BIOSTRATIGRAPHY

Calcareous nannofossil and planktonic foraminifer biostratigraphies indicate that a complete upper Pleistocene to upper Oligocene succession was recovered at Site 1237 (Table T10; Fig. F26). Most of the standard nannofossil and planktonic foraminiferal zonal markers as well as some nonstandard nannofossil markers can be used to reconstruct the biostratigraphic succession (Fig. F38). Calcareous nannofossils at the base of the hole suggest an age younger than 31.5 Ma. Diatoms provide additional biostratigraphic control down to ~136.7 mcd.

Calcareous nannofossils and foraminifers are generally abundant or common and well to moderately well preserved throughout. Diatoms are abundant and well preserved down to ~60 mcd, but abundance decreases and preservation deteriorates below ~69 mcd, and diatoms are absent below ~174 mcd.

Marked changes in the relative proportions of benthic foraminiferal species within the Pleistocene-late Pliocene assemblage indicate variations in carbon fluxes at the seafloor that are probably related to temporal and spatial fluctuations of the coastal upwelling system and/or shifts in subsurface water masses. In contrast, the early Pliocene to late Oligocene benthic foraminiferal assemblage characterizes an oligotrophic, pelagic environment. The diatom assemblage down to 59 mcd is typical of a coastal upwelling zone on a continental margin. Below this depth, typical upwelling forms occur only occasionally with some oceanic forms.

Calcareous Nannofossils

Calcareous nannofossils are generally very abundant and well to moderately preserved at Site 1237 (Table T11; Fig. F26). Most of the standard nannofossil zonal markers between late Pleistocene and late Oligocene time, as well as some nonstandard markers, have been recognized in the recovered cores. A relatively complete and detailed nannofossil biostratigraphy has been established for Site 1237. Given a similarly detailed magnetostratigraphy (see "Paleomagnetism"), this site will serve as a much-needed reference in the southeast Pacific.

The mudline sample (202-1237B-1H-1, 0 cm) is barren of calcareous nannofossils, presumably because of strong dissolution of carbonate at or near the surface of sediments during the Holocene. The first occurrence (FO) of Emiliania huxleyi was recognized between Samples 202-1237B-1H-4, 75 cm (5.26 mcd), and 1H-CC, 9 cm (5.51 mcd), and thus, the base of the nannofossil Zone NN21 (0.26 Ma) can be drawn between these two samples. Pseudoemiliania lacunosa is present from Sample 202-1237B-2H-7, 40 cm (14.09 mcd), downhole, and the base of Zone NN20 (0.46 Ma), as defined by the last occurrence (LO) of this species, is placed between Samples 2H-5, 75 cm (11.43 mcd), and 2H-7, 40 cm (14.94 mcd). Additional datums have been identified in Zone NN19: Reticulofenestra asanoi is present from Sample 202-1237B-3H-4, 60 cm, to 3H-CC (22.74-26.1 mcd), representing the time interval 0.88 to 1.08 Ma, and Gephyrocapsa spp. large (>5.5 µm) is present in Sample 4H-2, 75 cm (31.06 mcd), indicating an age of 1.24-1.45 Ma for this sample. The LO of Calcidiscus macintyrei occurs between Samples 202-1237B-4H-1, 75 cm, and 4H-2, 75 cm (29.55-31.06 mcd), indicating an age of 1.59 Ma, the base of Zone NN19.

The Pleistocene/Pliocene boundary, coinciding with the top of Zone NN18, is placed by the FO of Discoaster brouweri (1.96 Ma), between Samples 202-1237B-4H-CC and 5H-1, 75 cm (38.85-38.02 mcd). The LOs of Discoaster pentaradiatus and Discoaster surculus, markers of the tops of Zones NN17 and NN18, respectively, occur between Samples 202-1237B-5H-6, 75 cm, and 5H-CC (45.57-47.29 mcd). Current sampling resolution precludes separation of the two zonal boundaries. However, the LO of Discoaster tamalis (2.76 Ma), between Samples 202-1237B-6H-1, 75 cm, and 6H-2, 75 cm (49.97-51.48 mcd), provides an additional marker in Zone NN16. The boundary between the upper and lower Pliocene (top of Zone NN15) corresponds to the LO of Reticulofenestra pseudoumbilicus (>7 µm) (3.8 Ma). This event is identified between Samples 202-1237B-8H-1, 75 cm, and 8H-2, 75 cm (68.97-70.51 mcd). Slightly deeper, between Samples 202-1237B-8H-4, 75 cm, and 8H-5, 75 cm (73.5-75.01 mcd), both the FO of P. lacunosa (4.0 Ma) and the LO of Sphenolithus spp. occur. The LO of Sphenolithus spp. is slightly younger in the equatorial Pacific (ODP Leg 138) according to Raffi and Flores (1995).

The top of Zone NN18 is marked by the LO of Discoaster quinqueramus (5.56 Ma) and is used here to approximate the Miocene/Pliocene boundary because of the scarcity of other species traditionally used to characterize this boundary, such as Triquetrorhabdulus rugosus or Ceratolithus spp. The LO of D. quinqueramus is identified between Samples 202-1237B-11H-4, 75 cm, and 11H-5, 75 cm (105.95 and 107.46 mcd). This species and other five-rayed discoasters were absent at Site 1236 for the age-equivalent interval, confirming an anomalous biogeographic pattern of these taxa in the southeast Pacific.

The base of Subzone NN11b (FO of Amaurolithus primus; 7.24 Ma) is identified between Samples 202-1237B-15H-2, 75 cm, and 15H-3, 75 cm (143.81-145.32 mcd). The base of the Subzone NN11a is inferred by the FO of D. quinqueramus in the interval between Samples 202-1237B-16H-CC and 17H-1, 75 cm (163.55-160.91 mcd). The interval between Samples 202-1237B-15H-1, 75 cm (142.3 mcd), and 17H-1, 75 cm (163.55 mcd), is characterized by the absence of R. pseudoumbilicus (>7 µm), representing a time interval from 6.8 to 8.85 Ma.

Discoaster hamatus is scarce, but some specimens are present in Sample 202-1237B-17H-6, 76 cm (171.08 mcd), allowing identification of Zone NN9. Although markers for Zones NN8 to NN6 are not present at Hole 1237B, the LO of Coccolithus miopelagicus (10.4 Ma), between Samples 202-1237B-18H-4, 75 cm, and 18H-5, 75 cm (178.16-179.67 mcd), is useful to approximate the middle/late Miocene boundary. The LO of Sphenolithus heteromorphus, between Samples 202-1237B-20H-7, 40 cm, and 20H-CC (204.1-204.56 mcd), marks the top of Zone NN9.

In the interval corresponding with Zone NN6, other alternative events, such as the LO of Calcidiscus premacintyrei, the LO of Coronocyclus nitescens, and the FO of T. rugosus, provide additional and useful datums (Table T11). A significant increase in the number of specimens of Cyclicargolithus floridanus is observed in Sample 202-1237-19H-6, 75 cm (192.65 mcd); this event has an assigned age of ~11-~13 Ma but needs to be recalibrated in this region. The FO of S. heteromorphus (18.2 Ma) and the LO of Sphenolithus belemnos (18.3 Ma) occur in the same interval, from Sample 202-1237B-23H-CC through 24H-1, 75 cm (236.82-238.6 mcd). Zone NN3 is defined by the range of S. belemnos, which is observed between Samples 202-1237B-24H-6, 75 cm, and 24H-7, 40 cm (246.16-247.31 mcd).

Four datums are present in the interval between Samples 202-1237B-29H-4, 75 cm, and 29H-5, 75 cm (298.88 and 300.39 mcd): the FO of Sphenolithus disbelemnos (base of Zone NN2) and the LOs of Reticulofenestra bisecta (top of Zone NN1), Zygrhabdolithus bijugatus, and Sphenolithus ciperoensis (top of NP25). This could be interpreted as a hiatus of ~1 m.y., including the Miocene/Oligocene boundary. The assemblage identified below 300.39 mcd corresponds to upper Oligocene Zone NP25. Four additional events permit higher-resolution stratigraphy in this upper Oligocene interval: the LO of Sphenolithus distentus (27.5 Ma), between Samples 202-1237B-31H-7, 40 cm, and 31H-CC (325.88-326.47 mcd); the LO of Sphenolithus predistentus (27.5 Ma), between 31H-CC and 31H-1, 75 cm (326.47-329.06 mcd); the LO of Sphenolithus pseudoradians (29.1 Ma); and the FO of S. ciperoensis (29.1 Ma), between Samples 33H-5, 75 cm, and 33H-6, 75 cm (346.64-348.15 mcd). The deepest sample (202-1237B-34H-CC; 360.60 mcd) contains S. distentus but no Reticulofenestra umbilicus, suggesting a basal age younger than 31.5 Ma at the site.

Planktonic Foraminifers

The abundance of planktonic and benthic foraminifers varies markedly in the upper part of Hole 1237B (mudline to Sample 202-1237B-4H-CC; 0-38.85 mcd). Below this depth, foraminifers are common to abundant in Hole 1237B (Table T12; Fig. F26). The percentage of benthic foraminifers relative to total foraminifers shows significant variations in the upper part of Hole 1237B (mudline to Sample 202-1237B-5H-CC; 0-47.29 mcd), reaching up to 60% in Sample 4H-CC (38.85 mcd). Sample 202-1237B-10H-CC (100.51 mcd), from an ash layer, contains a high proportion of benthic foraminifers (50%). Preservation is generally moderate or good but deteriorates in two distinct intervals (Cores 202-1237B-6H through 8H [59.23-73.38 mcd] and Cores 15H through 20H [151.21-204.56 mcd]), where planktonic foraminiferal tests show increasing evidence of dissolution and are frequently fragmented. Standard marker species are present throughout the succession and can be used to establish a relatively detailed preliminary biostratigraphy for the whole of the Pleistocene-Oligocene interval recovered at this site.

The well-preserved Pleistocene planktonic foraminiferal assemblage includes Globigerina bulloides, Globigerina falconensis, Globigerina quinqueloba, Globigerinoides ruber, Globigerinita glutinata, Globorotalia scitula, Globorotalia tosaensis, Globorotalia truncatulinoides, Globorotalia tumida, Orbulina universa, Neogloboquadrina dutertrei, and Sphaeroidinellina dehiscens. An age younger than 0.45 Ma can be assigned to Sample 202-1237B-1H-CC (5.51 mcd), and the overlying section, based on the presence of Globorotalia hirsuta (upper Pleistocene Subzone Pt1b of Berggren et al. [1995]) (Fig. F12 in the "Explanatory Notes" chapter). The LO of G. tosaensis with an age of 0.65 Ma, marking the boundary between the upper and lower Pleistocene (Subzones Pt1b and Pt1a), can be placed between Samples 202-1237B-1H-CC and 2H-CC (5.51-14.62 mcd).

The LOs of Globigerinoides extremus (1.77 Ma) and Neogloboquadrina acostaensis (1.58 Ma) occur between Samples 202-1237B-2H-CC and 3H-CC (14.62-26.10 mcd). The LO of Globorotalia puncticulata (2.41 Ma; Zone Pl5) can be placed between Samples 202-1237B-3H-CC and 4H-CC (26.10-38.85 mcd). Two useful datums can be identified between Samples 202-1237B-5H-CC and 6H-CC (47.29-59.23 mcd), the LO of Sphaeroidinellopsis seminula (3.12 Ma) and the LO of Dentoglobigerina altispira (3.09 Ma), which define the base of upper Pliocene Zones Pl4 and Pl5, respectively. The FO of Sphaeroidinella dehiscens (3.25 Ma) occurs between Samples 202-1237B-6H-CC and 7H-CC (59.23-68.37 mcd). The LO of Globorotalia plesiotumida (3.77 Ma) can be placed between Samples 202-1237B-7H-CC and 8H-CC (68.37-78.38 mcd). The LO of Globoturborotalia nepenthes (4.20 Ma), between Samples 202-1237B-8H-CC and 9H-CC (78.38-89.23 mcd), characterizes the base of lower Pliocene Zone Pl2. The FO of G. tumida, between Samples 202-1237B-10H-CC and 11H-CC (100.51-110.77 mcd), indicates an age younger than 5.82 Ma, but shipboard sampling resolution does not define the Pliocene/Miocene boundary with precision.

The FO of Globorotalia margaritae (6.09 Ma), which indicates the upper part of upper Miocene Zone M13, occurs between Samples 202-1237B-11H-CC and 12H-CC (110.77-120.58 mcd). Other useful markers for Zone M13 are the FOs of Globigerinoides conglobatus (6.20 Ma) and Pulleniatina primalis (6.40 Ma), between Samples 202-1237B-12H-CC and 13H-CC (120.58-130.80 mcd), the FO of Globorotalia conomiozea (7.12 Ma), between Samples 13H-CC and 14H-CC (130.80-141.15 mcd), and the FO of G. plesiotumida (8.58 Ma), between Samples 15H-CC and 16H-CC (151.21-160.91 mcd). The FO of N. acostaensis (9.82 Ma), between Samples 1237B-17H-1, 98-99 cm, and 17H-CC (163.79-172.80 mcd), corresponds to the base of Zone M13.

The FO of G. nepenthes (11.19 Ma), between Samples 202-1237B-18H-CC and 19H-CC (182.62-194.16 mcd), defines the base of middle Miocene Zone M11. The LO of Globorotalia fohsi s.l. (11.68 Ma), between Samples 202-1237B-18H-CC and 19H-CC (182.62-194.16 mcd), and the FO of G. fohsi s.l. (13.42 Ma), between Samples 19H-CC and 20H-CC (194.16-204.56 mcd), allow identification of the middle Miocene Zone M9.

The LO of Globorotalia archeomenardii (14.2 Ma), between Samples 202-1237B-19H-CC and 20H-CC (94.16-204.56 mcd), indicates Zone M7. Another useful marker for Zone M7 is the LO of Globorotalia peripheroronda (14.6 Ma), between Samples 202-1237B-20H-CC and 21H-CC (204.56-214.99 mcd). The FO of Globorotalia peripheroacuta (14.8 Ma), between Samples 202-1237B-21H-CC and 22H-CC (214.99-225.79 mcd), marks the base of Zone M7.

The FO of Globorotalia praemenardii (14.9 Ma), indicating Zone M6, can be placed between Samples 202-1237B-21H-CC and 22H-CC (214.99-225.79 mcd). The FO of Globigerinoides diminutus (16.1 Ma), between Samples 202-1237B-22H-CC and 23H-CC (225.79-236.82 mcd), the LO of Globorotalia miozea (15.9 Ma), between Samples 21H-CC and 22H-CC (214.99-225.79 mcd), and the FO of Globorotalia birnagaea (16.7 Ma), between Samples 23H-CC and 24H-CC (236.82-247.52 mcd), all indicate Zone M5.

The LOs of Globorotalia semivera (17.3 Ma) and Catapsydrax dissimilis (17.3 Ma), between Samples 202-1237B-23H-CC and 24H-CC (236.82-247.52 mcd), are useful markers for lower Miocene Zone M3. The LO of Globoquadrina binaiensis (19.1 Ma) occurs between Samples 202-1237B-24H-CC and 25H-CC (247.52-259.48 mcd). The LO of Globoturborotalia angulisutularis (21.6 Ma) can be placed between Samples 202-1237B-25H-CC and 26H-CC (259.48-269.13 mcd). Other useful datums for the early Miocene are the FOs of G. binaiensis (22.1 Ma) and Globoquadrina dehiscens (23.2 Ma), between Samples 202-1237B-27H-CC and 28H-CC (281.26-291.54 mcd), and the FO of Globigerinoides trilobus (23.4 Ma), between Samples 28H-CC and 29H-CC (291.54-303.56 mcd).

The Miocene/Oligocene boundary is located in Core 202-1237B-29H but could not be determined with precision because of low-resolution shipboard sampling. The FO of Globigerinoides primordius common (24.3 Ma), between Samples 202-1237B-29H-CC and 30-CC (303.56-314.85 mcd), and the FO of G. primordius (26.7 Ma), between Samples 30H-CC and 31H-CC (314.85-326.47 mcd), indicate upper Oligocene Zone P22. The FO of G. angulisutularis (29.4 Ma), between Samples 202-1237B-31H-CC and 32H-CC (326.47-338.35 mcd), marks the base of Zone P21. The LO of Subbotina angiporoides (30 Ma) and the LO of Turborotalia ampliapertura, (30.30 Ma), which corresponds to the base of Zone P20, both occur between Samples 202-1237B-32H-CC and 33H-CC (338.35-349.95 mcd).

Benthic Foraminifers

The abundance of benthic foraminifers varies markedly in Hole 1237B. The higher proportion of benthic foraminifers in the upper part of the hole (mudline to Sample 202-1237B-5H-CC; 0-47.29 mcd) probably reflects the location of the site within an upwelling zone. Below this depth, the proportion of benthic foraminifers decreases significantly (except for Sample 202-1237B-10H-CC, from an ash layer; 100.51 mcd), and a marked change in assemblage composition occurs, which indicates changing food resources (Fig. F26). Preservation is generally good to moderate throughout. To evaluate assemblage composition and variability downhole, ~200 specimens from the >150-µm fraction were picked from the mudline and selected core catcher samples (Samples 202-1237B-1H-CC through 12H-CC [5.51-120.58 mcd], 21H [214.99 mcd], and 33H [349.95 mcd]) and counted. A total of 59 taxa were identified (Table T13)

The Pleistocene-late Pliocene assemblage in Cores 202-1237B-1H through 5H (5.51-47.29 mcd) is relatively diverse. However, the proportion of high carbon-flux indicators, such as Globobulimina affinis, Globobulimina pyrula, Uvigerina peregrina, and, in particular, Epistominella exigua (a species that rapidly exploits pulsed phytodetritus inputs (cf. Smart et al., 1994; Gooday, 1996), varies markedly. These variations in assemblage composition point to marked changes in carbon flux at the seafloor, probably related to fluctuations in upwelling regime. The Pliocene-Miocene assemblage below Core 202-1237B-5H (47.29 mcd), which generally reflects a lower food supply, is characterized by Chrysalogonium spp., Cibicidoides mundulus, Globocassidulina subglobosa, Gyroidinoides soldanii, Gyroidinoides orbicularis, Laticarinina pauperata, Melonis affinis, Oridorsalis umbonatus, Planulina wuellerstorfi, Pullenia bulloides, Pyrgo murrhina, Rectuvigerina striata, Siphonina tenuicarinata, Stilostomella abyssorum, Stilostomella subspinosa, and Vulvulina spinosa.

Diatoms

Acid-cleaned slides of all core catcher samples from Hole 1237B were analyzed, as well as smear slides of some additional layers from the split cores. Most samples from Core 202-1237B-1H through 18H (166.9 mcd) contain diatoms (Table T14) even though preservation and abundance decrease below Core 202-1237A-7H (68.4 mcd). Diatoms are absent from Core 202-1237A-19H (194.2 mcd) to the bottom of the hole (360.6 mcd). Preservation of diatoms varies from good to poor.

The Holocene diatom assemblage is characterized by the dominance of coastal upwelling taxa, such as Thalassionema nitzschioides and resting spores of the genus Chaetoceros, the common occurrence of large Coscinodiscus (Coscinodiscus gigas and Coscinodiscus oculus-iridus), and the presence of neritic and benthic species such as Actinocyclus senarius, Paralia sulcata, and Cocconeis spp. The Pleistocene diatom assemblage consists of a mixture of oceanic (e.g., Azpeitia nodulifera, Rhizosolenia spp., Pseudoeunotia doliolus, Nitzschia fossilis, Nitzschia reinholdii, and Hemidiscus cuneiformis) and nearshore coastal upwelling forms.

The presence of neritic and benthic diatoms and Miocene taxa (Crucidenticula spp., Denticulopsis spp., and Thalassiosira plicata) indicates reworking of nearshore sediment material to this site. An indication supported also by the observed increase in clay content (see "Lithostratigraphy"). Displaced shallow-water benthic and freshwater diatoms are found irregularly down to Core 202-1237B-8H (78.4 mcd) but more consistently between Cores 1H and 3H (0-26.1 mcd).

A noticeable change in diatom assemblage occurs in Sample 202-1237B-7H-2, 40 cm (60.5 mcd), below which the typical coastal upwelling forms are intermittently present.

The Pliocene diatom assemblage is characterized by heavily silicified diatoms, such as Thalassiosira convexa, Nitzschia jouseae, and Stephanopyxis spp.

Late Miocene diatoms are rare, and assemblages are characterized by the presence of Azpeitia nodulidera, Thalassiothrix spp., and T. nitzschioides. The species Nitzshia marina, N. fossilis, N. miocenica, N. cylindrica, N. porteri, T. convexa, and Rhizosolenia barboi are observed occasionally when diatom abundance increases.

On the basis of primary zonal markers and/or secondary markers, it is possible to identify the following diatom zones:

  1. The base of the Fragilariopsis doliolus Zone is defined by the FO of this species (2.00 Ma), between Sample 202-1237B-4H-5, 40 cm, and 4H-3, 40 cm (32.2-35.3 mcd). The presence of Rhizosolenia matuyamai in Samples 202-1237B-3H-5, 75 cm, and 3H-CC (22.9-26.1 mcd) places these samples within 1.05-1.18 Ma. However, the sporadic and rare presence of zonal boundary index species, such as N. reinholdii and Rhizosolenia praebergonii var. robusta, in the Quaternary section prevents a more detailed zonal assessment. The zonal boundary of the F. doliolus/N. reinholdii Zones, defined by the last continuous occurrence of N. fossilis, is tentatively placed between Samples 202-1237B-3H-1, 75 cm, and 3H-3, 75 cm (16.9-19.9 mcd).
  2. The absence of F. doliolus and N. jouseae in Samples 202-1237B-4H-1, 22 cm, and 6H-1, 75 cm (29.2-49.9 mcd), places this interval into the Nitzschia marina Zone (2.00-2.77 Ma). Subzone division is not possible because of the scarcity of T. convexa.
  3. The presence of N. jouseae in Samples 202-1237B-6H-2, 22 cm, and 9H-CC (50.9-89.2 mcd) places these samples into the N. jouseae Zone (2.77-5.12 Ma). The presence of R. praebergonii s.l. within the upper portion of this zone, above Sample 202-1237B-7H-1, 110 cm (59.6 mcd), suggests that the top part of this interval is younger than the FO of R. praebergonii (i.e., <3.17 Ma).
  4. The oldest identifiable diatom zone in Hole 1237B is the Nitzschia miocenica Zone (6.57-7.30 Ma), which is defined by the concurrent presence of N. miocenica and absence of T. convexa in Samples 202-1237B-13H-CC and 14H-4, 95 cm (130.8-136.7 mcd).

Below these samples, no age-diagnostic diatom species are observed, but diatoms are found again in Core 202-1237B-19H, where a monospecific Stephanopyxis spp. assemblage and abundant girdle bands are present in Sample 202-1237B-19H-1, 3 cm (184.4 mcd). The presence of the late Miocene-Pliocene species N. marina and N. reinholdii and the lack of middle Miocene diatom species within this sample suggests contamination. The late Quaternary diatom assemblage found in the surrounding olive-colored sediments (Sample 202-1237B-19H-1, 5 cm) also points to downhole contamination. No such level was found in the corresponding cores at Hole 1237C.

Abundant girdle bands are also observed within the late Pliocene N. marina Zone. Fragments of Hemiaulus spp. were found in Sample 202-1237B-6H-5, 70 cm (50.2 mcd) and are considered to be reworked from Eocene and Oligocene sediments.

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