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

P. C. Kemeny, E. R. Kast, M. P. Hain, S. E. Fawcett, F. Fripiat, A. S. Studer, A. Martínez-García, G. H. Haug, Daniel Mikhail Sigman

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4 Scopus citations

Abstract

In the Antarctic Zone of the Southern Ocean, the coupled observations of elevated diatom-bound 15 N/ 14 N (δ 15 N db ) and reduced export production during the ice ages indicates more complete nitrate (NO 3 ) consumption. This evidence points to an ice age decline in gross NO 3 supply from the deep ocean to the surface wind-mixed layer, which may help to explain the reduced CO 2 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 NO 3 supply implied by the data. When model parameters are varied to reflect reduced gross NO 3 supply, the concentration of wintertime upper ocean NO 3 is lowered, but with a much weaker increase in NO 3 δ 15 N 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 δ 15 N db rise of ~3‰–4‰ must be explained mostly by a greater summertime increase in NO 3 δ 15 N during the ice ages. The high degree of NO 3 consumption required to generate this summertime δ 15 N rise indicates a >80% reduction in gross NO 3 supply. Half or more of the modern gross NO 3 supply is from wind-forced Antarctic upwelling that drives the upper cell of Southern Ocean overturning. Thus, the decrease in NO 3 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.

Original languageEnglish (US)
Pages (from-to)1453-1471
Number of pages19
JournalPaleoceanography and Paleoclimatology
Volume33
Issue number12
DOIs
StatePublished - Dec 2018

All Science Journal Classification (ASJC) codes

  • Oceanography
  • Atmospheric Science
  • Palaeontology

Keywords

  • Antarctic Zone
  • LGM
  • diatoms
  • nitrate
  • seasonality

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    Kemeny, P. C., Kast, E. R., Hain, M. P., Fawcett, S. E., Fripiat, F., Studer, A. S., Martínez-García, A., Haug, G. H., & Sigman, D. M. (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. https://doi.org/10.1029/2018PA003478