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doi:10.2204/iodp.proc.342.109.2014

Biostratigraphy

Coring at Site U1408 recovered a 250 m thick sequence of Pleistocene to upper Paleocene clayey nannofossil ooze with varying amounts of clay and nannofossils. The uppermost Pleistocene section comprises foraminifer sand and clay. Minor nannofossil ooze and chert lithologies occur in the lower Eocene to Paleocene section. Nannofossils, planktonic foraminifers, and benthic foraminifers are present through most of the succession. Radiolarians are only present in a short interval of the middle Eocene and upper Paleocene. Thin Pleistocene, upper Miocene, and lower Oligocene intervals overlie a middle Eocene through upper Paleocene succession with significant hiatuses between the Oligocene and middle Eocene (~7 m.y.) and middle and lower Eocene (~3 m.y.) and a minor hiatus or condensed interval around the Paleocene/Eocene boundary. Sedimentation rates are high (~1.49–3.14 cm/k.y.) through the middle Eocene, low through the lower Eocene (~0.08 cm/k.y.), and relatively high through the upper Paleocene (~1.27 cm/k.y.).

Benthic foraminifers are generally rare (the “present” category) apart from the uppermost Eocene to Oligocene succession, where they become abundant to dominant. Benthic foraminifer preservation is good to very good through most of the recovered sequence, although poor preservation occurs in the Oligocene and lower Eocene.

An integrated calcareous and siliceous microfossil biozonation is shown in Figure F14. Datum and zonal determinations from nannofossils, planktonic foraminifers, and radiolarians are in close agreement. An age-depth plot including biostratigraphic and paleomagnetic datums is shown in Figure F15. A summary of calcareous and siliceous microfossil abundances and preservation is given in Figure F16.

Calcareous nannofossils

Calcareous nannofossil biostratigraphy is based on analysis of core catcher and additional working section-half samples from Hole U1408A. Depth positions and age estimates of biostratigraphic marker events are shown in Table T3. Calcareous nannofossil occurrence data are shown in Table T4. Note that the distribution chart is based on shipboard study only and is, therefore, biased toward age-diagnostic species.

At Site U1408, the preservation of calcareous nannofossils is generally good to exceptional in the middle Eocene but poor to moderate in the Oligocene, lower Eocene, and upper Paleocene. The uppermost sediment in Hole U1408A contains abundant nannofossils indicative of Pleistocene Zones NN21–NN18, as indicated by the presence of abundant Emiliania huxleyi in Sample 342-U1408A-1H-1, 75 cm (0.75 mbsf), top of Pseudoemiliania lacunosa in Sample 2H-3, 100 cm (8.30 mbsf), and top of Discoaster brouweri in Sample 2H-4, 100 cm (9.80 mbsf). The sample below this (Sample 3H-4, 100 cm; 11.30 mbsf) contains the short-ranging upper Miocene Zone NN9 marker species Discoaster hamatus. The short interval from Sample 342-U1408A-2H-7, 35 cm, to 4H-2, 150 cm (13.16–27.31 mbsf), can be assigned to Oligocene Zones NP25/NP23 to upper Zone NP23 based on the presence of Dictyococcites biscectus and the top and base of Sphenolithus distentus. Assemblages at the top of this interval are significantly etched. The identification of Zone NP23 in Sample 342-U1408A-4H-2, 150 cm (26.30 mbsf), and Zone NP17 in Sample 4H-3, 100 cm (27.30 mbsf), indicates the presence of a hiatus of ~8 m.y. representing the upper Eocene to lower Oligocene.

The sequence from Sample 342-U1408A-4H-4, 100 cm, through 27X-CC (28.81–246.59 mbsf) is assigned to the middle Eocene to upper Paleocene nannofossil Zones NP17–NP7/NP8. The majority of primary zonal marker species are present and listed in Table T3. The identification of Zone NP14 in Sample 342-U1408A-25X-5, 100 cm (224.70 mbsf), and Zone NP12 in Sample 25X-6, 80 cm (226.0 mbsf), indicates the presence of a short hiatus (~2 m.y.). The Paleocene–Eocene transition is indicated by the identification of Zone NP10 in Sample 342-U1408A-26X-2, 145 cm (229.44 mbsf), and Subzone NP9a in Sample 26X-2, 150 cm (229.49 mbsf), but the apparent absence of Subzone NP9b suggests the presence of a short hiatus (<1 m.y.) that may include most of the PETM. The PETM nannofossil excursion taxa, such as Discoaster araneus, were not observed.

The interval from Sample 342-U1408A-26X-2, 150 cm, to 27X-CC (229.49–246.59 mbsf) is assigned to Subzone NP9a and undifferentiated Zones NP7/NP8 based on the base of Discoaster multiradiatus (Sample 27X-5, 97 cm; 243.88 mbsf) and presence of Discoaster mohleri in the lowest Sample 27X-CC (246.59 mbsf).

Radiolarians

Radiolarian biostratigraphy is based on analysis of all core catcher samples from Hole U1408A and selected working half section samples from Cores 342-U1408A-16H, 17H, 26X, and 27X. No samples from Hole U1408B or U1408C were examined. Radiolarians are either absent or indeterminate through much of Hole U1408A but are abundant for a short interval in the middle Eocene and upper Paleocene. Sparse assemblages occur through the Paleocene–Eocene transition in Cores 25X and 26X. Depth positions and age estimates of biostratigraphic marker events are shown in Table T5, and the radiolarian distribution is shown in Table T6. Note that the distribution chart is based on shipboard study only and is, therefore, biased toward age-diagnostic species.

Samples 342-U1408A-1H-CC through 13H-CC (4.36–118.84 mbsf) are barren of radiolarians. Samples 14H-CC through 15H-CC (128.20–137.61 mbsf) contain only poorly preserved spumellarian radiolarians that cannot be assigned to species.

Samples 342-U1408A-16H-6, 78–80 cm, through 17H-3, 69–71 cm (144.29–150.50 mbsf), contain abundant and well-preserved middle Eocene radiolarians. The interval is correlated to radiolarian Zone RP13 based on the co-occurrence of rare Eusyringium fistuligerum, a key index species for the zone, and Calocyclas tripocha, a species that appears to be restricted to Zone RP13 (Kamikuri et al., 2012). The top of Podocyrtis dorus in Sample 342-U1408A-17H-3, 69–71 cm (150.50 mbsf), supports this correlation. Periphaena tripyramis triangula is present throughout this interval, although its last occurrence is recorded within underlying Zone RP12 in the equatorial Pacific (Kamikuri et al., 2012).

Samples 342-U1408A-17H-CC through 24X-CC (156.00–215.88 mbsf) contain only poorly preserved spumellarian radiolarians that cannot be assigned to species. Samples 25X-CC through 26X-1, 54–55 cm (227.29–227.85 mbsf), contain rare radiolarians of moderate preservation. The presence of rare specimens of the Eocene species Dictyoprora amphora indicates that this interval is no older than radiolarian Zone RP8 (Foreman, 1973). Samples 342-U1408A-26X-4, 85–86 cm, through 27X-CC (231.85–246.59 mbsf) are assigned to Zone RP7 based on the common occurrence of primary index species Bekoma bidartensis. As at Sites U1406 and U1407, this interval includes the lowermost Eocene indicator species Podocyrtis papalis, Theocorys? phyzella, and Theocorys? aff. phyzella (sensu Sanfilippo and Blome, 2001). However, nannofossils and benthic foraminifers indicate that this interval is of Paleocene age.

Planktonic foraminifers

Core catchers and additional samples from working section halves were examined in Hole U1408A. Samples contain diverse and well-preserved assemblages of planktonic foraminifers from Pleistocene to Paleocene age. Depth positions and age estimates of biostratigraphic marker events identified are shown in Table T7. The stratigraphic distribution of planktonic foraminifers is shown in Table T8.

The uppermost part of the interval between Sample 342-U1408A-1H-CC and 2H-4, 100–102 cm (4.33–9.80 mbsf), contains Globorotalia truncatulinoides and Globorotalia inflata, indicative of Pleistocene age. Samples 2H-5, 100–102 cm, and 2H-6, 100–102 cm (1.30–12.70 mbsf), contain poorly preserved assemblages containing no index markers except G. inflata and Neogloboquadrina pachyderma, suggesting an age of late Pliocene–Pleistocene. A poorly preserved Oligocene assemblage (mainly comprising Catapsydrax spp., Dentoglobigerina galavisi, Globorotaloides suteri, Paragloborotalia nana, and Subbotina corpulenta) was recovered from Samples 342-U1408A-2H-CC through 4H-3, 100–102 cm (13.82–27.30 mbsf).

Exceptionally well preserved and taxonomically diverse planktonic foraminifers of middle Eocene age were found in Samples 342-U1408A-4H-4, 100–102 cm, through 12H-CC (28.8–108.48 mbsf). The upper portion of this middle Eocene sequence is assigned to Zone E13. The top of O. beckmanni marks the top of Zone E12 in Sample 6H-CC (51.68 mbsf), with this species’ base marking the base of Zone E12 in Sample 7H-4, 100–102 cm (51.68 mbsf). O. beckmanni (e.g., Fig. F17) has a comparatively short (~1.6–1.0 m.y.) total range that is concurrent with the MECO (Edgar et al., 2010), a prominent global warming event that interrupted the long-term Eocene cooling trend (Bohaty et al., 2009; Edgar et al., 2010). In addition to O. beckmanii and its descendant Orbulinoides “superbeckmanni,” which is characterized by a completely spherical test (Fig. F17) (Sample 342-U1408A-6H-CC; 51.68 mbsf), Zone E12 also contains abundant and extremely well preserved specimens of Hantkenina spp. and globigerine forms with spines preserved in the apertural opening.

The top of Guembelitrioides nuttalli in Sample 342-U1408A-9H-3, 100–102 cm (74.8 mbsf), indicates the base of Zone E11. The top of the morphologically distinctive Morozovella aragonensis marks the base of Zone E10 in Sample 16H-2, 100–102 cm (138.51 mbsf). The base of Globigerinatheka kugleri occurs in Sample 342-U1408A-18H-6, 100–102 cm (163.33 mbsf), and marks the base of Zone E9. The base of G. nuttalli in Sample 24X-CC (215.82 mbsf) marks the base of Zone E8.

The interval from Sample 342-U1408A-25X-2, 100–102 cm, through 25X-CC (220.2–227.24 mbsf) contains planktonic foraminifers from Zones E7–E5. The poorly preserved planktonic foraminifers in Samples 25X-6, 80–82 cm, through 25X-CC (226–227.24 mbsf) do not include any key marker species. Samples 342-U1408A-26X-CC through 27X-CC (232.33–246.54 mbsf) contain poorly preserved assemblages and span upper Paleocene Zones P5–P4 based on the marker species Morozovella subbotinae alongside other biostratigraphically useful taxa such as Morozovella velascoensis, Morozovella occlusa, Morozovella aequa, Acarinina soldadoensis, and Subbotina velascoensis. The PETM interval is not recognized above this upper Paleocene sequence.

Benthic foraminifers

Benthic foraminifers were examined semiquantitatively in core catcher samples from Hole U1408A. Additional working section half samples taken from Cores 342-U1408A-2H through 27X were examined for preservation and relative abundance of benthic foraminifers. Benthic foraminifers at this site are predominantly rare (the “present” category) relative to total sediment particles >150 ?m in the Eocene and Paleocene and more abundant in the Oligocene and lower Eocene (Fig. F16; Table T9).

Preservation of benthic foraminifer tests is generally good to very good in the middle to upper Eocene to Paleocene, but the Oligocene and lower Eocene successions contain poorly to moderately preserved benthic foraminifers. The occurrences of benthic foraminifers at this site are shown in Tables T9 and T10.

Sample 342-U1408A-1H-CC yields a well-preserved Pleistocene fauna containing mainly Bolivina sp., Cibicidoides spp., Pullenia spp., and Pyrgo sp.

Samples 342-U1408A-2H-CC (13.87 mbsf) and 3H-CC (23.37 mbsf) include abundant Oligocene benthic foraminifers. The assemblage of Sample 2H-CC is dominated by the agglutinated taxon Bathysiphon sp., and calcareous taxa like Bulimina spp., Cassidulina subglobosa, Dentalina sp., and Gyroidinoides spp. Stilostomellids (including mainly Stilostomella lepidula and Stilostomella subspinosa) are subordinate (Table T10). The abundant occurrence of benthic foraminifers relative to total sediment particles and dominant occurrence of Bathysiphon sp. indicate intensive carbonate dissolution. Sample 342-U1408A-3H-CC (23.37 mbsf) is dominated by C. subglobosa and stilostomellids, indicating higher organic matter flux to the seafloor compared to Sample 2H-CC.

Samples 342-U1408A-4H-CC through 24X-CC (31.79–215.88 mbsf) yield a middle Eocene fauna characterized by the calcareous taxa Anomalina sp., Anomalinoides spp., Bulimina sp., Cibicidoides spp., Dentalina sp., Nuttallides truempyi, Oridorsalis umbonatus, and Pullenia spp. (including Pullenia bulloides, Pullenia cf. jarvisi, and Pullenia quinqueloba) and stilostomellids (Fig. F18). Agglutinated taxa are mainly represented by Dorothia trochoides, Karreriella sp., and Valvulina spinosa. Within the middle Eocene sequence, Samples 342-U1408A-7H-CC (61.94 mbsf) and 11H-CC (99.27 mbsf) show a dominance of infaunal taxa, such as C. subglobosa and stilostomellids, whereas epifaunal species are nearly absent. This pattern suggests high organic matter transport to the seafloor.

Sample 342-U1308A-25X-CC (227.29 mbsf) contains a lower Eocene assemblage represented by Anomalinoides sp., Cibicidoides praemundulus, Dentalina sp., Lenticulina sp., N. truempyi, and O. umbonatus. Benthic foraminifer preservation in this sample is moderate, probably because of the effect of dissolution related to the observed hiatus between lower and middle Eocene sediment.

Samples 342-U1408A-26X-CC (232.38 mbsf) and 27X-CC (246.59 mbsf) are represented by Paleocene taxa dominated by Anomalina sp., Gavelinella beccariiformis, and N. truempyi.

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