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doi:10.2204/iodp.proc.340.108.2013

Paleontology and biostratigraphy

Core catcher samples at Site U1398 contain calcareous nannofossils and planktonic and benthic foraminifers of varying abundances and at varying levels of preservation. Calcareous nannofossil and planktonic foraminiferal data both indicate ages within the late Pleistocene (Fig. F2), which suggests extremely high sedimentation rates (see “Planktonic foraminifers” for a discussion). Reworking of much older (early Pleistocene and late Pliocene) material is evident in several samples. Many of the core catcher samples at Site U1398 consist of very coarse grained material that contains numerous shallow-water benthic foraminifers and fragments of shell and coral. Well-preserved pteropod and heteropod shells, otoliths, and sponge spicules (Demospongiae) were also found in some hemipelagic samples.

Calcareous nannofossils

Nannofossils are moderately to well preserved throughout Site U1398, declining near the base. Throughout the entire sequence, species characteristic of the late Pleistocene were found. In both Holes U1398A and U1398B, Emiliania huxleyi, Gephyrocapsa oceanica, Gephyrocapsa caribbeanica, Gephyrocapsa parallela, Ceratolithus cristatus, and Ceratolithus telesmus were observed. Thus, the entire sequence was placed in Zone CN15, which has a maximum age of 0.25 Ma (Okada and Bukry, 1980). The presence of late Miocene to early Pliocene species in Samples 340-U1398A-14X-CC, 20X-CC, 21X-CC, 24X-CC, 7H-CC, 9H-CC, 15H-CC, 26H-CC, 32X-CC, and 34X-CC suggests extensive reworking.

In Hole U1398A, E. huxlyei is absent in Samples 340-U1398A-21X-CC to 24X-CC, whereas in Hole U1398B it is only absent in Sample 340-U1398B-7H-CC. Consequently, these samples were placed within Zone CN14 (Okada and Bukry, 1980). Younger sediment (Zone CN15) was found below this sample; it should be considered reworked.

Planktonic foraminifers

Of the 30 core catcher samples from Hole U1398A, 29 were analyzed for planktonic foraminiferal content, along with 33 of the 34 core catcher samples from Hole U1398B. The remaining cores (Samples 340-U1398A-16X-CC and 340-U1398B-25H-CC) did not retrieve any material. Planktonic foraminifers were present in all samples, although some were found at very low abundances, possibly because of the high volume of volcanic material. In samples with abundant specimens, the assemblage of planktonic foraminifers was diverse but dominated by Globigerinoides ruber (white and pink), Globigerinoides sacculifer, and Neogloboquadrina dutertrei (dextral). Other abundant species include Globorotalia truncatulinoides and Globorotalia tumida. The fauna does not change significantly throughout Site U1398, and all species present are indicative of warm subtropical waters.

Several datum species were found in both Holes U1398A and U1398B; however, datum species were generally not found in samples with low planktonic foraminiferal abundance. Globorotalia flexuosa (0.07–0.40 Ma) and Globigerinella calida (bottom occurrence at 0.22 Ma) were found in low numbers in both holes. The presence of G. calida at the base of both Holes U1398A and U1398B dates the sediments to younger than 0.22 Ma, within the Pleistocene. The presence of G. flexuosa suggests a minimum age of 0.07 Ma; however, this estimate should be considered very weak because of the reworking of this sediment. The weakness of this estimate makes a definitive minimum age inappropriate, especially as specimens of G. flexuosa are largely, but not completely, confined to intervals with turbidites. Counter to the prediction based on species ranges, G. calida extends past the range of G. flexuosa. This suggests either error in the calibration of G. calida, or a regional difference in the occurrence of either G. calida or G. flexuosa. A regional difference in G. flexuosa is perhaps the most likely explanation, as there is good agreement between the nannofossil biostratigraphy and the range of G. calida. However, paleomagnetic ages (see “Paleomagnetism”) do not agree with this biostratigraphic zonation.

The maximum biostratigraphic age suggests that sedimentation rates are much higher than originally anticipated. However, because of extensive reworking throughout this site and the small size of most G. calida specimens, further investigation will be needed to confirm this age. Several reworked coarse-grained samples from Holes U1398A and U1398B also contain datum species from the early Pleistocene and late Pliocene. Heavily abraded individuals of Globorotalia tosaensis (top occurrence [T] at 0.61 Ma) were found in Samples 340-U1398A-19X-CC and 23X-CC, whereas Globorotalia exilis (T 2.10 Ma) and Globorotalia multicamerata (T 2.99 Ma) were found in Samples 340-U1398B-7H-CC and 10H-CC, respectively. Because G. exilis and G. multicamerata unexpectedly co-occur here with G. flexuosa despite having nonconcurrent ranges, they cannot indicate the true age of the sediment but instead might indicate an age for the sediment source of the mass flows between ~43 to ~58 mbsf and 70 to ~74 mbsf in Hole U1398B.

Benthic foraminifers

A total of 35 genera and 32 species were identified at Site U1398 in the >150 µm size fraction. Benthic foraminifers examined in Holes U1398A and U1398B varied in abundance, diversity, and preservation (poor to moderate). Twenty-five species are present in Hole U1398A and sixteen in Hole U1398B. Rotaliids have low diversity and are present in low abundances (1–10 specimens per sample) overall in Holes U1398A and U1398B. Several samples (19) contain noticeable amounts of Amphistegina (1–30 specimens per sample) and Cibicides wuellerstorfi (1–30 specimens per sample). Amphistegina sp. is common in reef environments (≤100 m depth), and their poor preservation is indicative of reworking consistent with the volcaniclastic sedimentation in the area. Buliminids were nearly absent except for Samples 340-U1398A-25X-CC, 28X-CC, 30X-CC, and 34X-CC, which contain overall low abundances (1–10 specimens per sample). Sample 340-U1398A-25X-CC is dominated by Uvigerina peregrina (110 specimens) and U. peregrina f. parvula (14 specimens). Similarly, miliolids are nearly absent in most samples, with the exception that Pyrgo is the dominant genus in Holes U1398A and U1398B and Pyrgo murrhina is the dominant species. Agglutinated foraminifers are extremely rare, with only two genera (Bigenerina and Sigmoilopsis) present. Benthic foraminiferal density is generally low at Site U1398, ranging between 1 and 60 foraminifers/g of sediment, with the exception of Sample 340-U1398A-25X-CC, which has a density of 240 foraminifers/g of sediment.

At Site U1398, Siphonodosaria cooperensis, Siphonodosaria sargrinensis, and Siphonodosaria pomuligera are present in relatively low abundances (1–10 specimens per sample) in four samples. Single specimens of S. cooperensis are present in Samples 340-U1398A-25X-CC, 28X-CC, and 30X-CC, whereas S. sargrinensis and S. pomuligera are present in Sample 340-U1398B-34X-CC in low abundances (1–10 specimens per sample). The lack of dominant species in most samples makes it challenging to provide a robust paleodepth estimate. However, C. wuellerstorfi is common in middle–lower bathyal to abyssal settings and P. murrhina is common in middle to lower bathyal areas in the Grenada Basin (Galluzzo et al., 1990). Based on their very similar relative abundances and lack of other abyssal key taxa, a bathyal paleodepth is interpreted.

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