The effect of vertical nutrient redistribution on surface ocean δ13C

Abstract

Processes that shift nutrients from mid to lower ocean depths may or may not increase the ocean's vertical carbon pump. Redistribution of δ13C produced by these processes is compared in two box models: a vertical three‐box ocean and a thirteen‐box ocean. In the former there is no δ13C fractionation laterally between surface waters, and the ocean nutrient and δ13C distributions are linearly correlated. This correlation is not changed when these tracers become redistributed, and if the model surface ocean is nutrient limited, the surface δ13C cannot change. In the 13‐box ocean there is a transfer of negative δ13C from the cold surface water through the atmosphere into the warm ocean. Its δ13C is therefore lower than it would be as a result of the carbon pump by itself. If a middepth nutrient depletion occurs because of lower nutrients in Antarctic waters, the warm surface δ13C increases. If the vertical nutrient shift occurs because of ocean circulation or biological recycling changes, the warm surface water δ13C change depends on the ratio of its vertical CO2 fluxes, i.e., exchange of atmospheric CO2 versus upwelling total CO2 and net biological production. If this ratio remains about the same, then little change occurs in surface δ13C, and the δ13C of Pacific deep water decreases about 0.3%. In this case, no change in the average ocean δ13C is required to explain observations from sediment data. This would imply that the ice age land biota carbon mass was about the same as that of today. The vertical CO2 flux ratio could be an important consideration if greater wind‐driven upwelling is a factor in the nutrient redistribution.