Estimating the Glacial Atlantic Deep Water Circulation by Combining Simulations and Proxy Observations of Nutrient Distributions

Past climate changes provide an independent test of models used for the projections of future climate change. Simulations of the deep ocean circulation in the Atlantic with coupled ocean-atmosphere models show a large spread both for glacial climate as well as for greenhouse warming simulations. The glacial simulations display a range from a totally collapsed circulation to an increased circulation compared with present day. From decades of measurements on deep sea sediment cores a large database exists of the glacial ocean. Can we use this information to improve projections? Intuitively we would grant more confidence in a projection of a model that is consistent with the glacial proxy information than one that isn't. In order to assess if a model simulation is consistent (or not) with the proxy record it is necessary to translate the proxy record (e.g. tracer distributions like d13C) into quantities that are simulated by the models (e.g. mass flux). What was the mass flux of the glacial Atlantic overturning circulation? Here I compare different simulations of nutrient patterns using a coupled circulation-ecosystem-nutrient cycle model with a recent compilation of observations (Curry and Oppo 2005, Paleoceanography 20) of West Atlantic d13C data. Simulated nutrient patterns are converted to d13C using the present day linear relation. The results suggest that the mass flux of the glacial North Atlantic overturning was reduced by about 50% compared to present day and North Atlantic Deep Water was shallower (restricted to the upper 2 km). The best estimate (correlation of 0.87 with the observations) reproduces increased d13C concentrations (lower nutrient values) in the upper 2 km of the North Atlantic and a stronger vertical gradient at mid depths. None of the simulations reproduces the very low d13C values of glacial Antarctic Bottom Water. This suggests that changes in the air sea gas exchange of d13C in the Southern Ocean (which are not considered in the simulations) caused this depletion.