EGU2020-5924
https://doi.org/10.5194/egusphere-egu2020-5924
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Paleogene Polar Plankton and export productivity changes between the Eocene and Oligocene

Gabrielle Rodrigues de Faria, David Lazarus, Ulrich Struck, Gayane Asatryan, Johan Renaudie, and Volkan Ozen
Gabrielle Rodrigues de Faria et al.
  • Museum für Naturkunde, Leibniz-Institut für Evolutions- und Biodiversitätsforschung, Berlin, Germany.

Aiming to support the prediction of future climate developments, this project investigates the role on geological timescale of the ocean plankton in reducing atmospheric carbon concentration by exporting carbon to the deep-sea. While it is well-known that the transition from the Eocene to the Oligocene brought significant climate changes and, in connection, also a change of the oceans’ carbon export production, the important role of phytoplankton and the links to changing ocean circulation are still poorly understood, as is, similarly, the impact on those changes on the diversity of the plankton contributing to the carbon pump. Investigating the nature of this interaction will provide significant insight into the functions of the oceans as climate regulators.

To address those question, we are generating diversity and absolute abundance data for diatoms and radiolarians, biogeographic data for radiolarians, as well as oxygen and carbon isotope data on planktic and benthic foraminifera, and on the fine fraction (<45µm, i. e. coccoliths), as well as other proxies to estimate surface and deep ocean temperatures and export productivity. These will be generated as paired data from individual samples in various deep-sea drilling sites in and around the Southern Ocean (as it is the focal point of the climatic/oceanographic changes at that period). These data will then be compiled and confronted to an ocean circulation model.

Here we will present our results so far (oxygen and carbon isotope on the bulk fine fraction, as well as radiolarian and diatom diversity estimates), based on two main localities from the antarctic (ODP Site 689B from the Weddell Sea) and the subantarctic (ODP Site 1090B on the southern flank of the Agulhas ridge) South Atlantic. A comparison with a newly generated, database-driven diversity analysis of the same groups in the same region, using the Neptune (NSB) database, will also be shown. While the exhaustive taxonomical compilation made on these two sites for the diatoms records three times more species than what was recorded in the literature for the Southern Ocean biome, it still shows an evolutionary turnover at the Eocene-Oligocene, just as the classic, NSB-driven analysis does. The fine fraction oxygen isotope at both sites 689B and 1090B show a pattern similar to that recorded in planktonic foraminifera in neighbour sites, indicating a significant drop in SST close to the Eocene-Oligocene boundary, while the fine fraction carbon isotope signal in the antarctic site shows a subsequent decrease indicating changes in exported productivity 2Myr after the global cooling.

How to cite: Rodrigues de Faria, G., Lazarus, D., Struck, U., Asatryan, G., Renaudie, J., and Ozen, V.: Paleogene Polar Plankton and export productivity changes between the Eocene and Oligocene, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5924, https://doi.org/10.5194/egusphere-egu2020-5924, 2020

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Presentation version 1 – uploaded on 02 May 2020
  • CC1: Comment on EGU2020-5924, Luz Maria Mejia Ramirez, 11 May 2020

    Hello! Great work!

    I was just wondering if you have checked what is your <45 um fraction composed  of? Coccoliths are usually way smaller than 45 um and at least in sites we have worked at for coccolith size fraction separations the 10-11 um size fraction is largely concentrated in foraminifera fragments. Therefore, I would expect 11-45 um also to have some (maybe important?) fraction of foraminiferal calcite. I would suggest to carefully check how much of your <45 um size fraction could be influenced by foraminiferal calcite. The effect on the amount of foraminiferal calcite in your <45 um size fraction may be even larger (if you have them) if your site is enriched with small size clays, which are usually < 2um. Perhaps a better target would be <11 um, instead of 45 um? Not sure how big are your biggest coccoliths. Your size cut would depend on this size.  
    Also a final thing to always consider with oxygen and carbon isotopes in the coccoliths are potential vital effects between coccolith size fractions. Therefore, changing the assemblage from big to small or viceversa, could also potentially change your isotopes. 
    I hope this helps! Have a nice monday! 

    • AC1: Reply to CC1, Gabrielle Rodrigues de Faria, 11 May 2020

      Hello Luz Maria Mejia Ramirez,

      Thank you for your comment and suggestion. 

      We did check our samples and they are mainly composed of coccoliths. In fact, we decided analysing fine fraction because our samples were poor in foraminifera shells, so there were no foraminifera fragments in the fine fraction. Perhaps these smaller size cut will be important when there are lots of foraminifera shells.

      About the vital effects, some studies suggest the larger vital effects seem to appear around 7Ma (Check Bolton & Stoll, 2013).

      Have a nice monday too.

      Cheers,

      Gabrielle R. de Faria