BIOSTRATIGRAPHY

Preliminary age assignments were based on biostratigraphic analyses of calcareous nannofossils, planktonic foraminifers, and radiolarians. At multiply cored sites, efforts were focused on Hole A, thereby allowing greater sampling density and development of a detailed shipboard biostratigraphy. Age constraints of calcareous nannofossil datums were determined by observing from one to six samples per core section (sampling spacing = 0.25–1.5 m) as well as core catcher samples. Planktonic foraminifers were examined in one sample per core section from Hole A (sampling density = ~1.5 m) in addition to core catcher samples from all holes. Radiolarians were studied mainly in core catcher samples. The preservation, abundance, and zonal assignment for each microfossil group as well as for selected samples were recorded in the stratigraphic site summary sheets and entered into the Janus database.

Recognition of Cenozoic Critical Intervals

In the Cenozoic, much emphasis has been placed on two critical intervals: the Eocene/Oligocene (E/O) boundary and the Paleocene/Eocene (P/E) boundary. Biostratigraphic indicators for each of these events are discussed below.

Eocene/Oligocene Boundary

The lower Oligocene is zoned in most detail by calcareous nannofossils (Fig. F5), although the E/O boundary is recognized by the last occurrence (LO) of the planktonic foraminifer genus Hantkenina. The E/O boundary is present in uppermost Chron C13r at the planktonic foraminifer Zone P16/P18 boundary and in the middle of calcareous nannofossil Subzone CP16a. We did not recognize planktonic foraminifer Zone P17 of Berggren et al. (1995a) because of taxonomic and preservational problems associated with identification of the LO of Cribrohantkenina inflata. Calcareous nannofossil datum levels within 2 m.y. of the E/O boundary (33.7 Ma) include the first occurrence (FO) of Reticulofenestra umbilica (31.7 Ma), the FO of Ericsonia formosa (32.9 Ma), and the start of the Ericsonia subdisticha acme (33.3 Ma). The onset of the Ericsonia obruta acme (33.7 Ma) coincides with the E/O boundary. The planktonic foraminifer Pseudohastigerina (top = 32.0 Ma) is considered a reliable datum for the lowermost Oligocene. The uppermost Eocene is easily determined by the presence of the calcareous nannofossils Discoaster saipanensis (top = 34.0 Ma) and Discoaster barbadiensis (top = 34.2 Ma) as well as by planktonic foraminifers Turborotalia cerroazulensis (top = 33.8 Ma) and Globigerinatheka spp. (top = 34.3 Ma). The base of the Theocyrtis tuberosa radiolarian zone (RP20) has been approximated to 32.8 Ma (Sanfilippo and Nigrini, 1998).

Paleocene/Eocene Boundary

The P/E boundary has, until recently, been placed at the planktonic foraminifer Biozone P5/P6 boundary, which is found in the middle part of calcareous nannofossil Zone NP10 (Subzone CP9a) (Aubry et al., 1996). In 2000, the International Geological Correlation Programme Project 308 membership (Paleocene/Eocene Events in Space and Time) voted to recognize the carbon isotope excursion associated with the Paleocene–Eocene Thermal Maximum (PETM) as the defining criterion for identifying the P/E boundary. For Leg 207, we will adhere to this definition of the P/E boundary.

The P/E boundary, as defined by the carbon isotope excursion, can be approximated by a series of calcareous microfossil datums (Fig. F5). These events include a major extinction event among benthic foraminifers (including Gavelinella beccariiformis, Aragonia velascoensis, and Osangularia velascoensis) precisely at the boundary. The boundary can also be approximated by the presence of the planktonic foraminifer "excursion fauna," which includes Acarinina africana, Acarinina sybiaensis, and Morozovella allisonensis (Kelly et al., 1996, 1998). The planktonic foraminifer Pseudohastigerina wilcoxensis and large specimens of Chiloguembelina wilcoxensis also have FOs close to the boundary (Speijer et al., 1997). The interval ~500–700 k.y. above the P/E boundary is marked by the LOs of Morozovella velascoensis (marking the top of planktonic foraminifer Zone P5) and Morozovella occlusa as well as the FOs of Morozovella gracilis and Acarinina wilcoxensis.

The extinction of the calcareous nannofossil genus Fasciculithus occurs not far above the boundary, and the lineage Rhomboaster–Tribrachiatus evolves somewhat farther above the boundary; the FO of Campylosphaera eodela is observed shortly below. Thus, from a biostratigraphic point of view, the P/E boundary falls in planktonic foraminifer Biozone P5 and in calcareous nannofossil Biosubzone CP8b and Biozone NP9.

During drilling, however, it is desirable to know from core catcher samples when the P/E boundary is being approached and to confirm its capture in a core as accurately as possible in order to determine whether to take whole-round interstitial water samples. The planktonic foraminifers M. velascoensis and M. occlusa suggests proximity to the P/E boundary, whereas members of the excursion fauna indicate the sample comes from the ~200-k.y. interval immediately following the boundary. The benthic foraminifers A. velascoensis, G. beccariiformis, or O. velascoensis shows that the boundary has been passed and is either in the recovered core or is unconformable. For calcareous nannofossils we can detect the dominance change or crossover among the nannofossils downhole from Zygrhablithus bijugatus and Neochiastozygus junctus (whose high abundances extend some distance above the boundary in Zone NP9) to the abundant and varied fasciculiths below (see Bralower, 2002, regarding the evolutionary behavior of the latter two taxa). This technique worked so well that we have further subdivided Martini (1971) Zone NP9 into an upper NP9b and a lower NP9a Subzone based on this crossover in dominance/abundance (see Fig. F5). As further indicated in that figure, we have also made a similar division of the number-coded Okada and Bukry (1980) scheme in Subzone CP8b and divided it into Subsubzones CP8bB and CP8bA.

All of these biostratigraphic events are present in the long interval of Chron 24r. According to our timescale (Cande and Kent, 1995), this interval of uniform reversed polarity has a duration of 2.557 m.y. Recent cyclostratigraphic work (Norris and Rhl, 1999; Rhl et al., 2000) has shown that the carbon isotope excursion, and therefore the P/E boundary, is ~1 m.y. above the top of Chron C25n. Thus, it has an age of 55 Ma according to the Leg 207 timescale.

Hole 1051A (Leg 171B), in the western North Atlantic, is the only recovered P/E boundary section in which radiolarians have been studied (Sanfilippo and Blome, 2001). In this mid-latitude fauna, many tropical zonal markers are missing and others are diachronous with their tropical equivalents. There is no gross change in the composition of the fauna and only a minor increase in the number of FOs and LOs across the PETM and P/E boundary.

Sanfilippo and Nigrini (1998) established the stratigraphic sequence of the 70 lowest and highest radiolarian occurrences in a 10-m.y. interval spanning the P/E boundary from the Paleocene Bekoma campechensis Zone (RP6) to the upper part of the lower Eocene Buryella clinata Zone (RP8) and related them to the calcareous nannofossil zonation. Although none of their investigated tropical sequences contained the actual P/E boundary, they determined that there are six reliable, easily recognized, and potentially useful radiolarian FOs that approximate the P/E boundary: Calocyclas castum, Theocotylissa auctor, Lamtonium fabaeforme, Podocyrtis (Podocyrtis) papalis, Giraffospyris lata, and Phormocyrtis turgida.

Recognition of Cretaceous Critical Intervals

In the Cretaceous, "critical boundaries" include the following:

  1. The Cretaceous/Tertiary (K/T) boundary (65.5 Ma);
  2. The mid-Maastrichtian (~69 Ma); and
  3. Two oceanic anoxic events (OAEs), OAE 3 (~83.5–89.0 Ma) and OAE 2 (~97.6–93.5 Ma).

Biostratigraphic indicators for each of these events are discussed below.

Cretaceous/Tertiary Boundary

The K/T boundary is marked by one of the largest mass extinctions in the last 200 m.y. of Earth's history. The extinction level is about halfway through Magnetochron C29r and ~250 k.y. below the base of Chron C29n. The top of the Maastrichtian is recorded in the extinction of ~95% of the planktonic foraminifers, including the globotruncanids, rugoglobigerinids, and the large serially coiled planktonic foraminifers. The nearly exclusive presence of minute planktonic foraminifers, typically <63 m in diameter, are characteristic of the first Paleogene sediments above the K/T boundary. These Danian species include Parvulorugoglobigerina eugubina, Guembelina cretacea, and Heterohelix globulosa. Previous zonations have recognized a Zone P0 dominated by G. cretacea between the K/T boundary and the first appearance of P. eugubina (~64.97 Ma). However, reexamination of the classic K/T boundary section at el Kef, Tunisia, demonstrated that rare specimens of P. eugubina are present directly above the extinction level of the Cretaceous foraminifers, suggesting that Zone P0 reflects an increased abundance of P. eugubina rather than its evolutionary first appearance (Norris et al., 1999). Calcareous nannofossils also display a significant extinction across the K/T boundary, which is bracketed by the FOs of Cruciplacolithus tenuis (64.5 Ma) and Cruciplacolithus primus (64.8 Ma) above the boundary and the FO of Micula prinsii (65.4 Ma) below the boundary.

Mid-Maastrichtian Interval

Widespread and presumably related isotopic, sedimentological, and paleontological changes are concentrated in the middle of the Maastrichtian (e.g., Barrera and Savin, 1999; Frank and Arthur, 1999; MacLeod and Huber, 2001). However, there are currently no established criteria for defining the interval and erecting a definition poses some challenges. Some associated changes are graded over millions of years (e.g., high-latitude cooling), others are markedly diachronous (e.g., the LO of bathyal inoceramids), others are not expressed in all areas (e.g., carbon isotopic excursions among benthic foraminifers), and still others are poorly dated (e.g., the collapse of rudist reefs). These uncertainties notwithstanding, the decline in abundance among inoceramids can first be resolved in the upper part of Chron 31r in the subtropical North Atlantic and Tethys. Except for Tenuipteria, inoceramids disappear in Chron 31n. This interval coincides with diversification among Tethyan planktonic foraminifers, increased isotopic gradients among planktonic foraminifers on Blake Nose, and proposed changes in the North Atlantic carbonate compensation depth. Thus, during Leg 207, mid-Maastrichtian changes are expected to be seen in the upper portion of Chron 31r, in Zone CC25, and near the FO datum of the planktonic foraminifer Abathomphalus mayaroensis.

Santonian/Campanian and Turonian/Coniacian Boundaries (Oceanic Anoxic Event 3)

The interval between the base of the Coniacian and the base of the Campanian marks an interval of elevated deposition of organic carbon in the tropical Atlantic and South Atlantic that is sometimes referred to as OAE 3. The lower Campanian is defined by a series of nannofossil events that includes the LO of Marthasterites furcatus (~80.6 Ma) and the FOs of Ceratolithoides verbeekii (82.0 Ma), Aspidolithus parcus constrictus (82.5 Ma), and Aspidolithus parcus parcus (83.4 Ma). The top of the Santonian is approximated by the first appearance of the nannofossil Arkhangelskiella cymbiformis (~83.5 Ma). The planktonic foraminifer Dicarinella asymetrica (83.0 Ma) has its LO just above the base of the Campanian, whereas the FO of Globotruncana elevata (84.8 Ma) marks the middle of the Santonian. We did not find D. asymetrica at any Leg 207 site and resorted to using alternative markers to approximate Santonian Zone KS24. In particular, we used the presence of Rosita fornicata to mark Santonian-age black shales. However, R. fornicata is reported to range into the middle Coniancian by some authors (e.g., Premoli Silva and Sliter, 1999), whereas others extend its range only to the base of the Santonian (e.g., Robaszynski et al., 1984). Hence, it is entirely possible that samples we identify as Santonian may represent the middle or upper Coniacian.

The Turonian/Coniacian boundary (89.0 Ma), which approximates the onset of OAE 3 is slightly predated by the FO datum of M. furcatus (89.3 Ma). The FO of Dicarinella concavata (89.5 Ma) can also be used to recognize the uppermost Turonian. We also used the FO of Dicarinella primitiva to approximate the Turonian/Coniacian boundary. However, there is some disagreement in the lowest stratigraphic range of D. primitiva between recently published biostratigraphies, with some authors extending the FO datum into the upper Turonian (Premoli Silva and Sliter, 1999), whereas others infer a range only to the base of the Coniacian (Robaszynski et al., 1984). Hence, it is possible that any disagreements in age between planktonic foraminifers and calcareous nannofossils may be due to uncertainties in the published ages of various datums.

Oceanic Anoxic Event 2

OAE 2 falls in the lower part of calcareous nannofossil Zone CC11 and the upper part of Zone CC10. The boundary itself is marked by the FO of Quadrum gartneri (93.2 Ma). The upper Cenomanian range marker Lithraphidites acutus was observed only once on board the ship in Leg 207 sediments. Its LO is closely approximated, however, by the LOs of Corollithon kennedyi and Axopodorhabdus albianus. Although rare themselves in this part of the section, they, particularly the latter, were used in this study. As such, the LO of A. albianus provided a useful means to subdivide Zone CC10, which we propose here as a lower Subzone CC10a and an upper Subzone CC10b (see Fig. F5). The lowermost Turonian is marked by the LOs of the planktonic foraminifers Marginotruncana renzi (92.5 Ma) and Helvetoglobotruncana helvetica (93.0 Ma). Unfortunately, we did not find any specimens of H. helvetica at Leg 207 sites and so have approximated the OAE 2 interval based on the following alternate criteria: (1) the absence of rotaliporids; (2) the presence, sometimes exclusively, of Whiteinella archaeocretacea; and (3) the disappearance of virtually all planktonic foraminifers in the "dead zone," which is associated with what we assume are the most extreme anoxic conditions during the OAE. The upper Cenomanian is marked by the LO of Rotalipora cushmani (94.0 Ma) and the presence of Rotalipora greenhornensis and Whiteinella sp. Whiteinellids (mostly four-chambered Whiteinella baltica) range as low as the middle R. cushmani Zone (Zone KS19). At Leg 207 sites, it also appears that large inflated forms of H. globulosa range as low as the Cenomanian/Turonian boundary or perhaps into the lower Turonian part of the W. archaeocretacea Zone (KS20).

Zonal Schemes and Taxonomy

Calcareous Nannofossils

The zonal scheme of Martini (1971) was used for Cenozoic calcareous nannofossil biostratigraphy. This zonation represents a general framework for the biostratigraphic classification of mid- to low-latitude nannofossil assemblages and is presented in Figure F5. Ages and sources for Cenozoic calcareous nannofossil datums are presented in Table T1. The age estimates presented are all adjusted to the timescale from Leg 207. The zonation of Bukry (1973, 1975) (zonal code numbers CN and CP added and modified by Okada and Bukry, 1980) is also shown for reference. As explained previously in "Paleocene/Eocene Boundary" in "Recognition of Cenozoic Critical Intervals," we have further subdivided herein Zone NP9 of the Martini (1971) zonal compilation scheme and Subzone CP8b of the Okada and Bukry zonation.

The zonal schemes of Sissingh (1977) (CC Zones), as modified by Perch-Nielsen (1985) and Burnett (1999) (UC Zones), were used for the Upper Cretaceous (Table T2). Those of Roth (1978, 1983) (NC Zones) with subdivisions by Bralower et al. (1993) were used for the Lower Cretaceous (Table T2). As noted previously in "Oceanic Anoxic Event 2" in "Recognition of Cenozoic Critical Intervals," we have further subdivided Zone CC10 of the upper Cenomanian into Subzones CC10a and CC10b.

All of these zonations represent a general framework for the biostratigraphic classification of mid- to low-latitude nannofloral assemblages and are shown in Figure F5. Cenozoic nannofossil taxonomy follows that of Perch-Nielsen (1985) and Bown (1999). Cretaceous nannofossil taxonomy follows that used in Bown (1999), where full taxonomic lists can be found.

Planktonic Foraminifers

The tropical planktonic foraminifer zonal scheme (N and P zones) for the Cenozoic follows Berggren et al. (1995b) and is illustrated in Figure F5. Ages and sources for Cenozoic planktonic foraminifer datums are presented in Table T3. Cenozoic taxonomic concepts selectively follow Postuma (1971), Kennett and Srinivasan (1983), Bolli and Saunders (1985), Toumarkine and Luterbacher (1985), Spezzaferri and Premoli Silva (1991), Chaisson and Leckie (1993), Leckie et al. (1993), Spezzaferri (1994), Pearson (1995), Berggren and Norris (1997), Chaisson and Pearson (1997), Pearson and Chaisson (1997), Norris (1998), and Olsson et al. (1999). Genus-species combinations generally follow those rules used by Berggren et al. (1995b), with few modifications. The zonation used for Cretaceous planktonic foraminifers is based on the tropical zonal schemes of Caron (1985) and Sliter (1989) (KS Zones) with modifications by Bralower et al. (1993, 1995, 1997) and Premoli Silva and Sliter (1994, 1999) (Fig. F5). Age estimates for planktonic foraminiferal datum markers were obtained from Erba et al. (1995), Bralower et al. (1997), and Premoli Silva and Sliter (1999) (Table T4). Cretaceous taxonomic concepts are based on Longoria (1974), Robaszynski et al. (1979, 1984), Leckie (1984), Caron (1985), Nederbragt (1990, 1991), and Petrizzo (2000).

Radiolarians

Leg 207 Cenozoic radiolarian biostratigraphy is based largely on the radiolarian zonation and code numbers that are tied to the geomagnetic polarity timescale (GPTS) of Cande and Kent (1995) and documented by Sanfilippo and Nigrini (1998) (Fig. F5). Supplemental markers, also derived from Sanfilippo and Nigrini (1998), are used whenever possible and are correlated with the data supplied by calcareous nannofossils. Primary and supplemental datums are listed in Table T5 and illustrated in Figure F5. Upper Cretaceous radiolarian biostratigraphy (Table T6) is based on the zonation of Sanfilippo and Riedel (1985), O'Dogherty (1994), and Erbacher and Thurow (1998).

Methods of Study

Calcareous Nannofossils

Calcareous nannofossils were examined on smear slides using standard light microscope techniques under crossed nicols and transmitted light at 1000x magnification. The following abbreviations are used to describe nannofossil preservation:

G = good (little or no evidence of dissolution and/or recrystallization; diagnostic characteristics fully preserved).
M = moderate (dissolution and/or secondary overgrowth; partially altered primary morphological characteristics; most specimens were identifiable to the species level).
P = poor (severe dissolution, fragmentation, and/or overgrowth; primary morphological characteristics largely destroyed; specimens often cannot be identified at the species and/or generic level).

Abundances of calcareous nannofossil were estimated and their abundance levels recorded as follows:

A = abundant (the taxonomic category constitutes >10% of the total assemblage).
C = common (the taxonomic category constitutes from 1% to 10% of the total assemblage).
F = few (the taxonomic category constitutes from 0.1% to <1% of the total assemblage).
R = rare (the taxonomic category constitutes <0.1% of the total assemblage).
B = barren.

Foraminifers

Foraminifers from unlithified ooze were soaked in a 3% solution of hydrogen peroxide (with a small amount of Calgon added), warmed on a hot plate, and washed with tap water over a 45-m sieve. Semilithified ooze and chalk were first partially fragmented by hand then soaked in hydrogen peroxide and Calgon before washing. Hard chalk was pounded with a hammer into pea-sized chunks and boiled on a hot plate for 1 or 2 min in a combination of either peroxide and Calgon or peroxide and dishwashing detergent.

Repeated boiling and washing over a sieve was necessary to disaggregate many black shale samples. We tried the following methods, all with limited success:

  1. We placed samples in a freeze-dryer for ~12–24 hr and then soaked them in peroxide and detergent.
  2. We freeze-dried samples as above and soaked them in either methanol, acetone, or Quaternary-O (an industrial detergent).
  3. We boiled samples in detergent and peroxide and then scrubbed the rock fragments through a 710-m screen over a 45-m screen.

Of these methods, none produced consistently high-quality results. Freeze-drying worked extremely well with clay-rich Albian samples from Site 1258 but did little to improve sample preparation in most organic-rich black shale samples. Acetone proved to be the best solvent but was not effective in separating clay from foraminifers. Boiling in a detergent/peroxide mixture was used the most, but even this method often required five or six washing/soaking/boiling cycles to remove the majority of the clay fraction. The use of 30% peroxide was effective after several washing cycles but contributed to severe dissolution of the foraminifers. We had three major problems:

  1. The foraminifers are frequently coated with a tarlike organic fraction that is difficult to remove by standard washing procedures, even with repeated wetting/drying cycles.
  2. Many samples contain thin diagenetic calcite or phosphate seams that partly cement the foraminifers.
  3. Layers of foraminifer sands interbedded with black shales are strongly cemented with calcite spar.

We do not expect to be able to extract foraminifers in calcite or phosphate cements during postcruise work, but it may be possible to remove the organic fraction using turpentine, bleach, and dry-cleaning solution.

All samples were dried in a low-temperature oven at ~50C. Species identification for planktonic foraminifers was generally made on the >250- and >150-m size fractions. Benthic foraminifers were not examined in detail except for the interval around the P/E boundary, where the ranges of species that become extinct near the PETM were noted.

We did not attempt to estimate relative abundances except in a general nonquantitative fashion. Instead, we noted the presence or absence of the major marker species and the distinctive or dominant species in a sample. The pressure of processing samples in a timely fashion prevented detailed and comprehensive study of all samples. Some samples were examined only to identify the biozone marker species, whereas others were described more comprehensively to give the reader a sense for the species diversity and composition present in each zone.

Preservation status of the planktonic and benthic foraminifers was estimated as follows:

Glassy = very good (no evidence of overgrowth, infilling, dissolution, or abrasion; dry tests translucent. Specimens did not necessarily have to be completely clean in our samples to be described as "glassy").
G = good (little evidence of overgrowth, dissolution, or abrasion).
M = moderate (calcite overgrowth, dissolution, or abrasion were common but minor).
P = poor (substantial overgrowth, dissolution, or fragmentation).
B = barren (no planktonic foraminifers present).

We also use the informal designation "rocks" for samples where 80%–90% of the particles in a sample are unidentifiable pieces of rock.

Radiolarians

Core catcher samples were disaggregated by gentle boiling in a solution of 10% H2O2 and ~5 g of tetrasodium pyrophosphate. The solution was passed through a 63-m sieve. Calcareous components were dissolved by adding a 10% solution of hydrochloric acid and sieving again. A strewn slide was prepared by pipetting the microfossils onto a microscope slide, allowing the water to evaporate, adding a drop or two of xylene and some Norland optical adhesive to the slide, and covering the slide with a 22 mm x 40 mm glass coverslip.

Overall radiolarian abundances were determined based on strewn-slide evaluation at 100x, using the following conventions:

A = abundant (>100 specimens per slide traverse).
C = common (51–100 specimens per slide traverse).
F = few (11–50 specimens per slide traverse).
R = rare (1–10 specimens per slide traverse).
T = trace (<1 specimen per slide traverse).
B = barren (no radiolarians in sample).

Preservation was recorded as follows:

G = good (majority of specimens complete, with minor dissolution, recrystallization, and/or breakage).
M = moderate (minor but common dissolution, with a small amount of breakage of specimens).
P = poor (strong dissolution, recrystallization, or breakage; many specimens unidentifiable).

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