Environmental history of Lake Mariout at the ‘Marea’/Philoxenite archeological site, northern Egypt, during the Hellenistic–early Islamic periods as seen by fossil diatoms

Abstract

Diatom analysis of sediments obtained from five cores drilled in Lake Mariout and the ‘Marea’/Philoxenite archeological site southwest of Alexandria, Egypt, allows us to reconstruct the lake's environmental history during the Hellenistic–early Islamic periods. The distribution pattern and the variation in relative abundance of recognized diatoms differentiated each core section into characteristic zones. These zones are interrupted by periods of poor preservation or non-diatom deposition that coincide with fluctuating lake phases of freshwater flooding of the Nile during humid warm episodes and arid phases associated with a reduction in the Nile's water influx that provides Lake Mariout with fresh water. A rising water level of Lake Mariout is estimated from the great abundance of riverine, planktonic Aulacoseira species. A lowering of water level was linked to a high abundance of brackish water taxa and/or periods of non-diatom deposition. Furthermore, the absence of diatoms at two intervals in Core 1 is associated with silty clay with an abundance of gypsum crystals which point to falling water lake levels with increasing temperature during dry periods. However, the intervals of poor preservation or no diatoms in other cores at ‘Marea’/Philoxenite are presumably related to the coarser nature of the sediment that indicate a major lake level lowstand, a high-energy depositional environment together with increased salinity and alkalinity. By comparing the diatom data in the examined cores, it is clear that the Aulacoseira granulata assemblage zone is characteristic of the Roman and Byzantine periods, which reflects relatively rising water levels due to Nile water inflow via the Canopic Nile branch and thus the connection between the canal and lake during this time. Furthermore, the predominance of Aulacoseira granulata with some pollution-tolerant taxa indicates extensive human occupation during the Roman–Byzantine periods, which is evidenced by increased nutrient concentration with higher input of humic substances due to human activities and their influence on the lake ecosystem.