Site 1246 was drilled on the northern flank of the southern summit of Hydrate Ridge. Two holes were drilled, and one was cored. Hole 1246B, cored to 136.70 mbsf with 99% core recovery, recovered a Pleistocene-Holocene sedimentary sequence. Biostratigraphy for Site 1246 was based on examination of diatoms and calcareous nannofossils from all core catcher samples and from a few samples within cores from Hole 1246B.
Hole 1246B yields rare to common and poorly to moderately preserved diatoms. The interval from Samples 204-1246B-1H-CC through 3H-CC (4.69-23.95 mbsf) contains common diatoms, whereas diatoms are rare to few below this interval. The diatom assemblage in Hole 1246B is dominated by such species as Stephanopyxis dimorpha, Stephanopyxis spp., Neodenticula seminae, and Thalassionema nitzschioides.
The interval from Samples 204-1246B-1H-CC (4.69 mbsf) through 4H-CC (33.83 mbsf) contains N. seminae but does not contain Proboscia curvirostris. These samples were assigned to North Pacific Diatom Zone (NPD) 12 (N. seminae Zone). The last occurrence (LO) of P. curvirostris was placed between Samples 204-1246B-4H-CC (33.83 mbsf) and 5H-CC (43.22 mbsf).
The Pleistocene-Holocene sedimentary sequence in Hole 1246B generally yielded very few and poorly preserved calcareous nannofossils. However, samples from the middle part of the sequence in Hole 1246B (70.65-109.68 mbsf) contain rare to common and moderately to well-preserved calcareous nannofossils. Two late Pleistocene calcareous nannofossil events were determined based on the presence of bioevent marker species in these samples.
Emiliania huxleyi, in trace or rare abundance, was observed in a few core catcher samples of the top interval from 0 to 70.65 mbsf. Common and well-preserved, large-sized (~4 µm) E. huxleyi is present in Sample 204-1246B-9H-CC (80.22 mbsf). According to the study of Pujos-Lamy (1977a, 1977b), such large-sized forms of E. huxleyi are present in the early evolutionary stage of this species. The top interval (0-80.22 mbsf) was assigned to Zone NN21. Sample 204-1246B-10H-CC contains abundant Gephyrocapsa spp. but lacks E. huxleyi and Pseudoemiliania lacunosa and was assigned to Zone NN20. The LO of P. lacunosa observed in Sample 204-1246B-12H-CC (109.68 mbsf) marks the NN19b/NN20 zonal boundary. Rare to common, but not abundant, small Gephyrocapsa spp. are present in the bottom sequence of Hole 1246B from 122.57 to 136.71 mbsf, suggesting an age of younger than 1.0 Ma for the interval.
Based on an analysis of diatoms and calcareous nannofossils from the Pleistocene-Holocene sedimentary sequence in Hole 1246B, three microfossil events (i.e., one diatom and two nannofossil events) were recognized (Table T2). The LO of P. curvirostris (0.3 Ma) was found 40-50 m above the first occurrence (FO) of E. huxleyi (0.27 Ma) (Fig. F7), indicating a discrepancy between these two events in this hole. A very few P. lacunosa (LO at 0.46 Ma) found in the interval from 43.22 to 61.66 mbsf were considered to be reworked fossils. Similarly, the presence of P. curvirostris above 80.22 mbsf could also be caused by reworking. However, the presence of P. curvirostris is continuous in the interval from 43.22 mbsf to the bottom of the hole, and no indication of reworking was found in the diatom assemblage. Detailed postcruise studies on these two microfossil groups are needed to understand the discrepancy. In this report, we did not use the LO of P. curvirostris for estimating sedimentation rates because of the possibility of reworking. The age of the sequence at the bottom of Hole 1246B is suggested to be younger than 1.0 Ma based on the diatom and calcareous nannofossil assemblages.
We estimated the linear sedimentation rates using only two nannofossil events, as discussed above. The sedimentation rates are 31 cm/k.y. for the interval from the top of Hole 1246B to 85 mbsf and 10 cm/k.y. for the interval from 85 to 105 mbsf.