Sedimentological, micropaleontological and stable isotope investigation of the lower Oligocene in DSDP Core 40-363 from Walvis Ridge, southeastern Atlantic (Table 1)
The recurrence of Braarudosphaera chalks in the lower Oligocene sequences of the subtropical South Atlantic has been a long-standing conundrum, with many hypotheses having been advanced to explain the genesis of these exotic nannofossil assemblages. Here, we evaluate different paleoceanographic mode...
| Main Authors: | , , |
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| Format: | Dataset |
| Language: | English |
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PANGAEA
2003
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| Subjects: | |
| Online Access: | https://doi.pangaea.de/10.1594/PANGAEA.678291 https://doi.org/10.1594/PANGAEA.678291 |
| Summary: | The recurrence of Braarudosphaera chalks in the lower Oligocene sequences of the subtropical South Atlantic has been a long-standing conundrum, with many hypotheses having been advanced to explain the genesis of these exotic nannofossil assemblages. Here, we evaluate different paleoceanographic models within the context of stable isotope (delta18O, delta13C) data measured from bulk-sediment samples and well-preserved foraminifera. Two closely-spaced Braarudosphaera layers from a lower Oligocene (foram Subzone P21a, 29.4-28.5 Ma) section drilled in the southeastern Atlantic (DSDP Site 363) are investigated. Maximum durations for the blooms that deposited the lower and upper Braarudosphaera layers are estimated to be 1.1 and 2.2 k.y., respectively. Bulk-sediment samples enriched in braarudosphaerid carbonate exhibit pronounced delta18O increases on the order of 0.6-1.0‰ which we attribute to isotopic disequilibria driven by braarudosphaerid vital effects. The two Braarudosphaera layers straddle a single peak in benthic foraminiferal delta18O values, suggesting that these blooms may recur on glacial/interglacial timescales. This same pair of braarudosphaerid layers also occurs as a couplet bundled with prolonged (~6.7 k.y.) thermocline cooling, evidence that these stratigraphically distinct deposits may represent a 'split signal' for a single paleoceanographic/paleoclimatic event. Subsumed within this episode of subsurface cooling are two short-lived, negative excursions (~0.5‰) in the delta13C record of a thermocline-dwelling planktonic foraminifer that coincide with the braarudosphaerid layers. Thus, benthic-to-thermocline delta18O and delta13C gradients were reduced during the braarudosphaerid blooms, a hallmark signature for strengthened upwelling. Both braarudosphaerid layers are marked by transient divergences in the stable isotopic signals of two shallow-dwelling species of planktonic foraminifera. These transient delta18O offsets may reflect subtle differences in the depth ecologies of these two mixed-layer ... |
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