BIOSTRATIGRAPHY

ODP Leg 183 drilled two formerly united geological features, the Kerguelen Plateau and Broken Ridge, which rifted apart during the Eocene and now lie at rather different southern latitudes (~30S and 45-65S latitude, respectively). As a consequence, for the calcareous microfossil groups examined on board ship (calcareous nannofossils and planktonic foraminifers), rather different zonations were applied for age-dating sediments in these two regions. For instance, for the Neogene interval, the nannofossil zonation given in the revised Cenozoic geochronology and chronostratigraphy of Berggren et al. (1995) and the temperate planktonic foraminiferal zonation of Srinivasan and Kennett (1981) were in general directly applicable for Broken Ridge but not for the Kerguelen Plateau, where higher latitude schemes worked out by previous workers in that area were more suitable. For this reason, two somewhat different zonations for the calcareous microfossil groups, one for Broken Ridge and the other for the Kerguelen Plateau, are presented for Neogene sediments drilled during this cruise (Figs. F5 and F6, respectively). In these cases, we inserted the mid- or high-latitude zonations alongside the low-latitude schemes to aid in comparisons and interpretation.

In Figure F6, high-latitude species, groupings of species, or species concepts that are used for biostratigraphic datums are indicated in bold type to distinguish them from low-latitude datums (which are shown in plain type). The same is true of any differences in age calibrations between the high- and low-latitude datums. Foraminiferal biostratigraphic datums are underlined to distinguish them from nannofossil datums.

The shipboard palynomorph biostratigraphy was primarily concerned with nonmarine Cretaceous sequences. We discuss this separately below.

Preliminary age assignments made on board ship were based primarily on biostratigraphic analyses of calcareous planktonic microfossils from core-catcher samples, which were then supplemented as time permitted with additional samples taken within the cores. Estimates of biostratigraphic ages were calibrated against the magnetic polarity time scale of Cande and Kent (1995). As noted above, the time scale of Berggren et al. (1995) was applied to the Cenozoic where applicable. The time scale of Gradstein et al. (1994) was used for the Cretaceous. Age estimates for Cretaceous calcareous nannofossil datums traceable to the low latitudes were taken from Erba et al. (1995), which are calibrated by the Gradstein et al. time scale (1995). Cretaceous austral zonations are used where applicable for calcareous microfossils; however, none of these have yet been calibrated against the Gradstein et al. time scale, and, thus, their correlations with low latitude zonal schemes are highly tentative.

Calcareous Nannofossils

Biostratigraphy

For the Cenozoic, the nannofossil biostratigraphic framework was provided by the zonal schemes of Martini (1971; with modifications by Martini and Mller, 1986) and Bukry (1973, 1975; zonal code numbers added and modified by Okada and Bukry, 1980). As noted above, these schemes were applied directly for the Broken Ridge site but were condensed and/or modified as necessary for the Kerguelen Plateau sites, particularly for the Neogene, Oligocene, and Danian.

Pospichal et al. (1992) illustrated decreasing biostratigraphic resolution in nannofossil zonations from the low to high latitudes of the South Atlantic. The greatest loss occurs in the Neogene, where little stratigraphic control could be achieved between 51S and 65S. Wei and Wise (1992) summarized and calibrated to the paleomagnetic time scale a few usable Neogene high-latitude nannofossil datums detectable on the Kerguelen Plateau and elsewhere in the Southern Ocean. These are indicated on Figure F6 alongside the low-latitude zonation of Bukry for comparison, where they are labeled "Leg 183" and "Low Lat." respectively; in this figure the datums have been recalibrated for comparison with the Berggren et al. (1995) time scale (see discussion below). With so few datums, however, most Neogene nannofossil zones have had to be combined into about five total zones at these latitudes.

High-latitude nannofossil zonations with moderate resolution have been developed for the Oligocene to mid-middle Eocene (Wise, 1983; Wei and Wise, 1990; Wei and Thierstein, 1991), and these have been inserted into Figure F6. Ages for key datum levels have been calibrated in the region of the Kerguelen Plateau against magnetostratigraphy by Wei (1992); these are indicated in bold type in Figure F6, where they have been recalibrated against the Berggren et al. (1995) time scale.

As noted by Wei (1992), biomagnetostratigraphic correlations at several Southern Ocean sites may show considerably different ages relative to those compiled from the mid-latitudes by Berggren et al. (1985, 1995). Where such differences exist, we have, in most instances, chosen to use ages derived from the high-latitude calibrations against the magnetostratigraphy. As noted above, where such ages differ from those in the lower latitudes, the high-latitude ages are shown in bold type in Figure F6 following the corresponding datum level (similarly, high-latitude biostratigraphic datums are also indicated in bold type). For major differences in age assignment, arrows indicate where on the chart a datum has been repositioned for proposes of this study.

As Pospichal et al. (1992) indicate, biostratigraphic resolution increases down column from the mid-Eocene through the Paleocene because global climates were warmer then, nannofossil diversities higher, and the zonations for that interval were largely defined in temperate rather than tropical regions. Thus, the standard zonal compilations of Martini and Bukry cited above can be applied with relatively few modifications, although subzones may not be discernible. Unfortunately, few sections for that interval from the Kerguelen Plateau are available for age calibration because of poor core recovery (mostly a result of the presence of cherts or condensed intervals) or to the lack of detailed paleomagnetic studies. Thus, the biomagnetostratigraphic high-latitude correlations for the mid-Eocene-Paleocene given by Wei (1992, fig. 3) are from Broken Ridge or the Atlantic sector of the Southern Ocean only. For this reason, we indicate in Figure F6 no changes in the calibrations of Berggren et al. (1995) for this part of the column. Wei and Pospichal (1991) do, however, provide a useful lower Paleocene Antarctic zonation based on ODP Holes 690C (Leg 113: Maud Rise) and 738C (Leg 119: southern Kerguelen Plateau). Their zonation (nannofossil Zones NA1-NA6) has been inserted into Figure F6 with our best estimate for correlations to the current paleomagnetic time scale. We note, however, that little detailed paleomagnetic work is available on their sections.

The nannofossil zonation employed for the Upper Cretaceous during Leg 183 is that of Watkins et al. (1996), a high-latitude scheme developed for the Southern Ocean based on DSDP/ODP drilling throughout this region. In Figure F6, high-latitude zones used during Leg 183 have been inserted alongside the low-latitude zonal scheme compiled by Sissingh (1977) ("CC" zones) as modified or embellished by Perch-Nielsen (1985a). As Watkins et al. (1996) point out, the upper part of their zonation has the highest resolution and reliability, but is based nearly exclusively on high-latitude taxa; thus, it is highly provincial. For instance, only one of the uppermost eight nannofossil datum levels can be correlated directly to the low or mid-latitudes because of the marked endemism that characterizes the assemblages and biostratigraphic datums. Huber and Watkins (1992) discuss paleoceanographic scenarios that may account for this circumstance. On the other hand, there is some paleomagnetic control for this part of the Cretaceous section, and trial correlations with magnetic stratigraphy based primarily on ODP Site 690 in the Weddell Sea are indicated in Figure F6. We have also been guided in part by the correlation charts of Southern Ocean datums compiled by Cita et al. (1997). All of these correlations are highly tentative, however, and will be revised to the extent possible based on shore-based research on the Leg 183 materials.

Below the Campanian, the Upper Cretaceous zonation of Watkins et al. (1994) becomes more cosmopolitan in character, with more direct ties to low-latitude nannofossil zonations. No useful correlation with magnetostratigraphy is possible, however, because of the presence of the long Cretaceous normal superchron. Correlations with European stratotype stages in many cases are only approximate, particularly for the high-latitude index taxa; some of this uncertainty is indicated by the dashed zonal boundary lines and question marks attached to high-latitude biostratigraphic datums. Only their relative positions in the succession are known at this time.

The Lower Cretaceous (Albian) Austral zonation considers primarily the high-latitude zones of Wise (1983) within the context of the cosmopolitan nannofossil zonation by Bralower (1992; see also Bralower et al., 1993, 1995, and the more recent compilation by Bown et al., 1998). These authors include the "NC" zonal numbering scheme initiated by Roth (1978). None of these zonal schemes, however, have previously been calibrated against the Gradstein et al. (1995) time scale; thus, their correlation here with that chronology is only approximate.

Methods

Calcareous nannofossils were examined using standard light microscope techniques, under crossed polarizers, transmitted light, and phase contrast light at 1000 magnification. Preservation and abundance of calcareous nannofossil species may vary significantly because of etching, dissolution, or calcite overgrowth. It is not uncommon to find nearly pristine specimens occurring in the same sample as specimens exhibiting overgrowth or etching. Thus, a simple code system to characterize preservation has been adopted and is listed below:

Six calcareous nannofossil abundance levels are recorded as follows:

Planktonic Foraminifers

Biostratigraphy

High-latitude planktonic foraminifer faunas, as observed by previous workers during DSDP/ODP drilling (Berggren, 1992a, 1992b; Huber, 1990, 1991, 1992; Stott and Kennett, 1990; Quilty, 1992; Krashenninikov and Basov, 1983; Sliter, 1977), typically exhibit low diversity and high dominance, and, thus, many of the key index species useful for biostratigraphic correlation at low latitudes are absent. Cretaceous and Cenozoic zonal schemes developed specifically for the subpolar Southern Ocean regions have, therefore, been applied to all Kerguelen Plateau pelagic sediment sections. Figure F6, shows approximate correlations between high-latitude provincial ("Leg 183") planktonic foraminiferal and tropical ("Low Lat.") zonations.

In contrast to the low-diversity assemblages encountered on the Kerguelen Plateau, Neogene planktonic foraminifers recovered from Site 1141 on Broken Ridge (32.2S) are diverse and show affinities with subtropical regions as well as with the Southern Ocean. At this site we were, therefore, able to employ a more detailed temperate planktonic foraminifer zonation based on a largely different suite of planktonic foraminifers.

Cenozoic

Berggren's (1992) Neogene Kerguelen zonal scheme is used for Miocene planktonic foraminifers, and the Antarctic Paleogene (AP) zonal scheme of Stott and Kennett (1990; modified by Huber, 1991, and Berggren, 1992) is applied to the Paleocene-Oligocene sections. In descriptions of the fauna at Broken Ridge, we refer to the temperate planktonic foraminifer zonation of Srinivasan and Kennett (1981), which is based on the Austral Cenozoic zonation of Jenkins (1971; see also Jenkins and Srinivasan, 1986, and Kennett and Srinivasan, 1983). The generic classification of Kennett and Srinivasan (1983) is used throughout the Neogene. For most of the Paleogene, we base our taxonomic concepts on the work of Tourmarkine and Luterbacher (1985), and refer to Jenkins' (1971) and Berggren's (1992) discussions on the classification of Austral and high-latitude forms. For Paleocene and early Eocene forms, taxonomic usage has been modified according to Olsson et al. (1999).

Mesozoic

Zonation of Upper Cretaceous sections (Maastrichtian-Turonian) is based on Cita et al.'s (1997) Upper Cretaceous biostratigraphy for the Southern Ocean. We modified this scheme during Leg 183 drilling within the original framework of Huber's (1992) Upper Cretaceous Austral realm zonal scheme. The rather impoverished Lower Cretaceous planktonic foraminiferal fauna, encountered for the first time on the Kerguelen Plateau at Site 1136, was compared to Albian assemblages from the Falkland Plateau (DSDP Sites 327 and 511) described by Sliter (1977), Krasheninikov and Basov (1983), and Bralower et al. (1993), within the framework of the mid-Cretaceous zonal scheme of Leckie (1984). Our generic classification follows that of Caron (1985).

Methods

Core-catcher samples of ~20 cm3 (plus additional samples where necessary) were soaked in 3% hydrogen peroxide with a small amount of Calgon added, warmed on a hot plate and desegregated by washing in tap water over a 63-m mesh sieve. Each sieve was dipped in a solution of methyl blue dye to identify contaminants from previous samples. All samples were dried over a hot plate at ~50C. The dried samples were examined under a binocular microscope and planktonic foraminifer faunal composition was recorded in nonquantitative terms based on an assessment of forms observed in a random sample of 200-400 specimens from the >63-m size fraction. Relative abundances were reported using the following categories:

Preservation of planktonic foraminifer assemblages was recorded as

Palynology

The main interest for palynomorph studies during Leg 183 was the Cretaceous terrestrial palynoflora, which records the advent and radiation of angiosperms during the early Albian in Antarctica. The relatively fast dispersal of angiosperms during the Late Cretaceous provides insight into the biological and geological evolution of the Southern Hemisphere, especially for the pattern of Gondwana fragmentation.

The time calibration employed here is based on data from ODP Legs 113 and 120 by Mohr and Gee (1992). Dinoflagellate-based palynostratigraphy will also be used to improve marine biostratigraphic correlations.

To concentrate the organic material, samples were first treated with 10% HCl to dissolve out carbonates. After several washings and centrifuging, high-strength HF was added to dissolve silicates. After further washing and centrifuging, the concentrated material was sieved with 20-m sieves. Samples were mounted on slides, using glycerin jelly and wax.

Preliminary analyses were performed only on core-catcher material. The shipboard results do not represent the total palynomorph content of the sediments.

The following abundance categories of total palynomorphs per sample were used:

Diatoms

Fossil diatoms were used sparingly to provide age dates for Neogene siliceous sections where little or no biostratigraphic control could be provided by calcareous microfossils. For this purpose, the biostratigraphic zonation of Harwood and Maruyama (1992) as modified and calibrated to the time scale of Berggren et al. (1995) for Leg 178 (Shipboard Scientific Party, 1999b) was followed, using smear slides prepared from raw sediment. That zonal scheme, which extends down to the middle Miocene, is shown in Figure F7. Below the middle Miocene, we referred to the zonation of Harwood and Maruyama (1992) as calibrated by datums compiled for Leg 177 by Gersonde, Hoddell, Blum, et al. (1999). Also useful were species ranges compiled by Gersonde, Hoddell, Blum, et al. (1999) shown in Figure F8.

Silicoflagellates

Silicoflagellates are most commonly preserved in diatomites underlying modern or ancient ocean upwelling areas or in diatomites preserved by nearby volcanism. The investigation of silicoflagellates often allows the dating of sediment from high-latitude areas, where calcareous micro- and nannofossils are missing or nondiagnostic. It also allows some conclusions as to the paleoenvironmental conditions under which the sediments were deposited.

Smear slides of unprocessed material provided the simplest way to obtain an initial overview of the occurrence and abundance of silicoflagellates. For more detailed work, we processed samples.

Silicoflagellates usually make up only a small proportion of the siliceous microfossils in a sediment sample. Therefore, the preparation technique should concentrate the siliceous material. On board ship, we placed core catcher in beakers and treated them first with a few drops of hydrogen peroxide to desegregate the sediment and to remove the organic carbon. Then 10% HCl in various volumes (depending on the type of sediment and the percentage of calcium carbonate) was added to dissolve most of the carbonate. The residue was washed, centrifuged, and decanted. This procedure was repeated a minimum of three times. Samples were then strewn on a slide, covered with a 22 mm 30 mm cover slip, and mounted using Norland 61 mounting media. To determine the abundance of silicoflagellates, all specimens that consisted of more than half a skeleton were counted.

The silicoflagellate zonation followed was that utilized by McCartney and Wise (1990) and McCartney and Harwood (1992). The compilation by Perch-Nielsen (1985b) was also helpful.

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