Sediments recovered from Site 1174 provide a continuous sedimentary record from the Quaternary through the middle Miocene. The biostratigraphic outline was provided by calcareous nannofossils using 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 calcareouse nannofossil events recognized in Site 1174 are reported in Table T9. The epoch boundaries have been placed as in Table T10. For ranges of calcareous nannofossils at Site 1174 see Tables T11 and T12.
Hole 1174A was cored to 67.61 mbsf and contained upper Quaternary nannofossil assemblages assigned to Subzones NN21b and NN21a. Calcareous nannofossils are common to frequent throughout the sequence and mostly well to moderately preserved. The main assemblage is composed of placoliths (Emiliania huxleyi and Gephyrocapsa spp.) with a highly diverse subordinate assemblage. Reworked specimens, mainly discoasterids, sphenoliths, and reticulofenestrids of Pliocene to Miocene age, were encountered sporadically. The only event found in Hole 1174A was the onset of the E. huxleyi acme defining the base of Subzone NN21b (0.085 Ma), observed between Samples 190-1174B-7H-CC and 8H-2, 74-75 cm.
Hole 1174B was washed to 143.7 mbsf. The sediments recovered were assigned to Zone NN21 of the Pleistocene through Zone NN6 of the middle Miocene. Cores 190-1174B-1R to 39R (143.7 to 513.21 mbsf) yield mostly moderately to well-preserved nannofossil assemblages. Nannofossils are common throughout the Pleistocene sequence. Beginning in Sample 190-1174B-40R-CC (523.81 mbsf) nannofossils decrease drastically in abundance alongside a decline to poorer preservation. Nannofossil assemblages are strongly affected by mechanical breakage and dissolution, with several intervals barren of nannofossils (Samples 190-1174B-41R-CC to 42R-CC, 49R-CC, 64R-CC to 65R-3, 103-104 cm, 66R-CC, 68R-CC, 74R-CC, 77R-CC to 78R-CC, 80R-CC, 83R-CC, 87R-CC to 88R-CC, 97R-CC). A few samples exhibit strong overgrowth, which mainly affected discoasterids and reticulofenestrids in Samples 190-1174B-55R-CC to 56R-CC (668.08 to 671.61 mbsf), 59R-CC (706.45 mbsf), 71R-CC (815.49 mbsf), 82R-CC (922.46 mbsf), 84R-CC (946.55 mbsf) and 93R-CC (1031.08 mbsf). Therefore, the proper position of the biostratigraphic events defining zonal boundaries may actually be in the barren intervals. This could explain discrepancies observed between ages retrieved from paleomagnetic data and biostratigraphic ages. The biostratigraphic analysis of further samples may resolve these problems.
The Pleistocene includes the interval from 150.99 to 639.41 mbsf. Moderately to poorly preserved Pleistocene nannofossils were recovered from Sample 190-1174B-1R-CC to 52R-CC. The Pleistocene assemblages older than 0.26 Ma are characterized by the dominance of gephyrocapsids. Reworked Neogene taxa such as discoasterids, Reticulofenestra pseudoumbilicus, and Sphenolithus spp. were found in low numbers throughout the Pleistocene samples.
The base of Subzone NN21a, marked by the first occurrence of E. huxleyi (0.26 Ma), was placed between Samples 190-1174B-3R-CC and 4R-CC. The last occurrence of Pseudoemiliania lacunosa (0.46 Ma) was observed between Samples 190-1174B-16R-CC and 17R-CC bounding the negatively defined Zone NN20 to the bottom. The last occurrence of Reticulofenestra asanoi (0.8 Ma; between Samples 190-1174B-43R-CC and 44RH-3, 75-76 cm) and its first occurrence (1.06 Ma, between Samples 190-1174B-47-CC and 48H-CC) provide further datums to subdivide Zone NN19. The Pleistocene/Pliocene boundary was placed between Samples 190-1174B-52R-CC and 53R-3, 79-80 cm, according to the first appearance of Gephyrocapsa oceanica (1.77 Ma), which approximates the epoch boundary. Further analysis of relative abundances of the different Gephyrocapsa morphotypes may provide a higher biostratigraphic resolution for Zone NN19.
Cores recovered from 645.7 to 778.19 mbsf were assigned to the Pliocene. The Pliocene sediments contain mostly poorly preserved nannofossils in Samples 190-1174B-53R-CC and 67R-4, 75-76 cm. The abundance of nannofossils is generally lower than in the Pleistocene sediments. 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 190-1174B-53R-CC and 54R-CC. After an interval in which nannofossils were rare and poorly preserved (Samples 190-1174B-55R-CC and 56R-CC), the last occurrence of Discoaster pentaradiatus (2.52 Ma) was recorded between Samples 190-1174B-56R-2, 75-76 cm, and 56R-CC, assigning the level to Zone NN17. The top of Zone NN16 is marked by the last occurrence of Discoaster surculus (2.55 Ma) in Samples 190-1174B-56R-CC and 57R-2, 75-76 cm. The last occurrence of R. pseudoumbilicus (>7 µm) was used to define the boundary between Zones NN16 and NN15 (3.75 Ma) between Samples 190-1174B-62R-CC and 63R-CC. The last occurrence of Amaurolithus spp. (4.0 Ma, top of Zone NN14) in Sample 190-1174B-65R-5, 74-75 cm, was observed at the base of a barren sequence (Samples 190-1174B-64R-CC and 65R-3, 103-104 cm). The top of Zone NN14 (4.13 Ma) was indicated by the first appearance of Discoaster asymmetricus between Samples 190-1174B-66R-3, 75-76 cm, and 66R-4, 71-72 cm. The event that identifies zonal boundaries between Zones NN13 and NN12 was not recognized, that is, the first occurrence of Ceratolithus cristatus at the top of Zone NN12 (5.05 Ma). The first occurrence of this rarely observed species was recorded in Sample 190-1174B-63R-CC, which is above the last occurrence of Amaurolithus spp. (Zone NN14). 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 190-1174B-67R-4, 75-76 cm, and 67R-6, 75-76 cm.
Sediments recovered from Sample 190-1174B-67R-6, 75-76 cm (784.19 mbsf), to the bottom of Hole 1174B (1111.3 mbsf) contain Miocene nannofossils. The generally poorly preserved Miocene nannofossil assemblage mainly consists of placoliths, diverse discoasterids, and sphenoliths.
The top of Subzone NN11a, marked by the first occurrence of Amaurolithus spp. (7.2 Ma), was assessed between Samples 190-1174B-73R-CC and 74R-3, 41-42 cm. The first occurrence of D. quinqueramus was used to define the boundary between Zones NN10 and NN11 (8.6 Ma) between Samples 190-1174B-75R-CC and 79R-CC. The last occurrence of R. pseudoumbilicus (>7 µm) marking the top of Subzone NN10a (9.0 Ma) was observed between Samples 190-1174B-76R-CC and 79R-CC. The range of Discoaster hamatus defines the boundaries of Zone NN9 (9.63-10.7 Ma). Its top occurrence was observed in Sample 190-1174B-80R-4, 75-76 cm, and the first occurrence between Samples 190-1174B-81R-CC and 82R-2, 74-75 cm. The basis of Zone NN8 is marked by the first occurrence of Catinaster coalitus (10.83 Ma) between Samples 190-1174B-85R-4, 75-76 cm, and 85R-CC. This event approximates the late/middle Miocene boundary (11.2 Ma). The first occurrence of Discoaster kugleri marking the boundary between Zones NN7 and NN6 (11.8 Ma) was recorded between Samples 190-1174B-96R-CC and 98R-CC. The first occurrence of D. kugleri is problematic because intergrades with Discoaster deflandrei are commonly found below the first occurrence datum (Young, 1998). In the same interval, the last occurrence of Orthorhabdus serratus (12.3 Ma) was recorded. The last common occurrence of Cyclicargolithus floridanus (13.2 Ma) between Samples 190-1174B-99R-CC and 100R-CC provides a further datum to subdivide Zone NN6. The absence of Sphenolithus heteromorphus, the last occurrence of which defines the top of NN5 (13.6 Ma), and the first occurrence of large (>7 µm) R. pseudoumbilicus (13.4 Ma) between Samples 190-1174B-101R-CC and 102R-CC suggests the assignment to Zone NN6. Sample 190-1174B-102R-CC from the bottom of the hole does not yield any age diagnostic nannofossils.