Impact of the circulation on sapropel formation in the eastern Mediterranean

The role of the thermohaline circulation in controlling export production, oxygenation of deep waters, and hence possible sapropel formation in the eastern Mediterranean is examined using a simple nutrient-cycling model. The model is driven by velocity fields from a general circulation model and receives fluxes of nutrient from river run-off and atmospheric deposition. The model is used to study three scenarios: a strong anti-estuarine circulation, a weakened anti-estuarine circulation, and a weak estuarine circulation. Nutrient transports, ventilation of oxygen, and deposition of organic matter are investigated in each case. With a present-day circulation the model provides reasonable agreement with observed phosphate and oxygen profiles and for export production. With the weakened anti-estuarine circulation, consistent with surface salinity reconstructions for the most recent sapropel S-1, there is a modest increase in export production and reduced ventilation leading to anoxia in intermediate and deep waters. Sapropel formation is possible near the coastal margins, particularly if there is enhanced river run-off. With an estuarine circulation, there is significant increase in export production in addition to anoxia below a shallow winter mixed layer. While both the latter circulations allow sapropel formation in the model, the estuarine case is distinguished by higher organic carbon deposition and anoxia in the near-surface waters.