Holocene environmental change on the Atlantic coast of NW Iberia as inferred from the Ponzos wetland sequence

Holocene environmental change on the Atlantic coast of NW Iberia as inferred from the Ponzos wetland sequence LUIS G  OMEZ-ORELLANA , PABLO RAMIL-REGO , JAVIER FERREIRO DA COSTA AND CASTOR MU ~ NOZ SOBRINO G omez-Orellana, L.,Ramil-Rego, P., Ferreiro da Costa, J. & Mu~ noz Sobrino, C.: Holocene environmental change on the Atlantic coast of NW Iberia as inferred from the Ponzos wetland sequence. Boreas. https://doi.org/ 10.1111/bor.12535. ISSN 0300-9483. Theintertidalenvironment ofthePonzosbeach(NWIberianPeninsula)hostsasedimentarysequence(includinglarge wood fragments) deposited during the first half of the Holocene in a hygrophilous continental wetland. Pollen and macrofossildataalongsideradiocarbondating allowreconstruction of thechangesthat occurredduring the Earlyand MiddleHoloceneinthelandscapeoftheNWIberiacoastallowlands,aswellasthelocalwetlandplantcommunities, in response to the climate variations and the eustatic sea-level oscillations. The sequence represents the evolution of a coastal wetland from its initial phases as a hygrophilous wetland towards the subsequent installation of a freshwater lagoon.Pollendatashowthedominantroleof Atlantic(mainlydeciduous)woodytaxa,thescarcityofconifersandthe lackofMediterraneanelements in the coastallandscapesaround thePonzossite.Thepresenceandabundanceofsome taxasuchas deciduous Quercus,Castanea,Fagus,TiliaandUlmusduringtheEarly Holoceneprovidesfurthersupport fortheoccurrenceofglacialrefugesintheCantabrian-Atlanticareaduringthe LastGlaciation.Thediversevegetation that characterizes the modern landscapes in this territory established later, spreading from these glacial reservoirs of biodiversity. In this sense, the notable and early presence of Fagus at the beginning of the Holocene, a tree also previously recorded during several phases of the Last Glacial Cycle on the NW Iberia coasts, is noteworthy. In addition, during the Early and Middle Holocene are recorded other trees that are currently extirpated as natural taxa in the area, such as Pinus,Tilia and Carpinus. Luis G omez-Orellana (lgomezor[email protected]), Pablo Ramil-Rego and Javier Ferreiro da Costa, GI‑1934‑TB, Laboratorio de Bot anica e Bioxeograf ıa, IBADER, Universidade de Santiago de Compostela, Campus Terra s/n, 27002 Lugo (Galicia), Spain; Castor Mu~ noz Sobrino, CIM.UVigo, Departamento de Biolox ıa Vexetal e Ciencias do Solo, Facultadede Ciencias, Universidade de Vigo, Campus de Marcosende s/n, 36310Vigo, Spain; received 1st February 2021, accepted 1st May 2021. The palaeoecological information obtained in Europe over the last 30 years has confirmed that certain areas of the Iberian, Italian and Balkan Peninsulas would have acted as refuge areas for thermophilous and mesophilous flora during the Pleistocene. From these areas, during the Holocene, those taxa spread and expanded towards other nearby territories. The location of shelter areas for thermophilous and mesophilous taxa has been linked to low-altitude territories, located below the mountain altitudes affected by permanent or semi-permanent snow. The refuge areas are identified with systems of narrow valleys and depressions, where different species could accommodate their areas of distribution according to climate fluctuations, and also expand during the periods of more favourable conditions (Ramil-Rego et al. 1998a, 2000; Carri on et al. 2003; G omez-Orellana et al. 2007, 2012, 2013; Gonz alez-Samp eriz et al. 2010; Tzedakis et al. 2013; Birks 2019). In the Iberian Peninsula, the palaeoecological record shows three main patterns of vegetation history that can be identified, with at least three bioregions. Firstly, the Atlantic bioregion, enclosing the littoral, sublittoral and mountainous territories along the northern coast (Cantabrian littoral) and extending along the Atlantic coast. It covers areas that during the majority of the Holocene recorded mild winter temperatures and absence of summer droughts. Secondly, the southern part of the Atlantic littoral and its analogous territory on the Mediterranean coast belonging to the Mediterranean bioregion, with a climate during the Holocene marked by mild winters, and warm and dry summers. Thirdly, the Iberian Peninsula hinterlands, consisting of extensive plateaus and various mountainous ranges, forming the Iberian Continental bioregion that during the Holocene had climate conditions that were warm and dry in summer, and very cold in winter. The presence of refuge areas for thermophilous and sclerophyllous flora has been argued from various pollen sequences obtained in the Mediterranean bioregion (e.g. Carri on et al. 2003), linked to the persistence of taxa such as Quercus ilex, Olea,Phillyrea,Pistacia,Rhamnus and Buxus. Likewise, in studies carried out in the Atlantic bioregion, palynological evidence attests to the occurrence of refuge areas for various mesophilous elements such as deciduous Quercus,Corylus,Tilia,Ulmus, Castanea,Fagus,Carpinus, and even evergreen taxa such as Quercus ilex,Ilex aquifolium and Laurus nobilis (Ramil-Rego et al. 1998a, 2000; G omez-Orellana et al. 2012, 2013). The knowledge of the Iberian continental DOI 10.1111/bor.12535 ©2021 The Authors. Boreas published by John Wiley & Sons Ltd on behalf of The Boreas Collegium This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. bs_bs_banner bioregion is still scarce and fragmented and it does not allow in-depth assessment about the possible presence of refuges, even more so when the available sequences come from mountainous areas located in the transition between the Mediterranean and Atlanticbioregions (e.g. Morales-Molino & Garc ıa-Ant on 2014). Four main biogeographical units exist within the Atlantic bioregion. These have maintained, through the Lateglacial and the Holocene, common and differentiated characteristics in relation to the species present and the dominant landscapes (Ramil-Rego et al. 2006). The first unit comprises the littoral territories, from sea level to 200 m a.s.l. (Fig. 1). The second unit includes the sublittoral territories, far from the coastline and extending towards the main mountainous units. The third unit corresponds to the Atlantic mountains as awhole, with a main system, the Cantabrian Range (700–2650 m a.s.l.), stretching E–W, and other reliefs of lower entity and altitude located east andwest of that main range (Fig. 1). The fourth unit corresponds to an area of transition between the Atlantic mountains unit and the Iberian continental bioregion that runs through a set of river basins and valleys defined by the mountain reliefs (Ramil-Rego et al. 1998a). Thehigh-resolutionrecordsforthe Lateglacialandthe beginning of the Holocene in the Atlantic bioregion of the Iberian Peninsula correspond to deposits formed in different types of wetlands (blanket bogs, peatlands, lakes, lagoons, etc.). Most of them are located in the mountainunit, particularly in the Cantabrian Range and in other nearby mountains: the Northern Galician Mountains (Van Mourik 1986; Ramil-Rego et al. 1996, 1998a; Mu~ noz Sobrino et al. 2005), the Galician-Minho Mountains (Ramil-Rego et al. 1998a) and the GalicianDuero Mountains (Allen et al. 1996; Mu~ noz Sobrino et al. 2004, 2007, 2013) (Fig. 1). Most of these deposits are far from the coastal lowlands, where the main refuge areas for the mesophilous taxa during the Pleistocene would have mostprobably been located. Thus, at present, a great number of mesophilous flora located in other territories of the Atlantic biogeographical region are not present in these inner areas. The palaeobotanical analyses carried out on the sediments from the different types of littoral and sublittoral wetlands of the Iberian Atlantic bioregion have provided uneven and fragmented information. The cause of this unequal spread of data lies in the geomorphological characteristics (absence of great basins or settings favourable to the establishment of large wetlands, effects of erosion processes, changes in the superficial hydrological configuration and the coastal line, etc.). Besides, human action has been progressively destroying these deposits due to the increase of agricultural, forestry and urban activities. The end of the glacial period marked a turning point in the littoral ecosystems of the Cantabrian and Atlantic coasts of the Iberian Peninsula. The warmer and moister climate favoured the dissemination and expansion of several taxa, and the sea level rise caused the spatial reconfiguration of the ecosystems. Some littoral sections lost part of their coastal habitats (dunes, wetlands, cliffs), which were destroyed by the inland progression of coastline, while in other stretches the coastal habitats were reconfigured or even established on the old continental territories. In relation to these dynamics, different limnic remains have been identified as ancient organic deposits formed on continental wetlands in the littoral. Nowadays, these remains are located between the eulittoral and infralittoral areas, and record different periods of the last glacial–interglacial cycle. Only the palynological sequence of Area Longa covers the last glacial (G omez-Orellana et al. 2007). A number of pollen sites record different periods of the MIS 3 (Nonn 1966; Mary et al. 1977; Granja & De Groot 1996; G omez-Orellana et al. 2007, 2013); and other groups of sites record the second halfor the last third of the Holocene (Santos et al. 2001; Garc ıa Anton et al. 2006; Bao et al. 2007; Costas et al. 2009; Granja et al. 2010, 2016; G omez-Orellana et al. 2014; Mu~ noz Sobrino et al. 2016; S aez et al. 2018). Finally, pollen sequences recording the Lateglacial or the beginning of the Holocene are limited to the deposits of Moug as (G omez-Orellana et al. 1998) and Urdaibai (Iriarte Chiapusso et al. 2006). The presence of continental ancient wetland deposits that remain permanently or temporarily submerged in the sea also has been found in different Atlantic areas, such as in the case of the Doggerland area (North Sea). This area is a wide continental space that served as a bridge between the British Isles and the European continent, but that has remained below sea level since 8450–8150 BC (Gaffne et al. 2007; Coles 2014; Kr€ uger et al. 2017). This paper presents the palaeoecological data obtained from a fossil deposit accumulated in an ancient wetland located at the present-day beach of Ponzos, in the NW of the Iberian Peninsula (Galicia, Spain). The site is located in the northern end of the Atlantic littoral, in the coastal stretch designated as ‘Costa  Artabra’ (Fig. 1), only 50 km in a straight line from the area with the highest altitude of the northern Galician Mountains (Xistral mountains; Fig. 1). This mountain area provides a wide range of palaeobotanical information obtained from different types of peatlands and organic deposits located between 650 and 1050 m a.s.l. (RamilRego et al. 1996, 1998a; Mu~ noz Sobrino et al. 2005; Iriarte Chiapusso et al. 2016). Among this mountain rangeandthelittoral,thesublittoralunitincludesN–Sor W–E oriented small and narrow river valleys, where remains of ancient forests are located. Deciduous species (Quercus robur,Quercus pyrenaica,Corylus avellana, Betula alba,Acer pseudoplatanus,Fraxinus excelsior, Fraxinus angustifolia,Ulmus glabra,Frangula alnus) are dominant in these forests, with a scarce representation of evergreen species (Arbutus unedo,Ilex aquifolium,Taxus 2Luis G omez-Orellana et al. BOREAS baccata,Laurus nobilis). Besides, these ancient forests also host an important nemoral flora, including the presence of fern species considered as palaeo-relicts and that stand out for their high value for biodiversity conservation. The present-day distribution area of these fern species is not included in the Palaearctic ecozone: Culcita macrocarpa,Woodwardia radicans, Hymenophyllum tunbrigense,H. wilsonii,Vandenboschia speciosa (Jermy 1984; Sermolli et al. 1988; Rita 1990; Pausas & S aez 2000; Amigo et al. 2017). Besides, they show a clear stenoecious behaviour regarding the demands of their habitats (absence of frost, continuous high humidity, low direct incidence of sunlight) as well as a low dispersal capacity. The aim of this study is to reconstruct the landscape of the NW Iberia coastal lowlands during the phases of expansion and arboreal domination in the Holocene, as well as to decipher the dynamics of the coastal wetlands in relation to the Holocene marine transgression. Material and methods Sampling The Costa  Artabra littoral (Fig. 1) is dominated by granitic rocks (two-mica granites and leucogranites) combined with other geological materials. Geomorphologicalprocesseshaveledtotheemergenceoflongsectors of cliffs and pebblebeaches, in contrast to small bays that have, in their inner stretches, wide sand beaches, estuaries and different types of coastal wetlands (lagoons, marshlands, and hygrophilous and swamp environments). Ponzos beach (latitude 43°330N, longitude 8°150W) is 1.2 km long (Figs 1, 2). Behind the beach there is a wide Fig. 1. Location of the Ponzos wetland and other coastal lagoons currently existing in the area (1 =Ponzos deposit; 2 =Lagoon of Doni~ nos; 3 = Lagoon of Frouxeira; 4 =Lagoon of Pant ın). BOREAS Holocene environmental change on the Atlantic coast of NW Iberia 3 and complex dune system measuring more than 100 hectares, formed by embryonic dunes, mobile white dunes of Ammophila arenaria and fixed grey dunes with herbaceous vegetation and small shrubs (Daphne,Calluna,Erica,Ulex,Helichrysum,Othantus). The greydune includes small humid depressionswith ponding and high humidity between autumn and spring, decreasing or almost disappearing with the arrival of summer. The transition to the continental environment occurs over an area of fossil dunes covered by shrubs dominated by Erica vagans and small representations of dry heathlands on the dunes, hygrophilous wetlands with herbaceous and wooded formations, and small representations of rocky environments (Fig. 2). During spring tide periods, a thick organic sediment package appears at the western edge of the beach, in the intertidal area. This deposit normally remains covered by the sand of the present-day beach. This organic sediment wascontinuouslymonitoredfrom2009to2020totake advantage of the spring tides in order to document and collect different macro-remains. The outcropping surface of the organic sediment package reaches 900 m 2 , comprising a great number of wood fragments, among which the appearance of long trunks or branches (>5 m long and >40 cm in diameter) is noteworthy. They are located above the deposit or embedded, and chaotically distributed around different areas of the outcrop. When the sediment surrounding the wood remains was manually removed, it was determined that some of these are in living position and include both the main roots and the initial part of the trunk. During the sampling, some of these macro-remains were collected for botanical identification and dating. We drilled the surface of the deposit in different sites with Eijkelkamp percussion drilling equipment, using cores of 10 and 3 cm in diameter. All of the probes reached the rocky basement. The drill-cores recovered in the different tests were very similar. The main differences concern the thickness of the organic package and the characteristics of the upper levels(absenceorpresenceof sands and large wood remains). The thickest core (250 cm) obtained was chosen for the palynological and chronological analysis. The lithology of the core was described in the laboratory and then divided into 125 samples of 2 cm thick. All samples remained stored at 5 °C until analysis. Pollen analysis The samples were prepared for pollen analysis using standard methods (Fægri et al. 1989; Moore et al. 1991). The mounted slides were analysed using light Silty clay Bedrock Big trunks Organic silt Present-day beach/dune sand Sea water Inshore Offshore Foreshore Backshore Dune system 2 34 6 135 Present-day soil Location of cores A BC 7 Reworked trunks 0 10 cm Fig. 2. A. Idealized section of the Ponzos deposit, indicating the different facies present, the sampling points (the red arrow shows the location of the core analysed) and the configuration of the area. 1 =embryonic shifting dunes; 2 =shifting dunes along the shoreline with Ammophila arenaria (white dunes); 3 =fixed coastal dunes with herbaceous vegetation (grey dunes); 4 =humid dune slacks; 5 =Atlantic decalcified fixed dunes; 6 = herbaceous and woody wetlands; 7 =forest. B. Photograph of a trunk at Ponzos. C. Outcrop of peaty sediment at Ponzos. 4Luis G omez-Orellana et al. BOREAS microscopy. The total sum of pollen in all samples ranges between 411 and 789 grains. Terrestrial pollen percentages were calculated using a total terrestrial pollen sum (>300 grains), including trees, shrubs and upland herbs. The total sum of pollen was used to calculate percentages of the aquatics and cryptograms. The software TILIA 1.7.14 (Grimm 1990–2015) was used for calculations and graphic representation of results. Pollen diagrams were zoned using constrained incremental sum of squares (CONISS) cluster analysis (Fig. 3). Chronology Five radiocarbon dates were obtained for the core, four using AMS dating methods and one more using the conventional 14 C method (Table 1). 14 Cage determinations were carried out at the Centrum voor Isotopen Onderzoek, Groningen University and at the ICA Lab, Florida. Radiocarbon ages were converted to cal. a BP (2r) using Calib 8.2 (Stuiver et al. 2020; Table 1). Pinus (undiff) Pinus sylvestris type Pinus pinaster type 20 40 60 Quercus robur type Betula 20 40 Corylus Ulmus Fagus Carpinus Tilia Castanea Ilex Acer Frangula Arbutus Praia de Ponzos (Ferrol - A Coruña) alt. 0 m a.s.l. 100200300400 500 Terrestrial Pollen Sum 200 400 600 800 Total Pollen & Spores Sum 2468 Total sum of squares CONISS Ponzos Zones (PZ) Biozones 1 3b 2 3a He-2 He-3 Hd-1 4 5 6 7a 7b 7c 7d 7e 7f 8 9a 9b 10 Hd-2a Hd-2b Hd-3a Hd-3b Alnus Fraxinus Salix AP/NAP 20 40 60 80 100 Hd2a 6676 6629 7609 6573 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 Depth Lithology Silty clay with heterometric sand and gravel Silty clay with different brown tones Organic silt Present-day beach sand Large trunks 6227 Age (cal. a BP) 6200 6300 6400 6500 6600 6700 6800 6900 7000 7100 7200 7300 7400 7500 7600 7700 7800 7900 8000 8100 8200 8300 8400 8500 Analysis: L. Gómez-Orellana Age/depth model (cal. a BP) Fig. 3. Pollenpercentagesdiagramof the arborealtaxarecordedinthePonzos deposit.Biozones:He=Holocenearborealexpansion;Hd=Middle Holocene, from Mu~ noz Sobrino et al. (2005). Depth/age model is given on the left, based on the dating and the well-dated palyno-stratigraphical regional event, related to the expansion of Corylus (8500 cal. a BP). BOREAS Holocene environmental change on the Atlantic coast of NW Iberia 5 Results Biostratigraphy and radiocarbon dates The obtained cores include different organic and inorganic facies, their bases being directly placed on bedrock. The deepest core obtained in Ponzos beach achieved a depth of 250 cm and comprises several well-differentiated facies (Fig. 3). The basal part (250–154 cm depth) is formed by grey-brown silty clay, with different brown tones, probably related to changing organic matter content. The brown tone increases from the bottom up and it is possible to distinguish up to three facies (Figs 2, 3). Between 216 and 236 cm depth, there is ahighcontent ofquartzheterometricsands. Above,there is a very dark silty layer with high content of organic matter and presence of plant remains. It is 154 cm thick, with large wooden embedded fragments of Salix (Figs 2, 3). On top of this layer, there is an important set of largesized plant macro-remains (Figs 2, 3). The uppermost levelisapackofsand,75–100 cmthick,correspondingto the current beach. Five radiocarbon datings were obtained, three from bulk sediment and two from two wood fragments (Table 1). One of the fragments was recovered at 56 cm depth from inside the sampled core, embeddedwithin the sediment. The other was recovered within the macroremains that appear on the sediment surface. Palaeobotanical analyses The inorganic and organic sediments of the active halophilous wetlands (infralittoral and eulittoral environments of marshes and coastal lagoons) located in the Atlanticbioregion arecharacterizedbyhigh salt content, and they form efflorescences when dry. These usually have macro-remains of shells, carapaces, spicules or other carbonated structures belonging to marine invertebrates, as well as characteristic micro-remains of halophytic biocenosis. No efflorescence appears in the dried sediments obtained in Ponzos. Also, the analysis of the residues under 109and 309magnification after sieving through 0.5, 0.1 and 0.05 mm found no evidence of marine invertebrate remains, nor was any carbonic reaction after acid attack noted. The entirety of the recovered macro-remains corresponds to vascular plants, fibrous remains, small roots (<2 mm diameter), as well as pieces of leaves and stems of Poaceae and Cyperaceae. The different facies include abundant plant macroremains, especially at the most organic level (0–154 cm). Most of them correspond to fibrous tissues of herbaceous vascular plants and woody fragments of different sizes. The largest woody remains correspond to 25 large branchesand trunksover2 mlong(upto 5–7 m) and30– 40 cm in diameter. Some of these large fragments correspond to roots and basal fragments of the trunk in living disposition. However, most of them are located on the surface of the deposit, either embedded in it or free. The majority of our woody samples are from willow (according to the present-day regional flora, most probably correspond to Salix atrocinerea) and to a lesser extent, from deciduous oaks (according to the presentday regional flora, most probably correspond to Quercus robur/Quercus pyrenaica). The content of micro-remains consists of abundant pollen as well as moss and fern spores, in a good conservation status. In addition, several fungus remains (spores, hyphae), aswell asdiatoms and other non-pollen micro-fossils were found. The content in micro-remains is similar to the pollen signal obtained in different nonhalophilous wetlands in the littoral area (peatlands, wet heaths, high marshes, marsh areas surrounding coastal lagoons, etc). Figures 3, 4 and 5 show the pollen diagrams obtained. The identified pollen zones follow the results of the cluster analysis. Ten local pollen assemblage zones (LPAZ) were recognized, and designated as Ponzos Table 1. Radiocarbon and calibrated ages from Ponzos. All dates were calibrated by using the CALIB Rev 8.2 program and IntCal20 data set (Stuiver et al. 2020). 2-sigma (95.4%) confidence intervals and their relative areas were used as well as the median probability. Laboratory no. Depth (cm) Material Radiocarbon age uncal. a BP Method 2rcal. a BP age ranges (relative area) Cal. a BP median probability GrN 32293 0 Salix 607535 Conventional 7149–7126 (0.03) 6841–7017 (0.94) 6838–6833 (0.00) 6818–6798 (0.03) 6929 ICA-20OS/ 0901 20–22 Sediment 538040 AMS 6171–6283 (0.65) 6103–6158 (0.18) 6010–6081 (0.16) 6227 GrA-65937 52–54 Sediment 577540 AMS 6479–6668 (1) 6573 GrA 66804 56 Salix 586040 AMS 6778–6763 (0.03) 6596–6756 (0.91) 6594–6562 (0.06) 6676 GrA-65936 140–142 Sediment 672040 AMS 7555–7663 (0.85) 7510–7544 (0.15) 7609 6Luis G omez-Orellana et al. BOREAS Zones(PZ), butsomeof themweresubdividedinorderto highlight some particular facts related to the vegetation dynamics. PZ-1 (250–248 cm) marking the start of the sequence is characterized by dominance of trees, including a maximum of Quercus robur-type and around 20% of P Praia de Ponzos (Ferrol - A Coruña) alt. 0 m a.s.l. Total Pollen & Spores Sum Calluna Erica (undiff) Ulex type Cistaceae Daphne Lonicera 20 Poaceae Artemisia 20 Liguliflorae Tubiliflorae Chenopodium Centaurea Asphodelus Plantago Polygonaceae Cruciferae Leguminosae Geranium Caryophyllaceae Campanula Saxifraga Armeria Labiatae Succisa Centaurium Boraginaceae Rosaceae Varia 100200 300400 500 Total Pollen Sum 200 400 600 800 1 3b 2 3a He-2 He-3 Hd-1 4 5 6 7a 7b 7c 7d 7e 7f 8 9a 9b 10 6676 6629 7609 6573 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 Depth Lithology Hd-2a Hd-2b Hd-3a Hd-3b 6227 Silty clay with heterometric sand and gravel Organic silt Present-day beach sand Large trunks Ponzos Zones (PZ) Biozones Age (cal. a BP) Analysis: L. Gómez-Orellana Silty clay with different brown tones Fig. 4. Pollen percentages diagram of the herbs and shrubs recorded in the Ponzos deposit. Biozones: He =Holocene arboreal expansion; Hd = Middle Holocene, from Mu~ noz Sobrino et al. (2005). BOREAS Holocene environmental change on the Atlantic coast of NW Iberia 7 Corylus. Hereafter, PZ-2 (248–238 cm) starts with a slight decrease in total tree pollen, marked by the decline of the Quercus robur-type but the subsequent expansion and maximum of Corylus. PZ-3 (238–192 cm) evidences the recovery of the total tree pollen (>85%). The dominant deciduous tree species are Quercus robur-type and Corylus, with scarce presence of Pinus. In subzone 3a there is a slight increase of Poaceae while in subzone 3b Erica decreases and Compositae increase. PZ-4 (192–140 cm) reflects a slight total tree pollen detriment due to the decrease of Corylus. Fagus and Arbutus increase and the continuous curve of Ulmus starts. The percentages of Poaceae increase and the percentages of Erica and Compositae decrease. The top of the zone has been dated at 7609 cal. a BP (Table 1, Fig. 3). PZ-5 (140–134 cm) reflects a detriment in total tree pollen and represents the tree minimum of the sequence, together with the maximum of Poaceae (Fig. 4). In PZ-6 (134–120 cm) there is a rise of the tree pollen percentages that reflects the recovery of the deciduous forests, with the successive expansion of Quercus roburtype and Corylus, and Arbutus and Fagus increasing. Besides, a slight increase of Erica occurs, while Poaceae decline (Fig. 4). PZ-7 (120–44 cm) represents an initial decrease in tree pollen (PZ-7a) and its subsequent recovery (PZ-7b). Subzone PZ-7c reflects an increase of Quercus roburtype, followed by increases of Corylus and Betula and the minimum values of Arbutus (PZ-7d). Subzone PZ-7e shows a new increase in Quercus robur-type, Erica and Poaceae (Figs 3, 4). The top of subzone PZ-7e has been dated at 6573 cal. a BP (Table 1, Fig. 3). Finally, PZ-7f shows the replacement of Corylus by Quercus robur-type (Fig. 3) and the continuous increase of Poaceae (Fig. 4). PZ-8(44–26 cm) shows a moment of arboreal detriment, mainly due to decreases in Corylus and Quercus roburtype. PZ-9 (26–10 cm) reflects a subsequent recovery of the total tree pollen that is initiated with the increase of Betula and Corylus (PZ-9a) and then Quercus robur-type (PZ-9b). Subzone PZ-9a has been dated at 6227 cal. a BP (Table 1, Fig. 3). Quercus robur-typeand Corylus decline in PZ-10 (10–0 cm), while Betula,Fagus,Pinus pinastertype and Erica increase. The lowermost levels of the sequence could not be dated by radiocarbon techniques due to their scarce organic mattercontent. Above them, different dateswere obtained from bulk sediment and macro-remains (Table 1, Fig. 3). The interpretation and correlation of these pollen zones with the reference regional sequences for the Atlantic ecoregion (Ramil-Rego 1992; Mu~ noz Sobrino et al. 2005) and the 14 C dates obtained allow the attribution of the entire sequence to the beginning of the Holocene. The base of the sequence represents a phase of Corylus expansion and Quercus robur-type detrimentdatedbefore7069 cal.aBP.Thisallowsthe age of the base of the sequence to be estimated at some time between the regional expansions of Quercus (10 950 cal. a BP) and Corylus (8500 cal. a BP). The date of 6227 cal. a BP (Table 1) obtained at the top of the sequence (20 cm deep) would place the end of the sequence around 6000 cal. a BP (Fig. 3). Discussion The local ecosystem The sedimentological and palynological data (Fig. 5) allow us to consider that the depositional environment in the Ponzos site was a coastal wetland. Initially (250–120 cm depth) it would have been a freshwater wetland without a permanent body of water and formed by different tall herbaceous communities. Cladium mariscus and probably Phragmites australis would have been present, as they exist today in different wetlands of the area, forming part of estuaries and lagoons. Besides, other secondary herbaceous taxa (Iris, Caryophyllaceae, Umbelliferae, Osmunda regalis,Equisetum, etc.) existed, aswell as different small fern communities. These include Isoetes, with two species distributed at present in the hygrophilous wetlands of the area (Isoetes durieui and I. histrix)andOphioglossum, which in the Iberian Atlantic bioregion currently comprises two species, Ophioglossum lusitanicum and O. vulgatum (L opez Gonz alez 1986). Furthermore, different trees (Salix, Alnus,Corylus,Fraxinus,Betula) were also present in the wetland. In relation to the dominance of the hygrophilous species, some typical elements of swamp and peatland communities (Sphagnum) are noticeably absent in the first part of the sequence. Later, Sphagnum spores appear in the more organic facies, but noticeably underrepresented in relation to what happens in sequences from nearby peat bogs located at higher elevations (Ramil-Rego 1992). Between 120–0 cm depth hygrophilous environments persist, but different taxa indicative of fresh open waters also appear, which reveals significant changes in the ecological conditions of the site (Fig. 5). Particularly, the presence of Ranunculus and Potamogeton may be linked to the occurrence of shallow biotopes (<1–1.5 m depth). In the coastal lagoons of the Iberian Atlantic region, the native species Potamogeton pectinatus is characteristic of halophytic environments; but most of the other native species (Potamogeton natans,Potamogeton perfoliatus, Potamogeton polygonifolius,Potamogeton trichoides) inhabit subhalophytic environments and fresh waters. The species Nymphaea alba may be the only representative of Nymphaea, which usually configures dense biocenoses in freshwater wetlands and adjusts its seasonal changes to the flood levels. Finally, Myriophyllum is a hydrophyte that lives suspended close to the water surface, always in environments of reduced salinity. In coastal lagoons, as in inland lagoons, Myriophyllum 8Luis G omez-Orellana et al. BOREAS P Praia de Ponzos (Ferrol - A Coruña) alt. 0 m a.s.l. Myrica 100200 300400 500 Terrestrial Pollen Sum 200 400 600 800 Total Pollen & Spores Sum Ranunculaceae Umbelliferae 20 40 Cyperaceae Cladium Liliaceae Iridaceae Nymphaea Potamogetum Myriophyllum Polypodium Dryopteris Osmunda Hymenophylum Ophioglossum Spores trilete type Spores monolete type Sphagnum Equisetum Isoetes 1 3b 2 3a He-2 He-3 Hd-1 4 5 6 7a 7b 7c 7d 7e 7f 8 9a 9b 10 Hd-2a Hd-2b Hd-3a Hd-3b 20 Poaceae 6676 6629 7609 6573 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 Depth Lithology 6227 Silty clay with heterometric sand and gravel Organic silt Present-day beach sand Large trunks Ponzos Zones (PZ) Biozones Silty clay with different brown tones Age (cal. a BP) Analysis: L. Gómez-Orellana Fig. 5. Pollen percentages diagram of the taxa linked to freshwater environments recorded in the Ponzos deposit. Biozones: He =Holocene arboreal expansion; Hd =Middle Holocene, from Mu~ noz Sobrino et al. (2005). BOREAS Holocene environmental change on the Atlantic coast of NW Iberia 9