Low-sea-level emplacement of a very large Late Pleistocene 'megaturbidite' in the western Mediterranean Sea

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Large-volume turbidites, termed 'megaturbidites' or 'megabeds' [1], result from catastrophic slope failures and the associated down-slope transport of enormous quantities of sediment from continental margins to the deep sea. Such large sediment failures can generate tsunamis [2,3] and, in terrains underlain by gas hydrates (clathrates), may be associated with the release of substantial amounts of the greenhouse gas methane. It has been proposed that the megaturbidite events may be triggered by seismic activity [4], or may result from gas hydrate release itself [5,6], caused by a lowering of hydrostatic pressure on clathrates as a result of low sea level. Previous conclusions on the significance of sea-level change [7-9], however, have been conditional because of the lack of absolute times of turbidite emplacement. Here we use accelerator-mass-spectrometry radiocarbon dating in five widely spaced cores to constrain the date of emplacement of a large-volume ([approximately] 500 km3) bed in the Balearic Basin of the western Mediterranean. This turbidite is exceptional in its magnitude and represents the main sedimentation event in the Balearic Basin over the past 100 kyr. Our data provide an estimate of 22,000 calendar years before present for emplacement of the megabed, a time when sea level stood at its lowest level during the Last Glacial Maximum. The coincidence of these dates is consistent with emplacement due to clathrate destabilization caused by low sea level, although other triggering mechanisms, such as seismic shock, cannot be ruled out.

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