A "bright spot" located northwest of the southern Hydrate Ridge summit was observed by a deep-towed SeaMARC side-scan sonar survey (Johnson et al., in press). The objective of drilling at Site 1248 was to test whether this bright spot is a buried carbonate formation that may be an incipient chemoherm.
Three holes were drilled at Site 1248. Hole 1248A was drilled to make LWD measurements. Cores recovered from Holes 1248B and 1248C contain a 150-m-thick Quaternary sedimentary sequence. Abundant gas hydrate samples were recovered from Cores 204-1248C-1X through 5X. In this interval, core recovery is poor, ranging from 9% to 31%, probably as a result of massive gas hydrate close to the seafloor. Biostratigraphy for Site 1248 was based on examination of diatoms and calcareous nannofossils from all core catcher samples and a few samples from within cores in Hole 1248C.
Hole 1248C yields common and moderately preserved diatoms, except for the lower part of the hole. Diatoms are common and poorly to moderately preserved in the interval from 0 to 120 mbsf and are few to absent and poorly preserved in the interval from 120 mbsf to the bottom of Hole 1248C. Diatom assemblages from Hole 1248C are dominated by species such as Stephanopyxis dimorpha, Stephanopyxis spp., Neodenticula seminae, and Thalassionema nitzschioides. A few warm-water taxa, such as Hemidiscus cuneiformis and Fragilariopsis doliolus, are present sporadically. Neritic species, such as Actinoptychus senarius and Cocconeis spp., are present in most samples, whereas freshwater species, such as Aulacoseira spp., are present sporadically.
All core catcher samples examined contain N. seminae and Proboscia curvirostris but do not contain Actinocyclus oculatus. They are assigned to North Pacific Diatom Zone (NPD) 11 (P. curvirostris Zone; 0.3-1.0 Ma). The absence of A. oculatus in the interval from 120 mbsf to the bottom of Hole 1248C is possibly a result of the dissolution of the species, which is suggested by the poor preservation of diatoms in this interval. Therefore, the age estimated for the interval from 120 mbsf to the bottom of Hole 1248C is older than the last occurrence (LO) of A. oculatus (1.0 Ma). The age of the bottom of Hole 1248C is estimated to be younger than 1.6 Ma, based on the presence of P. curvirostris in Sample 204-1248C-17X-CC (148.5 mbsf).
The sequence above 94 mbsf in Hole 1245C contains very rare and poorly to moderately preserved calcareous nannofossils, except for Sample 204-1248C-6H-CC. Rare to common and moderately to well-preserved calcareous nannofossils were found in the lower sequence, from 94 mbsf to the bottom of the hole.
Emiliania huxleyi and Pseudoemiliania lacunosa are absent in Samples 204-1248C-1X-CC to 4X-CC (2.38-29.64 mbsf), assigning the interval to Zone NN20. The LO of P. lacunosa, which is observed in Sample 204-1248C-5X-CC (41.35 mbsf), marks the NN19b/NN20 zonal boundary. We are unable to reliably determine the small Gephyrocasa spp. Acme Zone in the interval between 66.33 and 94.51 mbsf, where calcareous nannofossils are barren or very rare. The LO of Calcidiscus macintyrei (1.59 Ma) was found in Sample 204-1248C-13H-CC, indicating an early Pleistocene age for the sediments. Abundant Gephyrocapsa lumina are present in Sample 204-1248C-17H-CC (148.47 mbsf); therefore, the sediments at the bottom of Hole 1248C were assigned to the lower part of Quaternary Zone NN19, with an age of younger than 1.67 Ma.
A number of samples from Site 1248 yielded common to rare diatoms and common to rare calcareous nannofossils, which allowed us to recognize one diatom and two nannofossil events for Site 1248 (Table T3). However, we are unable to further subdivide the zones of NN20 and NN21 and the zones of NPD 12 and 11 because of poor core recovery in this interval. It is also impossible to determine some of the mid-Pleistocene bioevents, such as the small Gephyrocapsa spp. Acme Zone and the LO of A. oculatus, because poorly preserved diatoms and very rare calcareous nannofossils were found in the middle sequence of the hole. The age of the bottom of Hole 1248C is estimated to be younger than 1.6 Ma, based on the presence of P. curvirostris in Sample 204-1248C-17X-CC (148.5 mbsf). This estimate is supported by the presence of G. lumina in the same sample. Based on diatom and two nannofossil events and assemblages, the sediment sequence in Hole 1245C was determined to be of early Pleistocene-Holocene age (Fig. F10).
The linear sedimentation rates at Site 1248 were calculated using the biostratigraphic data (Table T3). A low sedimentation rate was estimated for the top of Hole 1248C, based on the absence of E. huxleyi and presence of P. curvirstris in Sample 204-1248C-1X-CC (2.38 mbsf). This is probably due to the current position of this site on a topographic high. The sedimentation rate is estimated to be 21 cm/k.y. for the interval from 2.38 to ~40 mbsf and 7 cm/k.y. for the interval from ~40 to 120 mbsf. The age of the bottom of Hole 1248C is estimated to be younger than 1.6 Ma (diatoms) or 1.67 Ma (nannofossils). Therefore, a higher sedimentation rate is suggested for the interval from 120 mbsf to the bottom of the hole, although the precise rate is unknown (Fig. F11).