Sediments recovered from Site 1176 provide a sedimentary record from the Quaternary (Subzone NN21b) to the Pliocene. Calcareous nannofossils were used for developing the biostratigraphic framework according to the zonation schemes of Gartner (1977) and Martini (1971) with zonal modifications proposed by Young (1998) (Table T8). The interval (core and section) and depth (mbsf) constraints of calcareous nannofossil events recognized at Site 1176 are listed in Table T9. For the range of calcareous nannofossils at Site 1176, see Table T10.
Hole 1176A was cored to 440.35 mbsf recovering Quaternary (Subzone NN21b) to Pliocene nannofossil assemblages (Zone NN15?). Slump deposits within the Quaternary and common reworking of nannofossils in the Pliocene sediments may lead to misplaced first and last occurrences.
Sediments recovered from 7.4 to 286.37 mbsf (Samples 190-1176A-1H-CC to 32X-CC) yield Pleistocene nannofossil assemblages. Below an interval from Samples 190-1176A-1H-CC to 25X-CC yielding abundant and well-preserved nannofossils, there is a sharp decrease in abundance alongside a decline to poorer preservation. Small placoliths such as Emiliania huxleyi and Gephyrocapsa spp. dominate the assemblage. The composition of Pleistocene assemblages older than 0.26 Ma is characterized by the dominance of gephyrocapsids. Reworked Neogene taxa such as discoasterids, Reticulofenestra pseudoumbilicus, and Sphenolithus spp. were encountered sporadically throughout the Pleistocene samples. Based on counts of 300 specimens per sample, the onset of the E. huxleyi acme Subzone NN21b (0.085 Ma) could be determined between Samples 190-1176A-1H-CC and 2H-CC. A further event used to subdivide Subzone NN21a was the last occurrence of Helicosphaera inversa (0.14 Ma) between Samples 190-1176A-6H-CC and 7H-CC. The base of Subzone NN21a, marked by the first occurrence of E. huxleyi (0.26 Ma), was observed between Samples 190-1176A-9H-CC and 10H-3, 89-90 cm. Sample 190-1176A-10H-3, 89-90 cm, is placed in Zone NN20 because of the absence of E. huxleyi and Pseudoemiliania lacunosa. The following event recognized was the last occurrence of P. lacunosa that marks the top of Zone NN19 between Samples 190-1176A-10H-3, 89-90 cm, and 10H-CC. Because P. lacunosa is excellently preserved, reworking is not considered here. Samples 190-1176A-11H-4, 78-79 cm, to 12H-4, 75-76 cm, which are barren of P. lacunosa, were assigned to Zone NN20 again. The presence of P. lacunosa (0.46 Ma) in Sample 190-1176A-12H-4, 75-76 cm, assigns the level to Zone NN19. This unusual sequence of biostratigraphic events could be explained by local stratal disruption that characterizes the lithostratigraphic unit from 0 to 195.79 mbsf. The last occurrence of Reticulofenestra asanoi (0.8 Ma) (between Samples 190-1176A-22X-CC and 23X-2, 53-54 cm) and its first occurrence (1.06 Ma) (between Samples 190-1176A-26X-CC to 27X-2, 75-76 cm) provide further datums to subdivide Zone NN19. The first occurrence of Gephyrocapsa oceanica (1.77 Ma), which approximates the Pleistocene/Pliocene boundary, was observed between Samples 190-1176A-32X-CC and 33X-CC. Identification of different Gephyrocapsa acme zones may lead to a more precise biostratigraphy for the Quaternary.
Sediments cored from 296.90 mbsf to the bottom of the hole at 440.36 mbsf (Samples 190-1176A-42X-CC to 48X-CC) contain nannofossils of Pliocene age that are mostly poorly preserved and scarce. The late Pliocene assemblages are dominated by reticulofenestrids and Pseudoemiliania spp. Reworking of older Neogene taxa is common, making a biostratigraphic assignment for the lower part of the sequence difficult. The last occurrence of Discoaster brouweri defining the top of Zone NN18 (2.0 Ma) was observed between Samples 190-1176A-44X-CC and 45X-CC with fairly abundant and well-preserved D. brouweri specimens. The presence of Discoaster pentaradiatus and large R. pseudoumbilicus (>7 µm), of which the last occurrence defines the top of Zone NN15 (3.8 Ma), was observed between Samples 190-1176A-46X-CC and 47X-CC. This could represent reworking of early Pliocene fossils in younger nonfossiliferous sediments because nannofossils are rare throughout the Pliocene but obviously reworked specimens are common. Further analysis of additional samples may resolve those problems.