Previous   |    Next

doi:10.2204/iodp.proc.341.107.2014

Paleontology and biostratigraphy

Microfossil abundances are generally low in Hole U1421A (Fig. F13). Based on radiolarian and diatom biostratigraphy, the bottom of the hole (near 700 m CSF-A) is younger than 300 ka and older than 30 ka. Benthic and planktonic foraminifers are better preserved than the siliceous microfossils and occur throughout the record. Changes in the composition of the benthic foraminiferal fauna suggest changes in the degree of sediment transport from the shelf. Transport from the shelf is also supported by the presence of the neritic and coastal diatoms and neritic radiolarians.

Diatoms

In order to define the sediment depositional age and paleoenvironmental conditions, core catcher samples and samples from selected split core sections from Hole U1421A were investigated (Table T5). Of the 112 samples analyzed in Hole U1421A, 76 are barren of diatoms.

Zone NPD 12 (present–[0.3 ± 0.1] Ma) is recognized in most of the diatom-bearing samples from Hole U1421A. Although the base of Zone NPD 12 is defined by the last occurrence (LO) of Proboscia curvirostris (Jousé) Jordan et Priddle (D120; 0.3 ± 0.1 Ma), this taxon is not observed. However, one specimen of Proboscia barboi (Brun) Jordan et Priddle is found in Sample 341-U1421A-62X-CC (495.76 m CSF-A); the LO of P. barboi is at ~0.3 Ma (following Yanagisawa and Akiba, 1998). Neither P. curvirostris nor P. barboi are found deeper than Sample 62X-CC. Therefore, we consider all of the retrieved sediment to be within Zone NPD 12. For a detailed description of diatom zonal scheme and taxonomy, see the “Methods” chapter (Jaeger et al., 2014).

Diatom valve preservation varies strongly throughout the sediment column of Hole U1421A (Table T5) from poor to good (Fig. F13). The diversity of the diatom assemblage is low and exclusively consists of Pleistocene to Holocene species. In intervals where we observe higher diversity (e.g., >15 species in Samples 341-U1421A-1H-4W, 100 cm, and 56X-2W, 46 cm), the assemblages are mainly composed of cold-water species, including Neodenticula seminae (Simonsen et Kanaya) Akiba et Yanagisawa, Actinocyclus curvatulus Janisch in Schmidt, and Thalassiosira gravida Cleve (Sancetta, 1982; Medlin and Priddle, 1990). The presence of coastal and benthic diatoms, including resting spores of Chaetoceros, Thalassionema nitzschioides var. nitzschioides (Grunow) Mereschkowsky, and the tycoplanktonic diatom Paralia spp. (Hasle and Syvertsen, 1996) suggest that coastal waters influenced Site U1421 (Table T5; Figs. F13, F14).

Radiolarians

Radiolarian abundance and preservation are variable at Site U1421. Of 83 studied samples, 66 were barren of radiolarians. Where radiolarians occur (17 samples), their abundances range from rare to common and preservation is mostly poor to moderate (Fig. F13). Radiolarian species diversity is relatively low, and only 23 radiolarian species are encountered at this site (Table T6). The radiolarian faunas are consistent with those found in the Botryrostrobus acquilonaris Zone (0–0.5 Ma). The LO of Lychnocanoma sakaii Morley and Nigrini is encountered between Samples 341-U1421A-45X-CC (334.11 m CSF-A) and 46X-CC (343.59 m CSF-A), suggesting that sediments below 343.59 m CSF-A are older than 30 ka. The LO of Stylacontharium acquilonium Hays is not encountered at this site, suggesting that the bottom of the core is younger than 400 ka.

The radiolarian assemblages are dominated by cold- and surface-water (deeper than 200 m water depth) species in the uppermost ~200 m CSF-A, suggesting a neritic glacial environment according to Boltovskoy et al. (2010) (Fig. F14). Although cold-water neritic species continue to be abundant deeper than ~200 m CSF-A, the relative abundance of deepwater radiolarians (deeper than 500 m water depth) increases as well. This increase may indicate progressive environmental change or is caused by sediment transport from shallow-water environments to this continental slope site. Based on radiolarian and diatom biostratigraphy, the bottom of the core (near 700 m CSF-A) is between 30 and 300 ka.

Foraminifers

Core catcher and split core section samples from Hole U1421A were examined for planktonic foraminifers from the >150 µm size fraction in 44 samples (Table T7) and for benthic foraminifers from the >150 µm size fraction in 46 samples (Table T8). Unlike at other sites, the 63–150 µm size was not examined for benthic foraminifers because the siliciclastic component overwhelms the foraminiferal component in that size fraction. Nearly all samples contain lithic clasts >700 µm, and the 63–700 µm fraction is typically dominated by nonbiogenic material. Exceptions are Samples 341-U1421A-22X-CC (114.68 m CSF-A), 55X-CC (432.84 m CSF-A), and 56X-CC (442.59 m CSF-A), where biogenic components (planktonic and benthic foraminifers and radiolarians) dominate the sand-silt fraction.

Planktonic foraminifers

Planktonic foraminifers are present in 39 of the 44 samples examined from Hole U1421A. Group abundances of planktonic foraminifers are generally present, except for one sample where group abundance is dominant (Table T7; Fig. F13). Planktonic foraminiferal preservation is generally good, except for one sample where preservation is moderate.

The planktonic foraminiferal assemblages are dominated by polar-subpolar species including Neogloboquadrina pachyderma (sinistral), Globigerina bulloides, and Globigerina umbilicata (Table T7), suggesting a cold-water environment (Fig. F14). N. pachyderma (dextral) is also present at this site, but its presence is sporadic and this taxon is generally <10% of the total fauna (ranked as few).

Planktonic foraminifers examined in Samples 341-U1421A-75X-CC and 77X-CC contain specimens that resemble Neogloboquadrina inglei (LO 0.7 ± 0.1 Ma). The morphological features of these specimens agree with the original description of N. inglei (Kucera and Kennett, 2000), except for the coiling direction. In the California margin, N. inglei is exclusively dextral, whereas all specimens found at Site U1421 are sinistral. Moreover, the N. inglei datum event disagrees with the biostratigraphy provided by diatoms and radiolarians, if we assume the LO of sinistral form of N. inglei is the same as that of dextral form reported from the California margin (Kucera and Kennett, 2000). The validity of this bioevent needs to be tested by additional morphological investigation and further chronostratigraphic control.

Benthic foraminifers

Benthic foraminifers are present in 44 of the 46 samples examined. Abundances are generally low because of the dominance of siliciclastic material, but benthic foraminifers are dominant to abundant in six samples (Table T8). Preservation is generally poor to moderate; however, when benthic foraminiferal abundances are high, preservation is good to very good (Fig. F13).

Elphidium spp. is abundant to dominant in ~80% of the samples (Fig. F14; Table T8), suggesting that much of the sediment originated in littoral to inner neritic environments (<100 m water depth). Other samples are dominated by Cassidulina cushmani, Eubuliminella exilis, Epistominella pacifica, or Islandiella norcrossi, which is consistent with sediments from middle neritic to middle bathyal (~100–1500 m) water depths in the Gulf of Alaska (Bergen and O’Neil, 1979). When these taxa dominate, Elphidium spp. is often rare to absent, preservation is generally better, and abundances are generally higher. Specimens of Elphidium spp. are also more frequently fragmented than those of other taxa, but further research is necessary to quantify differences in preservation among taxa. Changes in the foraminiferal faunas and their preservation may reflect changes in sediment transport from shallower waters to Site U1421, with samples dominated by poorly preserved Elphidium spp. reflecting intervals of higher sedimentary input from the shelf. Alternatively, changes in faunal composition could partially reflect changes in water depth at the site.

Top of page   |    Previous   |    Next