Climatological influences on major storm events during the last millennium along the Atlantic coast of France

This paper reviews the climatological influences on major past storm events in the North-east Atlantic. Analyses are based on a millenary record of sedimentological and historical impacts affecting coastal societies. The effects of 20 past storms have been found from sedimentary deposits from the last 1,000 years. Historical archives confirmed these events. This paper highlights five major storms that have markedly impacted coastal populations. They date back to 1351–1352, 1469, 1645, 1711 and 1751 AD. The 1351–1352 AD event is defined as a millennium storm that was “likely apocalyptical”, provoking serious damage and long lasting floods on much of the European coast. Major storm impacts have mostly been recorded during positive North Atlantic Oscillation phases. Four decreasing temperature phases are concomitant with 1300–1355, 1420–1470, 1560–1590 and 1690–1715 AD periods, during which much of the northern Atlantic coast of France underwent severe storm damages.

. Characteristics of the two studied sites. The two aerial photographs were obtained from Google Earth. Map was generated with QGIS 2.18.28 from the QGIS Development Team (2020), QGIS Geographic Information System, Open Source Geospatial Foundation Project (https ://qgis.osgeo .org).
Scientific RepoRtS | (2020) 10:12059 | https://doi.org/10.1038/s41598-020-69069-w www.nature.com/scientificreports/ Sedimentological evidence of past storm impacts. Storm conditions can cause overwash processes over the protecting dune of both back barrier depositional environments 5,25 . Marine allochthonous layers detected after coring sampling can be considered as "washover" deposit. They testify to sedimentological parameters which are distinct from the coastal lagoonal sequences 15 . The marine or coastal origin of each layer identified is obtained from radiography, grain size, geochemical, colour, organic matter and Magnetic Susceptibility (MS) analyses. Sediments are then dated by radiocarbon ( 14 C), 210 Pb and 217 Cs isotopes to assess the deposition date of each extracted washover deposit. High values of marine proxies [Mean Grain Size (MGS), colour (and especially lightness from SCI), strontium (Sr), calcium (Ca) and silicon (Si)] combined with low values of coastal or continental indicators such as Organic Matter (OM: CO 2 values), Zn (zinc), Fe (iron), Ti (titanium) and MS indicate a marine origin of the sediment. In the PMG and the TDC, 8 and 15 marine incursions are extracted; respectively (Figs. 2, 3). The complete lithostratigraphy analyses of both coastal lagoons are detailed in the Supplementary S1 section. The crossing with historical archives testifies to the storm origin of each washover deposit extracted in the sedimentological cores 17 . Both dating sequences reach good overall agreement indices. Each radiocarbon sample has been dated with a low uncertainty (Table 1), resulting in two accurate age-depth models (Fig. 4). The TDC2017-3 and the PMG2017_3 cores show low uncertainties in the upper 230 cm and 100 cm; respectively, which correspond to the last millennium. The transition between 210 Pb/ 137 Cs and 14 C dating at 27 cm depth is well defined in TDC2017-3. In the PMG2017_3 core, however, a higher margin of error can be found at the 800-1000 cal y BP (950-1150 AD, for Anno Domini) period. It corresponds to the limit of two different layers. The 70-80 cm depth section also shows higher uncertainties (reaching ± 80 years) due to the 210 Pb/ 137 Cs until 14 C dating methods transition. Nevertheless, these two curves provide precise information to date recorded environmental changes during the last millennium. From the combination of these dating methods, a mean sedimentation rate of 0.22 cm/year (TDC) and 0.12 cm/year (PMG) is estimated since 1000 AD. These results are consistent with other sedimentation rates found in the western French back-barrier environments, which are mainly assessed between 0.1 and 0.5 cm/year [26][27][28] .
Twenty past storm events have been extracted in the last 1,000 years. Fifteen storms present local impacts. The recent 1896, 1924, 1940, 1972 (or 1977), 1990 and 1999 AD storms have impacted north-west France 17 29 . The four thirteenth century events are part of a common phase of stormy activity dated around 1250-1350 AD, which also includes the 1351 AD storm. The 1083 AD marine incursion can be linked to a stormy period which impacted north-west France around   Table 2 and mapped in Fig. 5, is presented in historical documents and detailed in the following sections. Dates are expressed in the Gregorian calendar and the new style is used for pre-October 1582 AD historical dates.
Reconstructing environmental and socio-economic impacts of five major historical storms. Two notable impacts can be found in both cores around 1350 AD. They can be detected because of the 1325 ± 80 AD PMG high increases of MGS and marine elements (186 cm) and the 1315 ± 35 AD TDC important marine intrusion (162 cm). Other sedimentological evidence has been found in the European Atlantic coast: a Yeu Stormy period (1350-1450 AD), during a European Atlantic Stormy Event 6 (1350-1650 AD EASE 1), a storm found at the Baie d' Audierne near 1335 AD 20 and a "coarse grained sedimentation pulse" (CSP) detected www.nature.com/scientificreports/ near 1300 AD in the Pertuis Charentais 19 . At a larger scale, this impact concurs with a 1350-1650 AD European high storminess period 7 . In the British isles, damage was recorded between 1250-1400 AD 31-34 , with a peak of sand mobilisation between 1258-1446 AD 35 . Historical documents concur since they provide records of a damaging storm during winter 1351-1352 AD (Fig. 5). Numerous lowlands of Noirmoutier Island were flooded for nearly half a century 9 . Marshes were also destroyed in Olonne, and 15 years later, salt production was still impacted. This event has been defined as "likely apocalyptical" and as a major submersion of Ré Island 13 . Athimon presented this storm as "one of the most violent and dramatic meteorological events of the last millennium along the French Atlantic coast" 29 . Sedimentological and historical evidence highlight impacts from Brittany to the Pertuis Charentais. They also imply that this storm may have hit the British Isles, which would mean that much of the European Atlantic coast was affected. Two other sandy inputs are reported in the PMG (1445 ± 40 AD at 79 cm depth) and the TDC lagoonal sequence (1470 ± 25 AD at 118 cm depth). A 3 cm diameter pebble of vein quartz has been extracted from this marine layer in the PMG core. This StE is part of two European Stormy Periods 6,7 . Impacts found in the TDC and PMG are linked to the storm that hit the French Atlantic coast during 27-28 January 1469 AD (Fig. 5). The tide coefficient is estimated around 106 (spring tide) during 28 January 29 . This 1469 AD storm seriously damaged dikes and salt marshes in Bouin. This former island was submerged overnight. Afterwards, historical records reported that 1500 tonnes of salt had probably been lost, thus inducing major economic losses 9 . Thirty metre long breaches appeared in the salt marsh causeways. Numerous ridges and roads were destroyed, and a salt merchant drowned. In the Retz region, marine flooding is recorded thanks to the taxes merchants had to pay to their lord (seigniorial taxes). Prices fell because of the impact of this storm 13,36 . The lands became sterile because of brutal marine incursions 8 . Windy impacts were recorded near Angers, where the bell tower of Saint-Aubin toppled, and "numerous trees" were uprooted 9 . The impacts were, for the most part, recorded on the coast (from mid-Vendee to southern Brittany). This is why the supposed trajectory of this event is more uncertain than in other cases. The damage reported around Angers attests to its possible eastern direction.
At 66 cm depth in the TDC core, a strong geochemical signal testifies to a marine incursion. It has been dated at 1665 ± 30 AD, which refers to the storm that hit much of France during 28 January 1645 AD (Fig. 5). It can be part of the two European Stormy Periods 6,7 . Marine flooding was reported in the marshes of Ré, Aix and Oléron islands, and in the cities of La Rochelle, Marennes, Arvert and Saint-Sornin 29 . A large ship collapsed into the lowlands situated at 3 m NGF (Nivellement général de la France) elevation, and nearly 30 ships sank. The losses in the salt marshes came to nearly "500,000 écus" (a French currency in the Middle-Ages) of salt lost. Inland, the Cathedral of Saintes was destroyed and the Notre Dame la Grande church of Poitiers was damaged.  Athimon 29 , these impacts are extracted from the departmental archives of Charentes, Charentes-Maritimes, Gironde and Sarthe and historical local books [37][38][39][40] . This storm is qualified as a "disorder", "horrible", or "terrible" and much of the population hit by this event characterised it as "exceptional" 29 . Concerning the economic cost, it took three months for the Ré Island inhabitants to repair the damage, which impacted agricultural activities 41 . Overall, the spatial extent of this storm is considerable and includes much of the French Atlantic coast. An important concentration of storm impacts is identified between La Rochelle, Poitiers and Bordeaux. Bearing in mind the reports of wind destruction and river flooding in Le Mans, this event presents a SW-NE trajectory. An important geochemical marine input, dated at 1678 ± 35 AD, is recorded at 64 cm depth in the TDC sequence. This environmental disturbance is linked to a storm that hit the central French Atlantic coast on 9 December 1711 AD (Fig. 5). A powerful storm surge was observed at La-Faute-sur-Mer 10 . Historical archives reported important damage, such as the destruction of seawalls and several saltmarsh flooding. The total cost is estimated at 1.19 million Euros at current currency values, mainly because repairing the dikes took a considerable amount of time. Ré island was seriously flooded 8 . After the storm, salt production tools were discovered near the forecourt of the church of The Portes en Ré 42 . According to local parish registers, this storm and its linked marine flooding made a deep impression on coastal families, which lasted for several centuries 8 . A written source called this event a "hurricane" after Thouars's bell tower collapsed on the church's vault: "a stone from the belfry was detached from the top, which fell on the vault of the choir (…) A few minutes after the top of the bell tower crossed the vault of the choir. Half of the bell tower collapsed falling on the arches of the choir and chapel of St. Margaret of Scotland" 43 . The accident resulted in one casualty, named as "a beggar". Similar bell tower damage is also reported in Parthenay, and this storm produced the most important "windthrow" damage of the eighteenth century in Fontainebleau 43 . This extratropical depression seems to have crossed central France in a SW-NE trajectory, without impacting Brittany. This hypothesis is attested by damage recorded in Fontainebleau forest 44 .
A last important historical storm was recorded in the TDC (1720 ± 35 AD at 57 cm depth) and the PMG (1775 ± 30 AD at 63 cm depth) cores. These two marine inputs are linked to the storm on 14-15 March 1751 AD www.nature.com/scientificreports/ (Fig. 5). At La Bruffière, a local parish register, written between 1751-1760 29 , presents the damage as: "A hurricane ripped out more than half of the church, (…), destroyed the nave of the church, (…) destroyed a large part of the houses, and ripped out and uprooted some stronger and taller oak trees (…). Finally, it is almost incredible the damage caused by this storm in this parish and others around. " Marine flooding was reported on the former island of Bouin and the Sables d'Olonnes. Wind impacts were recorded in Nantes and Thouars and according to the Francheteau family registers, Legé was severely impacted 9 . An ancient written source explains that "(…) about one hour after midnight, a storm began, or hurricane, that continued for a few hours with such violence that we did not know where to go, nor where to get to safety" 45 . Seuilly, Rennes, Le Mans, Poitiers and Angers churches or abbeys were partially destroyed 29 . Near Tours, the church of Saint-Croix of Montrichard was damaged (La Nouvelle République, August 2016). At La Ploueze, damage was so impressive that people thought this event happened at the same time as an earthquake 46 . Finally, this storm also had important impacts offshore. On 17 March 1751 AD, at the request of the Consulates, the Admiralty of Nantes issued an order directing all boats owners to unload their cargo within 24 h in order to leave as quickly as possible to Paimboeuf 8 . Their mission was to retrieve the goods and assist ships stranded by the storm. According to this source, the Admiralty wanted to reduce the damage of this event on trade 47 . The Parish register of Nantes reported that only three of the 60 ships remained unscathed, and many people were injured or died 29 . The numerous impacts recorded generally underline the eastern direction of this event.

Discussion
Two main storm paths can be extracted from the spatial survey of environmental, societal and economic impacts recorded in the various archives and mapped on Fig. 5. The first one, regarding the 1469 and 1751 AD events, is estimated from west to east in a restricted part of the western Atlantic coast (from Vendee to southern Brittany). The second one seems to be more extended, with a SW-NE trajectory impacting a larger part of this coast. This second hypothesis is based on the analyses of the 1645 and 1711 AD events. These results concur with the study of Lozano et al. 48 . Four main stormy trajectories are exposed at the European scale, and two main storm paths are presented on the French Atlantic coast. Study cases of 1469 and 1751 AD seem to correspond to the trajectory "Zone 3" presented in the Fig. 4 of Lozano et al. 48 . The 1645 and 1711 AD storms are linked to the main extratropical cyclone track of "Zone 4". Zone 3 events followed an eastward path nearly 45° N, and most of these storms occur west of 0°. The average lifespan of storm events is ∼ 4.5 days 49 in the north-east Atlantic Ocean and ∼ 4.1 days in Zone 3 48 . Zone 4 extratropical storms follow a north-eastward direction from 15° W, for a mean ∼ 4.6 days lifespan 48 . Zone 3 and 4 storms present higher pressure values than Zones 1 and 2, which occur in northern Europe 48 . Extratropical cyclones trajectories vary with climate change, and a northward position has been observed due to recent global warming 3,4,50 .
Seven sedimentological studies presenting historical storm records in north-western France are summarised in Fig. 6. We recorded storm impacts in northern Brittany and west Cotentin 51 (Fig. 6A). In Brittany, two previous studies recorded sedimentological storminess 52,20 , reported in Fig. 6B,C. Sedimentological storm impacts detected in the Petite Mer de Gâvres and the Traicts du Croisic are presented in Fig. 6D,E. The "Yeu Stormy Phase" A and a recent storm impact (Fig. 6F) are extracted from Yeu Island 6 . Finally, three "Coarse Grained Sedimentation Pulses" of the Pertuis Charentais, which are induced by storm activity 19 , are presented in Fig. 6G. These chronologies have been compared with the NAO variations over the last millennium. Three NAO reconstructions [53][54][55] have been synthesised into an overall tendency (Fig. 6H,I,J). Phases of positive NAO state are assessed at 1000-1030, 1100-1430, 1450-1560, 1585-1600, 1640-1660, 1690-1710, 1870-1910AD and 1990 AD to present. Storm impacts detected along the north of the French Atlantic coast presented in Fig. 6A-G are mostly recorded during NAO positive phases. Stormy phases have also been compared with four temperature curves of the northern hemisphere [56][57][58][59] , which were presented in the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment 50 (Fig. 6K-N). Four periods present storm impacts in several sites reviewed in Fig. 6: 1300-1355 AD (Baie d' Audierne, TDC, PMG, Yeu Island and the Pertuis Charentais), 1420-1470 AD (Brittany-Cotentin, PMG and TDC), 1560-590 AD (the three northern references) and 1690-1715 AD (Brittany and TDC). From the IPCC Fifth Assessment temperature curves used, we noticed that these four storm phases are all concomitant with decreasing temperature periods. Storm impact may be influenced by a positive NAO state and a decreasing temperature phase in north-western France.

Methods
Sampling and sediment analysis. Both cores were collected using an Eijkelkamp 50 mm Ø, motordriven percussion corer in the two coastal depositional environments (Fig. 1). A Trimble Differential Global Positioning System (DGPS) was used to survey core positions. Locations were linked to Institut national de l'information géographique et forestière (IGN) benchmarks and levelled according to the NGF datum. Cores were covered in long plastic tubes and stored at + 4 °C. Sediments were characterised by visual litho-microstratigraphic analysis to identify the main sedimentological changes, and to extract each detected macrofossil. Cores were cut into 1 cm long slices and oven-dried at 35-40 °C. A LECO carbon analyser estimated CO 2 percentages after burning at 1,400 °C 60 . Grain size was measured using a laser granulometer (Malvern Mastersizer 2000). Geochemical elemental analyses were evaluated using an Avaatech© XRF core scanner at the UMR CNRS 5805 Environnements et Paléoenvironnements Océaniques et Continentaux. (EPOC) Laboratory (University of Bordeaux). Element intensities were normalised out of the total intensity 61 . Three marine elements (calcium, strontium, and less prominent silicon), and three terrestrial proxies (iron, titanium and less prominent zinc) were retained to interpret geochemical data 17,62 . The Scopix© system was used to take X-ray radiographs 63 . Each cm of both cores was meticulously examined. Lightness was estimated thanks to colorimetric analyses with a Minolta© Cm-2600d spectrometer 64  Seven samples were dated in the Gliwice Absolute Dating Methods Center of the Institute the Silesian University of Technology (GADAM), by the accelerator mass spectrometry (AMS) technique. Organic sediment samples of high organic matter content were selected based on the CO 2 percentage, and two well-formed marine shells were found in the TDC core. Organic samples were subjected to standard preparation to extract total organic carbon, which included treatment with 0.5 M hydrochloric acid, washing in demineralised water and drying. After CO 2 combustion, graphitization took place in an automated graphitization system AGE-3 66 . Determinations of 14 C content were carried out in the Direct AMS Laboratory, Bothell, USA. Radiocarbon ages were calibrated using the IntCal13 atmospheric curve 67 for core PMG2017_3. As the TDC2017-3 is mainly composed of shell samples, the Marine13 67 curve was used, with the local reservoir effect of 151 ± 45 years. The mean of the five nearest available data set from the Marine Correction Database points was calculated 68 . Results were introduced into the P_Sequence algorithm 69 thanks to the OxCal software v.4.3.2 70 in order to obtain the age-depth models. The collection year 2017 AD has been included at 0 cm depth, and the models were extrapolated to reach 280 cm depth for both cores (Fig. 4). The upper parts were precisely dated thanks to the 210 Pb and 137 Cs dating calculation in Pouzet et al. 17 . The two 1 m deep cores dated by 210 Pb and 137 Cs are localised a few metres from the two 3 m deep cores analysed in this study. According to Pouzet et al. 17 , a calculated 0.42 cm/year sedimentation rate was included in the upper 63 cm of the PMG age depth model. The 0.24 cm/year sedimentation rate calculated in the upper 27 cm of the TDC has also been included in the age-depth model.

Data availability
The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.