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doi:10.2204/iodp.proc.344.103.2013

Paleontology and biostratigraphy

Calcareous nannofossil and radiolarian assemblages observed at Site U1381 provide biostratigraphic information for the cored sediment. Benthic foraminifer assemblages are used to characterize paleoenvironmental changes throughout the depositional history of the cored sediment.

Calcareous nannofossils

Two main sequences have been identified based on nannofossil biostratigraphy. The first is a Pleistocene to recent sequence that corresponds to Unit I. The second is a Miocene sequence that corresponds to Unit II. The hiatus between Units I and II is tentatively estimated to be ~11 m.y.

Nannofossils were observed in all core catcher samples from Cores 344-U1381C-1H through 12H. Overall preservation is good to moderate. The interval from 0 to 18.11 mbsf is assigned to nannofossil Zone NN21, based on the presence of Emiliana huxleyi in Sample 344-U1381C-2H-CC. Calcidiscus leptoporus, Gephyrocapsa oceanica, Gephyrocapsa caribbeanica, and Helicosphera carteri are species indicative of this assemblage. The presence of the planktonic foraminifer Globigerinoides ruber (pink) in this sample indicates that this section is older than 120 ka (last occurrence [LO] of G. ruber [pink]) and younger than 290 ka (first occurrence of E. huxleyi). A single Discoaster sp. specimen identified in this sample suggests reworking of older material. The interval from 18.11 to 46.62 mbsf is constrained to Zones NN19–NN21, based on the presence of C. leptoporus, G. oceanica, G. caribbeanica, and H. carteri and the absence of E. huxleyi.

The interval from 46.62 to 56.13 mbsf is tentatively assigned to nannofossil Zone NN19, based on the occurrence of Pseudoemiliania lacunosa in Sample 344-U1381C-6H-CC. Further biostratigraphic data for this sample are provided by benthic foraminifers. The extinction of the Stilostomella group has been assigned to the late early and middle Pleistocene (~1.2–0.6 Ma; Hayward, 2002). Sample 344-U1381C-6H-CC contains the LO of representatives of this group (Table T3), suggesting that the minimum age of the sediment is >0.6 Ma and the maximum age of the sediment, just above the lithologic boundary between Units I and II, is 1.89 Ma. However, according to the sediment accumulation rate estimated for Unit I (80–100 m/m.y.), the lower interval may be closer to 1 Ma.

A major change in nannofossil assemblage occurs between Samples 344-U1381C-6H-CC and 7H-CC, represented by the disappearance of the genus Gephyrocapsa and the appearance of Sphenolithus. Samples 344-U1381C-7H-CC through 12H-CC are assigned to Zone NN5 (13.53–14.91 Ma), based on the presence of Sphenolithus heteromorphus, Sphenolithus moriformis, Coccolithus miopelagicus, Discoaster exilis, and Discoaster deflandrei.

In order to better estimate the hiatus between Units I and II, 10 samples were taken across the lithologic boundary. Preliminary results indicate that a 60 cm zone of reworked material with a mixed assemblage is present across the unit boundary, consistent with a zone of bioturbation (see “Lithostratigraphy and petrology”). The lowermost sediments of Unit I contain similar assemblages to those found in Sample 344-U1381C-6H-CC and are indicative of Zone NN19. However, a number of genera, such as Discoaster spp. and some radiolarians (Didymocyrtis antepenultima and Didymocyrtis laticonus), are also found in these samples and indicate reworking of older sediments. The uppermost portion of Unit II is also heavily bioturbated, but the undisturbed (Zone NN5) pelagic sedimentation resumes 30 cm below the boundary. A hiatus of ~11 m.y. is recorded between Units I and II.

Radiolarians

Radiolarians were present in all recovered cores with the exception of Samples 344-U1381C-11H-CC and 12H-CC. Overall, the siliceous fraction was dominated by radiolarians and diatoms, with a minor proportion of sponge spicules. Radiolarians were generally well preserved.

Observations of radiolarians indicate two distinct assemblages in Hole U1381C (Fig. F15; Table T4). The upper assemblage (Samples 344-U1381C-1H-CC through 6H-CC) represents a Pleistocene–Holocene sequence that corresponds to Unit I. Detailed biostratigraphic zonations could not be assigned to this assemblage because of the lack of biostratigraphically important species. However, the presence of Amphirhopalum ypsilon in Samples 344-U1381C-1H-CC through 4H-CC suggests a depositional age <1 Ma (Zone RN14; Sanfilippo and Nigrini, 1998).

The lower assemblage (Samples 344-U1381C-7H-CC through 10H-CC) is representative of the early to middle Miocene and corresponds to Unit II. The presence of D. antepenultima and D. laticonus in Samples 344-U1381C-7H-CC and 8H-CC is indicative of the early late Miocene (Zones RN6 and RN5). Samples 344-U1381C-9H-CC and 10H-CC are constrained to the late early Miocene (Zone RN5) because of the presence of Didymocyrtis violina (Sanfilippo and Nigrini, 1998). Radiolarian biostratigraphy suggests a hiatus of ~9 to 11 m.y. between Units I and II, consistent with the hiatus seen in nannofossil biostratigraphy. Using the estimates from nannofossil and radiolarian biostratigraphy, a range of 9–11 m.y. is proposed for the duration of the hiatus.

Benthic foraminifers

Benthic foraminifers (B) were studied in core catcher samples from Cores 344-U1381C-1H through 11H (Table T5). All samples, with the exception of Sample 344-U1381C-11H-CC, which is well indurated, consist of loose sediment dominated by planktonic foraminifers (P) (P/[P + B] > 0.8). Benthic foraminifer assemblages indicate that two different environments are represented in Hole U1381C, as also indicated by biostratigraphic and lithostratigraphic data (Fig. F16).

The upper interval (Samples 344-U1381C-1H-CC through 6H-CC) corresponds to Unit I and is dominated by species of the genera Uvigerina (Uvigerina peregrina, Uvigerina auberiana, and Uvigerina cf. senticosa) and Siphouvigerina (Siphouvigerina ampullacea), with minor relative abundances of Chilostomella oolina, Cibicidoides pachyderma, and Fontbotia wuellerstorfi. This assemblage is characteristic of bathyal environments with a moderate to high flux of organic carbon to the seafloor (Gooday and Rathburn, 1999).

The lower interval (Samples 344-U1381C-7H-CC through 11H-CC) contains a substantially different assemblage than Unit I. Characteristic species found in this interval include C. oolina, F. wuellerstorfi, Uvigerina sp., Gyroidinoides sp., and Globocassidulina subglobosa, together with a diverse group of elongated foraminifers belonging to genera such as Stilostomella, Plectofrondicularia, Pleurostomella, Siphonodosaria, and Crysalogonium. The presence of species that occupy a wide microhabitat range (F. wuellerstorfi, C. pachyderma, and C. oolina) might suggest a bathyal environment with low to moderate flux of organic carbon and suboxic to anoxic conditions present at the time the deeper sediment layers were deposited (Gooday and Rathburn, 1999; Corliss, 1985).

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