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Maria Marino, José-Abel Flores, Leg 177 Shipboard Scientific Party:
Middle Eocene-Early Oligocene calcareous nannofossils at Site 1090 (ODP Leg 177)
(Poster)

Site 1090 is located in the central part of the Subantarctic Zone, on the southern flank of the Agulhas Ridge (42°54.8212'S, 8°53.9842'E). The water-depth (3702m) places it near the boundary between the North Atlantic Deep Water and underlying Circumpolar Deep Water, and above the CCD.

Hole 1090B was drilled to 397.5mbsf, spanning the Holocene to Middle Eocene. Semiquantitative analyses were carried out on Early Oligocene-Middle Eocene calcareous nannofossil assemblages in order to obtain detailed biozonal attributions and nannoflora variation through the Eocene/Oligocene boundary. Excellent palaeomagnetic data are available for this time interval, even cores which were recovered by XCP, promising for detailed correlation of biostratigraphic datums to the geomagnetic polarity time-scale.

The calcareous nannofossil assemblages are characterised by the presence of both high-latitude species (Reticulofenestra oamaruensis) and lower- to mid-latitude species (Ericsonia spp. and Discoaster spp.). Species diversity in the nannofossils ranges from 3 to 30 taxa, depending on a severe dissolution phenomenon which prevented the recognition of Chiasmolithus at species level.

Standard zonations of Martini and Okada & Bukry, as well as that of Wei & Wise were adopted, allowing recognition of several events from CP14 (or NP15) to CP16 (or NP22). The bottom of Site 1090 shows a Middle Eocene nannofossil assemblage with Reticulofenestra umbilica; the NP15/NP16 boundary is approximated by the FO of Reticulofenestra bisecta rather than the LO of Rhabdosphaera gladius that was absent from the samples. The rarity and scattered presence of Discoaster bifax, and the low reliability of the LO of Chiasmolithus solitus, caused by unclear taxonomic criteria for this marker and by poor preservation of Chiasmolithus specimens, imposed the combination of both CP14a-CP14b (Okada & Bukry), NP16-NP17 (Martini), as well as the Discoaster saipanensis-Reticulofenestra reticulata biozones of Wei & Wise. The FO of Reticulofenestra reticulata and the LO of Nannotetrina cristata occur just below the FO of R. bisecta, and the LO of Chiasmolithus grandis occurs at the top of the Middle Eocene interval.

The Late Eocene assemblage is characterised by diversified and well-preserved nannofloras, so that several events were recognised for standard biozonal attribution, from NP18 to NP19-20 (CP15a-CP15b): FO of Chiasmolithus oamaruensis, LO of R. reticulata, FO of Isthmolithus recurvus and Reticulofenestra oamaruensis, LO of Discoaster saipanensis and R. oamaruensis. These events also allowed us to identify the C. oamaruensis, I. recurvus and R. oamaruensis Biozones of Wei & Wise.

The end of the acme of Clausicoccus, LOs of I. recurvus and Ericsonia formosa, and LO of R. umbilica in the Early Oligocene interval at Site 1090 allowed us to recognise the standard biozones NP21 and NP22, CP16a-b-c, and the Blackites spinosus and Reticulofenestra daviesii biozones of Wei & Wise. Moreover, a clear acme of Chiasmolithus, Clausicoccus and Blackites occurs at the base of the Oligocene; comparable abundance patterns for these genera have previously been shown at different latitudes.

Site 1090 represents a good opportunity to improve nannofossil biostratigraphy at southern high-latitude and to verify diachronism of events relative to the middle and low latitudes, even if the correlation of events to the palaeomagnetic record seems to show no diachronism for most nannofossil datums. Moreover, the Eocene/Oligocene boundary is characterised by continuous sedimentation, reliable nannofossil events (LO of R. oamaruensis and D. saipanensis) and excellent magnetic data.

No abrupt change was observed in nannoflora abundance and species diversity just across the Eocene/Oligocene boundary; moreover, both cold, warm as well as temperate taxa show comparable abundance patterns, probably as effect of dissolution which may have modified the original nannofloral composition.

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 [Division of Micropalaeontology] [Department of Geosciences] [Bremen University]

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