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doi:10.2204/iodp.proc.344.106.2013

Paleontology and biostratigraphy

The microfossil content of core catcher samples from Hole U1380C was examined. Because radiolarians were not present in core catcher samples, biostratigraphic zones were determined by nannofossil content. Benthic foraminifers were used to characterize paleoenvironmental changes at Site U1380.

Calcareous nannofossils

Calcareous nannofossils were observed in 49 of the 52 examined core catcher samples. Three core catcher samples are barren (Samples 344-U1380C-18R-CC, 31R-CC, and 41R-CC). Overall nannofossil preservation is moderate to poor, with abundances ranging from few to rare.

Four biostratigraphic zones were identified. The interval from 439.47 to 460.26 mbsf is assigned to nannofossil Zones NN19–NN21, based on the presence of Gephyrocapsa oceanica in Sample 344-U1380C-2R-CC. The second interval, between Samples 344-U1380C-5R-CC and 8R-CC, is assigned to Zone NN19, based on the presence of Pseudoemiliania lacunosa. The third zone, NN18, is defined by the first downhole occurrence of Discoaster brouweri, which appears in Sample 344-U1380C-9R-CC. Calcidiscus leptoporus, D. brouweri, G. oceanica, P. lacunosa, and Helicosphaera carteri are indicative species of this assemblage (Table T3). A fourth biostratigraphic zone is tentatively proposed and encompasses Samples 344-U1380C-51R-CC and 52R-CC (791–797 mbsf). It is assigned to Zones NN15–NN17, based on the last occurrence of Discoaster pentaradiatus, the first occurrence of P. lacunosa in Sample 344-U1380C-51R-CC, and the first occurrence of Discoaster asymmetricus in Sample 52R-CC. However, as the boundary between Zones NN18 and NN15–NN17 is located between Samples 344-U1380C-50R-CC and 51R-CC, the oldest sediment is assumed to be closer to the upper Zone NN17 boundary (~2.4 Ma). Based on this assumption, the sediment accumulation rate for Zone NN18 (272 mbsf) is estimated to be 590 m/m.y.

Benthic foraminifers

Benthic foraminifers were studied in 44 of the 52 core catcher samples (Table T4). All samples were indurated and treated with H₂O₂ before sieving (see “Paleontology and biostratigraphy” in the “Methods” chapter [Harris et al., 2013b]). Despite the large amount of material examined, the abundance of benthic foraminifers varies from few (Samples 344-U1380-2R-CC through 9R-CC) to present (Samples 31R-CC through 52R-CC). Benthic foraminifers are absent in Sample 344-U1380C-32R-CC. Planktonic foraminifers are rare throughout all samples; therefore, the planktonic to benthic ratio could not be accurately determined.

Preservation ranges from moderate where abundance is high to poor where foraminifers are present or rare (Table T4). Several samples are very poorly preserved. Samples 344-U1380C-10R-CC and 11R-CC show clear signs of carbonate dissolution. Samples 344-U1380C-13R-CC and 14R-CC contain ~95% broken and/or abraded benthic foraminifers, corresponding to Subunit IIA (Fig. F14).

Overall, benthic foraminifer assemblages are characterized by several indicative species, including Uvigerina peregrina, Cibicidoides pachyderma, Epistominella smithi, several species of the genus Brizalina (Table T4), and Hansenisca altiformis. These species are also present in samples from Hole U1381C (see “Paleontology and biostratigraphy” in the “Input Site U1381” chapter [Harris et al., 2013a]). Less common species include Globocassidulina sp., Planulina ornata, Stainforthia complanata, and Sphaeroidina bulloides. Benthic foraminifer percentages were only calculated on samples that contained at least 100 individuals (Fig. F14). However, because of the low abundance and poor preservation of individuals, these percentages should be treated with caution. Benthic foraminifer assemblage changes downhole are relatively subtle. U. peregrina and E. smithi are more frequent in Unit I, whereas C. pachyderma and H. altiformis are more common in Unit II. E. smithi is a species extensively described in oxygen minimum zones around the world (Sen Gupta and Machain-Castillo, 1993), and U. peregrina is known to live in environments with moderate to high organic carbon fluxes at the seafloor (Fontanier et al., 2003). The presence of these species, together with the relative high abundance of Brizalina spp., also typical of high productive settings (Sen Gupta and Machain-Castillo, 1993), suggests an environment rich in organic carbon with periods of low bottom water oxygenation. In the upper part of Unit II, the increase of the epifaunal–shallow infaunal C. pachyderma and the relatively oligotrophic H. altiformis may suggest a change in oxygen levels at the seafloor coupled with less organic carbon. Changes in the water column depth cannot be inferred with confidence from the data reported here because of the low abundance and poor preservation of foraminifers, coupled with the subtle assemblage changes downhole.

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