Middle- to late-Holocene storminess in Brittany (NW France): Part I – morphological impact and stratigraphical record

Abstract

Our study aims to understand the recurring climatic conditions prevailing during the largest storms reaching NW France (Brittany). These storms are responsible for the breaching of coastal barriers and major flooding of lowlands. In a first part of our work, we examine the morphological impact and stratigraphic record of storm events along Western Brittany rocky coasts, with a special focus on the southern coast of the Bay of Audierne, the most exposed coast of the region. In a second paper ('Middle-to Late-Holocene Storminess in Brittany (NW France): Part II'), we shall focus on the chronology of storm events and their climate forcing conditions. Drilling transects and stratigraphic analyses were first undertaken to constrain chronology, strength and wind direction during the main Holocene storm events. New dates, observations and a relative sea-level (RSL) curve were then used to inform discussion of the necessary climatic and morphologic conditions leading to destructive storm events. Most recorded events appear to be linked with cooling episodes of the Holocene and a RSL close to present. Some storms are clearly responsible for breaching and dune building or remobilisation. We demonstrate that storm frequency and intensity appear to rise in a stepwise manner during the late Holocene. Maximum efficiency is reached during the 'Little Ice Age' with clustered events probably lasting several days, but major storms also occurred immediately prior to the 'Medieval Warm Period'. We suggest that recent coastal dune building from c. ad 1100 until now, despite a sea level close to present and continuously rising, may be a direct consequence of the restoration of beaches after periods of recurrent storminess. This building activity often occurred during dry negative North Atlantic Oscillation (NAO) events, in connection with the available sedimentary supply.

Changes in Holocene meridional circulation and poleward Atlantic flow: the Bay of Biscay as a nodal point

“…The Atlantic Meridional Overturning Circulation (AMOC) and its dynamics are critical regarding the modulations of climate (amplitude and frequency) over Europe (westerlies, droughts and/or stormy periods, e.g. Clark et al, 2002;Bryden et al, 2005;Dawson et al, 2004;Magny et al, 2003;Sorrel et al, 2009;Trouet et al, 2012;Van Vliet-Lanoe et al, 2014a, b;Jackson et al, 2015). The two connected North Atlantic gyres, the subpolar gyre (SPG) and the subtropical gyre (STG) are fundamental for these processes as they transfer heat and salt toward the Nordic seas (e.g.…”

Formation and evolution of back‐barrier perched lakes in rocky coasts: An example of a Holocene system in north‐west Spain

“…(, ) identified a pulse of storm‐related aeolian activity in the neighbouring coast of Portugal at 1·2 ka cal bp , corresponding to the Rapid Climate Change event of 1·0 to 1·2 ka cal bp (Mayewski et al ., ). Increased storm activity has also been detected in the coastal sedimentary archives of the English Channel and Brittany (north‐west France) (Sorrel et al ., ; Van Vliet‐Lanoë et al ., ,b) and in the lacustrine records of the Mediterranean Iberian region (Corella et al ., ; Sánchez‐López et al ., ) at this time interval, which roughly corresponds to the Dark Ages Cold Event (DA)/Medieval Climatic Anomaly (MCA) transition. A combination of high tides and storm surges in the north‐east Atlantic under this favourable palaeoclimatic situation could explain the breaching of the outlet channel and the catastrophic erosive event in Doniños.…”

“…The architecture of the resulting transgressive sequence basically depends on the balance between the rate of sediment accumulation and the rate of increase in accommodation space driven by sea-level change, as well as by the nature of sediments supplied from the sea and the continent (Curray, 1964;Cattaneo & Steel, 2003;Tesson et al, 2011;Cooper et al, 2012;Hein et al, 2016). In addition to changes in sea-level and sedimentary budgets, climate variability can also play a significant role, especially in determining the stability of barriers, which are usually vulnerable to strong storm regimes (Andrade et al, 2004;Van Vliet-Lano€ e et al, 2014a). This stability is also a function of changes in the vegetation cover (Jackson & Cooper, 2011;Gonz alez-Villanueva et al, 2013) or direct human intervention (Dinis et al, 2006).…”

Holocene formation and evolution of coastal dunes ridges, Brittany (France)

“…1a and b) completed by vibrocores and drilling transects at Audierne Bay, Kermor-Tudy (Bé nodet Bay), Grand Loch de Guidel and Kerzine (Lorient), Le Conquet, Pors Milin (Brest), Le Vourch and Guisseny (Ploudalmé -zau). Stratigraphic descriptions with AMS-14 C dates have been undertaken on these sections (Goslin, 2014;Van Vliet-Lanoë et al, 2014a) and completed in March 2014. The stratigraphical data of Meurisse-Fort (2009) were used for Normandy and Picardy.…”

“…The sedimentary record of RSL rise and particularly the High Stands (HS) were perturbed by the increased strength of storm surges, mostly after Roman times (Meurisse-Fort, 2009). Large-scale (pluridecimetric to metric) pseudo-oscillations observed in SL reconstructions (e.g., Morzadec-Kerfroun, 1974) are the expression of disturbances of the sedimentary record induced by storms (Goslin et al, 2013;Van Vliet-Lanoë et al, 2014a). Extreme storms or several-day gales could be responsible for major morphological changes in the former ridge and back-ridge zones.…”