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doi:10.2204/iodp.proc.342.107.2014

Biostratigraphy

Coring at Site U1406 recovered a 281 m thick sequence of Pleistocene to middle Paleocene nannofossil ooze with varying amounts of clay and biosiliceous material (mainly radiolarians). Nannofossils and planktonic and benthic foraminifers are generally present throughout. Radiolarians are present in the lower Miocene–upper Oligocene and in the middle Eocene–upper Paleocene but are absent from the ~105 m thick interval spanning the EOT (Cores 342-U1406A-15H through 28X). The lower Miocene to middle Eocene succession appears to be stratigraphically complete at the resolution of shipboard biostratigraphy, but a significant hiatus is apparent between the middle Eocene and uppermost Paleocene. Sedimentation rates are relatively consistent around ~3 cm/k.y. for the lower Miocene to upper middle Eocene but are lower (~0.5 cm/k.y.) through the lower middle Eocene and the Paleocene.

The uppermost brown foraminifer sandy clay and nannofossil ooze (Cores 342-U1406A-1H through 2H; 0–2.25 mbsf) contain nannofossils and planktonic foraminifers that indicate Pleistocene ages (nannofossil Zones NN18–NN19). From 2.25 mbsf, nannofossils, planktonic foraminifers, and radiolarians provide a well-defined biostratigraphy indicating upper lower Miocene to middle Eocene sediment. A major unconformity at 255.30 mbsf is inferred to span the entire lower Eocene and uppermost Paleocene, including the PETM. Below this unconformity, a complete succession of uppermost to middle Paleocene microfossil zones was identified. Benthic foraminifers are generally rare (the “present” category) but increase in abundance in upper Eocene to upper Oligocene sediment (Cores 342-U1406A-13H through 26X; 113.20–221.74 mbsf). Benthic foraminifer preservation is generally good to very good.

Below the Pleistocene veneer, a complete sequence of nannofossil zones from Zone NN4 to NP15 (upper middle Miocene to middle Eocene) and then from Zone NP9 to NP5 (upper to middle Paleocene) is apparent. The three radiolarian-rich intervals are assigned to Zones RN3–RP21, RP11–RP12, and RP6–RP7. The planktonic foraminifer zones from Subzone PT1a (Pleistocene), Zones M4–E7 (lower Miocene to upper Eocene), and Zones P5–P4 (Paleocene) are recognized.

An integrated calcareous and siliceous microfossil biozonation is shown in Figure F15. 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 F23. 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 Holes U1406A and U1406B. 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 U1406, the preservation of calcareous nannofossils is generally moderate and good. The uppermost sediment in Hole U1406A contains abundant nannofossils indicative of Pleistocene Zones NN19 and NN18, as indicated by the presence of Pseudoemiliania lacunosa in Sample 342-U1406A-1H-1, 75 cm (0.75 mbsf), the top of Discoaster brouweri in Sample 1H-2, 75 cm (2.26 mbsf), and the absence of Discoaster pentaradiatus.

The interval from Sample 342-U1406A-1H-3, 75 cm, to 30X-CC (3.76–254.68 mbsf) contains abundant calcareous nannofossils ascribed to upper lower Miocene to middle Eocene Zones NN4–NNP14. All primary zonal marker species are present and listed in Table T3. The Oligocene/Miocene boundary is approximated by the top of the short-ranging species Sphenolithus delphix and the top of Sphenolithus capricornutus, which are both identified in Sample 342-U1406A-9H-5, 88 cm (79.72 mbsf). The Eocene/Oligocene boundary falls within nannofossil Zone NP21, which is identified by the top of Coccolithus formosus and top of Discoaster saipanensis in Samples 342-U1406A-21H-CC (191.51 mbsf) and 22H-CC (199.29 mbsf), respectively. Carbonate is continuously present across the boundary interval and through the upper to middle Eocene section, and preservation of calcareous nannofossils is good and moderate to good. The base of Dictyococcites bisectus occurs in Sample 342-U1406A-28X-1, 100 cm (233.41 mbsf), and equates with a level just prior to the Middle Eocene Climatic Optimum (Fornaciari et al., 2010). Middle Eocene nannofossil preservation deteriorates slightly below 245.11 mbsf through Zones NP14 and NP15, and the presence of Chiphragmalithus spp. is evidence of lower Eocene reworking in the lower half of Core 342-U1406A-29X (Sections 342-U1406A-29X-5 through 29X-CC). The oldest Eocene at this site is tentatively assigned at the upper part of Zone NP13 based on the absence of Tribrachiatus orthostylus, Nannotetrina spp., or any specimens ascribable to Discoaster sublodoensis and the co-occurrence of Coccolithus crassus, Discoaster lodoensis, Discoaster kuepperi, and Zygrhablithus bijugatus. A hiatus between Cores 342-U1406A-30X and 31X represents a roughly 7 m.y. interval, with nannofossil Zones NP13 and NP9 identified above and below. Moderate to good nannofossil preservation is seen in the Paleocene section, and Zones NP9–NP5 are identified based on the occurrence of base Discoaster multiradiatus in Sample 342-U1406A-32X-2, 34 cm (262.84 mbsf), the base of Heliolithus riedelii in Sample 32X-CC (267.19 mbsf), the base of Discoaster mohleri in Sample 33X-CC (278.63 mbsf), the base of Heliolithus kleinpellii in Sample 34X-1, 83 cm (279.43 mbsf), and the presence of Fasciculithus tympaniformis in the lowermost sample (34X-CC; 281.03 mbsf).

Radiolarians

Radiolarian biostratigraphy is based on analysis of all core catcher samples from Hole U1406A and selected section samples from Cores 342-U1406A-29X through 33X. No samples from Hole U1406B were examined. Radiolarians are absent from uppermost Sample 342-U1406A-1H-CC (6.23 mbsf) but are abundant to common and of good to moderate preservation in Sections 342-U1406A-2H-CC through 15H-CC (16.04–139.68 mbsf). Below this radiolarian-rich interval, radiolarians are absent over a ~100 m thick interval that encompasses the Eocene/Oligocene boundary (Samples 342-U1406A-16H-CC through 28X-CC; 148.04–242.30 mbsf). Middle Eocene radiolarians are common in underlying interval 342-U1406A-29X-3, 20–22 cm, through 29X-4, 16–18 cm (245.11–246.57 mbsf), but are rare and poorly preserved in Sample 30X-CC (254.68 mbsf). Nannofossil evidence suggests that a significant hiatus is situated between Cores 342-U1406A-30X and 31X and that sediment below Section 342-U1406A-30X-CC is of Paleocene age. Radiolarians are abundant and well preserved in this Paleocene interval. Depth positions and age estimates of biostratigraphic marker events are shown in Table T5, and 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.

Sample 342-U1406A-1H-CC (6.23 mbsf) is barren of radiolarians. Radiolarians are abundant and well preserved from Sample 342-U1404A-2H-CC through 10H-CC (16.04–91.65 mbsf), and the assemblages can be correlated to lower Miocene–uppermost Oligocene radiolarian Zones RN3–RP22. In contrast to Sites U1404 and U1405, diatoms are relatively rare in the lower Miocene–upper Oligocene at Site U1406. Sections 342-U1406A-2H-CC through 3H-CC (16.04–25.49 mbsf) are assigned to Zone RN3 based on the range overlap of Lychnocanoma elongata and Dorcadospyris dentata. Sections 342-U1406A-4H-CC through 5H-CC (35.07–44.43 mbsf) are assigned to Zone RN2 based on the primary marker for the base of the zone, the top of Theocyrtis annosa, in Sample 342-U1406A-5H-CC (44.43 mbsf). Another key datum, the top of Dorcadospyris ateuchus, occurs in Sample 4H-CC (35.07 mbsf). Sections 342-U1406A-6H-CC through 8H-CC (53.93–72.63 mbsf) are assigned to Zone RN1 based on the primary datum for the base of the zone, the base of Cyrtocapsella tetrapera, in Sample 8H-CC (72.63 mbsf). Sections 342-U1406A-9H-CC through 10H-CC (82.64–91.65 mbsf) are assigned to Zone RP22 based on the primary datum for the base of the zone, the base of L. elongata, in Sample 10H-CC (91.65 mbsf). Sections 342-U1406A-11H-CC through 13H-7 (101.52–120.06 mbsf) are assigned to recently defined Subzone RP21b (Kamikuri et al., 2012) based on the primary datum for the base of the zone, the base of Lychnocanoma apadora, in Section 13H-7 (120.06 mbsf). Sections 342-U1406A-14H-CC through 15H-CC (128.55–139.68 mbsf) are assigned to Subzone RP21a (Kamikuri et al., 2012) based on the primary datum for the base of the zone, the base of D. ateuchus, in Sample 15H-CC (139.68 mbsf).

Samples 342-U1406A-16H-CC through 28X-CC (148.04–242.03 mbsf) are barren of radiolarians. A comparable barren interval spanning the lower Oligocene and upper Eocene was encountered at Site U1404. At Site U1405, the barren interval extends lower into the Eocene to Zone RP12 (~45 Ma), whereas at Site U1404 the barren interval terminated in Zone RP16 (~40 Ma).

Samples 342-U1406A-29X-3, 20–22 cm, through 29X-3, 98–100 cm (245.11–245.89 mbsf), are assigned to Zone RP12 based on the co-occurrence of primary index species Eusyringium lagena and Theocotyle conica, a species that last occurs at the top of the zone. Sample 342-U1406A-29X-4, 16–18 cm (246.57 mbsf), is assigned to Zone RP11 based on the presence of primary index species Dictyoprora mongolfieri and the absence of E. lagena. An unconformity is inferred to lie between this interval and Sample 342-U1406A-31X-1, 15–17 cm (255.96 mbsf), which contains a poorly preserved assemblage of late Paleocene or early Eocene age. Nannofossils indicate that Section 342-U1406A-32X-2, 34 cm, through 34X-CC (262.84–281.03 mbsf) is late to middle Paleocene in age, and radiolarians are abundant and well preserved. Samples 342-U1406A-31X-2, 99–101 cm, through 32X-CC (258.30–266.19 mbsf) are assigned to Zone RP7 based on the presence of primary index species Bekoma bidartensis. In the absence of other fossil evidence, this interval would be assigned the Eocene part of the zone based on the presence of Podocyrtis papalis, Theocorys? phyzella, and Theocorys? aff. phyzella (sensu Sanfilippo and Blome, 2001). These three species first occur directly above or within the PETM at Blake Nose ODP Site 1051 (Sanfilippo and Blome, 2001) and at Mead Stream, New Zealand (Hollis et al. 2005). However, as there is strong evidence from calcareous nannofossils and foraminifers for a Paleocene age, it seems likely that we have sampled an upper Paleocene interval that was absent from Blake Nose and perhaps poorly represented at nearby Site 384, where there is an exceptional record of radiolarians of late and middle Paleocene age (Nishimura, 1992). This implies that there are currently no unequivocal markers to separate Eocene and Paleocene components of Zone RP7, at least in the North Atlantic. The stratigraphically deepest sample examined, Sample 342-U1406A-33X-4, 102–103 cm (275.89 mbsf), is assigned to Zone RP6 based on the presence of primary index species Bekoma campechensis.

Planktonic foraminifers

Core catchers and additional samples from working-half sections were examined in Hole U1406A. Samples contained diverse and generally well preserved assemblages of planktonic foraminifers from the lower Miocene through Paleocene. 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 and Figure F15.

The uppermost interval from Sample 342-U1406A-1H-1, 134 cm (1.35 mbsf), contains Globorotalia truncatulinoides, indicative of Pleistocene age. Samples 1H-2, 130–132 cm, through 9H-4, 100–102 cm (2.81–78.22 mbsf), contain diverse and well-preserved lower Miocene assemblages containing the marker species Praeorbulina glomerosa, Praeorbulina sicana, Catapsydrax dissimilis, Paragloborotalia kugleri, and Globoquadrina dehiscens, indicating a continuous succession at the zonal level from Zone M5 through Subzone M1a. The base of P. sicana indicates Zone M5 in Sample 342-U1406A-2H-CC (16.04 mbsf). The top of C. dissimilis marks the top of Zone M3 in Sample 3H-CC (25.49 mbsf). The base of G. dehiscens in Sample 8H-4, 100–102 cm (68.71 mbsf), denotes the Subzone M1a/M1b boundary. The top and base of P. kugleri occur in Samples 5H-6, 100–102 cm (43.21 mbsf), and 9H-4, 100–102 cm (78.22 mbsf), respectively, marking the top of Subzone M1b and base of Subzone M1a. The base of P. kugleri has been calibrated to occur just 70 k.y. above the Oligocene/Miocene boundary (23.03 Ma).

Samples 342-U1406A-9H-6, 88–90 cm, through 22H-6, 50–52 cm (81.12–198.97 mbsf), contain abundant and well- to moderately preserved planktonic foraminifers. The base of Paragloborotalia pseudokugleri (Sample 342-U1406A-13H-6, 75–77 cm) indicates the base of Zone O7. The bases of Zones O4 and O3 are marked by the base of Globigerina angulisuturalis in Sample 342-U1406A-17H-3, 46–48 cm (151.57 mbsf), and the top of Turborotalia ampliapertura in Sample 18H-4, 60–62 cm (162.25 mbsf), respectively. The top of Pseudohastigerina naguewichiensis occurs in Sample 20H-2, 100–102 cm (175.12 mbsf), marking the top of Zone O1. Although the absence of Hantkenina in the upper sediment at Site U1406 means that the Eocene/Oligocene boundary cannot be defined sensu stricto, the evidence from nannofossils and sedimentology indicates that the Eocene/Oligocene boundary occurs between Samples 342-U1406A-22H-6, 50–52 cm, and 22H-CC (198.97–199.29 mbsf).

Relatively diverse planktonic foraminifers of Eocene age occur in Samples 342-U1406A-22H-CC through 30X-CC (199.29–254.67 mbsf), where preservation varies from good to moderate. The tops of Globigerinatheka semiinvoluta and Morozovella crassatus mark the base of Zones E15 and E14 in Samples 342-U1406A-26X-2, 100–102 cm (220.21 mbsf), and 27X-6, 100–102 cm (231.31 mbsf), respectively. The top of Acarinina species, calibrated to 37.75 Ma, occurs in Sample 27X-2, 100–102 cm (225.31 mbsf). The comparatively short (400 k.y.) Zone E12 is not recognized, probably because of the relatively low sedimentation rates (Fig. F23). The top of Guembelitrioides nuttalli demarcates the base of Zone E11 in Sample 342-U1406A-28X-4, 100–102 cm (237.91 mbsf). The comparatively short (~900 k.y.) Zone E9 is not recognized at Site U1406, again probably because of the low sedimentation rates. The boundary between Zones E8 and E7, marked by the base of G. nuttalli, is found in Sample 342-U1406A-29X-2, 50–52 cm (243.91 mbsf).

Below Sample 342-U1406A-30X-CC (254.67 mbsf), planktonic foraminifers indicate sediment of late Paleocene age (Zone P5) indicative of an unconformity spanning the lower Eocene. The top of Zone P4 is in Sample 342-U1406A-31X-3, 10–11 cm (258.91 mbsf). Below Sample 34X-CC (281.25 mbsf), planktonic foraminifers are rare or absent, preventing zonal demarcation.

Benthic foraminifers

Benthic foraminifers were examined semiquantitatively in core catcher samples from Hole U1406A. Additional working section half samples taken from Cores 342-U1406A-2H through 34X were examined for preservation and relative abundance of total benthic foraminifers and individual morphogroups. Benthic foraminifers at this site are rare (the “present” category) relative to total sediment particles >150 μm in the Miocene and Paleocene to middle Eocene and more abundant in the upper Eocene to upper Oligocene (Fig. F16).

Preservation of benthic foraminifer tests is generally good to very good except for lithostratigraphic Unit IV, which contains poorly and moderately preserved samples (Fig. F16). The occurrences of benthic foraminifers at this site are shown in Tables T9 and T10.

Sections 342-U1406A-1H-CC through 23H-CC (6.20–207.93 mbsf) are dominated by an uppermost Eocene to lower Miocene assemblage of the calcareous taxa Cassidulina subglobosa, Gyroidinoides spp., Pullenia bulloides, Pullenia quinqueloba, and stilostomellids (including Stilostomella gracillima, Stilostomella lepidula, Stilostomella subspinosa, and Stilostomella sp.). Agglutinated benthic foraminifers are minor constituents only. Overall, this assemblage shows a high abundance of infaunal taxa, indicating high organic matter flux to the seafloor, comparable to deeper sites at J-Anomaly Ridge (Sites U1404 and U1405).

The lower to upper Eocene sediment from Sections 342-U1406A-25H-CC through 30X-CC (217.60–254.64 mbsf) shows a distinct change in benthic foraminifer assemblages compared to the overlying sediment (Table T9). Infaunal taxa are less abundant, and assemblages are mainly characterized by Cibicidoides subspiratus, Nuttallides truempyi, and Oridorsalis umbonatus, indicating a lower organic matter flux to the seafloor at this time.

Samples 342-U1406A-31X-CC through 34X-CC (260.44–281.03 mbsf) are dominated by a Paleocene assemblage. Abundant calcareous taxa include Aragonia velascoensis, Bulimina sp., gavelinellids (Gavelinella beccariiformis, Gavelinella hyphalus, and Gavelinella sp.), Guttulina sp., Gyroidinoides globosus, N. truempyi, and Pullenia coryelli. The agglutinated benthic foraminifer Gaudryina pyramidata occurs frequently in these samples (Table T9). Preservation of benthic foraminifer individuals is poor or moderate in these samples, and calcite infill is commonly observed.

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