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

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.