Abstract:
Glaciomarine sediments found around the Antarctic continental margin coupled with worldwide increases in foraminiferal delta (super 18) O records and marked fluctuations in global sea level indicate that Earth plunged into an unipolar, "ice-house" mode during the earliest Oligocene. As the Australo-Antarctic and Drake passages continued to widen, rapid expansion of Antarctic ice sheets caused dramatic changes in Southern Ocean circulation that triggered a major paleobiogeographic reorganization among the calcareous nannoplankton. Perhaps the most conspicuous of these biogeographical shifts occurred in the South Atlantic where numerous Braarudosphaera oozes and chalks blanketed large tracts of the seafloor. These deposits are enigmatic in nature as they are composed of near monospecific fragments of Braarudosphaera debris that exported significant carbonate to the seafloor. The iterative recurrence of Braarudosphaera deposits indicates that atypical hydrographic conditions prevailed repeatedly throughout the subtropics in the South Atlantic during the Early Oligocene (Rupelian age). Detailed study of calcareous nannofossil assemblages preserved within a lower Oligocene section recovered from atop the Rio Grande Rise (DSDP Site 516) has revealed the presence of four previously unidentified Braarudosphaera layers that contain nannofossil assemblages composed of up to 78% braarudosphaerid debris. Bulk-carbonate stable isotope records through these braarudosphaerid-enriched layers yields unique delta (super 18) O and delta (super 13) C enrichments that are attributed to early stages of marine "chalky" diagenesis and disequilibrium vital effects. High-resolution benthic foraminiferal (Cibicidoides spp.) stable isotope records through this section record rhythmic delta (super 18) O and delta (super 13) C oscillations that reflect orbitally paced ( approximately 41 k.y.) changes in Antarctic ice-sheet dynamics and thermohaline circulation. Moreover, benthic foraminiferal delta (super 18) O ratios systematically increase through all four braarudosphaerid layers, indicating that these enigmatic blooms coincided with the steady buildup of Antarctic ice sheets. Thus, all four of the braarudosphaerid-enriched layers are confined to half of one obliquity cycle, constraining their maximum duration to approximately 20 k.y. Parallel thermocline (Subbotina spp.) isotope records indicate that the water column over the Rio Grande Rise remained thermally stratified during the braarudosphaerid blooms. A strong component of glaciogenic intermediate waters analogous to modern Antarctic Intermediate Water flowed over the study site resulting in a sharp pycnocline that impeded the downward transport of organic-rich phytodetritus. This set of hydrographic conditions would allow for increased oxidation of organic matter within the water column, and may have fostered the development of subsurface braarudosphaerid blooms.