Previous   |    Next

doi:10.2204/iodp.proc.340.107.2013

Paleontology and biostratigraphy

Core catcher samples at Site U1397 are largely composed of coarse-grained sediment containing low abundances of calcareous nannofossils and planktonic and benthic foraminifers at varying levels of preservation. For some sections, additional nannofossil sampling was carried out from the working half of the core, where fine-grained sediment was available. Where obtainable, biostratigraphic dates are generally nonsequential; however, throughout the site, a trend to older material with depth can be seen (see “Age models”). These factors, accompanied by the presence of poorly preserved shallow-water reef benthic foraminifers and coral fragments in many core catcher samples, suggest persistent reworking of the sediment. Biostratigraphic data derived from both calcareous nannofossils and planktonic foraminifers show that Site U1397 contains many levels of reworked sediment from the early Pleistocene and late Pliocene amid a background of late Pleistocene sedimentation (Fig. F2).

Calcareous nannofossils

Calcareous nannofossils from 21 core catcher samples from Hole U1397A were investigated, along with 18 from Hole U1397B. Generally, investigation of the core material found extreme mixing of late Miocene to early Pliocene sediment within a background of late Pleistocene sedimentation (Samples 340-U1397B-24X-CC, 29X-CC, 31X-CC, and 32X-CC). Because of the coarse nature of the material retrieved in some core catcher samples in Hole U1397A, additional sampling from the working half of the core was carried out (Samples 340-U1397A-10H-2W, 96 cm; 12H-1W, 141 cm; and 12H-3W, 120 cm). In contrast, Hole U1397B core catcher samples were much more suitable for calcareous nannofossil analysis. Preservation of samples is generally good. Samples show a range of abundances, from abundant to common, with one sample completely barren.

Sample 340-U1397A-1H-CC is characterized by an upper Pleistocene species assemblage of Gephyrocapsa oceanica, Calcidiscus leptoporus, Helicosphaera hyalina, Ceratolithus cristatus, and small gephyrocapsid species, such as Gephyrocapsa sinuosa, Gephyrocapsa ericsonii, and Gephyrocapsa parallela. Small forms, such as Emiliania huxleyi, were more abundant than gephyrocapsid species. Thus, this sample was assigned to Zone CN15, with a maximum age of 0.085 Ma (Thierstein et al., 1977; Okada and Bukry, 1980). Samples 340-U1397A-3H-CC to 10H-CC were also assigned to Zone CN15; however, based on the relative decrease in E. huxleyi to gephyrocapsid species, these samples were assigned ages ranging from 0.085 to 0.25 Ma (Okada and Bukry, 1980; Kameo and Bralower, 2000).

Samples 340-U1397A-12H-CC to 35X-CC, including two additional non–core catcher samples (Samples 340-U1397A-12H-1W, 141 cm, and 12H-3W, 120 cm), contain common G. oceanica, Helicosphaera carteri var. wallichii, C. cristatus, C. leptoporus, and G. parallela. Specimens of E. huxleyi were hard to distinguish because of their small size, delicate construction, and moderate preservation. Because of the absence of Pseudoemiliania lacunosa, these samples were assigned to Zone CN14b. Scanning electron microscopy is needed to verify with certainty the presence or absence of E. huxleyi.

At the base of Hole U1397B (Samples 340-U1397B-24X-CC, 29X-CC, 31X-CC, and 32X-CC), the effects of reworking are noticeable, suggesting a number of sediment emplacement episodes. Preservation of the reworked species is moderate to good, which may indicate a source close by. When analyzing these samples, care was taken in determining which species represent the true age of the background sediment. This sediment would have been assigned to Subzone CN13a because of the presence of Crenalithus doronicoides; however, because these specimens are clearly reworked, these dates are unlikely to represent background sedimentation and most likely represent the age of the transported material. Recovery in the remainder of Hole U1397B provided a very similar record to that of Hole U1397A, including abundance and preservation.

Planktonic foraminifers

Of the core catcher samples collected from Hole U1397A, 31 of 35 were analyzed for planktonic foraminiferal content, along with 29 of the 33 collected from Hole U1397B. The remaining cores retrieved no sediment. Planktonic foraminifers were present in most samples, although some were found at very low abundances, possibly because of the high volume of volcanic material (Samples 340-U1397A-20H-CC and 32X-1 and 340-U1397B-4H-CC and 22X-CC). In samples with abundant specimens, the assemblage of planktonic foraminifers was diverse but dominated by Globigerinoides ruber (white and pink) and Globigerinoides sacculifer. Other abundant species include Globigerinoides elongatus, Globorotalia tumida, and Neogloboquadrina dutertrei (dextral). The fauna does not change significantly throughout Site U1397, and all species present are indicative of warm subtropical waters.

Datum species were not found in many of the samples; however, in both holes the datum species Globorotalia flexuosa (0.07–0.40 Ma) and Globigerinella calida (bottom occurrence at 0.22 Ma) were found in low numbers. The presence of these datum species at the top and base of both Holes U1397A and U1397B dates the sediment to younger than 0.4 Ma, within the Pleistocene. Other datum species were encountered toward the base of Hole U1397A (no equivalent datum species were found in Hole U1397B). Globorotalia tosaensis (top occurrence [T] at 0.61 Ma), Globorotalia limbata (T 2.39 Ma), and Globoturborotalia apertura (T 1.63 Ma) were found in Sections 340-U1397A-30X-1 and 32X-1, which suggests the presence of much older transported material. Unfortunately, it is likely that postcruise sampling of the material recovered at this site would not yield better results because of the persistent reworked nature of the retrieved sediment. It should also be noted that this site is nearly past the resolution of foraminiferal biostratigraphy, as there are only two true datum species located within the roughly 240 m of sediment.

Benthic foraminifers

A total of 40 genera and 33 species were identified at Site U1397 in the >150 µm size fraction. Benthic foraminifers from Holes U1397A and U1397B varied in abundance, diversity, and preservation (moderate). Fourteen species are present in Hole U1397A and 26 in Hole U1397B. Rotaliids have low diversity and were present in low abundances (1–10 specimens per sample) overall in Holes U1397A and U1397B. Numerous samples (28) are dominated by Amphistegina (1–30 specimens per sample). This genus is common in reef environments (≤100 m depth), and their moderate to poor preservation is indicative of reworking associated with volcaniclastic sedimentation in the area. Buliminids are practically absent, except for Samples 340-U1397B-24X-CC, 25X-CC, and 29X-CC, which have low abundances (1–10 individuals per sample) overall. Similarly, miliolids are nearly absent in most samples with the exception of Quinqueloculina, which is dominant in Hole U1397A, and Pyrgo in Hole U1397B. Agglutinated foraminifers are extremely rare, with only two genera (Ammobaculites and Vulvulina) and one species (Vulvulina pennatula) present in one sample (340-U1397A-1H-CC). Benthic foraminiferal density is generally low at Site U1397, ranging between 1 and 63 foraminifers/g of sediment.

At Site U1397, Orthomorphina perversa, Pleurostomella alternans, Proxifrons inaequalis, Siphonodosaria cooperensis, Stilostomella pomuligera, and V. pennatula are present in relatively low abundances (1–10 specimens per sample) in most samples. S. cooperensis has a low abundance (1–10 specimens per sample) in Samples 340-U1397B-25X-CC and 27X-CC, whereas S. pomuligera has a moderate abundance (1–30 specimens per sample) in Sample 25X-CC. Based on the presence and persistence of the above group, a bathyal paleodepth is interpreted.

Age models

Three age models have been constructed from several biostratigraphic datums and the presence of a particular clast of volcanogenic origin that can be tied to the Piton du Carbet Volcano from ~330 to 350 ka. Figures F3 and F4 represent the most likely estimates for ages from Holes U1397A and U1397B, respectively, using only robust biostratigraphic information. To construct these age models, any age relying on the top occurrence or absence of a fossil species has been removed, again because of the persistent and prolific reworking of sediment. This leaves only a few datums from which to construct a crude age model. This has been done using the planktonic foraminifers G. calida and G. flexuosa and the coccolithophorid E. huxleyi. The age models (biostratigraphic data only) for Holes U1397A and U1397B include the top of G. flexuosa (0.07 Ma); however, it has been removed from consideration from the composite age model (see below).

Figure F5 represents a composite age model, using biostratigraphic data from Holes U1397A and U1397B (red line) and the presence of volcanic clasts from Piton du Carbet within Hole U1397B (dark blue line, see “Lithostratigraphy” for further discussion). The resulting age model suggests an age younger than 220 ka for the top ~25 mbsf, younger than 250 ka from ~50 to ~100 mbsf, younger than ~350 ka from 100 m to nearly the base, and 400 ka in the bottom core catcher sample. It should be noted that these ages are only a maximum age estimate and that a minimum age estimate is inappropriate given the prolific reworking of sediment.

Top of page   |    Previous   |    Next