Environmental changes and historical occupation at Oued Laksar (Ksar Seghir), Morocco

Laksar is a small river near the village of Ksar Seghir, located between Tangier and Ceuta (Morocco), strategic points to control the Strait of Gibraltar over centuries. The river outlets at the western part of a sandy beach that can block the fluvial discharge in years of low precipitation/river flow. The area is densely occupied since the 5th century BC, during the roman period and Early Middle Age in the archaeological site of Dhar d’Aseqfane, located around 1 km from the mouth of the river. The coastal town of Ksar Seghir was founded on the eastern margin and was used as a shipyard for Moroccan dynasties after the 12th century. During the 15th and 16th centuries, Portugal occupied Ksar Seghir to control the Strait and to increase its power in North Africa. After the Portuguese left the site remained abandoned. The present-day Moroccan village developed in the 20th century, first expanding on the western river slope and gradually encompassing the archaeological site. This work aims to i) characterize the environmental evolution of the Laksar River; ii) identify potential records of extreme events such as floods or tsunamis; and iii) explore the relationship between the landscape dynamics and human occupation. To achieve these goals, a ∼5.4 m long sediment core - KS1 - was studied applying high-resolution multiproxy analyses (X-Ray, magnetic susceptibility, sedimentology, organic chemistry and micropalaeontology) together with 14 C dating to establish a chronological model. The sedimentary sequence corresponds to intercalations of fine sand and mud. Results point to deposition in subtidal conditions between the core base (ca. 2500 cal BP) and the 15th century AD (388 cm below surface). The dominance of marine foraminifera assemblages with high number of juveniles and high diversity index suggests an environment under open marine conditions during this period. Conversely, the low δ 13 C values, coupled with the presence of charophytes and Cretaceous, Paleogene and Neogene reworked calcareous nannofossils reflect the contribution of fluvial processes. The sedimentary record underscores fluctuations in fluvial discharge, beach morphology and marine influence. The accumulation of magnetic (heavy) minerals at the core base is the likely result of at least one high-energy event. Following the onset of Portuguese occupation of Ksar Seghir (15th century), the sedimentation rate increases, silting-up the channel margins. Deposition occurs under low subtidal to intertidal conditions. Foraminifera assemblages point to a low salinity environment; coarse materials are mostly constituted by minerogenic particles transported by the fluvial network. Changes in the sedimentary succession (heavy-minerals accumulation, higher marine influence and rip-up clasts) were detected between 219 cm and 140 cm depth below surface that, according to the age-model, overlap temporally with the Lisbon 1755 AD tsunami event (Great Lisbon earthquake).