BIOSTRATIGRAPHY

We used the timescale of Berggren et al. (1995b) during Leg 202. The biostratigraphic zones/subzones of calcareous nannofossils, planktonic foraminifers, and diatoms are summarized in Figure F12. Detailed descriptions of the shipboard methods used for different groups of microfossils follow.

Calcareous Nannofossils

During Leg 202, we referred to the zonal schemes of Martini (1971) and Bukry (1973, 1975), code numbered by Okada and Bukry (1980). These zonations are regarded as a standard framework for the biostratigraphic subdivision of low-latitude Cenozoic marine sediments based on calcareous nannofossils, and some of these events are also identifiable in middle to high latitudes. In addition to the classical concept of first occurrences (FOs) and last occurrences (LOs) of index species, we used abundance variations of some taxa to improve the stratigraphic resolution of the Pleistocene interval. The top of the Gephyrocapsa caribbeanica acme zone, dated at 260 ka, is synchronous with the FO of Emiliania huxleyi (Pujos, 1988) and, thus, provides a useful alternative to identify the base of Martini's NN21 Zone. The ages of most calcareous nannofossil data employed to construct the Leg 202 age model for the Pliocene-Pleistocene interval come from the work of Raffi et al. (1993), Wei (1993), and Raffi and Flores (1995). For the Miocene, we followed the biochronology proposed by Raffi and Flores (1995) and made comparisons with the proposals of Backman and Raffi (1997) and Young (1998) (Table T4).

Methods

Standard smear slides were made for all samples using Norland optical adhesive as a mounting medium. Calcareous nannofossils were examined by means of a light-polarized microscope, at 1000x magnification. Unless otherwise noted, we followed taxonomic concepts summarized in Perch-Nielsen (1985). For morphometric concepts concerning the Gephyrocapsa group, we mainly followed Raffi et al. (1993). We also considered (1) G. caribbeanica (3-4 Ám), whose acme is coincident with the FO of E. huxleyi, and (2) Gephyrocapsa sp. 3, whose FO is coincident with the reentrance of Gephyrocapsa medium (4-5.5 Ám) in low-latitude regions (Raffi et al., 1993).

To assess nannofossil preservation, etching and overgrowth are the most important features. We ranked preservation with the following code system:

G = good (little or no evidence of dissolution and/or secondary overgrowth of calcite; diagnostic characters fully preserved).
M = moderate (dissolution and/or secondary overgrowth; partially altered primary morphological characteristics; however, nearly all specimens can be identified at the species level).
P = poor (severe dissolution, fragmentation, and/or secondary overgrowth with primary features largely destroyed; many specimens cannot be identified at the species level and/or generic level).

Regarding relative abundance of individual species or morphotypes, five levels were considered:

D = dominant (>10 nannoliths per field of view).
A = abundant (1-10 nannoliths per field of view).
C = common (1 nannolith per 2-10 fields of view).
F = few (1 nannolith per 11-50 fields of view).
R = rare (1 nannolith in >50 fields of view).

Total abundance of calcareous nannofossils for each sample was estimated as follows:

VA = very abundant (>100 nannoliths per field of view).
A = abundant (10-100 nannoliths per field of view).
C = common (1-9 nannoliths per field of view).
F = few (1 nannolith per 2-10 fields of view).
R = rare (1 nannolith in >10 fields of view).
B = barren.

Planktonic Foraminifers

Preliminary ages were assigned to core catcher samples. The zonal schemes of Berggren et al. (1995a, 1995b) were applied for the Cenozoic. The astrochronologically tuned planktonic foraminiferal ages of Berggren et al. (1995a, 1995b) were used from the Holocene to 4.70 Ma. The tuned ages of Chaisson and Pearson (1997), Pearson and Chaisson (1997), and Curry, Shackleton, Richter, et al. (1995) were used for 4.7-13.42 Ma. Ages older than 13.42 Ma were untuned and taken from Berggren et al. (1995b) and Berggren et al. (1995a) (Table T5). In addition to the above ages, we also used the LO (0.12 Ma) (Thompson et al., 1979) and FO (0.40 Ma) (Li, 1997) of pink Globigerinoides ruber as biostratigraphic references. Taxonomic concepts for Neogene and Paleogene taxa are illustrated in Kennett and Srinivasan (1983), Bolli and Saunders (1985), and Toumarkine and Luterbacher (1985).

Methods

Unlithified to semilithified core sediment samples were soaked in tap water and then washed over a 63-Ám sieve. Lithified material was crushed to pea size, boiled in a solution of Calgon diluted to 1% by weight, then sieved and dried as before. Cleaned sieves were put into a sonicator for several minutes to avoid contamination between successive samples. Washed samples were examined under a binocular microscope.

Planktonic foraminiferal abundance in relation to total residue was categorized as follows:

A = abundant (>30%).
C = common (10%-30%).
F = few (5%-10%).
R = rare (<5%).
P = only a few broken tests recorded in a sample.
B = barren.

Preservation was categorized as follows:

VG = very good (no evidence of breakage or dissolution).
G = good (dissolution effects are rare; >90% of specimens unbroken).
M = moderate (dissolution damages, such as etched and partially broken tests, occur frequently; 30%-90% of specimens unbroken).
P = poor (strongly recrystallized or dominated by fragments or corroded specimens).

Planktonic foraminiferal species abundance in a random sample of 200-400 specimens from the >150-Ám size fraction was defined as follows:

D = dominant (>30%).
A = abundant (10%-30%).
F = few (5%-30%).
R = rare (1%-5%).
P = present (<1%).
B = barren.

Benthic Foraminifers

Benthic foraminifers provide limited biostratigraphic age control as currently applied to Leg 202 samples. Whenever possible, the LO of the benthic foraminifer Stilostomella was noted. Most species of this genus disappeared from the global ocean at different latitudes during the interval of 1.0-0.6 Ma (Hayward, 2001). For the Leg 202 sites, we used 0.65 Ma as the LO of Stilostomella (Hayward, 2001).

Benthic foraminifers were identified mainly to determine past changes in oxygenation and carbon flux, as these are main factors controlling abundance and species composition in deep-sea assemblages (Jorissen and Rohling, 2000). The generic classification of Loeblich and Tappan (1988) was used and updated in some instances. Taxonomic assignments follow Tjalsma and Lohmann (1983), van Morkhoven et al. (1986), Miller and Katz (1987), Thomas (1990), Katz and Miller (1991), Holbourn and Henderson (2002), and Kuhnt et al. (2002).

Methods

Benthic foraminifers were examined from core catcher samples used for planktonic foraminiferal studies. Relative percentages of benthic to planktonic tests were determined by counting specimens in four adjacent quadrants in three different locations on the tray. To assess assemblage composition and variability downhole, ~200 specimens from the >150-Ám fraction were picked from each core catcher sample and mounted onto slides prior to identification and counting, whenever time allowed. The fine fraction was cursorily checked for small taxa.

Preservation was categorized as follows:

VG = very good (no evidence of breakage or dissolution).
G = good (>90% of specimens unbroken).
M = moderate (30%-90% of specimens unbroken).
P = poor (strongly recrystallized or dominated by fragments or corroded specimens).

Diatoms

Diatoms characterize fertile surface waters and are abundant and diverse both within 5░ of the equator and along the western margin of South and Central America. Numerous diatom biostratigraphic studies have been completed for the equatorial Pacific (Kanaya, 1971; Muchina, 1971; Burckle, 1972, 1977a, 1977b, 1978; Bukry and Foster, 1973; JousÚ, 1973; Burckle and Trainer, 1979; Barron, 1980, 1981, 1983, 1985a, 1985b, 1992; Sancetta, 1983; Baldauf, 1985; Baldauf and Iwai, 1995). The Neogene and Quaternary diatom zonation used for the low-latitude sites of Leg 202 was that proposed by Barron (1985a, 1985b) and modified by Baldauf and Iwai (1995). Table T6 lists diatom biostratigraphic events, paleomagnetic calibration, and age estimates based on Barron (1985a, 1992) and Shackleton et al. (1995) and used during this leg.

In the marginal upwelling regions, the input of new nutrients (Dugdale and Goering, 1967) generates extensive diatom blooms (Margalef, 1978) and an uncoupling between the primary and secondary producers (Hutchings et al., 1995) that results in significant export flux to the sediments (Berelson et al., 1987). As such, the diatom record should preserve a great deal of information related to past productivity conditions (Schrader and Sorknes, 1990; Abrantes, 2000). Besides the works of Schuette (1980) and Abrantes (1988), Abrantes and Moita (1999) have shown that upwelling assemblages are marked by the dominance of the genus Chaetoceros, which spores are generally well preserved in the sediments. Variations in diatom abundance, and in particular of the genus Chaetoceros, may be used to trace upwelling variability along the most marginal sites. Indications of cold- and warm-water masses are based on the presence of oceanic/pelagic species with specific temperature distribution. The presence of benthic diatoms will be noted in order to assess the amount of displacement of shallow (<100 m water depth) material.

Methods

Smear slides were examined on a routine basis for stratigraphic markers. When required (because of low concentration of specimens), samples were processed by boiling them in hydrogen peroxide and hydrochloric acid, then cleansing them of acid by centrifuging (at 1200 rpm for 2-4 min), decanting off the liquid, washing in distilled water, and repeating that cycle four or more times, following the procedures of Baldauf (1984).

To remove the clay-sized particles, the residue was carefully shaken in a mixture of 0.5% sodium pyrophosphate and the procedure of centrifuging and decanting was repeated three times. Strewn slides of the clean material were prepared using Norland optical adhesive and cured in ultraviolet light for 3 min.

The entire microscope slide was routinely examined on a Zeiss Axioplan microscope equipped with differential interference contrast (DIC) at 630x magnification to check for the presence of biostratigraphic marker species. Taxonomic identifications were routinely checked at 1000x magnification.

Abundance of diatoms was based on the number of specimens observed per field of view at 630x and noted as follows:

A = abundant (10 or more valves per field of view).
C = common (1-9 valves per field of view).
F = few (>1 valve per each vertical traverse but <1 specimen per field of view).
R = rare (>3 valves on slide but <1 specimen per each vertical traverse).
T = trace (<3 valves observed on slide and/or appearance of fragments).
B = barren (no valves observed on slide).

Preservation of diatoms was determined qualitatively and recorded as follows:

G = good (both thinly and heavily silicified forms; robust forms are present and no alteration of the frustules was observed).
M = moderate (thinly silicified forms are present but exhibit some alteration).
P = poor (thinly silicified forms are rare or absent; robust forms dominate the assemblage).

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