Late-Holocene dust accumulation within the ancient town of Marea (coastal zone of the South Mediterranean Sea, N Egypt)

The ruins of the Marea ancient town are located on the southern shore of Maryut Lake (ancient Mareotis), 45 km to the southwest of Alexandria, Egypt. Marea was a very important trade, religious and agricultural centre in the 5th–7th century A.D. The area, occupied by a town in the past, has diversified relief. Local hills are built of sandstones constituted by calcified dunes. The hills are separated by flat depressions of irregular shape. The uppermost layer in the depressions covering loam and sand sediments is made up of structureless aeolian dust of 0.6–1.2 m thickness. The following analyses of the sediments were conducted: granulometric composition, the mineralogical composition of samples, the loss on ignition, the total organic carbon (TOC), organic matter (OM), and calcium carbonate (CaCO3), clay minerals and geochemistry.Results of OSL dating indicate that the filling of the depressions with silt deposits started probably about 2450 years ago, and that it is correlated with dust deposition. The rate of the dust accumulation was about 0.49 mm/year. However, diversified humus content in the deposits suggests that it was not a permanent process. The accumulation took place during intensive dust storms. The sediments in the depressions are fine silt and clay, with uni- and bi-modal particle size distribution likely caused by both the local and the regional origin of the material. Most dust accumulated within the ancient town of Marea originated from the border between Sahel and Sahara. Local and even regional sources of material were of lower importance.Geochemical ratios (Mg/Ca, Fe/Mn and Na/K) distinguished in the deposits of the depressions show that the beginning of the dust accumulation took place during environmental conditions more humid than the contemporary ones. That indicates a slight increase in the humidity of climate, dated to ca. 10th–12th century A.D., followed by its significant drying, initiated ca. 15th–16th century A.D. In addition, results of analysis of trace elements suggest that already in the 5th century BC the town of Marea, or its closest vicinity, may have been the site of development of industrial activity, much earlier than the age of the oldest artefacts found within the town.