RECURRENCE OF ANOMALOUS BRAARUDOSPHAERA CHALKS IN EARLY OLIGOCENE SEQUENCES OF THE SOUTHWESTERN ATLANTIC OCEAN (DSDP SITE 516)

Abrupt increases in foraminiferal d¹⁸O records, marked falls in global sea level, and glaciomarine sediments peripheral to the Antarctic continental margin indicate that Earth climate plunged headlong into its unipolar "ice house" mode during the early Oligocene. This climatic shift resulted in steeper latitudinal temperature gradients, which in turn led to increased provincialism among the calcareous nannoplankton. A classic case of this provincialism is the numerous Braarudosphaera oozes/chalks blanketing vast tracts of the South Atlantic seafloor. Prolific blooms of these enigmatic protists repeatedly deposited extraordinary nannofloras composed almost entirely of braarudosphaerid pentaliths and fragments. The recurrence of these deposits in such prodigious fashion in open-ocean settings reflects rhythmic oscillations in sea-surface conditions. Foraminiferal d¹⁸O records spanning braarudosphaerid layers preserved in early Oligocene sequences of the southeastern Atlantic (Walvis Ridge) suggest that these subtropical deposits recurred on glacial/interglacial timescales (10³ – 10⁴ years) in response to intense ocean mixing immediately following episodes of Antarctic glaciation. Efforts are underway to confirm this relationship by generating parallel bulk-carbonate and foraminiferal stable isotope stratigraphies through part (~1.2 m.y.) of an early Oligocene (foram zones P18-P19) section recovered from the southwestern Atlantic (DSDP Site 516). Inspection of this material reveals the presence of at least three prominent braarudosphaerid layers. A bulk-carbonate d¹⁸O record through one of these layers shows a ~1.0‰ increase, a geochemical signature also seen in the southeastern deposits. We attribute much of this bulk d¹⁸O increase to braarudosphaerid vital effects, an interpretation that awaits testing with complementary foraminiferal d¹⁸O data. The recurrence of braarudosphaerid layers on both sides of the South Atlantic indicates that these unusual nannofossil deposits are of regional importance. Hence, understanding the paleoceanographic conditions that paced the formation of these deposits will shed light on the tempo and mode of ocean/climate/cryosphere evolution during the early Oligocene.