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  A Seasonal Model of Nitrogen Isotopes in the Ice Age Antarctic Zone: Support for Weakening of the Southern Ocean Upper Overturning Cell

Kemeny, P. C., Kast, E. R., Hain, M. P., Fawcett, S. E., Fripiat, F., Studer, A. S., et al. (2018). A Seasonal Model of Nitrogen Isotopes in the Ice Age Antarctic Zone: Support for Weakening of the Southern Ocean Upper Overturning Cell. Paleoceanography and paleoclimatology, 33(12), 1453-1471. doi:10.1029/2018PA003478.

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 Creators:
Kemeny, P. C.1, Author
Kast, E. R.1, Author
Hain, M. P.1, Author
Fawcett, S. E.1, Author
Fripiat, Francois2, Author           
Studer, Anja S.2, Author           
Martinez-Garcia, Alfredo2, Author           
Haug, Gerald H.2, Author           
Sigman, D. M.1, Author
Affiliations:
1external, ou_persistent22              
2Climate Geochemistry, Max Planck Institute for Chemistry, Max Planck Society, ou_2237635              

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 Abstract: In the Antarctic Zone of the Southern Ocean, the coupled observations of elevated diatom‐bound 15N/14N (δ15Ndb) and reduced export production during the ice ages indicates more complete nitrate (NO3−) consumption. This evidence points to an ice age decline in gross NO3− supply from the deep ocean to the surface wind‐mixed layer, which may help to explain the reduced CO2 levels of the ice age atmosphere. We use a seasonally resolved, two‐layer model of the N isotopes in the Antarctic Zone upper ocean to quantify the ice age decline in gross NO3− supply implied by the data. When model parameters are varied to reflect reduced gross NO3− supply, the concentration of wintertime upper ocean NO3− is lowered, but with a much weaker increase in NO3− δ15N than predicted by analytical models such as the Rayleigh and steady state models. Physical mixing is the dominant cause, with a modest contribution from foodweb dynamics. As a result, the observed δ15Ndb rise of ~3‰–4‰ must be explained mostly by a greater summertime increase in NO3− δ15N during the ice ages. The high degree of NO3− consumption required to generate this summertime δ15N rise indicates a >80% reduction in gross NO3− supply. Half or more of the modern gross NO3− supply is from wind‐forced Antarctic upwelling that drives the upper cell of Southern Ocean overturning. Thus, the decrease in NO3− supply cannot be achieved solely by a decline in vertical mixing or wintertime convection; rather, it requires an ice age weakening of the upper cell.

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Language(s): eng - English
 Dates: 2018
 Publication Status: Issued
 Pages: -
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 Table of Contents: -
 Rev. Type: -
 Identifiers: ISI: 000455877000009
DOI: 10.1029/2018PA003478
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Title: Paleoceanography and paleoclimatology
Source Genre: Journal
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Publ. Info: Hoboken, NJ : Wiley
Pages: - Volume / Issue: 33 (12) Sequence Number: - Start / End Page: 1453 - 1471 Identifier: ISSN: 1944-9186
CoNE: https://pure.mpg.de/cone/journals/resource/1944-9186