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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

Author(s): Kemeny, Preston C; Kast, Emma R; Hain, Mathis P; Fawcett, Sarah E; Fripiat, François; et al

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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.
Publication Date: 30-Nov-2018
Citation: Kemeny, Preston C., Emma R. Kast, Mathis P. Hain, Sarah E. Fawcett, François Fripiat, Anja S. Studer, Alfredo Martínez‐García, Gerald H. Haug, and Daniel M. Sigman. "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, no. 12 (2018): 1453-1471. doi:10.1029/2018PA003478.
DOI: doi:10.1029/2018PA003478
EISSN: 2572-4525
Pages: 1453 - 1471
Type of Material: Journal Article
Journal/Proceeding Title: Paleoceanography and Paleoclimatology
Version: Final published version. Article is made available in OAR by the publisher's permission or policy.

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