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Orbitally forced hyperstratification of the Oligocene South Atlantic Ocean

Orbitally forced hyperstratification of the Oligocene South Atlantic Ocean
Orbitally forced hyperstratification of the Oligocene South Atlantic Ocean
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‐ky and 405‐ky paced eccentricity cycles. The longest lasting acme event corresponds to a pronounced minimum in the ~2.4‐My eccentricity cycle. In the modern ocean, Braarudosphaera occurrences are limited to shallow marine and neritic settings, and the calcified tests of Braarudosphaera probably represent a resting stage in its life cycle. Therefore, we hypothesize that the Oligocene acmes point to extensive and episodic (hyper)stratified surface water conditions, i.e., a shallowly situated pycnocline that may have served as a virtual sea floor, which (partially) prevented the tests from sinking in the pelagic realm. We speculate that hyperstratification was either ocean‐basin‐wide, 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.
2572-4525
Liebrand, Diederik
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Raffi, Isabella
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Laxenaire, Remi
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Bosmans, J.H.C.
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Wilson, Paul A.
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Batenburg, Sietske J.
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Beddow, Helen M.
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Bohaty, Steven M.
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Brown, Paul R.
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Crocker, Anya J.
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Huck, Claire
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Lourens, Lucas J.
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Sabia, Luciana
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Fraguas, Angela
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Liebrand, Diederik
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Raffi, Isabella
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Fraguas, Angela
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Laxenaire, Remi
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Bosmans, J.H.C.
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Wilson, Paul A.
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Batenburg, Sietske J.
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Beddow, Helen M.
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Bohaty, Steven M.
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Brown, Paul R.
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Crocker, Anya J.
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Huck, Claire
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Lourens, Lucas J.
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Sabia, Luciana
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Liebrand, Diederik, Raffi, Isabella, Laxenaire, Remi, Bosmans, J.H.C., Wilson, Paul A., Batenburg, Sietske J., Beddow, Helen M., Bohaty, Steven M., Brown, Paul R., Crocker, Anya J., Huck, Claire, Lourens, Lucas J. and Sabia, Luciana , Fraguas, Angela (ed.) (2018) Orbitally forced hyperstratification of the Oligocene South Atlantic Ocean. Paleoceanography and Paleoclimatology. (doi:10.1002/2017PA003222).

Record type: Article

Abstract

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‐ky and 405‐ky paced eccentricity cycles. The longest lasting acme event corresponds to a pronounced minimum in the ~2.4‐My eccentricity cycle. In the modern ocean, Braarudosphaera occurrences are limited to shallow marine and neritic settings, and the calcified tests of Braarudosphaera probably represent a resting stage in its life cycle. Therefore, we hypothesize that the Oligocene acmes point to extensive and episodic (hyper)stratified surface water conditions, i.e., a shallowly situated pycnocline that may have served as a virtual sea floor, which (partially) prevented the tests from sinking in the pelagic realm. We speculate that hyperstratification was either ocean‐basin‐wide, 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.

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Liebrand_Raffi_et_al_Pal_Pal_2018_8_ - Accepted Manuscript
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Accepted/In Press date: 15 March 2018
e-pub ahead of print date: 24 March 2018

Identifiers

Local EPrints ID: 419021
URI: http://eprints.soton.ac.uk/id/eprint/419021
ISSN: 2572-4525
PURE UUID: c4b99bf8-9494-4fb8-b62a-a59a9742be18
ORCID for Steven M. Bohaty: ORCID iD orcid.org/0000-0002-1193-7398

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Date deposited: 28 Mar 2018 16:30
Last modified: 18 Feb 2021 17:08

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Contributors

Author: Diederik Liebrand
Author: Isabella Raffi
Editor: Angela Fraguas
Author: Remi Laxenaire
Author: J.H.C. Bosmans
Author: Paul A. Wilson
Author: Sietske J. Batenburg
Author: Helen M. Beddow
Author: Paul R. Brown
Author: Anya J. Crocker
Author: Claire Huck
Author: Lucas J. Lourens
Author: Luciana Sabia

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