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

Abstract

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 6300 and 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 2100 to 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.

Alexander the Great's tombolos at Tyre and Alexandria, eastern Mediterranean

“…At this time, Stanley and Bernasconi (2006) have described an open marine environment inside the eastern bay. Faced with this problem, the Romans constructed artificial marinas in this area (Goddio et al, 1998;Goddio and Bernand, 2004;Bernasconi et al, 2006). After 2085 ± 45 14 C BP (200 to 410 cal.…”

Late Holocene palaeoenvironmental evolution of the northern harbour at the Elaiussa Sebaste archaeological site (south-eastern Turkey): evidence from core ELA6

“…These environmental fluctuations are clearly recorded by the changes of both the abiotic characters (texture and organic C content) and the different biodiversity of the foraminifer, ostracod, and mollusc assemblages. According to the method currently used in the description of the Neogene to Recent Mediterranean palaeoenvironments (Pugliese and Stanley, 1991;Bernasconi and Robba, 1993;Bernasconi et al, 2006Bernasconi et al, , 2007, high and low species richness recorded in the examined samples is interpreted as indications of favourable and degraded environmental conditions, respectively.…”

“…Geoscientific methods (mainly sedimentology, palaeontology, pollen analysis, and geophysics) are increasingly applied to archaeological research (e.g., Bernasconi et al, 2006Bernasconi et al, , 2007Marriner and Morhange, 2007;Algan et al, 2011;Di Rita et al, 2011;Stock et al, 2013;Schneider et al, 2014;Seeliger et al, 2014). They can be used in various fields of research since they can be very interesting tools in helping to define ancient environments, the origin of human settlements, and the natural and human processes that have controlled the evolution of the territory.…”

Response of mollusc assemblages to climate variability and anthropogenic activities: a 4000‐year record from a shallow bar‐built lagoon system

“…A comparably long period of apparent stability in molluscan faunal associations has also been observed in Holocene studies by Gordillo (), Bernasconi et al . (), and Poirier et al . (), suggesting that this is a fairly common feature of coastal ecosystems.…”

“…One exception to this pattern was noted by Bernasconi et al . () who found that construction in Alexandria's Eastern Harbour during 2100–1800 years bp altered circulation patterns, grain size, and the molluscan fauna at one core location. Poirier et al .…”