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. ©2018. American Geophysical Union. All Rights Reserved.