Journal of Marine Science and Engineering Article Coastal Retreat and Sedimentation during the Last 3000 Years. Atlantic Coast of NW Spain Ramón Blanco-Chao * , Manuela Costa-Casais , Daniel Cajade-Pascual and Gonzalo Gómez-Rey Departamento de Geografía, Universidad de Santiago de Compostela, 15782 Santiago, Spain * Correspondence: [email protected]; Tel.: +34-8818-11000 (ext. 12675) Received: 12 July 2019; Accepted: 22 September 2019; Published: 24 September 2019 Abstract: During the last glaciation, thick nival and periglacial sediments buried large sectors of the NW coast of Spain. The sediments were mostly eroded by the rising sea level during the Holocene, but in several places they remain, forming sedimentary cliffs. Radiocarbon dates obtained at the topmost layers of these cliffs prove that continental sedimentation was active until very recent times, followed by a retreat of the cliffs. During the first stages of the transgression, the erosion of the cliffs and the changes in the coastal system were controlled by the rising sea-level. Once the sea-level stabilized, the exhumation of inherited landforms, the supply of sediments, and a continuous continental sedimentation became the main factors. The last stages of cliff retreat were almost synchronous with the sedimentation of the upper layers of the deposits. Keywords: sedimentary cliffs; coastal change; late Holocene; human activity 1. Introduction The eustatic, isostatic, and tectonic components of relative sea-level, as well as sediment availability, are the main factors driving the morphodynamics of transgressive coastal systems, but in the evolution of a sedimentary cliff under a rising sea level the intervening factors are quite complex. The retreat of the cliff-front is caused by wave action and slope instability, with changes in the retreat rate modulated by sea-level oscillations, wave action, sediment availability, and the exhumation of rock landforms. The climatic changes that took place during the Holocene have been accompanied by changes in wave environment, which is the main energy input in coastal dynamics. The sediment supply depends largely on the geomorphological processes that dominated during the previous regressive stage in the areas that were inundated by the rising sea-level. In high and mid latitudes, many present parts of the coasts with coarse clastic beaches are located in areas that were subjected to periglacial and glacial processes. Paraglacial coasts are those in which glacial landforms and sediments have a strong influence in present morphodynamics [1]. These glacioisostatically uplifted coasts are characterized by a high availability of sediments derived from the erosion of glacial tills. In contrast, coast which experienced periglacial processes are usually isostatically stable and are covered by sediments with a higher variability of facies. [2]. We defined para-periglacial coasts as those in which Holocene evolution has been influenced by the erosion of periglacial and fluvio-nival deposits by the Holocene rising sea-level [3]. In the northwestern coast of the Iberian Peninsula these deposits were formed since the middle Weichselian glaciation to the beginning of the Holocene, although there are sedimentological variations related to local topographic conditions [4–8]. One of the most important factors in the para-periglacial coastal systems of the NW Spain is that the erosion of the continental deposits by the Holocene rising sea-level leaded to the exhumation of ancient coastal landforms, shaped during the last interglacial [3,8,9]. J. Mar. Sci. Eng. 2019, 7, 331; doi:10.3390/jmse7100331 www.mdpi.com/journal/jmse J. Mar. Sci. Eng. 2019, 7, x FOR PEER REVIEW 2 of 12 There is no precise data on the changing relative sea-level in NW Spain, but the available data suggests that the sea-level reached its present position after 3500 yr BP. Studies conducted in coastal lagoons in the Atlantic coast of the Iberian Peninsula confirmed that in most of them marine conditions were established around 6-5 ka BP and that from 5 to 3 ka BP the rate of sea-level rise J. Mar. Sci. Eng. 2019, 7, 331 2 of 12 decreased and the lagoons were closed by sand barriers [10–14]. More recently, it was confirmed that sea-level was still below the present one by 3500 yr BP and that the present sea-level was probably stabilizedThere after is no that precise date data[15,16]. on Research the changing conducted relative on sea-level marine sediments in NW Spain, in the but Atlantic the available rias and data on suggeststhe continental that the platform sea-level identified reached its several present changes position inafter the marine 3500 yr conditions BP. Studies during conducted the inlast coastal 3 ka. lagoonsThese changes in the Atlantic were coastsynchronous of the Iberian with Peninsulaoscillations confirmed of the thatNAO in mostindex, of suggesting them marine consequent conditions werevariations established in the upwelling around 6–5 system ka BP [17–19]. and that Recent from 5 investigations to 3 ka BP the found rate of that sea-level a pronounced rise decreased change and in the energetic lagoons were conditions closed in by the sand Gali barrierscian coast, [10– from14]. Morea storm recently, regime it to was a low-energy confirmed environment, that sea-level may was stillhave below happen the between present 2850–2200 one by 3500 cal. yr yr BP BP and [17,19]. that theFor present more recent sea-level periods, was probablythe reconstruction stabilized of after the thatnorthern date [hemisphere15,16]. Research troposheric conducted circulation on marine suggests sediments an inabrupt the Atlantic change rias around and on1400 the AD continental from a platformweaker atmospheric identified several circulation changes during in the marinethe Mediev conditionsal Warm during Period the last to 3a ka. period These of changes increasing were synchronousstorminess during with oscillationsthe Little Ice of Age the [20,21], NAO index, which suggesting was also identified consequent in the variations wave regime in the upwelling[19]. systemThis [17 work–19]. focuses Recent on investigations the evolution found of para-perig that a pronouncedlacial coastal change systems in thesince energetic the stabilization conditions of insea-level the Galician in the coast,late Holocene from a storm in NW regime Spain. to In a pr low-energyevious works environment, we highlighted may havethe fact happen that there between was 2850–2200evidence of cal. very yr recent BP [17 episodes,19]. For of more retreat recent in one periods, cliff-deposit the reconstruction located in the of Ria the of northern Muros-Noia hemisphere [3,8,9]. troposhericIn order to verify circulation if this suggestswas a local an or abrupt more changeregional around feature, 1400 we studied AD from the a previous weaker atmosphericcliff-deposit circulationand a new duringone located the Medieval far from Warm it but Period in the to same a period region of increasingin more detail. storminess We aimed during to the determine Little Ice Agewhether [20,21 late], which Holocene was coastal also identified retreat is in regional, the wave and regime that [there19]. was an almost continuous supply of sedimentThis workfrom focusesthe continental on the evolution areas. The of results para-periglacial suggest that coastal the coastal systems systems since the studied stabilization attained of sea-levelequilibrium in thewith late the Holocene present in conditions NW Spain. very In previous recently. works The weLate highlighted Holocene theevolution fact that was there more was evidencecomplex than of very a simple recent linear episodes cliff of retreat retreat as in the one se clia-levelff-deposit rose. located Consequently, in the Ria the of factors Muros-Noia involved [3,8, 9in]. Inthis order evolution to verify arise if thisas important was a local controls or more in regional the coastal feature, response we studied to future the sea-level previous rise. cliff -deposit and a new one located far from it but in the same region in more detail. We aimed to determine whether late Holocene2. Regional coastal Setting retreat is regional, and that there was an almost continuous supply of sediment from the continentalThe Caamaño areas. and The Oia results cliff-deposits suggest thatare located the coastal in the systems Atlantic studied coast attainedof the NW equilibrium of the Iberian with thePenisula, present at conditionsthe southern very margin recently. of the The Ria Late of HoloceneMuros and evolution Noia and was between more complex Cape Silleiro than aand simple the linearmouth cli offf retreatthe Miño as the river, sea-level respectively; rose. Consequently, the distance the between factors involvedthem is in80 this km evolution (Figure arise1). The as importantsedimentology controls of these in the cliff-deposits coastal response was pr toesented future sea-level in previous rise. studies [4,5,8,9]. Both coastal sectors are characterized by high wave energy. The coast of Caamaño is exposed to 2. Regional Setting waves from the NW and W but more protected from SW waves. Sixty percent of waves range betweenThe 1 Caamaño and 2 m andof significant Oia cliff-deposits height and are their located period in the varies Atlantic from coast 9 to 12 of s. the The NW coast of theat Oia Iberian is a Penisula,more energetic at the southern sector because margin ofit theis exposed Ria of Muros to waves and Noia coming and between from the Cape NW, Silleiro W, and and theSW, mouth with ofsignificant the Miño height river, respectively;between 1 and the 3 distance m and periods between of them 10 to is 14 80 kms representing (Figure1). The 40% sedimentology of the total. The of thesespring cli tidalff-deposits range is was 3.75 presented m, reaching in previous 4 m unde studiesr storm [ 4surge,5,8,9 ].conditions. Figure 1. Location of the studied sedimentary cliffs. cliffs. Both coastal sectors are characterized by high wave energy.