Seismically triggered anoxia and brine spillover during the CE 365 Crete mega-earthquake in the eastern Mediterranean Sea

• Sedimentary processes and inception of brines in the Hephaestus basin. • Age of seafloor anoxia is in agreement with the Mw > 8 CE 365 Crete earthquake. • Seismic shaking triggered large scale sediment remobilization. • Submarine landslides and large amplitude waves at the brine interface. • Catastrophic events trigger seafloor anoxia and brine spillover. The Hellenic Arc subduction system hosts unique brine-filled depressions that represent the world’s deepest and most hypersaline basins currently characterized by anoxic sedimentation and gypsum precipitation. Their origin is intimately linked to active tectonics and to the presence of Messinian evaporites in the sub-seafloor, but the formation processes are not fully understood. Understanding how they developed and the triggering mechanism for brine formation can provide valuable information about their evolution and the past history of such extreme environments on Earth and extraterrestrial analogues. We conducted sediment coring and direct sampling of the brine lake Hephaestus to reconstruct the sedimentary and environmental characteristics of the basin. We found that the basin preserves a stratigraphy spanning at least 200 kyr and contains a combination of pelagic sediment and resedimented deposits. Sediment stratigraphy records the transition from oxic sediment at the core bottom to recent anoxic conditions marked by black viscous sediment. This transition is punctuated by multi-sourced slumped material that was emplaced during a catastrophic event capable of simultaneously triggering deep sea slope failures and turbidity currents from the coastal environment. Radiometric dating and age modeling suggest that sediment anoxia resulted from catastrophic sediment remobilization that occurred sometime in the time interval CE 155-439. This is consistent with the CE 365 M>8 Crete earthquake and related mega-tsunami. We propose that seismic shaking triggered slope failures, turbidity currents and large-amplitude waves of the brine interface in the neighboring anoxic Kryos basin with brine spillover from one basin into deep depression(s).