28 Jul 2020

28 Jul 2020

Review status: a revised version of this preprint is currently under review for the journal CP.

The Atmospheric Bridge Communicated the δ13C Decline during the Last Deglaciation to the Global Upper Ocean

Jun Shao1, Lowell D Stott1, Laurie Menviel2, Andy Ridgwell3, Malin Ödalen4,5, and Mayhar Mohtadi6 Jun Shao et al.
  • 1Department of Earth Science, University of Southern California, Los Angeles, CA 90089, USA
  • 2Climate Change Research Centre and PANGEA, University of New South Wales, NSW 2052, Sydney
  • 3Department of Earth Sciences, University of California, Riverside, CA 92521, USA
  • 4Department of Meteorology, Bolin Centre for Climate Research, Stockholm University, 106 91 Stockholm, Sweden
  • 5Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA
  • 6MARUM-Center for Marine Environmental Sciences, University of Bremen, 28359 Germany

Abstract. During the early last glacial termination (17.2–15 ka) atmospheric δ13C declined sharply by 0.3–0.4 ‰ as atmospheric pCO2 rose. This was the initial part of the atmospheric δ13C excursion that lasted for multiple thousand years. A similar δ13C decline has been documented in marine proxy records from both surface and thermocline-dwelling planktic foraminifera. The foraminiferal δ13C decline has previously been attributed to a flux of respired carbon from the deep ocean that was subsequently transported within the upper ocean (i.e. bottom up transport) to sites where the signal is recorded. Here, we provide modeling evidence that when respired carbon upwells in the Southern Ocean, negative δ13C anomalies in the global upper ocean were instead transferred from the atmosphere (i.e. top down transport). Due to this efficient atmospheric bridge, the pathway of δ13C transport was likely to be different from nutrient transport during the early deglaciation. This implies that the usage of planktic δ13C records for identifying the carbon source(s) responsible for the atmospheric pCO2 rise during the early deglaciation is limited. The model results also suggest that thermocline waters in upwelling systems like the eastern equatorial Pacific, and even upper deep waters above 2000 m, can be affected by this atmospheric bridge during the early deglaciation. Our results imply that caution must be applied when interpreting early deglacial marine δ13C records from depths that are potentially affected by the atmosphere.

Jun Shao et al.

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Jun Shao et al.


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Short summary
planktic foraminiferal stable carbon isotope (δ13C) data show a rapid decline during the early last deglaciation. This widespread signal was linked to respired carbon released from the deep ocean and its transport through the upper ocean circulation. Using numerical simulations where a stronger flux of respired carbon upwells and outcrops in the Southern Ocean, we find that the depleted δ13C signal is instead transmitted to the rest of the upper ocean through air-sea gas exchange.