The transformation of two intertidal environments from northern Spain during the last 150 years shows an evolution from a tidal flat into a salt marsh environment, with an intermediate transitional stage. The environment of deposition was reconstructed based on benthic foraminifera and sand content. Sediments were put into a temporal framework using short-lived radioisotope activities and heavy metal concentrations. The observed natural evolution responds to the availability of abundant sediment and the current sea-level rise scenario, where intertidal environments are trying to adapt to increasing flooding periods by accreting sediment rapidly. © 2019 Sociedad Geologica de Espana. All rights reserved.This research was funded by the Mi nistry of Economy and Competitiveness of Spain (CGL2013-41083-P), the University of the Basque Country UPV/EHU (UFI11/09), and the Basque Government (IT976-16). Ane García-Artola was funded by the Basque Government (BFI08.180). Miriam Torrontegui Aguado carried out the micropalaeontological analysis of the PR core and Eduardo Leorri (East Carolina University, USA) helped in the field. We thank Juan Usera (Universitat de Valèn-cia), an anonymous reviewer, and Manuel Díaz Azpiroz (Assistant Editor) for their valuable comments. This work represents contribution #29 of the Geo-Q Zentroa Research Unit (Joaquín Gómez de Llarena Laboratory)

Cearreta, A.

Corbett, D. R.

Garciá-Artola, A.

English

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IntroductionGeological records from northern Spainshow an average relative sea-level rise rateof 2 mm/yr in the past century (Leorri et al.,2008), in agreement with regional tidegauge data (Chust et al., 2009). This sup-poses an important departure from back-ground Holocene values (0.53 ± 0.48mm/yr; García-Artola et al., 2018). Unless salt marshes are able to keep pacewith sea level, these and other intertidal areasare at risk of submergence. Salt marsh envi-ronmental regeneration studies in northernSpain showed that the abundant sedimentavailable in the regional estuaries allowedrapid sedimentation rates (up to 18 mm/yr)to take place (García-Artola et al., 2016).Consequently, intertidal areas, such as saltmarshes from northern Spain, could poten-tially adapt to future sea-level rise.The survival of intertidal environmentsis determined by a complex equilibrium bet-ween biotic and abiotic processes. Sedimentavailability is a key factor. Sedimentation insalt marshes occurs from deposition of sus-pended detritic sediment and in situ plantorganic matter (Cahoon, 2006; Allen, 2009;Schile et al., 2014). Moreover, the sedimentcapture efficiency by vegetation is an essen-tial factor in the determination of the drow-ning or adaptation of a salt marsh (Day etal., 2011). This efficiency might be affectedby sea-level rise that increases inundation,which in turn affects plant productivity (Kir-wan and Guntenspergen, 2012; Voss et al.,2013). In fact, the type of vegetation in-fluences the adaptation capacity (Janousekand Mayo, 2013). Tidal flats lack halophytic vegetation(Pratolongo et al., 2019), which is knownto be of vital importance for the evolutionof salt marshes in the current context ofsea-level rise (Morris et al., 2002; Maraniet al., 2013). While numerous regionalstudies have analysed the behaviour ofsalt marshes (see above), none has focu-sed on tidal flats.GEOGACETA, 66, 201979Copyright© 2019 Sociedad Geológica de España / www.geogaceta.comRecepción: 1 de febrero de 2019Revisión: 25 de abril de 2019Aceptación: 24 de mayo de 2019Geogaceta, 66 (2019), 79-82ISSN (versión impresa): 0213-683XISSN (Internet): 2173-6545Recent transformation of intertidal environments under a sea-levelrise scenario: examples from northern SpainTransformación reciente de ambientes intermareales bajo un escenario de ascenso del nivel marino:ejemplos del norte de EspañaAne García-Artola1, Alejandro Cearreta1,2 and D. Reide Corbett31 Departamento de Estratigrafía y Paleontología, Facultad de Ciencia y Tecnología, Universidad del País Vasco UPV/EHU, Apartado 644, 48080 Bilbao, Spain ane.garcia@ehu.eus;alejandro.cearreta@ehu.eus2 Basque Centre for Climate Change-BC3, Leioa, Spain3 Department of Coastal Studies, Integrated Coastal Programs, East Carolina University, Wanchese, NC, 27981, USA. corbettd@ecu.eduABSTRACTThe transformation of two intertidal environments from northern Spain dur-ing the last 150 years shows an evolution from a tidal flat into a salt marsh en-vironment, with an intermediate transitional stage. The environment ofdeposition was reconstructed based on benthic foraminifera and sand content.Sediments were put into a temporal framework using short-lived radioisotopeactivities and heavy metal concentrations. The observed natural evolution re-sponds to the availability of abundant sediment and the current sea-level risescenario, where intertidal environments are trying to adapt to increasing flood-ing periods by accreting sediment rapidly.Key-words: foraminifera, intertidal environment, natural transformation,sea-level rise, Santoña estuary.RESUMENLa transformación de dos ambientes intermareales en el norte de Españadurante los últimos 150 años muestra la evolución desde una llanura interma-real a una marisma, con una etapa de transición intermedia. El paleoambientese reconstruyó mediante el contenido en foraminíferos bentónicos y en arena.Los sedimentos fueron datados utilizando radioisótopos de vida corta y con-centraciones de metales pesados. La evolución natural observada responde ala disponibilidad de abundante sedimento y al escenario actual de ascenso enel nivel del mar, donde los ambientes intermareales están tratando de adaptarsea los crecientes períodos de inundación acrecionando sedimento rápidamente.Palabras clave: foraminíferos, ambiente intermareal, transformaciónnatural, ascenso del nivel marino, estuario de Santoña.Here, we study the evolution of twotidal flat environments from the Santoñaestuary (northern Spain) over the past cen-tury. These environments evolved recentlyinto salt marshes as a response to the20th century sea-level rise. As for saltmarshes, sediment availability seems tocontrol the regional transformation ofthese lower elevation environments thataccrete sediment very fast to reach equili-brium with the tidal frame.Materials and Methods The Santoña estuary (northern Spain)is formed by the tidal part of the Asónriver with an extension of 11 km lengthand 0.5 km width in the upper estuaryand 3 km width in the lower estuary, co-vering a total area of 200 ha (García-Ar-tola et al., 2016).The Justreda (JU: X=461353.594;Y=4807556.389; Z=2.737 m; two 50-cmlong replicates) and Primosto (PR: X=462252.9; Y= 4804325.367; Z=2.683 m;two 46-cm long replicates) cores were ex-tracted by hand introducing two 50-cm longand 12.5-cm diameter PVC tubes in eachsalt marsh characterized by Spartina mari-tima (Curtis) Fernald vegetation (Fig. 1). Pre-cise location and elevation of the coreswere measured using a RTK-GPS, with a ho-rizontal precision of ±20 mm and a verticalprecision of ±35 mm. The Z coordinate wasreferenced to the local ordnance datum(LOD: lowest tide at the Bilbao Harbour onSeptember 27, 1878; located 1.73 m belowthe Spanish national levelling datum or MSLin Alicante). Sediment compaction duringsampling was negligible due to the minero-genic nature of the sediment and the largediameter of the tube. In both cases, histori-cal aerial photography does not show anyevidence of human impact in the recentpast (Fig. 1).Each replicate was longitudinally divi-ded into two halves. The first half was usedto analyse the grain size and the foramini-feral content in order to interpret the pa-laeoenvironment (see García-Artola et al.,2016 for methodology). The second halfwas employed to measure the 137Cs activityand the Pb concentration to develop a chro-nology for the recovered sediments (seeCorbett et al., 2006 and García-Artola etal., 2016 for methodology).Results and DiscussionThe lower half of the JU core is formedof grey sand (with abundant mollusc shellsin the lowermost 8 cm) and the upper halfis made of grey mud with plant roots at thetop 15 cm (Fig. 2). According to the fora-miniferal content, three depth intervals(DIs) were distinguished in this core. The lo-wermost 22 cm (DI3) are characterised byhigh numbers of foraminiferal tests (mean2846 tests/50 g). The assemblage is domi-nated by calcareous hyaline species (ave-rage 98%), where Ammonia tepida(Cushman) (average 39%), Haynesina ger-manica (Ehrenberg) (average 39%), andCribroelphidium williamsoni (Haynes) (ave-rage 16%) are dominant. This interval is in-terpreted as a tidal flat environment basedon its similarity with modern assemblagesdescribed by Cearreta (1988), Cearreta etal. (2002) and Leorri et al. (2008). Theupper 4 cm (DI2) contain moderate num-bers of foraminiferal tests (mean 1040tests/50 g) characterised by a mixture ofcalcareous hyaline (average 65%) and up-ward increasing agglutinated (average35%) forms. The assemblage is dominatedGEOGACETA, 66, 2019 A. García-Artola, A. Cearreta and D.R. Corbett80 Paleontología / PalaeontologyFig. 1.- On the left, geographical location (green square) of the Santoña estuary in northern Spain and position of the JU and PR cores (red dots). Onthe right, detailed historical (1946: left) and modern (2011: right) aerial photographs with the position of the JU and PR cores (red dots). See colorfigure in the web.Fig. 1.- A la izquierda, localización geográfica (cuadrado verde) del estuario de Santoña en el norte de España y la posición de los sondeos JU y PR(puntos rojos). A la derecha, fotografías aéreas histórica (1946: izquierda) y moderna (2011: derecha) con la posición de los sondeos JU y PR (puntosrojos). Ver figura en color en la web.by H. germanica (average 40%), A. tepida(average 23%), Entzia macrescens (Brady)(average 23%) and Trochammina inflata(Montagu) (average 11%). This intervalmight represent a transitional environmentfrom the lower tidal flat environment to-wards the upper DI1. In fact, the increaseof agglutinated species towards the top ofthis interval is interpreted as the shallowingof the environment. The uppermost 24 cmformed DI1 that is divided into two sectionsbased on differences in foraminiferal den-sities. The lower 16 cm (DI1b) are represen-ted by very low numbers of agglutinatedforaminiferal tests (mean 62 tests/50 g). Fi-nally, the upper 8 cm (DI1a) are dominatedby agglutinated forms (average 99%) anda moderate foraminiferal density (mean1584 tests/50 g), except for the uppermost1 cm that contains abundant calcareoustaxa. Trochammina inflata (average 72%)and E. macrescens (average 27%) are do-minant in DI1a. This interval represents ahigh salt marsh environment because theforaminiferal assemblages are almost ex-clusively formed by agglutinated species,except for the top sample that is indicativeof a low salt marsh environment. The verylow foraminiferal density in the lower DI1bcould be explained by the very high sedi-mentation rates that must have occurredso that the higher elevation salt marsh inDI1a could develop. Sand content decrea-ses from the core bottom, where it is veryhigh (mean 75% in DI3), to the top, whereit achieves very low values (3% in DI1a).This pattern suggests that as the environ-ment was gaining elevation it becamemore restricted and, therefore, the entranceof estuarine and marine sand was more li-mited. Sediment containing 137Cs is first re-corded at 28 cm depth (DI2). The 137Csrecord indicates the transition between thetidal flat and the salt marsh occurred after1954 when the atmospheric nuclear wea-pon testing record in sediments begun (Rit-chie and McHenry, 1990). Additionally, theAl-normalized Pb concentration peak at 24cm depth (DI1b) suggests this transition oc-curred prior to the 1970s when maximumemissions of Pb in Europe and the main re-gional industrialization period coincide(Pacyna et al., 2007; Irabien et al., 2015).The lower 24 cm of the PR core wasmade of brown mud and the top 22 cm arecomposed of soft black mud, with Scrobi-cularia plana (da Costa) bivalve all over andHediste diversicolor (O.F. Müller) polychaetein the top half (Fig. 2). The core was dividedinto three DIs according to the foraminiferalcontent. The lowermost 14 cm (DI3) containa moderate number of foraminiferal tests(mean 480 tests/50 g), almost exclusivelycomposed of hyaline species (average99.7%). Haynesina germanica (average49%) and A. tepida (average 46%) are themain species. This interval is interpreted asa tidal flat environment because it was for-med almost exclusively of calcareous hya-line species. The upper 7 cm (DI2) arecharacterized by a mixture of upwards in-creasing agglutinated (average 41%) anddecreasing calcareous hyaline (average59%) forms. The assemblage displays mo-derate numbers of foraminiferal tests (meanGEOGACETA, 66, 201981Paleontología / PalaeontologyRecent transformation of intertidal environments under a sea-level rise scenario: examples from northern SpainFig. 2.- Identified depth intervals (DIs) in the JU and PR cores based on the sand content (%) and main foraminiferal species (%). 137Cs activity (Bq/kg)and Al-normalised Pb distribution with depth (cm) are also shown for chronological purposes (see text). Fig. 2.- Intervalos de profundidad identificados en los sondeos JU y PR en función del contenido en arena (%) y las especies de foraminíferos dominantes(%). También se muestra la actividad de 137Cs (Bq/kg) y la distribución de Pb normalizado respecto a Al en función de la profundidad (cm) con fines cro-nológicos (ver texto).GEOGACETA, 66, 2019 A. García-Artola, A. Cearreta and D.R. Corbett82 Paleontología / Palaeontology410 tests/50 g), dominated by A. tepida(average 33%), H. germanica (average24%), Scherochorella moniliformis (Sid-dall) (average 19%) and E. macrescens(average 16%). As in the JU core, this in-termediate interval represents a transitio-nal environment from the lower tidal flatenvironment towards the upper saltmarsh setting. The top 25 cm (DI1) arecharacterised by a moderate foraminiferaldensity (mean 379 tests/50 g), where cal-careous hyaline species increases up-wards (average 61%). This upper part isdominated by H. germanica (average42%), S. moniliformis (average 23%) andA. tepida (average 19%). This interval re-presents a low salt marsh environmentthat is losing elevation through time asthe increase of calcareous hyaline speciesin detriment of agglutinated species to-wards the core top indicates. Sand con-tent is rather constant throughout(average 11%) except for the top 10 cmwhere it increases up to 30%. This evolu-tion of the sand content supports the ideaof a topographically lower salt marsh en-vironment towards the top, which makesit more prone to tidal inundation. In Pri-mosto, the bottom tidal flat evolved intoa high salt marsh but, unlike Justreda, thesalt marsh apparently did not receiveenough sediment to maintain the increa-sing elevation and quickly turned into alow marsh. This could be related to varia-tions in local hydrodynamic conditions ordifferential river flow inputs to the Jus-treda and Primosto salt marshes. The Jus-treda salt marsh receives sediment fromthe main Asón river through the Limpiaschannel as well as from the Clarín stream,a tributary of the former, through theRada channel (Fig. 1). The location of thePrimosto salt marsh, separated from theLimpias channel by the Carasa salt marsh(Fig. 1), which has recently been regene-rated (García-Artola et al., 2016), couldrestrict sediment input to the smaller Cla-rín stream. Furthermore, the limited spaceavailable for salt marsh migration orchanges in plant productivity could havealso been responsible for the observedevolution (Schile et al., 2014). The PR corepresents a similar temporal evolution tothe JU core, with the transitional zone be-ginning prior to 1954 (137Cs is first recor-ded at 27 cm depth) and ending between1954 and the 1970s (the Al-normalizedPb maximum is at 21 cm depth).ConclusionsThe sediment records extracted fromtwo intertidal areas from northern Spainshowed the natural transformation of atidal flat environment into a salt marshsetting, located higher in the tidal frame.The former tidal flats accreted sedimentaiming to reach equilibrium with the newtidal frame as sea level was rising. Thiswas indicated by the upwards increase ofagglutinated species, suggesting that theenvironment was gaining elevation. Boththese records show the importance ofabundant sediment availability for theseenvironments to keep pace with increa-sing sea level. When sediment input wasnot sufficient, the otherwise high marshenvironment was transformed into a lowmarsh environment, dominated by a mix-ture of calcareous and agglutinated fo-raminifera. These conclusions are ofinterest for the development of predic-tions regarding the adaptive response ofthese coastal ecosystems and their po-tential use as natural defences for on-going sea-level rise.AcknowledgmentsThis research was funded by the Mi-nistry of Economy and Competitiveness ofSpain (CGL2013-41083-P), the Universityof the Basque Country UPV/EHU(UFI11/09), and the Basque Government(IT976-16). Ane García-Artola was fundedby the Basque Government (BFI08.180).Miriam Torrontegui Aguado carried outthe micropalaeontological analysis of thePR core and Eduardo Leorri (East CarolinaUniversity, USA) helped in the field. Wethank Juan Usera (Universitat de Valèn-cia), an anonymous reviewer, and ManuelDíaz Azpiroz (Assistant Editor) for their va-luable comments. 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Recent transformation of intertidal environments under a sea-level rise scenario: Examples from northern Spain

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Recent transformation of intertidal environments under a sea-level rise scenario: Examples from northern Spain

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