BIOSTRATIGRAPHY

The 283-m-thick sedimentary sequence recovered from the three holes cored at Site 1240 is of Pleistocene-late Pliocene age (Table T10; Fig. F32).

Persistent reworking of late Miocene microfossils is noted throughout the sequence; however, the three microfossil groups provide a well-constrained biostratigraphic model, which indicates an expanded Pleistocene-upper Pliocene sequence with a sediment accumulation rate of ~80 m/m.y (Table T10).

Calcareous nannofossils are generally abundant and well preserved but show a slight decline in preservation between 20 and 46 mcd and below 120 mcd. Foraminifers are common, except between 163 and 194 mcd, where the percentage of benthic foraminifers relative to total foraminifers also increases. Preservation is moderate to good. Diatoms are common throughout the section but have lower abundance and poorer preservation between 120 and 170 mcd (Fig. F22).

Calcareous Nannofossils

Calcareous nannofossils are abundant, and their preservation is good to moderate in all samples examined (Table T11; Fig. F22). All the well-known nannofossil datums for the Pleistocene have been determined within a sample spacing of ~1.5 m. The nannofossil datums show a relatively high and remarkably uniform sedimentation rate (~8 cm/k.y.) for the Pleistocene. Florisphaera profunda, a well-established nannofossil proxy for upwelling intensity, is few to abundant at the site. Thus, this site offers an excellent opportunity to use nannofossil assemblages to reconstruct centennial- to millennial-scale upwelling variability in the east equatorial Pacific and also to investigate variations on orbital or longer timescales.

The placements of all the nannofossil datums are listed in Table T10. All the Pleistocene nannofossil datums are well determined without any apparent complications. The late Pliocene datums are less well constrained and deserve some discussion. Two factors limited the nannofossil biostratigraphy of the upper Pliocene: (1) evidence of reworking of nannofossils and (2) the fact that the well-established nannofossil zonation relies exclusively on last occurrence (LO) datums for this interval. When there is indication of persistent reworking, it is generally difficult to precisely determine where the true LOs of marker species are. We tentatively considered the rare specimens of Discoaster brouweri above Sample 202-1240A-20H-CC (205.9 mcd) as reworked because they are less abundant and not consistently present. Similarly, we considered the rare and sporadic presence of Discoaster surculus above Sample 202-1240A-28H-3, 40 cm (282.5 mcd), as reworked. We placed the LO of D. surculus (2.61 Ma) between Samples 202-1240A-28H-2, 75 cm (281.3 mcd), and 28H-3, 40 cm (282.5 mcd), based on the more abundant and consistent presence of the species from the latter sample downhole. The deepest sample, 202-1240A-28H-CC (282.8 mcd), is just slightly below the LO of D. surculus and has an extrapolated age of ~2.7 Ma. It is clearly younger than 3.6 Ma, as Sphenolithus spp. are sporadic and considered reworked in these samples.

Planktonic Foraminifers

Planktonic foraminifers are generally common to frequent in samples from the upper part of Hole 1240A (mudline to Sample 202-1240A-15H-CC; 0-47.22 mcd). Between Samples 202-1240A-16H-CC and 21H-CC (163.03-215.29 mcd), the abundance of foraminifers decreases significantly (Table T11; Fig. F22) and radiolarians are particularly abundant in the >63-µm coarse fraction. In the lower part of Hole 1240A (Samples 202-1240A-22H-CC to 27H-CC; 224.97-278.67 mcd), foraminifers are common, except for the lowermost sample above the basalt (Sample 28H-CC; 282.84 mcd), where they are rare. Preservation is moderate overall, and planktonic tests show some evidence of dissolution (etching and fragmentation in 5%-30% of tests).

Diversity is relatively low, and assemblages tend to be dominated by typical upwelling taxa such as Neogloboquadrina dutertrei or Neogloboquadrina pachyderma. The Pleistocene-early Pliocene assemblage also frequently includes Globigerina bulloides, Globorotalia menardii, Globorotalia scitula, Globorotalia tumida, Globigerinoides sacculifer, Globigerinoides trilobus, Neogloboquadrina acostaensis, Orbulina universa, and Sphaeroidinella dehiscens. A preliminary biostratigraphy can be established for the Pleistocene-upper Pliocene interval recovered at Site 1240, based on the presence of some standard marker species (Table T10).

The LO of Globigerinoides ruber (pink) (0.12 Ma) is placed between Samples 202-1240A-2H-CC and 3H-CC (13.97-24.82 mcd), and the overlying section can be assigned to upper Pleistocene Subzone Pt1b of Berggren et al. (1995) (Fig. F12 in the "Explanatory Notes" chapter). The first occurrence (FO) of G. ruber (pink) (0.9 Ma) is recognized between Samples 202-1240A-5H-CC and 6H-CC (47.22-59.73 mcd).

The boundary between upper Pleistocene Subzone Pt1b and the lower Pleistocene Subzone Pt1a is marked by the LO of Globorotalia tosaensis (0.65 Ma; Zone Pl5), which can be placed between Samples 202-1240A-5H-CC and 6H-CC (47.22-59.73 mcd). The LO of Globigerinoides obliquus (1.77 Ma), which can be identified between Samples 202-1240A-13H-CC and 14H-CC (130.80-140.60 mcd), indicates Subzone Pt1a.

The Pliocene/Pleistocene boundary could not be recognized because of the absence of the standard zonal markers. However, the LO of Globorotalia exilis (2.15 Ma), between Samples 202-1240A-19H-CC and 20H-CC (195.24-205.92 mcd), indicates upper Pliocene Zone Pl6, and the LO of Globorotalia pseudomiocenica (2.30 Ma), between Samples 22H-CC and 23H-CC (224.97-235.31 mcd), marks the base of this zone. The LO of Globorotalia limbata (2.38 Ma), between Samples 202-1240A-22H-CC and 23H-CC (224.97-235.31 mcd), and the LOs of Glorotalia puncticulata (2.40 Ma) and of Globorotalia pertenuis (2.60 Ma), identified between Samples 23H-CC and 24H-CC (235.31-245.88 mcd), all provide useful markers for upper Pliocene Zone Pl5.

Sample 202-1240A-28H-CC (282.84 mcd), at the base of Hole 1240A, contains only a sparse planktonic foraminiferal assemblage, which does not include any age diagnostic index species. However, the presence of S. dehiscens in Sample 202-1240A-27H-CC (278.67 mcd) indicates an age no older than 3.25 Ma for this sample.

Benthic Foraminifers

Benthic foraminifers are generally rare in Hole 1240A samples, and the percentage of benthic foraminifers relative to total foraminifers is consistently low (~1%), except for Samples 202-1240A-17H-CC (174.04 mcd), 19H-CC (195.24 mcd), and 20H-CC (205.92 mcd), where benthic foraminifers represent 3%-10% of the total foraminiferal assemblage (Table T12). Benthic foraminifers, overall, show better preservation than planktonic foraminifers and are only rarely fragmented as a result of dissolution.

The Pleistocene-late Pliocene assemblage is characterized by Chilostomella ovoidea, Eggerella bradyi, Ehrenbergina serrata, Eubulimina exilis, Globocassidulina subglobosa, Globobulimina affinis, Globobulimina pyrula, Gyroidinoides soldanii, Laticarinina pauperata, Melonis pompilioides, Oridorsalis umbonatus, Planulina wuellerstorfi, Plectofrondicularia vaughani, Pullenia bulloides, Pyrgo murrhina, Pyrgo serrata, and Uvigerina peregrina. Preliminary shipboard study of core catcher samples suggests that the proportion of Bolivina, Bulimina, Globobulimina, and Uvigerina, which represent useful indicators of high carbon fluxes at the seafloor, vary significantly downhole. However, it was not possible to quantify such changes or to evaluate to what extent the changes may be related to variations in upwelling intensity and/or circulation patterns or merely reflect a preservation bias (e.g., change in benthic foraminiferal accumulation rate).

Diatoms

All core catcher samples from Hole 1240A, as well as smear slides from some additional layers of the split cores, were analyzed.

Diatoms are generally few to abundant, but abundance decreases and preservation deteriorates between 120 and 170 mcd (Table T13; Fig. F22). Diatom assemblages are either dominated by Thalassiothrix and Thalassionema spp. or Azpetia nodulifer, and Thalassiothrix oozes were observed at the following levels: intervals 202-1240A-15H-2, 12-100 cm (143.6-144.5 mcd); 15H-2, 135 cm (144.8 mcd); 15H-2, 139 cm (144.9 mcd); 18H-CC (184.5 mcd); and 19H-CC (190.6 mcd).

Diatoms recovered from Site 1240 span a continuous stratigraphic interval from the Holocene Fragilariopsis doliolus Zone to the Nitzschia marina Zone (Table T10). The absence of Nitzschia jouseae at the bottom of the hole (Sample 202-1240-28H-CC; 282.8 mcd) indicates an age <2.77 Ma (LO age for this species).

Late Miocene diatoms are observed in trace abundances in most of the studied samples, and the presence of the marker species Thalassiosira miocenica and Nitzschia miocenica indicates reworking from a 6.55- to 6.08-Ma formation.

Two mid-Pleistocene diatom datums were identified; the LO of Nitzschia reinholdii (0.62 Ma) is placed between Samples 202-1240A-4H-CC and 5H-CC (36.4-47.2 mcd), and the LO of Nitzschia fossilis (0.70 Ma) between Samples 7H-4, 75 cm, and 7H-5, 75 cm (73.7-65.2 mcd).

The following early Pleistocene diatom datums were recognized: the LO of Rhizosolenia matuyamai (1.05 Ma) between Samples 202-1240A-9H-4, 75 cm, and 9H-5, 75 cm (84.4-85.9 mcd); the FO of R. matuyamai (1.18 Ma) between Samples 11H-2, 75 cm, and 11H-3, 75 cm (101.3-102.8 mcd); the LO of Rhizosolenia praebergonii var. robusta (1.73 Ma) between Samples 14H-CC and 15H-1, 10 cm (140.6-142.1 mcd). The presence of F. doliolus within samples at depths above 194 mcd places its FO (2.00 Ma) between Samples 202-1240A-19H-6, 75 cm, and 1240A-19H-7, 40 cm (193.6-194.8 mcd). Of the late Pliocene diatom datums, only the LO of Thalassiosira convexa s.l. (2.41 Ma) could be recognized between Samples 202-1240A-22H-4, 75 cm, and 22H-5, 75 cm (220.2-221.7 mcd).

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