Palaeoceanography and numerical modelling: the Mediterranean Sea at times of sapropel formation

Abstract

AbstractPalaeo-circulation concepts based on (micro-)palaeontological records are compared with results of a general circulation model (GCM), in a review of the eastern Mediterranean conditions during sapropel S1 deposition (7–8ka bp). We discuss two conceptual models, based on proxy data, which integrate bottom water anoxia and high productivity during sapropel formation. One infers a weakened version of the present-day anti-estuarine circulation, whereas the other invokes a reversal of the circulation to an estuarine type. Although both seem reasonable, in view of the available palaeoceanographic proxy data, they imply completely different states of the Mediterranean fresh-water budget. Numerical modelling has been based on three available reconstructions of changes in the west-east salinity gradient compared with present-day conditions (small decrease, intermediate decrease and reversing). The three scenarios give surprisingly similar results. The interface between the surface and intermediate water has shoaled significantly compared with the present, which fuels a deep chlorophyll maximum. All three simulations show stagnant deep waters, which could cause bottom water anoxia. Reversal of circulation is not observed, not even in the reconstruction of most extreme salinity changes. The combined approach of palaeoceanographic and numerical modelling reviewed in this paper shows that modelling provides an indispensable feedback on the viability of the various proxy-based conceptual models.