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Orbitally Forced Hyperstratification of the Oligocene South Atlantic Ocean
Author(s) -
Liebrand Diederik,
Raffi Isabella,
Fraguas Ángela,
Laxenaire Rémi,
Bosmans Joyce H. C.,
Hilgen Frederik J.,
Wilson Paul A.,
Batenburg Sietske J.,
Beddow Helen M.,
Bohaty Steven M.,
Bown Paul R.,
Crocker Anya J.,
Huck Claire E.,
Lourens Lucas J.,
Sabia Luciana
Publication year - 2018
Publication title -
paleoceanography and paleoclimatology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.927
H-Index - 127
eISSN - 2572-4525
pISSN - 2572-4517
DOI - 10.1002/2017pa003222
Subject(s) - geology , pelagic zone , bathyal zone , seafloor spreading , oceanography , paleontology , thermohaline circulation , tropical atlantic , ocean current , north atlantic deep water , mesopelagic zone , deep sea , benthic zone , sea surface temperature
Pelagic sediments from the subtropical South Atlantic Ocean contain geographically extensive Oligocene ooze and chalk layers that consist almost entirely of the calcareous nannofossil Braarudosphaera . Poor recovery and the lack of precise dating of these horizons in previous studies has limited the understanding of the number of acmes, their timing and durations, and therefore their likely cause. Here we present a high‐resolution, astronomically tuned stratigraphy of Braarudosphaera oozes (29.5–27.9 Ma) from Ocean Drilling Program Site 1264 in the southeastern Atlantic Ocean. We identify seven episodes with highly abundant Braarudosphaera . Four of these acme events coincide with maxima and three with minima in the ~110 and 405‐kyr paced eccentricity cycles. The longest lasting acme event corresponds to a pronounced minimum in the ~2.4‐Myr eccentricity cycle. In the modern ocean, Braarudosphaera occurrences are limited to shallow marine and neritic settings, and the calcified coccospheres of Braarudosphaera are probably produced during a resting stage in the algal life cycle. Therefore, we hypothesize that the Oligocene acmes point to extensive and episodic (hyper) stratified surface water conditions, with a shallow pycnocline that may have served as a virtual seafloor and (partially/temporarily) prevented the coccospheres from sinking in the pelagic realm. We speculate that hyperstratification was either extended across large areas of the South Atlantic basin, through the formation of relatively hyposaline surface waters, or eddy contained through strong isopycnals at the base of eddies. Astronomical forcing of atmospheric and/or oceanic circulation could have triggered these conditions through either sustained rainfall over the open ocean and adjacent land masses or increased Agulhas Leakage.