis (T 2.1 Ma), Globigerinoides extremus (T 2.1 Ma), G. miocenica (T 2.39 Ma), G. pertenius (T 2.60 Ma), Globorotalia multicamerata (T 2.99 Ma), S. seminulina (T 3.16 Ma), Pulleniatina primalis (T 3.65 Ma), Globorotalia margaritae (T 3.84 Ma), and G. nepenthes (T 4.36 Ma). Although several more first appearance datums (B) are calibrated through this interval as secondary markers, when compared against the datums listed above they appear to be largely unreliable as age determinations at this site and were not used.

Benthic foraminifers

A total of 41 genera and 44 species were identified at Site U1396 in the >150 µm size fraction. Benthic foraminifers examined in Holes U1396A, U1396B, and U1396C varied in abundance, diversity, and preservation (moderate). Hole U1396A has the greatest number of species (40), followed by Hole U1396B (25) and then Hole U1396C (4). Rotaliids have low diversity and are present in low abundances (1–10 specimens per sample) overall in Holes U1396A and U1396C. Cibicides and Cibicidoides are the dominant genera in Hole U1396A but are nearly absent in Holes U1396B and U1396C. Cibicides wuellerstorfi is the most abundant species in Hole U1396A. Similarly, buliminids have low diversity with low abundances (1–10 specimens per sample) overall in Holes U1396A and U1396C. However, buliminids dominate the assemblage in Samples 340-U1396A-4H-CC, 5H-CC, 8H-CC, and 9H-CC and 340-U1396C-3H-CC, 4H-CC, and 8H-CC. Miliolids are practically absent in most samples, with the exception of Pyrgo murrhina, the dominant species. Agglutinated foraminifers are represented by the genera Sigmoilopsis, Bigenerina, and Vulvulina, with Karreriella bradyi as the dominant species. Benthic foraminiferal density is generally low at Site U1396, ranging between 1 and 67 foraminifers/g of sediment. However, Samples 340-U1396A-5H-CC and 10H-CC and 340-U1396C-4H-CC and 13H-CC have densities of 214, 111, 156, and 113 foraminifers/g of sediment, respectively.

At Site U1396, Myllostomella costai, Myllostomella huygaensis, Orthomorphina jedlitschkai, Orthomorphina perversa, Pleurostomella alternans, Proxifrons inaequalis, Siphonodosaria cooperensis, Siphonodosaria insecta, Siphonodosaria pomuligera, Siphonodosaria sargrinensis, Stilostomella fistuca, and Vulvulina pennatula are present in relative low abundances (1–10 specimens per sample) in most samples. This group has a last appearance datum (T) at 0.58 Ma (Hayward et al., 2006). S. cooperensis has a moderate abundance (1–30 specimens per sample) in Samples 340-U1396A-4H-CC, 5H-CC, and 6H-CC. Based on the presence and persistence of the above group, a bathyal paleodepth is interpreted.

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