Abstract
Outer shelf sedimentary records are promising for determining the recurrence intervals of tsunamis. However, compared to onshore deposits, offshore deposits are more difficult to access, and so far, studies of outer shelf tsunami deposits are scarce. Here, an example of studying these deposits is presented to infer implications for tsunami-related signatures in similar environments and potentially contribute to pre-historic tsunami event detections. A multidisciplinary approach was performed to detect the sedimentary imprints left by the 1755 CE tsunami in two cores, located in the southern Portuguese continental shelf at water depths of 58 and 91 m. Age models based on 14C and 210Pbxs allowed a probable correspondence with the 1755 CE tsunami event. A multi-proxy approach, including sand composition, grain-size, inorganic geochemistry, magnetic susceptibility, and microtextural features on quartz grain surfaces, yielded evidence for a tsunami depositional signature, although only a subtle terrestrial signal is present. A low contribution of terrestrial material to outer shelf tsunami deposits calls for methodologies that reveal sedimentary structures linked to tsunami event hydrodynamics. Finally, a change in general sedimentation after the tsunami event might have influenced the signature of the 1755 CE tsunami in the outer shelf environment.
Highlights
Research related to tsunamis has increased significantly in the last two decades, focusing on retrieving return periods, and on understanding tsunami event dynamics [1]
The onset of 137 Cs is observed early 50s, as expected, and the maximum activities in subsurface are in agreement with the known inputs events, the low sedimentation rate in core M107 do not permit to identify the two peaks (Figure 4a,d)
We presented a detailed analysis of the 1755 CE tsunami sediments in outer shelf settings applying a multi-proxy approach
Summary
Research related to tsunamis has increased significantly in the last two decades, focusing on retrieving return periods, and on understanding tsunami event dynamics [1]. It might be less likely to have significant sedimentary imprints of tsunamis in deeper offshore areas (i.e., mid to outer shelf) [5], the controversial differentiation between tsunami and storm deposits [11] can be neglected when studying sediments beneath the storm wave base [12]. Few studies have focused on analyzing deeper offshore sedimentary records [2,3,8,13,14,15,16,17,18,19,20,21,22] and even fewer have involved possible historical/palaeotsunami deposits in such environments [2,3,4,13,17,22,23,24,25,26]
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