The magnetic fabric of fine-grained sediments laid from the Cretan 365 CE tsunami on the SE coast of Sicily

Along Sicily's southeastern coast, sandy barriers and geological features isolate coastal lagoons known locally as "pantani." This study examines the anisotropy of magnetic susceptibility (AMS) in 69 oriented samples from the Pantano Morghella wetland, north of Portopalo. Collected in 2 cm³ plastic cubes from a single trench ~700 m from the present shoreline, samples follow two parallel profiles (alternated by 1 cm) spanning 134 cm of stratigraphic sequence. This includes sediments from one of antiquity's most devastating tsunamis, which struck offshore Crete on July 21, 365 CE (Gerardi et al., 2012, https://doi.org/10.5194/nhess-12-1185-2012).

The basal tsunami deposit at the trench site consists of a ca. 5 cm thick yellowish bioclastic sandy layer, whose abrupt and probably erosive lower boundary is at about 101.5 cm stratigraphic depth. The rest of the analysed stratigraphic sequence consists of massive grey to blackish muds and grey to pale brown muds, respectively below and above the tsunami sand.

Magnetic susceptibility profiles and AMS data divide the sequence into six zones, revealing distinct depositional environments.

Zone VI (134–101.5 cm): Lowest susceptibility (~500 × 10⁻⁶ SI) and oblate fabric, with a minimum susceptibility axis (k_min) close to the vertical and the maximum (k_max) and intermediate (k_int) susceptibility axes scattered in the horizontal plane. This indicates undisturbed, low-energy lagoon/wetland deposition.

Zone V (101.5–94 cm): The sandy tsunami layer, with only three samples showing scattered AMS axes, reflecting chaotic high-energy deposition.

Zone IV (94–63 cm): High susceptibility (1000–2500 × 10⁻⁶ SI), prolate fabric, clustered E-W horizontal k_max, and k_int and k_min axes scattered in the N-S vertical plane. This indicates deposition under the action of high-energy currents almost perpendicular to the coast.

Zone III (63–53 cm): Similar fabric to Zone IV but lower susceptibility, decreasing upward to ~500 × 10⁻⁶ SI, suggesting waning high-energy influence.

Zone II (53–35 cm): Returns to Zone VI-like low susceptibility and oblate fabric, typical of calm lagoon conditions.

Zone I (35 cm upward): High susceptibility (>2000 × 10⁻⁶ SI in top 20 cm) and triaxial fabric, linked to human salt pan activities – that started in the XIX century - altering sedimentation.

The trends of magnetic fabric are then compared to CT-scan data (X-ray microtomography) providing statistics of size, shape, and orientation of the sand grains. Overall, AMS analyses provide a robust proxy for paleoenvironmental reconstruction in Pantano Morghella. They distinguish intervals of undisturbed, low-energy sedimentation typical of a lagoon/wetland environment from those disrupted by natural catastrophes, such as the 365 CE tsunami, and later anthropogenic activities over the past two centuries.