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Identification of suitable areas for high-resolution sea-levelstudies in SW Europe using commonly applied 210Pb models

Identificación de aéreas validas para la realización de estudios de alta resolución sobre variaciones del nivelmarino en el SO de Europa mediante modelos comunes de 210Pb

Eduardo Leorri (1), Alejandro Cearreta (2), Reide Corbett (1), William Blake (3), Francisco Fatela (4),Roland Gehrels (5) and María Jesús Irabien (6)

(1) Department of Geological Sciences. East Carolina University. Graham Building, Room 101. Greenville, NC, 27858. USA. leorrie@ecu.edu;corbettd@ecu.edu(2) Departamento de Estratigrafía y Paleontología. Facultad de Ciencia y Tecnología. Universidad del País Vasco/EHU. Apartado 644, 48080 Bilbao.alejandro.cearreta@ehu.es.(3) School of Geography, Earth and Environmental Sciences and the Consolidated Radioisotope Facility. University of Plymouth. Plymouth, UK.william.blake@plymouth.ac.uk.(4) Faculdade de Ciências, Centro e Departamento de Geologia, Universidade de Lisboa, Campo Grande, 1749-016, Lisboa, Portugal. ffatela@fc.ul.pt(5) School of Geography, Earth and Environmental Sciences. University of Plymouth. Drake Circus, Plymouth, PL4 8AA. UK. W.R.Gehrels@plymouth.ac.uk(6) Departamento de Mineralogía y Petrología. Facultad de Ciencia y Tecnología. Universidad del País Vasco/EHU. Apartado 644, 48080 Bilbao.mariajesus.irabien@ehu.es.

RESUMEN

Se ha analizado la distribución vertical de 210Pb y 137Cs en siete nuevos sondeos cortos (50 cm) decinco estuarios diferentes de la costa Atlántica de la Península Ibérica con el fin de identificar posiblesáreas donde desarrollar estudios de alta resolución sobre cambios en el nivel marino. De los siete sondeos,dos presentaron excelentes perfiles sugiriendo que esas zonas podrían ser adecuadas para este tipo deestudios (Urdaibai y Miño). Por otro lado, dos sondeos parecen reflejar las actividades humanas en lasáreas colindantes, con elevadas tasas de sedimentación (Urdaibai y Mira). Un sondeo presenta unainterrupción en la sedimentación (Mira), invalidando su uso, mientras que los dos restantes (Plentzia ySado) presentan cronologías que dependen del modelo aplicado y requieren, por tanto, de indicadoresadicionales antes de poder ser utilizados en este tipo de estudios.

Key words: Sea-level; marsh environment; 210Pb; 137Cs; SW Europe.

Geogaceta, 48 (2010), 35-38 Fecha de recepción: 15 de febrero de 2010ISSN: 0213-683X Fecha de revisión: 21 de abril de 2010

Fecha de aceptación: 28 de mayo de 2010

Introduction

Global estimates of sea-level rise forthe 20th and 21st centuries are ca. 1.8 mmyr-1 (Church and White, 2006; IPCC,2007). These values represent anacceleration in comparison to previouscenturies. However, these estimates arebased on geographically limitedinstrumental data that in most cases spanshort periods of time (Douglas, 2008).Furthermore, they do not capture localand regional sea-level variability(Katsman et al., 2008). Geological datacan usefully complement instrumentaldata, e.g., in northern Spain sea-levelobservations have been extended back tothe late 17th century. (Leorri et al., 2008;Garcia-Artola et al., in press). Thesestudies depend strongly on a precisechronology (Leorri et al . , 2008).Typically, to cover the last 100-150 yearschronologies are based on 210Pb andsupported by 137Cs and other chrono-horizons (Leorri et al., 2008). We analyze

here the vertical distribution of 210Pb and137Cs in seven new short (50 cm) cores inorder to identify possible areas suitablefor sea-level studies.

Study area

We collected seven 50-cm sedimentcores for geochemical analyses in fivedifferent estuarine areas along thenorthern and western Iberian coast: twocores (Kanala and Mape) in the Urdaibaiestuary (northern Spain), one core in thePlentzia estuary (north of Spain), onecore in the Minho estuary (northernPortugal), one core in the Sado estuary(southern Portugal) and two final cores(Xisto and Vilanova de Milfontes) in theMira estuary (southern Portugal) (Fig.1). Sampling sites were chosen in thehigh marsh environment (above themean high water level). The size of theestuaries ranges from < 200 ha (Plentzia)to more than 23,500 ha (Sado). Tidalranges are mesotidal and the mean tidal

range varies from 1.8-2.0 m in Minhoand Sado to 2-2.5 m in Plentzia,Urdaibai and Mira (Fig. 1).

Materials and Methods

Dating recent marsh sedimentsusually relies on the determination of thevertical distribution of unsupported 210Pb(half life 22.3 years), a naturallyoccurring fallout radionuclide, whichthen allows ages to be ascribed tosedimentary layers based on the knowndecay rate of 210Pb (Appleby andOldfield, 1992). On the other hand, 137Cs(half life 30 years) is an artificiallyproduced radionuclide present in theenvironment due to atmospheric falloutfrom nuclear weapons testing, reactoraccidents, and discharges from nuclearfacilities. Global dispersion and fallout of137Cs occurred from 1954 onwardsfollowing the detonation of high-yieldthermonuclear weapons. In the northernhemisphere, distinct maxima in fallout

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occurred in 1958 and 1963 (from nuclearweapons testing) and in 1986 (from theChernobyl incident). In favorableconditions, these periods of peak falloutprovide subsurface activity maxima inaccumulating sediments, which can beused to derive rates of sediment accretionand support 210Pb derived chronologies.The remote position of the study areasfrom major nuclear facility discharges(e.g. Sellafield, La Hague) and theChernobyl plume means that 137Cs inthese marshes is likely to be dominantlyderived from nuclear weapons testing,with peak fallout in 1963 (Cearreta et al.,2002).

Samples from the cores recovered inUrdaibai, Plentzia and Sado wereanalyzed following the methodologydescribed by Appleby (2001). Allsamples were packed and sealed in gastight containers and stored for 21 days toallow equilibration between 214Pb and itsparent radioisotope 226Ra prior tomeasurement by gamma spectrometry.Activities of the target radionuclides weremeasured using an EG&G Ortec planar(GEM-FX8530-S N-type) HPGe gammaspectrometry system built to ultra-lowbackground specification for 210Pbdetection. Total 210Pb was measured by itsgamma emissions at 46.5 KeV and its

unsupported component (210PbExcess

)calculated by subtraction of 226Ra activity,which in turn was measured by thegamma emission of 214Pb at 295 and 352KeV.

In the Minho and Mira estuaries, total210Pb was measured by alpha spectroscopyfollowing the methodology of Nittrouer etal. (1979). Approximately 1.5 g ofsediment was spiked with 209Po, as a yielddeterminant, and then was partiallydigested with 8 N nitric acid (HNO

3) by

microwave heating. Polonium-209 and210Po in solution was then electroplatedonto nickel planchets in a dilute acidsolution (modified from Flynn, 1968).210Pb

Excess was determined by subtracting

the 210Pb activity supported by 226Ra fromthe total 210Pb activity, where the supported210Pb activity for a given core was assumedto be equal to the uniform backgroundactivity found at depth (Nittrouer et al.,1979).

In all cores, 137Cs was determined by itsgamma emissions at 662 KeV (withcorrections for 214Bi emissions). Samplesfrom each core were counted for the orderof 24 hours to the depth of limit of fallout210Pb or 137Cs, i.e., until activityconcentrations of both radionuclidesdropped below the minimum detectableactivity. Sample resolution was 1 cm in all

cases and no granulometric changes wereidentify within each core for the analyzedsections.

The processes affecting sedimentdeposition can vary greatly from place toplace and there is not a unique model toprovide a chronology based on 210Pb(Irabien et al., 2008). Most models arebased on a series of assumptions amongwhich the most common are: 1- steady-state system, 2- closed system, 3-continuous 210Pb supply; and 4- nopostdepositional vertical redistribution of210Pb. From the possible models, we haveused here the simple model (Robbins,1978), constant initial concentration(CIC; Robbins and Edgington, 1975) andconstant rate of supply (CRS; Applebyand Oldfield, 1978) and when possiblecompared them.

We additionally measured watercontent and bulk density to account forpossible changes in the radiometricactivity due to these factors, e.g., possiblechanges of density due to autocompactiondowncore.

Results and Discussion

Figure 2 summarizes the verticaldistribution of both 210Pb

Excess and 137Cs in

all seven cores. With the exception of onecore (Xisto), the agreement between 210Pbderived chronologies and 137Cs could beclassified as good or excellent.

All cores, except Mape, showed anexponential, although variable, decline in210Pb

Excess with depth and plots of the

natural logarithm of 210Pb were fairlylinear. This suggested relatively constantrates of deposition, allowing us to use thesimple and CIC model. We alsocalculated sedimentation rates based onCRS for Kanala, Isuzkiza and Sado.However, irregularities in the plot of thenatural logarithm of 210Pb

Excess at Mape

suggested that there could have been ashort-term fluctuation in net depositionand therefore we used only CRS sincethese changes prevent the use of the othertwo methods (Marshall et al., 2007).

In northern Spain, Irabien et al. (2008)showed the strong variability ofsedimentation rates in response toenvironmental changes, spatialdistribution and human impact. In thissame area, several low-resolution sea-level reconstructions have been based onchronologies derived from 210Pb (Leorri etal., 2008; García-Artola et al., in press). Itis therefore essential to understand thesedepositional environments before weundertake any high-resolution sea-levelstudy.

Fig. 1.- Location map. Key: 1-Urdaibai; 2-Plentzia; 3-Minho; 4-Sado; 5-Mira.

Fig. 1.- Mapa de localización. Clave: 1-Urdaibai; 2-Plentzia; 3-Miño; 4-Sado, 5-Mira.

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Identification of suitable areas for high-resolution sea-level studies in SW Europe

At the Kanala site, 210Pb activityshowed a general decline with depth. Thisprofile did not suggest any major mixingor disruption in the sedimentation.Estimates derived from the simple, CICand CRS models are in agreement andindicate that the uppermost 10 cm havebeen deposited over 100 years. The maindifference occurred between 8.5 and 10.5cm depth, where CRS indicates aninflexion in the sedimentation rates, i.e.,lower sedimentation rates at the bottomof the core. On the other hand, 137Csshows a clear subsurface maximum inactivity at 3.5 cm depth, declining withdepth to negligible values at 18 cm (Fig.2). Ascribing this subsurface activitymaximum to AD 1963 supports all threeestimates; the disagreement with thesimple and CIC model estimates is lessthan 2 years. While all models are in goodagreement, Marshall et al . (2009)indicated that the CRS model tended toproduce dates that were progressively tooold when compared to ages derived fromlocal metal mining histories in four saltmarshes of SW England. While theinflexion indicated by the CRS modelcould be an artifact rather than a realchange in the sedimentation rates, theactivity at these levels is very low, theassociated uncertainty very large andtherefore the difference betweenestimates is not statistically significant.Based on 210Pb and 137Cs profiles and thelack of compaction in the core, Kanala isan excellent site for high-resolution sea-level studies.

In the same estuary, the corerecovered in Mape strongly differed fromKanala. In fact, the 210Pb

Excess profile

showed greater values in the lowersection of the core as it happens with the137Cs profile. In this case, we onlyattempted to provide a chronology basedon the CRS model that indicates that the50 cm were deposited in less than 100years, but with variable rates up to 20 mmyr-1 in some sections. This is in generalagreement with the presence on 137Cs upto 50 cm depth that indicates that the coresection has been deposited in ca. 50years. These results are supported byhistorical aerial photography that showthat the environmental change occurredin the area over the last 50 years inresponse to the deforestation of thenearby hills and increase of sediment loadtransported by one of the Oka rivertributaries.

In the Plentzia estuary, the Isuskizacore showed some inflexions in the210Pb

Excess profile which prevented the use

of the CIC model. Also, the 137Cs showed

high mobility downcore that seems to becharacteristic of most marsh cores. Wetried to use additional chronologicalmarkers, such as 241Am but thisradioisotope did not present sufficientactivity to permit measurement. Therefore,the chronology derived from this coreshould be considered carefully. As inKanala, CRS-derived ages tend to showdecrease in the sedimentation ratesdowncore, in this case, below 27 cm. Thiseffect together with a better agreementbetween the simple model and 137Csderived chronologies (by 1 yr) suggeststhat the simple model is appropriate,providing a sedimentation rate of 4.2 mmyear-1.

In northern Portugal, the corerecovered in the Minho estuary showed avery similar profile to Kanala, althoughwith a significantly lower sedimentationrate. It presented, in fact, the lowest rateof all the study sites, recording the last100 years in the top 4-6 cm. In this core,the limiting factor was the samplingresolution (1 cm samples). CIC estimatesbetter agree with the 137Cs (by 4 yrs)suggesting a sedimentation rate of 0.6mm yr-1. As in Kanala, this core did notpresent sedimentological changes andonly below 33 cm there was a change in

the sample density that might be relatedto autocompaction. This suggests that themarsh has been accumulating very slowlyat least over the last 100 years andsuitable sea-level reconstructions mightbe possible for the last 500 years or more,albeit at low resolution.

In the south of Portugal, the corerecovered in the Sado estuary sharedsome similarities with Isuskiza. In fact,both cores exhibit some inversion in the210Pb

Excess profile. Also in this case, we

should consider carefully the derivedchronology. As in previous cases, CRSderived sedimentation changes indicate adecrease in the rates downcore. CRS andCIC provided rates between 1.2 and 1.0mm yr-1, while the simple model indicatesmuch larger sedimentation rates (1.6 mmyr-1) in better agreement with the maximaindicated by 137Cs.

In Mira, further south, we found thesame contrasting patterns between thetwo cores studied here as in Urdaibai.While Vilanova de Milfontes presentedhigh sedimentation rates, Xisto had lowrates. In the case of Vilanova deMilfontes, the 210Pb

Excess profile shows a

top area of mixing (upper 7.5 cm) andboth the simple model and CIC are ingood agreement (4.4 and 4.0 mm yr-1,

Fig. 2.- 210Pb (upper axis) and 137Cs (lower axis) in depth (cm) profiles for all cores analyzed.

Fig. 2.- Perfiles de 210Pb (eje superior) y 137Cs (eje inferior) según la profundidad (cm) en todoslos sondeos analizados.

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respectively). Here, 137Cs is in betteragreement with the simple model (4.3 mmyr-1). While in this case we do not haveevidence of environmental change fromhistorical data as happened in Mape, fromthe sedimentological evidence and thechanges in density downcore it seems thatthe rapid sedimentation rates reflect theinfilling of the marsh area, most probablyin response to anthropogenic activities asoccurred in Mape.

Finally, the core recovered in theXisto marsh has one of the lowestsedimentation rates in all seven cores.Both the simple and CIC model estimatesare in good agreement (0.7-0.8 mm yr-1).However, they do not agree with the 137Csprofile that indicates rates that are twicethe 210Pb estimates (1.5 mm yr-1). The factthat the 137Cs peak is concomitant withlow 210Pb

Excess values suggests that the

chronology from this core is unreliable.In fact, the 210Pb

Excess profile shows a

significant jump between 2.5 and 3.5 cmdepth. A similar effect was reported inPlentzia, where it was associated with adisruption in the sedimentation (Cearretaet al., 2002), these events are fairlycommon in marsh environments.

The recent sediment accretion ratesobtained from these estuarine areas aretypical of many mesotidal marsheselsewhere in NW Europe (e.g. Cundy andCroudace, 1995). However, they arehighly variable and strongly influencedby local factors. While Kanala and Minhoapparently reflected naturally accretingmarshes over the last 100 years, Mapeand Vilanova de Milfontes seemed toreflect the human activities in these areas,although this has been proved only inMape. Xisto on the other hand, showed adisruption in the sedimentation veryrecently. Isuskiza and Sado showedprofiles that yield very differentchronologies depending upon thedifferent model used and therefore shouldbe considered with caution.

Conclusions

The results derived from this studysuggest that marshes from the northern andwestern coasts of the Iberian Peninsula canbe used for high-resolution sea-levelstudies. Careful sedimentological andstratigraphic analysis should be performedprior to the selection of the different sitesand cores should be selected from highmarsh environments. During this study wefound two sites that responded to otherfactors rather than natural accretion,thought to be of anthropogenic origin.

Furthermore, we were able to identify adiscontinuity in the sedimentation in oneof the cores. In two other cores, additionaltemporal markers should be required tosupport the chronology over the intervalstudied. In fact, the use of additionalchronological marker should be stronglyrecommended in all cases.

While all three models used here arecommonly used for dating recent saltmarsh sediments, in general, CRS modelestimates presented a greater offset inrelation with the 137Cs derivedchronologies than the simple and CICmodel estimates. Furthermore, CRSmodel presented slower sedimentationrates in the oldest (i.e., 100-120 years)layers in all four cores where we used thismodel. However, the discrepanciesbetween models tend to be minimal if theassociated errors are considered. Thiseffect should be further investigatedusing additional chronological markers asit can have bearing on the interpretationof accelerations in sea-level rise from the19th to the 20th century.

Acknowledgements

We gratefully thank to Drs Zazo andLario for their comments and suggestionsthat improved greatly the originalmanuscript. This research has beenfunded by East Carolina UniversityCoastal-Maritime Council, Ralph E.Powe Junior Faculty EnhancementAwards (ORAU), Acción IntegradaHispano-Portuguesa HP2007-0098/Acção Integrada Luso-Espanhola nº E-20/80, WesTLog - Recent evolution ofPortuguese W coast estuaries: highresolution studies from marshesgeological record (FCT - PTDC/CTE/105370/2008), Convenio UPV/EHU-Ihobe SA en el marco del proyecto K-Egokitzen-Cambio climático: impacto yadaptación (Etortek) and TANYA project(MICINN, CGL2009-08840). Work inthe Sado Estuary was undertaken as partof HOLSMEER, a European project ofthe Fifth Framework (contract EVK2-CT-2000-00060). This is a contribution toIGCP Project 495 and Northwest Europeworking group of the INQUACommission on Coastal and MarineProcesses.

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