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doi:10.2204/iodp.proc.344.104.2013

Paleontology and biostratigraphy

The microfossil content of core catcher samples from Hole U1414A was examined and described. Biostratigraphy was based on calcareous nannofossils and radiolarians. Benthic foraminifers were used to characterize paleoenvironmental changes at Site U1414 and provide additional biostratigraphic constraints.

Calcareous nannofossils

Calcareous nannofossils are present in all 39 core catcher samples, with good to moderate preservation throughout. However, an overall increase in recrystallization was observed downhole, becoming more pervasive below Sample 344-U1414A-34X-CC. This site provides a continuous record of sedimentation from the Miocene to the present day.

Samples 344-U1414A-1H-CC through 8H-CC are assigned to Zone NN21, based on the presence of Emiliania huxleyi in Sample 344-U1414A-8H-CC. The first downhole occurrence of Pseudoemiliania lacunosa, which constrains the transition to Zone NN19, was identified in Sample 344-U1414A-11H-CC. The last downhole occurrence of Gephyrocapsa spp. in Sample 344-U1414A-13H-CC, coupled with the first downhole appearance of Discoaster spp. in Sample 14H-CC, indicates that the boundary between Zones NN19 and NN18 (1.93 Ma) is located in Core 344-U1414A-14H. The first appearance of the genus Sphenolithus was observed in Sample 344-U1414A-15H-CC, which corresponds to Zone NN15 (3.70–3.92 Ma).

Pleistocene assemblages are dominated by Gephyrocapsa spp., whereas Discoaster spp. and Sphenolithus spp. characterize the Pliocene. Miocene assemblages are dominated by Reticulofenestra pseudoumbilicus, Discoaster spp., and Sphenolithus spp. Sample 344-U1414A-37R-CC is moderately preserved because of recrystallization, but the presence of Cyclicargolithus floridanus, Calcidiscus tropicanus, and R. pseudoumbilicus constrains the base of the hole to the middle Miocene (Table T5).

Sediment accumulation rates were calculated over two intervals in Hole U1414A. The estimated accumulation rate for the upper interval (0–97.15 mbsf) is 221 m/m.y. The transition from Subunit IB (calcareous nannofossil–rich clay) to Subunit IIA (nannofossil-rich calcareous ooze) coincides with a drastic decrease in the sediment accumulation rate, with an average of 17 m/m.y. from 97.15 mbsf to the bottom of the hole.

Radiolarians

Radiolarians are present in all core catcher samples processed, with the exception of Samples 344-U1414-18H-CC and 21H-CC. The siliceous fraction was dominated by radiolarians and diatoms, with some sponge spicules also present. Radiolarian preservation and abundance was very good to good and abundant to common, respectively.

Radiolarians were exceptionally abundant and diverse (as many as 50 different species) in the upper section of Hole U1414A (Samples 344-U1414A-1H-CC through 6H-CC). This section was estimated to be no older than Zone RN16 because of the presence of Collosphaera tuberosa. Samples 344-U1414-8H-CC through 13H-CC are assigned to be no older than Zones RN13–RN14 because of the presence of Pterocorys minythorax, a taxa which first appears in Zone RN13 (Kamikuri, 2009). Abundance decreases, and deterioration in preservation is observed in Samples 344-U1414-14H-CC through 21H-CC, rendering age assignment difficult. Despite the low abundance and poor preservation throughout this interval, the radiolarian assemblage in Sample 344-U1414A-16H-CC is tentatively assigned to Zone RN9.

An apparent increase in the biosiliceous fraction is observed in Samples 344-U1414-24H-CC through 29H-CC, where radiolarians are very abundant and well preserved. The planktonic to benthic foraminiferal ratio decreases substantially in this interval (see next section). This interval contains both Diartus hughesi and Diartus petterssoni and is assigned to Zones RN6–RN7 (Table T6).

Benthic foraminifers

Benthic foraminifers were studied in 34 core catcher samples from Hole U1414A. Benthic foraminifer abundance ranges from common to few and preservation varies from good to moderate in Samples 344-U1414A-1H-CC through 27X-CC, below which the preservation varies between moderate and poor. Some specimens from Samples 344-U1414A-27X-CC through 34X-CC show signs of recrystallization, and consequently, identification to species level is very difficult.

Planktonic/benthic (P/B) ratios exhibit high values (average = 77.1%) in Samples 344-U1414A-1H-CC through 21H-CC. P/B ratios show an overall decrease to <5%–25% from Sample 344-U1414A-23H-CC to Sample 26X-CC. The ratio then recovers to values of ~90% in Samples 344-U1414A-30X-CC through 34X-CC (Table T7). Low P/B ratios cannot be explained by carbonate dissolution because of the dominance of nannofossils and the presence of benthic foraminifers in these samples. The decrease in the P/B ratios coincides with an increase in the abundance of radiolarians, which suggests that low planktonic foraminifer abundance could be caused by a changing environment.

Benthic foraminifers in Hole U1414A are tentatively divided into two assemblages with a transitional zone. The upper assemblage (Samples 344-U1414A-1H-CC through 15H-CC) is characterized by the presence of Uvigerina auberiana, Cassidulina carinata, and various species of the Globobulimina group (including Globobulimina affinis, Globobulimina auriculata, Globobulimina spp., Praeglobobulimina ovata, Praeglobobulimina spinescens, and Chilostomella oolina) (Fig. F24). The lower assemblage (Samples 344-U1414A-21H-CC through 34X-CC) is characterized by high abundances of Oridorsalis umbonatus, Globocassidulina subglobosa, and various species of the Cibicides group (Cibicides spp., Cibicidoides bradyi, Cibicidoides pachyderma, Cibicidoides mckannai, Cibicidoides spp., and Fontbotia wuellerstorfi) and the Pullenia group (including Pullenia bulloides, Pullenia quinqueloba, Pullenia subcarinata, and Pullenia sp.). The transitional zone is characterized by an increased abundance in Melonis affinis, Uvigerina cf. senticosa, and the Pullenia group. The presence of Planulina renzi (a species that disappears in the middle Miocene) in the lower sections of Hole U1414A (Fig. F24) correlates well with biostratigraphic estimates for this sedimentary interval.

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