Sediments recovered from Hole 1173A provide a continuous sedimentary record from the Pleistocene through the middle Miocene. Calcareous nannofossils were used for developing the biostratigraphic framework using the biostratigraphy of Gartner (1977) and Martini (1971) with zonal modifications proposed by Young (1998) (Table T8). Abundance and preservation of calcareous nannofossils vary throughout the sequence. The interval (core and section) and depth (mbsf) constraints of calcareous nannofossil events recognized at Site 1173 throughout the succession are reported in Table T9. The epoch boundaries have been placed as in Table T10. For nannofossil ranges see Tables T11, T12, and T13.
Hole 1173A recovered core to 724.90 mbsf. In addition to the core-catcher samples, samples from core sections have been examined for calcareous nannofossils. The sediments recovered from Hole 1173A range in age from Subzone NN21b of the Pleistocene to Zone NN5 of the middle Miocene. The nannofossil assemblages decrease in abundance with greater depth. The nannofossils recovered from the sediments display a wide range of preservation states from good to poor. Some intervals are even barren of nannofossils. Cores 190-1173A-2H to 25X (7.14 to 235.33 mbsf) bear mostly moderately to well-preserved nannofossil assemblages. This interval is followed by an interval from Cores 190-1173A-26X to 39X (235.24 to 369.38 mbsf) that is affected by dissolution and is therefore characterized by poor preservation except for Core 32X (292.65 to 302.24 mbsf) and Cores 37X through 39X (340.74 to 369.83 mbsf), which have moderately to well-preserved specimens. From Core 190-1173A-40X to the bottom of the hole (369.64 to 724 mbsf), strong overgrowth and etching affected the nannofossil assemblages severely, with some intervals (Cores 190-1173A-40X, 41X, 53X, 54X, 60X, 61X, and 68X to 71X) barren of nannofossils. Thus, zonal assignments were very difficult, leading to displaced first and last appearance datums. Additional shore-based work may resolve these problems.
Moderately to well-preserved Pleistocene nannofossils were recovered from Sample 190-1173A-2H-CC (17.14 mbsf) to 25X-CC (235.33 mbsf). The Pleistocene interval is characterized by the dominance of placoliths (i.e., Emiliania huxleyi and Gephyrocapsa spp.). Reworked Neogene taxa such as discoasterids, Reticulofenestra pseudoumbilicus, and Sphenolithus spp., were found in low numbers throughout the Pleistocene samples.
The relative abundance of E. huxleyi out of 300 specimens per sample was counted in selected samples to determine the onset of the E. huxleyi acme (0.085 Ma), which defines the base of Subzone NN21b. This event is placed between Samples 190-1173A-6H-1, 75-76 cm, and 7H-CC. The first occurrence of E. huxleyi (0.26 Ma) was observed between Samples 190-1173A-13H-4, 75-76 cm, and 13H-6, 75-76 cm. The last occurrence of Pseudoemiliania lacunosa (0.46 Ma) was recorded between Samples 190-1173A-14H-4, 75-76 cm, and 14H-CC. The last occurrence of Reticulofenestra asanoi (0.8 Ma) (between Samples 190-1173A-19H-3, 75-76 cm, and 19H-CC), its first occurrence (1.06 Ma) (between Samples 190-1173A-21-CC to 22H-3, 75-76 cm) and the last occurrence of Helicosphaera sellii (1.46 Ma) (between Samples 190-1173A-22H-CC and 23H-4, 70-71 cm) provide further datum levels to subdivide Zone NN19. The first occurrence of Gephyrocapsa oceanica (1.77 Ma), which approximates the Pleistocene/Pliocene boundary, was observed between Samples 190-1173A-25X-CC and 26X-4, 96-97 cm.
The Pliocene sediments retrieved from Hole 1173A contain mostly poorly preserved nannofossils (Samples 190-1173A-26X-4, 96-97 cm, [240.7 mbsf] and 43X-4, 66-67 cm [403.68 mbsf]). The abundance of nannofossils is generally lower than in the Pleistocene assemblages. The Pliocene nannofossil assemblages are dominated by different morphotypes of reticulofenestrids and discoasterids; associated are mainly Coccolithus spp., Calcidiscus spp., and sphenoliths. Reworked specimens were encountered sporadically.
The last occurrence of Discoaster brouweri (1.95 Ma) was used to determine the top of Zone NN18 between Samples 190-1173A-28X-4, 71-72 cm, and 28X-CC. The top of Zone NN17 as marked by the last occurrence of Discoaster pentaradiatus was recorded between Samples 190-1173A-31X-2, 75-76 cm, and 31X-4, 75-76 cm. Because the interval above (Samples 190-1173A-29X-CC and 30X-CC) contains only few poorly preserved specimens or is depleted of nannofossils, the top of Zone NN16 cannot be precisely identified. The last occurrence of Discoaster surculus (2.55 Ma), which defines the top of Zone NN16, was observed between Samples 190-1173A-31X-4, 75-76 cm, and 31X-6, 75-76 cm. The last occurrence of R. pseudoumbilicus (>7 µm) defines the top of Zone NN15 (3.75 Ma) between Samples 190-1173A-41X-4, 39-40 cm, to 41X-4, 82-83 cm. In the same interval, the last occurrence of Sphenolithus abies, an additional event indicating the top of Zone NN15, was recorded. The last appearance of Amaurolithus spp. (4.0 Ma) was used to determine the top of Zone NN14 between Samples 190-1173A-42X-2, 75-76 cm, and 42X-4, 75-76 cm. Because of the low abundance and poor preservation of nannofossils in the interval above from Samples 190-1173A-41X-6, 30 cm, to 41X-CC, the actual last occurrence of Amaurolithus spp. might occur higher in the sequence. Neither the last common occurrence of Discoaster asymmetricus at the top of Zone NN13 (4.13 Ma) nor the last occurrence of Ceratolithus cristatus, which defines the top of Zone NN12 (5.05 Ma), were assessed because of poor preservation and thus impoverished nannofossil assemblages. The last occurrence of Discoaster quinqueramus (5.54 Ma) at the top of Subzone NN11b, which approximates the Pliocene/Miocene boundary (5.32 Ma), was observed between Samples 190-1173A-43X-5, 75-76 cm, and 43X-CC.
Miocene sediments (403.68-724.90 mbsf) recovered from Hole 1173A yield poorly preserved nannofossils diminishing in abundance toward the base of the sequence. The Miocene assemblage is characterized by diverse discoasterids, sphenoliths, and placoliths, mainly Calcidiscus spp. and Coccolithus spp.
The top of the Subzone NN11a, marked by the first occurrence of Amaurolithus primus (7.2 Ma), was assessed between Samples 190-1173A-47X-CC and 48X-2, 73-74 cm. The first occurrence of Discoaster quinqueramus was used to define the boundary between Zones NN10 and NN11 (8.6 Ma) between Samples 190-1173A-50X-CC to 53X-4, 70-71 cm. The absence of nannofossils or zonal marker species precludes a precise zonal assignment for the interval between Samples 190-1173-51X-4, 33 cm, to 55X-2, 115 cm, except from rare and scattered specimens of R. pseudoumbilicus (>7 µm) and strongly altered discoasterids. The last occurrence of R. pseudoumbilicus (>7 µm), marking the top of Subzone NN10a, was observed between Samples 190-1173A-50X-CC and 51X-6, 21-22 cm. Bounding the barren interval, the last occurrence of Discoaster hamatus (9.63 Ma), a species characteristic of Zone NN9, was recorded between Samples 190-1173A-54X-4, 94-95 cm, and 55X-CC. Its first occurrence (10.7 Ma) was observed between Samples 190-1173A-57X-4, 75-76 cm, and 57X-CC. The basis of Zone NN8 is marked by the first occurrence of Catinaster coalitus (10.83 Ma) between Samples 190-1173A-58X-4, 77-78 cm, and 58X-CC. This event approximates the late/middle Miocene boundary (11.2 Ma) as placed at 549.34 mbsf. Discoaster kugleri, marking the boundary between Zones NN7 and NN6 (11.8 Ma), could not be determined precisely. Its first occurrence was recorded between the Samples 190-1173A-59X-CC and 64X-CC. The last occurrence of Orthorhabdus serratus (12.3 Ma) between Samples 190-1173A-66X-CC and 67X-CC and the first occurrence of large (>7 µm) R. pseudoumbilicus (13.4 Ma) between Samples 190-1173A-67X-CC and 68X-CC were used as further events to subdivide Zone NN6. The last occurrence of Spenolithus heteromorphus (13.6 Ma), strongly altered by overgrowth, was recorded as the lowermost biostratigraphic zonal event in the sequence defining the top of Zone NN5 between Samples 190-1173A-67X-CC and 69X-CC. Additional datums were provided by the presence of two species confined to an interval (14.0 to 15.3 Ma) within Zone NN5: the last occurrences of Helicosphaera waltrans and Discoaster musicus, recorded between Samples 190-1173A-75X-CC and 76X-CC. Helicosphaera ampliaperta, the last occurrence of which marks the top of Zone NN4 (15.6 Ma), was absent in the lowermost Sample (190-1173A-77X-CC). Therefore, the basal part of the cored sequence is assigned to Zone NN5 of the middle Miocene.