Evaluating mechanisms of nutrient depletion and 13C enrichment in the intermediate-depth Atlantic during the last ice age -: art. no. 1072

[1] Using an ocean box model, we have studied the effect of altered circulation on the oceanic distributions of phosphate (PO4(-3)) and the C-13/C-12 and C-14/C-12 of dissolved inorganic carbon to evaluate competing hypotheses for the cause of observed nutrient depletion and C-13 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 C-13/C-12 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., C-13 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 PO4(-3) relative to the model's interglacial control, in agreement with consensus paleoceanographic evidence. This Indo-Pacific PO4(-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.