Deciphering the paleoceanographic significance of early Oligocene Braarudosphaera chalks in the South Atlantic

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doi: 10.1016/S0377-8398(03)00027-6
Author(s): Kelly, D. C.; Norris, R. D.; Zachos, J. C.
Author Affiliation(s): Primary:
University of Wisconsin, Department of Geology and Geophysics, Madison, WI, United States
Scripps Institution of Oceanography, United States
University of California at Santa Cruz, United States
Volume Title: Marine Micropaleontology
Source: Marine Micropaleontology, 49(1-2), p.49-63. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0377-8398 CODEN: MAMIDH
Note: In English. 60 refs.; illus., incl. 1 plate, 1 table, sketch map
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 (δ18O, δ13C) 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 δ18O increases on the order of 0.6-1.0 per mil which we attribute to isotopic disequilibria driven by braarudosphaerid vital effects. The two Braarudosphaera layers straddle a single peak in benthic foraminiferal δ18O 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 per mil) in the δ13C record of a thermocline-dwelling planktonic foraminifer that coincide with the braarudosphaerid layers. Thus, benthic-to-thermocline δ18O and δ13C 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 δ18O offsets may reflect subtle differences in the depth ecologies of these two mixed-layer species. If so, then braarudosphaerid depositional events may represent "subsurface blooms" that took place within the lower parts of the euphotic zone. Alternatively, these transient δ18O offsets may reflect periods of pronounced seasonality, with braarudosphaerid blooms occurring during spring upwelling. The recurrence of Braarudosphaera blooms on both sides of the South Atlantic is believed to reflect rhythmic changes in the vigor and configuration of gyre circulation. We speculate that the termination of Braarudosphaera blooms in the South Atlantic near the end of the Early Oligocene may be related to paleoceanographic change caused by the crossing of a critical threshold in the tectonic opening of the Drake Passage and the development of the Antarctic Circum-Polar Current. Abstract Copyright (2003) Elsevier, B.V.
Year of Publication: 2003
Research Program: DSDP Deep Sea Drilling Project
Key Words: 12 Stratigraphy, Historical Geology and Paleoecology; Algae; Algal blooms; Antarctic Circumpolar Current; Atlantic Ocean; Biochemistry; Biostratigraphy; Braarudosphaeridae; C-13/C-12; Carbon; Carbonate rocks; Cenozoic; Chalk; Cores; DSDP Site 363; Deep Sea Drilling Project; Drake Passage; Foraminifera; Geochemistry; Glacial environment; Glaciomarine environment; Interglacial environment; Invertebrata; Isotope ratios; Isotopes; Leg 40; Lower Oligocene; Marine environment; Microfossils; Nannofossils; O-18/O-16; Oligocene; Oxygen; Paleo-oceanography; Paleocirculation; Paleoclimatology; Paleocurrents; Paleoecology; Paleogene; Paleogeography; Plantae; Protista; Sedimentary rocks; South Atlantic; Stable isotopes; Subtropical environment; Tertiary; Thermocline; Upwelling
Coordinates: S193845 S193845 E0090248 E0090248
Record ID: 2005071050
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands