1. A Seasonal Model of Nitrogen Isotopes in the Ice Age Antarctic Zone: Support for Weakening of the Southern Ocean Upper Overturning Cell.
- Author
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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., and Sigman, D. M.
- Subjects
NITROGEN isotopes ,GLACIAL Epoch ,WINTER ,DIATOMS ,FOOD chains - 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. Key Points: A two‐layer model of Antarctic Zone seasonality that includes upper ocean N cycling captures key N isotopic patternsThe model indicates that an ice age rise of ~3‰‐4‰ in diatom‐bound δ15N requires a >80% reduction in gross NO3− supply from deep watersThis large change implies a weakening of the upper cell of global ocean overturning that passes through the Antarctic surface [ABSTRACT FROM AUTHOR]
- Published
- 2018
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