Benthic biofacies to interpret Holocene environmental changes and human impact in Alexandria’s Eastern Harbour, Egypt

Holocene molluscan and foraminiferal faunas, sensitive ecological markers, are statistically treated to define their temporal and spatial distributions in radiocarbon-dated cores in Alexandria’s Eastern Harbour. Up-core variations of these benthic biofacies record the evolution of depositional environments during the past 8000 years in this long-occupied Eastern Mediterranean port. The most important change occurred in the mid-Holocene between about 6300 and about 4300 yr ago, a time of lower sea level and minimal human activity. This faunal change, indicating a transition from a somewhat protected to a more open and higher-energy marine environment, resulted primarily from rise of sea level, lowering of the harbour floor and diminished shielding by Pleistocene coastal ridges and reefs near the bay mouth and within the harbour. Since that time, faunal assemblages have remained generally constant, except during the period from about 2100 to about 1800 yr ago when Greeks and Romans built the Heptastadion that connected Alexandria to Pharos Island; this construction effectively separated the Eastern from the Western Harbour. Absence of a well-marked faunal change during the past 18 centuries is unexpected,particularly in light of the much-increased population and associated human effects. The harbour’s overall large size, rise in relative sea level and continued exchange of marine water between the open shelf and port have maintained an active circulation pattern. The strong current regime and reworking of bottom sediments have generally masked short-term human effects and altered the harbour’s late-Holocene sediment stratigraphy. These factors help explain why benthic biofacies primarily record influences of natural processes that prevailed in this relatively unconfined, high-energy marine setting rather than those of a more constricted and anthropogenically impacted port.