Orbitally Forced Hyperstratification of the Oligocene South Atlantic Ocean

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 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.

Climate, cryosphere and carbon cycle controls on Southeast Atlantic orbital-scale carbonate deposition since the Oligocene (30–0 Ma)

“…Liebrand et al (2016) Ma), where they range between 8 and 31 m/Myr. The Oligocene-early Miocene (30-17 Ma) and Plio-Pleistocene (5.3-0.0 Ma) shipboard LSR were confirmed by previous studies on Sites 1264 and 1265 (Liebrand et al, 2011(Liebrand et al, , 2016(Liebrand et al, , 2017(Liebrand et al, , 2018Bell et al, 2014Bell et al, , 2015, which also support the shipboard notion that these sites are excellent recorders of orbital-scale climate dynamics.…”

“…At Site 1264, a continuous ~316 m shipboard composite section back to the early Oligocene was developed using magnetic susceptibility and 600/450nm colour reflectance data (Shipboard Scientific Party Leg 208, 2004a). Oligocene-early Miocene XRF ln(Ca/Fe) data was used to verify the shipboard splice and produce a revised composite depth (rmcd) scale (Liebrand et al, 2016(Liebrand et al, , 2018. Liebrand et al (2016) Ma), where they range between 8 and 31 m/Myr.…”

“…The composite core images, and the ln(Ca/Fe) data, together with the shipboard 600/450nm colour reflectance and MS data (Shipboard Scientific Party Leg 208, 2004a) were also used to map off-splice intervals of Holes 1264A and 1264B onto the splice between 0 and 196.13 rmcd (Supplementary Table 4). Liebrand et al (2016Liebrand et al ( , 2018 Tables 2-5). Furthermore, the line-scan composite core photos showed that the mapping of Core 1264B-29H to the splice, erroneously corrected in Liebrand et al (2018) from original mapping in Liebrand et al (2016), should be adjusted (see Supplementary Figure 3 and Table 4).…”

“…Understanding past changes in surface water productivity and deep-sea dissolution can inform about past climate development, and vice versa, how global processes affected regional production and deposition of biogenic carbonates. Primary and export productivity is a sensitive recorder of past climate variability, responding to changes in solar insolation and nutrient availability (Coxall and Wilson, 2011;Pälike et al, 2012;Lyle and Baldauf, 2015;Carter et al, 2016;Liebrand et al, 2018).…”

The Eocene‐Oligocene Transition in the South‐Western Neo‐Tethys (Tunisia): Astronomical Calibration and Paleoenvironmental Changes

“…Blooms of B. bigelowii also occur in open-ocean conditions at times of higher nutrients and/or increased environmental stress (Kelly et al, 2003;Siesser et al, 1992;Wade & Bown, 2006). The occurrence of the Oligocene Braarudosphaera oozes in the open South Atlantic ocean has been possibly related to the orbitally paced formation of hyposaline surface waters (Liebrand et al, 2018). Here, B. bigelowii blooms during the EOT and remains a consistent component in the assemblage during the early Oligocene, which is likely related to shallower conditions.…”