Orbitally Forced Hyperstratification of the Oligocene South Atlantic Ocean

• Main Authors: Liebrand, D, Raffi, I, Fraguas, Á, Laxenaire, R, Bosmans, JHC, Hilgen, FJ, Wilson, PA, Batenburg, SJ, Beddow, HM, Bohaty, SM, Bown, PR, Crocker, AJ, Huck, CE, Lourens, LJ, Sabia, L
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2018
• Subjects: Braarudosphaera acmes; astronomical forcing of atmospheric and oceanic fronts; surface ocean stratification; Oligocene; monsoons; eddies; South Atlantic Ocean
• Online Access: https://discovery.ucl.ac.uk/id/eprint/10047463/1/Bown_Liebrand_et_al-2018-Paleoceanography_and_Paleoclimatology.pdf; https://discovery.ucl.ac.uk/id/eprint/10047463/

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.