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doi:10.2204/iodp.proc.341.105.2014

Paleontology and biostratigraphy

Microfossil abundance and preservation at Site U1419 varies depending on skeletal composition. Calcareous microfossils (planktonic and benthic foraminifers) are well preserved and continuously abundant at Site U1419. Siliceous microfossil preservation and abundance are less consistent. Radiolarian faunas are moderately preserved in the upper ~100 m CCSF-B, and their abundances fluctuate from rare to abundant. Deeper than 100 m CCSF-B, almost all samples are barren of radiolarians. The diatom preservation and abundance trends are similar to the radiolarian trends; however, diatoms are less well preserved, and their abundance is lower shallower than 100 m CCSF-B. From ~80 to 90 m CCSF-B, diatom resting spores have an abundance peak, and the benthic foraminifer Eubuliminella exilis dominates the benthic foraminifer assemblage, suggesting a notable environmental change in the water column and at the seafloor during the time represented by that interval.

Diatoms

In order to define the sediment depositional age and paleoenvironmental conditions, core catcher samples and samples from selected split core sections from Holes U1419A–U1419E were investigated (Table T5; Fig. F12). Approximately two samples per core were analyzed in Holes U1419A–U1419E; however, 71 of the 149 samples investigated are barren of diatoms. Most of the barren samples are deeper than 113 m CCSF-B. Diatom occurrence and valve preservation vary strongly throughout the sediment column of Site U1419.

The only biozone recognized at Site U1419 is Zone NPD 12 (present–[0.3 ± 0.1] Ma) because we did not observe the last occurrence (LO) of Proboscia curvirostris (Jousé) Jordan et Priddle (D120; 0.3 ± 0.1 Ma). 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., 2014a).

Diatom assemblages at Site U1419 are generally dominated by coastal and neritic species, mainly Thalassionema nitzschioides (Grunow) Mereschkowsky, Paralia sulcata (Ehrenberg) Cleve, and resting spores of Chaetoceros (Fig. F13). Although Chaetoceros resting spores are present throughout the cores, they are especially abundant in the interval from 75 to 90 m CCSF-B. High relative abundances of Chaetoceros resting spores are treated as indicative of high-productivity continental shelf and margin environments (e.g., Sancetta, 1982; Suto et al., 2012). The co-occurrence of other coastal and neritic species with Chaetoceros resting spores could also imply offshore transport.

Cold-water species, defined following Hasle and Syvertsen (1996) and Koizumi (2008), are also dominant at Site U1419 and show particularly high abundances in the uppermost 10 m CCSF-B and in the interval from 75 to 90 m CCSF-B (Fig. F13). Sea ice–related warm and temperate species are also continuously present downcore; however, these species are not a significant component of the assemblage and range between present and rare.

Radiolarians

Radiolarians are well preserved in the upper 110 m CCSF-B, and their abundances fluctuate between rare to abundant. However, deeper than 110 m CCSF-B, almost all samples are barren (Fig. F12). Based on observations of 43 species, all samples from Site U1419 are located inside the Botryostrobus acquilonaris Zone (0–0.5 Ma). The LO datum of Stylocontharium acquilonium Hays (0.4 Ma) is not encountered, suggesting that the sediments are younger than 0.4 Ma. The occurrence of Lychnocanoma sakaii is not continuous, and therefore it is difficult to precisely define the LO of this species (0.03 Ma). The last observed occurrence of L. sakaii is between Samples 341-U1419D-16H-CC (87.98 m CCSF-B) and 341-U1419B-12H-CC (88.45 m CCSF-B). However, based on the sporadic occurrence of L. sakaii, the sediment between those two intervals may be older than 30 ka (Table T6).

Generally, cold-water taxa dominate the assemblages (Fig. F13). This group of environmentally sensitive taxa is composed of Stylochlamydium venustum Bailey and Stylodictya vallisdispina Jørgensen following Kamikuri et al. (2008). Cold-water taxa are abundant almost continuously as deep as 110 m CCSF-B. Warm-water taxa fluctuate from barren to common in the upper 110 m CCSF-B.

Foraminifers

Core catcher samples from Hole U1419A were examined for planktonic foraminifers from the >125 µm size fraction in 26 samples (Table T7) and for benthic foraminifers from the >63 µm size fraction in 26 samples (Table T8).

Planktonic foraminifers

Planktonic foraminifers are present in all examined samples except for Sample 341-U1419A-19H-CC (118.53 m CCSF-B). Their abundances range from present to abundant, and their preservation ranges from very good to moderate (Fig. F12). Fourteen planktonic foraminifer species are encountered at Site U1419; faunal assemblages are dominated by polar to subpolar species Neogloboquadrina pachyderma (sinistral) and subtropical to temperate species Globigerina bulloides and Globigerina umbilicata (Table T7; Fig. F14). N. pachyderma (dextral) is also present at this site; abundances range from rare to present. The extinct species Neogloboquadrina kagaensis (LO 1.9 ± 0.02 Ma) is present in Samples 341-U1419A-16H-CC and 26H-CC, suggestive of occasional input of reworked sediment.

At Site U1419, the total group abundance of planktonic foraminifers shows recurring changes downhole (Figs. F12, F14), which coincide with changes in the faunal assemblage. High abundances of planktonic foraminifers occur between the intervals of 6.30–13.37 m CCSF-B (Samples 341-U1419A-1H-CC to 2H-CC) and 80.44–94.36 m CCSF-B (Samples 10H-CC to 12H-CC). Concurrently, the relative abundances of G. umbilicata and G. bulloides decrease from abundant to few between 80.44 and 94.36 m CCSF-B. The concurrent changes in group abundance and in planktonic foraminifer faunal composition suggest that upper ocean conditions changed during the deposition of that sediment interval, but further study is necessary to confirm this hypothesis.

Benthic foraminifers

All core catchers examined contained benthic foraminifers (Fig. F12). Abundances were highest from ~80.46 to 94.34 m CCSF-B and lowest between 118.53 and 149.58 m CCSF-B. Preservation was generally moderate to very good, with poor preservation deeper than 178 m CCSF-B. Thirty-six species or species groups were identified (Table T8). The number of genera per sample ranges from 4 to 17.

The taxonomic composition of benthic foraminiferal samples varies downhole (Table T7; Fig. F15). Shallower than 100 m CCSF-B, samples commonly contain abundant to dominant Epistominella pacifica and/or Uvigerina akitaensis and rare to dominant E. exilis. Deeper than 100 m CCSF-B, samples more frequently contain abundant to dominant Elphidium spp. This change in taxonomic composition roughly corresponds to the transition between lithostratigraphic Units I and II (~90 m CCSF-B; Fig. F15), suggesting that increased productivity (Unit I is richer in biosiliceous muds), a decline in the input of coarse sediment (Unit II is richer in clasts), and/or a change in a correlated environmental factor contributed to the faunal change. Shallow-water Elphidium species may also have been more frequently transported to the site during the deposition of Unit II. Species that frequently occur at abundances from few to dominant throughout the record include Cassidulina cushmani and Islandiella norcrossi; Nonionella labradorica is also a major constituent, with abundance fluctuations from rare to abundant (Fig. F15).

E. exilis is dominant in Samples 341-U1419A-10H-CC (~80.46 m CCSF-B) and 11H-CC (~86.52 m CCSF-B), where preservation is also at its highest (Figs. F12, F15). This species may indicate low-oxygen conditions or high export of organic carbon to the seafloor. These samples contain a very low sand fraction compared to rest of the samples, and this faunal change could also be related to processes that influence the sedimentary grain size.

Calcareous nannofossils

Calcareous nannofossil abundances in Hole U1419A range from barren to common and are poorly to well preserved. Nannofossil abundance is greatest shallower than ~25 m CCSF-B in the hole, with the exception of 0–5 m CCSF-B, where the core is almost barren of nannofossils. Nannofossil abundance gradually decreases from 25 to 80 m CCSF-B, from few to barren. From 80 to 100 m CCSF-B, nannofossils are rare to few, and deeper than 100 m CCSF-B, nannofossils are very rare. The most common species observed are Coccolithus pelagicus, Cruciplacolithus neohelis, Gephyrocapsa oceanica, reticulofenestrids, and Braarudosphaera spp.

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