Evaluating mechanisms of nutrient depletion and 13 C enrichment in the intermediate‐depth Atlantic during the last ice age

Using an ocean box model, we have studied the effect of altered circulation on the oceanic distributions of phosphate (PO 4 −3 ) and the 13 C/ 12 C and 14 C/ 12 C of dissolved inorganic carbon to evaluate competing hypotheses for the cause of observed nutrient depletion and 13 C enrichment at intermediate depths of the Atlantic during the last ice age. Because of “nutrient trapping” and limited air‐sea carbon isotopic equilibration, the simple imposition of an intense meridional overturning cell in the Atlantic fails to simultaneously lower nutrient concentrations and raise 13 C/ 12 C to observed glacial levels. Export of intermediate water out of the Atlantic causes a basin‐to‐basin nutrient transfer, thus providing a more efficient mechanism of intermediate‐depth Atlantic nutrient depletion and improved carbon isotopic equilibration at low temperatures (i.e., 13 C enrichment). Although this export adds nutrients to the intermediate depths of the Pacific and Indian Oceans, the simulated glacial intermediate‐depth Indo‐Pacific is nevertheless moderately depleted in PO 4 −3 relative to the model's interglacial control, in agreement with consensus paleoceanographic evidence. This Indo‐Pacific PO 4 −3 depletion results from our use of a “glacial base case” in which nutrient‐rich Antarctic Intermediate Water formation is absent as part of the elimination of the modern North‐Atlantic‐Deep‐Water‐based “conveyor” circulation.