Late Holocene marine incursions and hydrological extremes recorded in the Oued El Fedj delta (Boughrara Lagoon, SE Tunisia)

ABSTRACT: The Mediterranean basin is a climatically sensitive transition zone between mid-latitude and subtropical circulation regimes, where modest forcing can produce pronounced hydroclimatic and coastal impacts (Ben Ameur et al., 2019 and 2022 and 2024). Here we investigate Late Holocene marine and hydrological variability along the arid Tunisian margin using a sediment archive from the Oued El Fedj delta (western shore of the semi-enclosed Boughrara Lagoon, Gulf of Gabès). A 268 cm long sediment core was retrieved from the downstream deltaic plain and analysed using magnetic susceptibility, grain-size distribution (modal grain size, sorting and frequency-curve partitioning), and carbonate content, all measured at 1 cm sampling intervals, together with XRF geochemical scanning of major and trace elements on 16 representative samples.

Two intervals show a clear increase in carbonate and marine influence, expressed by upward shifts in CaCO₃ and elevated Ca/Ti, Ca/Si, Ca/Al, and Ca/Fe ratios: (i) a carbonate-rich unit at 173–200 cm, tentatively correlated with the Late Holocene highstand/transgressive phase reported for southern Tunisia (~2.5–2.0 ka BP; Paskoff and Sanlaville (1983)), and (ii) an upper unit at 30–57 cm reflecting the ongoing historical-to-modern marine ingression. Both intervals are characterized by poor sorting and polymodal grain-size distributions spanning namely silt (4- 63 µm), very fine (63-125 µm), fine (125- 250 µm), medium (250- 500 µm), and coarse sands (500 µm- 1 mm), consistent with mixed depositional processes in a lagoon–delta setting.

Episodes of intensified fluvial activity are identified by prominent magnetic-susceptibility peaks at 95–140 cm and 20–35 cm, interpreted as enhanced detrital input during high-discharge/flood phases. The older pulse postdates the ~2.5–2.0 ka marine interval and may align with increased hydrological variability during the Dark Ages Cold Period (~1.4–1.0 ka BP) documented in Tunisian sebkha archives, whereas the younger pulse occurs during the recent marine ingression, suggesting coupled relative sea-level rise and flood-driven sediment delivery.

Keywords: Southern Mediterranean; Southeastern Tunisia; Boughrara Lagoon; Late Holocene; relative sea level; flooding event; magnetic susceptibility.

References:

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Ben Ameur, M., Masmoudi, S., Omar, H., Ouameni, I., Medhioub, M., & Yaich, C. (2022). Middle to late Holocene sedimentary filling history of the Sebkha el Melah in south-eastern Tunisia. Sedimentology, 69(5), 2348–2366. https:// doi. org/ 10. 1111/ sed.

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Ben Ameur, M., Masmoudi, S., Omar, H., & Yaich, C. (2024). Centennial‑scale flood cyclicity in the second half of Holocene in the southeast Tunisian sediments: paleoenvironmental region gauge between the western and eastern Mediterranean. Journal of Sedimentary Environments, 9, 625–644. https://doi.org/10.1007/s43217-024-00185-7.

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