Previous   |    Next

doi:10.2204/iodp.proc.341.106.2014

Paleontology and biostratigraphy

Microfossil abundances are generally low in Hole U1420A (Fig. F12). Diatoms and radiolarians are only observed in three intervals. Thus, age constraints are few but collectively suggest that all recovered sediments are younger than 0.7 Ma. Benthic and planktonic foraminifers are better preserved and occur throughout the record. Changes in the composition of the benthic foraminiferal fauna suggest changes in water depth ranging from inner neritic to upper bathyal.

Diatoms

In order to define the sediment depositional age and paleoenvironmental conditions, core catcher samples and samples from selected split core sections from Hole U1420A were investigated (Table T5). Of the 119 samples analyzed in Hole U1420A, 66 are barren of diatoms.

The only biozone recognized in Hole U1420A is Zone NPD 12 (present–[0.3 ± 0.1] Ma). The base of this zone is defined by the last occurrence (LO) of Proboscia curvirostris (Jousé) Jordan et Priddle (D120; 0.3 ± 0.1 Ma), which was not observed. Thus, 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).

Diatoms are generally absent to rare, and when present, valve preservation is poor to moderate (Table T5; Fig. F12). The diversity of the diatom community at Site U1420 is low and mainly consists of Pleistocene to Holocene species. Cold-water species, including N. seminae (Simonsen et Kanaya) Akiba et Yanagisawa, Actinocyclus curvatulus Janisch in Schmidt, and Rhizosolenia hebetata f. hiemalis Gran (Sancetta, 1982; Medlin and Priddle, 1990), are the most abundant (Table T5; Fig. F12). The influence of coastal waters at Site U1420 is suggested by 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).

Radiolarians

All but three samples are barren of radiolarians (Table T6). In Sample 341-U1420A-16R-1W, 0–5 cm (138.13 m CSF-A), radiolarians are common but poorly preserved. The fauna is characterized by the presence of cold- and shallow-water radiolarians such as Stylochlamydium venustum Bailey and Stylodictya validispina Jørgensen (Kamikuri et al., 2008; Boltovskoy et al., 2010) (Table T6). In Sample 341-U1420A-80R-2W, 100–101 cm (761.41 m CSF-A), we observe the datum species Lychnocanoma sakaii Morley and Nigrini, suggesting an age older than 0.03 Ma. The datum species Stylacontharium acquilonium Hays is not encountered, suggesting that the sediment of this interval is younger than 0.4 Ma, thereby bracketing the age to between 0.03 and 0.4 Ma. A faunal change marked by relatively high abundances of the deepwater species Cycladophora davisiana (>500 m CSF-A; Matul et al., 2011) and Spongopyle osculosa (>500 m CSF-A; Boltovskoy et al., 2010) also occurs in this interval (Table T6), suggesting middle to bathyal water depths (500–1000 m). The time represented by sediments shallower than 100 m CSF-A is interpreted as a neritic environment (<200 m) based on the radiolarian faunas.

Foraminifers

Core catcher and section samples from Hole U1420A were examined for planktonic foraminifers from the >125 µm size fraction in 42 samples (Table T7) and for benthic foraminifers from the >63 µm size fraction in 43 samples (Table T8). Nearly all samples contain grains >700 µm; sediment in the sand and silt fractions is also abundant in all samples.

Planktonic foraminifers

Planktonic foraminifers are present in 26 of the 42 samples examined, with 10 taxonomic groups represented (Table T7). Total abundances of planktonic foraminifers are generally ranked as present, with an exceptionally high abundance (ranked as abundant) in Sample 341-U1420A-16R-1W, 0–5 cm (Fig. F12). Preservation is good except for two samples, which are ranked as moderate (Table T7). The occasional presence of orange or brown foraminifers (comments on Table T7) suggests possible diagenetic processes.

Only one specimen of Neogloboquadrina inglei (LO 0.70 ± 0.1 Ma) is present in Sample 341-U1420A-96R-CC. Judging from the shell texture of that specimen, which lacks the prominent structures found in other neogloboquadrinids in the same sample, it was most likely transported through reworking processes.

Planktonic foraminifers at Site U1420 are dominated by Neogloboquadrina pachyderma (sinistral), Globigerina umbilicata, and Globigerina bulloides, suggesting cold-water conditions (Fig. F13). The dextral form of N. pachyderma is also present at this site, but its abundance is generally rare to abundant (Table T7), also suggesting a cold-water environment.

Benthic foraminifers

Benthic foraminifers are present to abundant in 42 of the 43 samples examined, and 38 species or species groups are identified (Table T8). Abundances are generally very low, likely due to the dominance of siliciclastic material in the 63–250 µm size fraction, and abundances are greater than present in only six samples. Benthic foraminiferal preservation varies between good and poor, with poor preservation recorded for only ~33% of the samples (Fig. F12; Table T8).

Elphidium spp. dominates 71% of the samples (Fig. F14), suggesting that a majority of the record was deposited in inner neritic (~0–100 m water depth) environments (Bergen and O’Neil, 1979). Middle neritic to upper bathyal (~100–500 m water depth) sedimentation is suggested between 138.13 and 177.16 m CSF-A and between 487.54 and 867.04 m CSF-A by increased relative abundances of taxa found at those depths in the modern Gulf of Alaska, such as Islandiella norcrossi, Uvigerina spp., and Epistominella pacifica (Fig. F14). Variation in the relative abundance of Elphidium spp. versus deeper water taxa could also be controlled by sediment transport from shallower water environments without a change in water depth.

Top of page   |    Previous   |    Next