Megatsunami deposits and range expansion of cold-temperate marine species towards the tropics in glacial times

Over geological time, glacial-interglacial cycles affected the geographical range of marine species. Typically, this has been documented by tracking the long-distance dispersal of tropical, shallow-water mollusc species in archipelagos during the last phase of glacial terminations or the early phase of an interglacial episode. Many studies conducted in the Macaronesian archipelagos (i.e., the Azores, Madeira, Selvagens, Canaries and Cabo Verde) support this view. To date, however, such studies exclude data from full glacial periods, owing to difficulties in accessing the geological record of lower sea-level glacial episodes. Here we demonstrate, for the first time, the range expansion into the tropics of cold-water/temperate species during two glacial episodes (MIS 4 and MIS 6), using the Macaronesian region as a case study. For that, we innovate by using megatsunami deposits to unveil biogeographic processes and patterns noticed in the conglomerates of Tarrafal (Santiago Island, Cabo Verde) and of Teno Bajo (Tenerife, Canary Islands), which are interpreted to have been emplaced by megatsunamis triggered by volcanic flank collapses occurred at ~68 ka (MIS 4) and ~170 ka (MIS 6), respectively. Our results detect that not only latitudinal, long-distance dispersal of marine molluscs occurred toward the tropics (mainly between archipelagos, and between European and African shores towards the Macaronesian archipelagos), but also longitudinal range expansion. Moreover, both MIS 4 and MIS 6 megatsunami deposits yielded a high biodiversity (expressed both by species richness and diversity indices) when compared with raised beach sediments. This new finding must be added to the distinctive sedimentological and textural characteristics of tsunami deposits. Finally, we demonstrate that four mollusc species reported from the Teno glacial MIS 6 tsunami deposits, and several temperate and sub-tropical bivalve and gastropod species reported from the Tarrafal glacial MIS 4 tsunami deposits spread to the Canaries and Cabo Verde, respectively, establishing viable populations in those archipelagos. Thus, these species provide evidence of geographical range expansion of marine species from mid-latitudes to low latitudes by means of long-distance, equatorward dispersal of benthic, shallow-, cold-water/temperate marine molluscs between archipelagos and from continental shores to oceanic islands during glacial periods. Megatsunami deposits are used to unveil biogeographic patterns and processes in the Macaronesian archipelagos. Megatsunami deposits record the geographical range expansion of cold-water/temperate species towards the tropics during glacial times. Our data demonstrates both latitudinal and longitudinal range shifts of marine molluscs, by means of long-distance dispersal. Both MIS 4 and MIS 6 megatsunami deposits analysed yielded a high biodiversity when compared with MIS 5e raised beach sediments. A high biodiversity (expressed both by species richness and diversity indices) is a new finding to characterize the distinctive sedimentological and textural characteristics of tsunami deposits.