Abstract:
Four prominent Braarudosphaera-rich layers were identified in an early Oligocene (planktic foraminiferal Zone P21a, 28.5-29.4 Ma) section recovered from atop the Walvis Ridge in the southeastern Atlantic at Ocean Drilling Program (ODP) Site 1263. Continuous core recovery and good preservation of the foraminifera made it possible to generate three depth-stratified, high-resolution stable isotope records. In addition, parallel records of planktic foraminiferal abundance counts, weight % coarse fraction (> 63 mu m), magnetic susceptibility, and bulk-carbonate stable isotopes records were compiled through the study section. Using these data, two new models are proposed to explain the initiation, recurrence and decline of the Braarudosphaera blooms in the South Atlantic during the early Oligocene. Dramatic increases in the bulk delta (super 18) O record ( approximately 1ppm) are a hallmark of abundant Braarudosphaera debris, indicating post-depositional diagenesis and/or isotopic disequilibria due to vital effects. The Braarudosphaera layers are also coincident with decreases in both magnetic susceptibility and weight % coarse fraction values, indicating the blooms provided a large amount of fine-fraction carbonate to the sea floor. Negative excursions in the Catapsydrax (deep-thermocline) delta (super 13) C record, along with brief increases in Catapsydrax abundances, and step-wise negative excursions in the P. nana (supra-thermocline) delta (super 13) C record, occur through several of the Braarudosphaera layers. Collectively, these coincident changes are taken to indicate that Braarudosphaera blooms were fostered by the development of oxygen minimum zones (OMZs) over both the Walvis Ridge and Rio Grande Rise. Flow of Antarctic Intermediate Water (AAIW) throughout the South Atlantic basin may have generated a strong density contrast at depth, aiding in the development of the OMZs, which would account for the presence of prominent Braarudosphaera layers on either side of the South Atlantic. A collapse in the isotopic gradients between the deep- and supra-thermocline delta (super 13) C records, coeval with a divergence in the complementary delta (super 18) O records, a gradual decline of the planktic foraminiferal species P. opima (> 250 mu m), and an overall increase in coarse fraction values following the third Braarudosphaera layer ( approximately 28.8 Ma) indicate long-term changes to water column stratification. A southward shift in the Intertropical Convergence Zone (ITCZ) is invoked to explain these changes. Increased flow of the Agulhas Current into the eastern boundary of the South Atlantic Basin is also proposed, in particular to explain the divergence in the delta (super 18) O isotopic gradients. Both mechanisms are invoked to explain the gradual demise of the Braarudosphaera blooms at the Rupelian/Chattian boundary.