Biostratigraphic assignments were made to core-catcher and other selected samples using calcareous nannofossils and planktonic foraminifers. Figure F29 shows a summary of biozonal assignments. Paleobathymetry estimations were made using benthic foraminifer assemblages (see "Biostratigraphy" in the "Explanatory Notes" chapter).
Samples from Core 180-1108B-1R contain common to abundant, well-preserved nannofossils. Core 180-1108B-2R was not recovered. Samples 180-1108B-3R-CC and 4R-CC contain rare to few, moderately to poorly preserved, nannofossils. Samples 180-1108B-5R-CC, 8R-CC, and 9R-CC are barren, and there was no recovery of sediment in Cores 180-1108B-6R and 7R. Core 180-1108B-10R contains a rare, moderately to poorly preserved assemblage. There was no recovery in Cores 180-1108B-11R, 12R, and 13R; and Sample 180-1108B-14R-CC is barren. Nannofossils in Cores 180-1108B-15R to 19R range from rare to abundant (mostly few) and are moderately preserved. Nannofossil abundance and quality of preservation decline markedly from Core 180-1108B-20R downward. Samples containing nannofossils are mostly rare to few in abundance, with an occasional sample containing common nannofossils. Preservation is moderate to poor. Numerous barren samples, or samples with only trace amounts of nannofossils, occur in the interval from Core 180-1108B-20R downward.
We noted a normal amount of reworking of older species throughout the sampled interval (Cores 180-1108B-1R through 51R).
Table T5 shows the distribution of the index species and other selected species. This table was prepared during shipboard investigation and does not list all species present in a given sample. The uppermost core, 180-1108B-1R, is in nannofossil Zone NN21, based on the presence of Emiliania huxleyi (Fig. F29). Sample 180-1108B-3R-CC contains Gephyrocapsa oceanica and Pseudoemiliania lacunosa, indicating the Pleistocene Subzonal interval NN19B-F. We found single specimens of Discoaster brouweri (Pliocene), Sphenolithus abies (Pliocene), and Calcidiscus macintyrei (early Pleistocene) in this sample, which we consider to be reworked. Sample 180-1108B-4R-CC is in the Subzone NN19B-C/D interval based on the last occurrence (LO) of Helicosphaera sellii.
Calcidiscus macintyrei and G. oceanica co-occur in Sample 180-1108B-10R-1, 94-96 cm, indicating Subzone NN19B. We assign Samples 180-1108B-15R-CC to 17R-CC to the subzonal interval NN19A-B, based on the presence of C. macintyrei, Gephyrocapsa caribbeanica, G. aff. G. oceanica (early, nontypical forms), and the absence of D. brouweri. The first downhole D. brouweri is found in Sample 180-1108B-18R-1, 99-102 cm. This LAD of D. brouweri marks the top of Zone NN18 (late Pliocene). Zone NN18 is present from Section 180-1108B-18R-1 down to Sample 180-1108B-19R-3, 97-99 cm. Specific zonal indicators are absent below that level because of generally poor preservation of the assemblages.
Planktonic foraminiferal assemblages varied from well preserved and abundant in Sample 180-1108B-1R-CC to recrystallized, poorly preserved, and rare in Sample 180-1108B-31R-CC, with the majority of the assemblages size sorted toward the larger specimens, moderately preserved, and common to few in abundance. The core catchers of Cores 180-1108B-2R, 4R, 5R, 19R, 22R, 23R, 36R, 38R, 42R, 43R, 45R, and 49R through 51R were barren of foraminifers. Cores 180-1108B-6R, 7R, and 11R through 13R were not recovered.
Table T6 shows the stratigraphic distribution of species. The planktonic foraminiferal zonation is presented in Figure F29 and described below:
Zone N23 (late Pleistocene-Holocene) is recognized in Sample 180-1108B-1R-CC by the presence of Bolliella calida and B. adamsi. Globorotalia truncatulinoides is rare in Sample 180-1108B-1R-4, 4-6 cm.Zone N22, lower part (early Pleistocene or late Pliocene) is recognized in Sample 180-1108B-3R-CC through 18R-1, 99-102 cm, by the spotty occurrences of Globorotalia crassaformis hessi with G. tosaensis and G. truncatulinoides. The last occurrence of Globigerinoides fistulosus (LAD 1.6 Ma) in Sample 180-1108B-16R-CC, 7-10 cm, lies just above the Pliocene/Pleistocene boundary.
Zone N21 (middle to late Pliocene) is recognized from Sample 180-1108B-18R-CC through Sample 180-1108B-51R-1, 49-50.5 cm, the bottom of the hole. The upper boundary is drawn below the first occurrence of Globorotalia truncatulinoides. Globorotalia tosaensis occurs throughout this interval to the bottom of the hole.
Hole 1108B was drilled in a water depth of 3177 m. Coring revealed a benthic foraminiferal assemblage consistent with this lower bathyal depth in the youngest foraminiferal zone, N23 (Sample 1108B-1R-CC). Species represented include Fontbotia wuellerstorfi, Melonis affinis, Neouvigerina ampullacea, Nuttallides umbonifera, Parrelloides bradyi, Pseudoparrella exigua, and Pullenia bulloides. This full complement of lower bathyal species was not found in the remainder of the cores from this site, principally because of size sorting and the admixture of shallower dwelling species transported in turbidites, and possibly a different paleoenvironment in the lowest part of the cored section.
Interspersed with samples in which the benthic foraminifers are predominantly shelf (neritic) or higher slope (upper and middle bathyal) species are samples containing lower bathyal species, which indicates that the turbidites were deposited in deep water. The planktonic assemblages, which are mostly size sorted, contain species such as Sphaeroidinella dehiscens and Globorotalia tumida that live at 200-300 m depth in the upper water column. These species do not occur in inner neritic deposits, thus supporting the interpretation presented here that no part of the recovered section is indicative of shallow-water paleodepths.
A brief summary of the in situ and derived assemblage paleobathymetry of samples recovered from Hole 1108B and the key species upon which the interpretation is based follows:
Sample 180-1108B-1R-CC (N23). Lower bathyal (>2000 m) in situ benthic foraminifers as described above, with no admixture of species from shallower water.Sample 180-1108B-10R-CC (lower N22). Bolivina robusta and B. karreriana transported from middle bathyal (~500-1500 m).
Samples 180-1108B-14R-CC through 16R-CC (lower N22). Species transported from inner neritic depths (<50 m) include those of Amphistegina, Baculogypsina, Calcarina, Elphidium, Heterostegina, and Operculina.
Samples 180-1108B-17R-CC, 18R-1, 99-102 cm (lower N22), and 18R-CC (N21). In situ lower bathyal species of Fontbotia, Hoeglundina, Laticarinina, and Oridorsalis.
Sample 180-1108B-20R-CC (N21). Species transported from inner neritic depths, including those of Amphistegina, Calcarina, Elphidium, and the miliolids.
Samples 180-1108B-21R-CC, 24R-CC, and 25R-CC (N21). Rare benthic foraminifers.
Samples 180-1108B-26R-CC through 33R-CC (N21). Transported inner neritic Amphistegina, Elphidium, Operculina, miliolids, Pseudorotalia, and Lugdunum schwagerianum (Samples 180-1108B-26R-CC and 32R-CC). Admixture with bathyal species Globocassidulina subglobosa in Sample 180-1108B-27R-CC.
Sample 180-1108B-34R-2, 91-92 cm (N21). In situ lower bathyal species, including Ceratobulimina pacifica, Fontbotia wuellerstorfi, Gyroidinoides sp., Pullenia bulloides, and Uvigerina hispida.
Samples 180-1108B-36R-CC, 37R-CC, and 40R-CC (N21). Species transported from inner neritic depths including those of Amphistegina, Elphidium, and Operculina, admixed with bathyal Fontbotia, Globocassidulina, and Oridorsalis in Sample 180-1108B-40R-CC.
Sample 180-1108B-44R-1, 84-85 cm (N21). Low-oxygen/high-organic carbon environment dominated by globobuliminids.
Sample 180-1108B-44R-CC (N21). Middle to lower bathyal benthic assemblage with Ceratobulimina pacifica, Gyroidinoides, and Pyrgo.
Sample 180-1108B-48R-2, 6-8 cm (N21). Low-oxygen/high-organic carbon environment dominated by globobuliminids.
Figure F30 shows the estimated sediment accumulation rate at Site 1108, based on foraminiferal and nannofossil datums and magnetic chrons and subchrons. The calculated sedimentation rate for the interval from the present to 1.25 Ma is 15 m/m.y. However, much of the section may be missing in this interval (removed by erosion or slumping; see "Erosion Estimates" ("Thematic Overview" in the "Leg 180 Summary" chapter), making the actual sedimentation rate higher. The rate from 1.25 to 1.71 Ma is estimated to be 138 m/m.y.; the rate from 1.71 Ma to 1.95 is estimated to be 324 m/m.y. The apparent sedimentation rate from 1.95 to 2.58 Ma is 18 m/m.y., but the presence of a normal fault at ~165 mbsf between Cores 180-1108B-18R and 19R (inferred from structural, temperature, and inorganic geochemistry measurements) and the high sedimentation rates surrounding this interval, make the interpretation that ~200 m of section has more likely been displaced by the fault. Excluding this discontinuity, the sedimentation rates progressively increase downsection. From 2.58 to 3.04 Ma, the rate is estimated to be 409 m/m.y.; from 3.04 to 3.22 Ma, it is estimated to be 378 m/m.y.