Impact of wind direction variability on hazard assessment in Martinique (Lesser Antilles): The example of the 13.5  ka  cal  BP Bellefontaine Plinian eruption of Mount Pelée volcano

International audienceThe threat posed by powerful Plinian explosive eruptions, which inject large quantities of ash into the atmosphere and produce pyroclastic density currents (PDC) on ground, is mainly controlled by eruptive parameters and by the direction and strength of the wind field during the eruption. In most studies, mean wind profiles are used to investigate potential tephra deposit dispersion and to assess volcanic risk. Here we present a detailed reconstruction and reinterpretation of a poorly-understood eruption of Mount Pelée volcano (Martinique), and use it to demonstrate that exclusive use of the average trade wind profile can lead to a misrepresentation of the volcanic risk. The great interest of this eruption stems from its unusual southward dispersion, which encompasses areas that are considered to be safe in current hazard maps and that host major infrastructure. Our new field study and radiocarbon dating show that these deposits are not part of the 2010 BP P3 eruptive sequence as previously thought, but define a so far unknown eruptive event dating back to 13,516 cal BP, which we propose to name the Bellefontaine eruption. The Bellefontaine sequence consists of a basal grey lithic-rich layer resulting from an explosive opening phase that destroyed a pre-existing lava dome, immediately followed by a much thicker, slightly reverse-graded white pumice-fall layer. Their dispersal, thickness, and grain-size distribution are used together with physical models of a volcanic plume to reconstruct the time evolution of the eruption. We find that the mass eruption rate reached 5 × 10^7 kg s-1, producing a 20-km-high Plinian plume, and that the minimum volume of pyroclastic deposits was 0.18 km3 DRE. 2D simulations of tephra dispersion in the atmosphere performed with HAZMAP show that, unlike the recent eruptions at Mount Pelée volcano, mean seasonal wind profiles cannot explain the southward dispersal of the Bellefontaine deposits. To understand the origin of this unusual dispersion axis, we retrieve forty years of wind data over Martinique by using global atmospheric reanalyses from 1979 by the European Center for Medium-Range Weather Forecasts ERA Interim (hereafter ERA-Interim) and ERA5 (hereafter ERA5). We find that, contrary to previous assumptions, this eruption did not necessarily occur during extreme weather conditions associated with the passing of a hurricane. Looking in detail into the ERA Interim datasets, we observe that the wind direction variability over the past 40 years is very low during the dry season (from December to May), and much larger during the wet season (from June to November), even in the troposphere (≈0 to 18 km), which can occasionally result in northerly winds in the mid- to high troposphere over Martinique. As a similar eruption today would spread volcanic material as far as the prefecture of Fort-de-France and its international airport, a zone classified as safe in current hazard maps, this study highlights the importance of including daily variability of winds in hazard assessment models when considering Plinian eruptions