-1

Post-depositional137

Cs mobility in the sediments of three shallow coastal

lagoons, SW England

I.D.L. Foster1,5,*, T.M. Mighall2, H. Proffitt1, D.E. Walling3 and P.N. Owens41Centre for Environmental Research and Consultancy, Coventry University, Priory St., Coventry CV1 5FB,UK; 2Department of Geography and Environment, University of Aberdeen, Elphinstone Road, Aberdeen,AB24 3UF, UK; 3Department of Geography, University of Exeter, Rennes Drive, Exeter, EX4 4RJ, UK;4National Soil Resources Institute (NSRI), Cranfield University, North Wyke Research Station,Okehampton, Devon EX20 2SB, UK; 5Geography Department, Rhodes University, Grahamstown 6140,Eastern Cape, South Africa; *Author for correspondence (e-mail: i.foster@coventry.ac.uk)

Received 14 June 2005; accepted in revised form 14 December 2005

Key words: 137Cs remobilisation, 210Pb, 7Be, Coastal lagoons, Salinity changes

Abstract

We present 137Cs profiles for three low lying coastal lagoons in Southwest England that show a decline inactivity with sediment depth. 137Cs inventories are lower than expected by comparison with local referenceinventories despite the fact that sampling was undertaken in the deep-water zone of each lake wheresediment and 137Cs focusing would be expected. At all three locations, lake sediment 7Be and unsupported210Pb (210Pbun) inventories are not significantly lower than the local reference inventory. 137Cs inventoriesin the study cores range from 38 to 95% of local reference inventories. The standing water level and mud:water interface at two sites are below maximum tide level and, at all three sites, salinity increases signifi-cantly in the water columns between low and high tide and in the pore waters of the underlying sediments.We suggest that the difference in hydrostatic pressure between sea level and standing water levels in thelagoons forces salt water up through the sediment column and that monovalent cations (especially Na+

and K+) replace 137Cs on exchange sites leading to the upward migration and loss of 137Cs. Rising sea levelsmay therefore contribute to remobilisation and release of 137Cs to the aquatic environment from thesediments of coastal lagoons.

Introduction

Since the pioneering work of Pennington et al.(1973) and Ritchie et al. (1973), 137Cs has beenused extensively to date lake and floodplain sedi-ment sequences and to provide independentmarkers to confirm chronologies derived from210Pb dating (see Appleby 2001) in both thenorthern and southern hemispheres. Because 137Csfallout from thermonuclear weapons testing be-tween the early 1950s and 1963 occurred globally,

the 137Cs technique can be used for dating andfingerprinting studies, although activities in theSouthern Hemisphere are generally much lowerthan in the North (see Longmore 1982 andLoughran and Campbell 1995 for Australia:Owens and Walling 1996 and Foster et al. 2005 forSouthern Africa). Additional 137Cs inputs from the1986 Chernobyl nuclear accident were spatiallymore variable and produced regional fallout con-fined to the Northern Hemisphere (Appleby 2001).Where Chernobyl fallout occurred, however, an

Journal of Paleolimnology (2006) 35: 881–895 � Springer 2006

DOI 10.1007/s10933-005-6187-6

additional chronological marker is evident inmany sedimentary sequences (see Foster and Lees1999; Foster 2006).

The form of the 137Cs profile in lowland UKlakes is typified by that derived from an analysisof sediment deposits in the Old Mill reservoir,Devon, UK (Figure 1; for site location seeFigure 2a). The form of the profile shows evidencefor the first occurrence of 137Cs in 1954 and rises toa peak that is usually associated with maximumweapons testing fallout in 1963. High 137Cs activi-ties are sustained up-core, largely as a result of theerosion and transport of 137Cs-bearing topsoil tothe reservoir over the subsequent ca. 30 years(Foster and Walling 1994) and the good preserva-tion of the shape of the profile is probably a func-tion of rapid sedimentation rates at this location(He and Walling 1993). No Chernobyl fallout wasrecorded in this area of Southwest England and a1986 marker horizon is therefore absent.

Although 137Cs is frequently used to date lakesediments (e.g., Foster 1995; He et al. 1996; Zhangand Walling 2005), many published 137Cs profilesdo not approach the idealised pattern of Figure 1or support an independent 210Pb chronology. Thismay be because 137Cs is more mobile than Pb,especially in organic-rich sediments (Davis et al.1984; Appleby 2004). Higher mobility can result ina ‘tail’ of 137Cs extending to depths substantiallybelow 1954 although in many cases the location ofthe 1963 peak remains reasonably well fixed (Smol2002). As shown by Walling and He (1992) theprecise shape of the 137Cs profile in a lake will alsoreflect the relative importance of the direct fall-out and catchment-derived inputs of 137Cs. Manydifferent profile shapes can occur in response to thiscontrol. Bioturbation may also be a factor inredistributing 137Cs within slowly sedimentingbasins (He andWalling 1993) although this processwould displace other radionuclide and geochemicalsignatures at the same time. Furthermore, Appleby(2001) suggests that the higher solubility of 137Cswith respect to Pb leads to an imbalance ofinventory ratios of these two isotopes in the lakesediments of several UK Lake District lakes.Longmore (1982) identified four factors thatcontrolled 137Cs adsorption and mobility. Thesewere:

1. Size of sediment particles. 137Cs is adsorbedmore strongly by smaller particles.

2. Variations in mineralogy and ion exchangecapacity. Clay minerals, especially illite, have ahigh affinity for 137Cs (see Comans et al. 1998).

3. Presence of competing ions in the same groupof the periodic table, especially Na+ and K+,in the interstitial water or overlying water col-umn (see Coleman et al. 1963; Longmore et al.1986).

4. pH and available oxygen levels. These factorsaffect 137Cs adsorption and stability. Increasingacidity provides more competition for exchangesites with H+ (Longmore et al. 1983) and lowoxygen conditions appear to favour 137Csadsorption.

More recent research has emphasised the role ofNa+ but has questioned the role of acidity inreleasing 137Cs (e.g., Davison et al. 1993) while

Figure 1. A typical 137Cs profile in a lowland UK reservoir

(after Foster and Walling 1994) (See text for explanation). The

total inventory relates to the year of sampling and counting

errors are typically <±10%. The local reference inventory is

similar to that for Slapton Ley: see Table 1.

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Figure 2. Site locations (a) and details of the South Devon (b) and Scilly Isles (c) sampling locations.

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others have suggested that NH4+ under anoxic

conditions may also compete with 137Cs on theexchange complex (Pardue et al. 1989). 137Cs has,however, been used to date marine and estuarinesedimentary sequences (e.g., Roman et al. 1997),suggesting that salinity levels in the overlying wa-ter column may be less significant than the salinityof the pore waters. Laboratory experimentsundertaken by Sholkovitz et al. (1983) suggestedthat 137Cs might be mobilised in salt-water envi-ronments.

Three research projects were undertaken inSouthwest England that attempted to use 137Cs todevelop a chronology or support 210Pb chronolo-gies in shallow coastal lagoons, but they met withlittle success, despite the apparent reliability of210Pb profiles from the same locations. In addition,137Cs measurements have been used successfully tounderstand soil redistribution and sediment stor-age within one of the contributing catchments atSlapton Ley (Foster et al. 1996; Owens et al. 1997).All 137Cs profiles show similar patterns to theexamples cited by Longmore et al. (1983, 1986)with 137Cs activities increasing upcore towards themud:water interface. Two of the sites, WiddicombeLey and Slapton Ley, are freshwater lagoons withconductivities ranging from ca. 350 to 550 ls cm�1

and have circum-neutral to alkaline pH, whilethe third site, Big Pool on St Agnes, Isles of Scilly,is brackish, with conductivity approaching3000 ls cm�1 and circum-neutral to alkaline pH.One site (Widdicombe Ley) was re-sampled in 2004to establish whether the 137Cs profile shape hadchanged since the lake was initially sampled in1990. Slapton Ley was re-sampled in 2005 and anadditional reference inventory was obtained from alocation immediately adjacent to the lake.

This study explores and attempts to explain theatypical pattern in the 137Cs profiles from the threesites, in the light of the factors outlined above thatare known to affect 137Cs mobility in sedimentsequences. A brief introduction to the researchlocations and analyses is provided to explain aplausible mechanism for the 137Cs profiles.

Site descriptions and methods

The late Holocene evolution of the three sites isimportant because the stratigraphy of each loca-tion appears to have a bearing on the mechanism

that could explain post-depositional up-coremigration of 137Cs. The following section thereforeconsiders the late Holocene evolution and sedi-mentation in these three lagoons.

Site descriptions

The three sites from which 137Cs profiles wereobtained include two lakes in Southwest Devon,UK (Widdicombe Ley and Slapton Lower Ley)and a small shallow coastal lagoon, Big Pool, onSt Agnes, Isles of Scilly (Figure 2). Slapton LowerLey (77 ha, max depth ca. 2.6 m) and WiddicombeLey (9 ha, max depth ca. 1 m) are shallow waterbodies that developed as a result of the shorewardmovement of sediment from the offshore SkerriesBank as Holocene sea levels rose (Robinson 1961;Hails 1975). Both are separated from the sea by ashingle ridge that rises to ca. 6 m above UKOrdnance Datum (Newlyn) and at its maximumdepth is ca. 4 m below Ordnance Datum. Theshingle overlies terrestrial peat and/or marine clay(Hails 1975). Radiocarbon dating suggests thatthe barrier at Widdicombe was closed to thedirect effects of fully marine conditions at ca.4767±100 year BP while the barrier at Slaptonwas not fully closed until 2889±100 year BP(Morey 1976), suggesting a northerly migration ofthe shingle barrier as Holocene sea levels rose.Geologically, the contributing catchments areunderlain by Lower Devonian slates and shales.Small outcrops of Permian sandstone are found inthe Slapton Ley catchment (Hails 1975). Thecatchments contributing to these two shallowcoastal lagoons comprise steeply sloping agricul-tural land and narrow, often wooded valleys.

Once enclosed as a freshwater lagoon, Widdi-combe Ley began to infill with silt and organicmatter, turning the lagoon into a reed swamp andcarr (Morey 1976). A key event in the recent his-tory of the Ley was the artificial raising of waterlevels in 1880 to provide fishing for the landlord ofWiddicombe House (Anonymous 1882). This wasachieved by laying a thin (5–10 cm) clay liner andconstructing a sluice and overflow in the southerncorner of the Ley. Slapton appears to have been anopen water environment for much of its lateHolocene history, and prior to the 1850s was ashallow, clear, productive and well oxygenatedlake with abundant macrophytes (O’Sullivan

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1994). Between the basal marine clay and thecontemporary diatom mud, however, lie occa-sional layers of coarse sand, that may reflectovertopping by pre-historic storm surges, andlayers of phragmites peat suggesting occasionallowering of water levels (Morey 1976). Stormsurges, generally associated with easterly gales,happened several times during the 20th century(1917, 1979 and 1989: Job 1993) and the mostrecent storm of January 2001 destroyed the roadon the strand and deposited large quantities ofshingle in the Ley. Slapton Ley water levels wereartificially raised in 1856 and in the 1920s when aspillway was constructed at its southern end todeepen and regulate water levels (O’Sullivan 1994).A major road was also constructed along theshingle barrier in 1856. Extremely low water levelshave been observed in the recent past, especially in1976 (Chell pers. comm.).

The late Holocene history of Big Pool, St Agnesis less well documented, although the lake isknown to have existed in approximately the samelocation since the late 17th century (Borlase 1756).It comprises a small (ca. 0.38 ha), shallow (maxdepth ca. 60 cm) brackish pool located 30–40 mfrom the sea and surrounded on three sides bymajor beach embayments. The maximum height ofthe coastal barrier prior to 1996 was ca. 3.5 mabove local datum. The sediments comprise a50–60 cm thick organic gyttja, overlying a dis-continuous, thin (5–10 cm) peat layer. Gyttja orpeat then overlie at least 60 cm of organic, gradingto inorganic, medium to coarse sand that mayhave been deposited by the 1755 Lisbon tsunami.Peat accumulations above and below the sandlayer and the sand itself have been dated toapproximately this time using both 14C and OSLdating methods (Foster et al. 1991; Banerjee et al.2001). Thomas (1985) suggests that the rapid lateHolocene relative sea level rise on Scilly has led tothe drowning of large parts of the landscape andhis analysis suggested that at 2000 BP the lakewould probably not have existed. This view wasbased on sea level change curves derived fromsubmerged archaeological remains, although morerecent estimates of sea level change, based on da-ted intertidal peats (Ratcliffe and Straker 1996),suggest that sea level rise in the late Holocene mayhave been much less rapid. Over the last century,several major storm surges have been recorded onScilly that are known to have inundated the pool,

including those of 1924, 1925, and 1962 (Thomas1985) and, more recently, 1989. In 1996, increasingrisk of flooding led to the raising of the sand banksand sea defences surrounding the Pool by 1–2 m.Occasionally, as in 1909 (Mothersole 1919) and in1976, the pool completely dried out duringdrought periods, although there are no obviousdiscontinuities in the sedimentary record. It has nosurface water catchment and water levels appear tobe controlled by local groundwater fluctuations.Observations made between the lowest and highestsea levels during the highest spring tide of the yearin August 2004 showed that water levels in thePool rose by 3–4 cm. No precipitation had fallenfor at least 3 weeks prior to this observation.Water level in the Pool is artificially regulated witha drainage sluice through the sea defences to thewest. However, this is built with a non-return valveto prevent sea water incursion at high tide.

Field and laboratory analysis

137Cs activities and inventories were establishedfor the sediments of three lagoons in 1987 and2005 (Slapton Ley), 1990 and 2004 (WiddicombeLey) and in 2003 (Big Pool) on ca. 6 cm diametermini-Mackereth (Mackereth 1969) cores sliced at0.5 or 1 cm intervals, dried and measured usinggamma spectrometry (see Appleby et al. 1986;Wallbrink et al. 2002; Foster et al. 2003). The210Pb profile was not measured on the core col-lected from Slapton Ley in 1987, but a core froma similar location was analysed for 210Pb byO’Sullivan et al. (1991). 210Pb and 7Be activitieswere also measured directly by gamma spectrom-etry on the cores collected from Widdicombe Leyin 2004, Slapton Ley in 2005 and Big Pool in 2003.Local 137Cs reference inventories for the SouthDevon sites were reported by Foster et al. (1996)and Owens et al. (1997). An additional referenceinventory was obtained from a site on the northernend of Slapton Lower Ley (Figure 2b) in 2005 toconfirm the earlier published 137Cs inventory andto provide inventories for 210Pb and 7Be. A refer-ence inventory for all three radionuclides wasobtained in 2003 on a flat area some 500 m southof Big Pool. At all sites, inventories were estab-lished for 7Be, 137Cs and 210Pbun (unsupported210Pb). 137Cs inventories have been decay correctedto December 2004.

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Levelling surveys of the lakes were used todetermine the elevation of the mud:water interfaceand standing water surfaces relative to OrdnanceDatum (Devon), Local Datum (Scilly) and Admi-ralty Chart Datum at the three sites between 2003and 2005. Predicted Admiralty Tide Data (relativetoAdmiralty Chart Datum) were obtained from thesoftware package Neptune Tides: UK tidal pre-dictions (www.neptune-navigation.com) for Scillyand South Devon, but these predictions do notaccount for any possible increases or decreases inabsolute sea surface elevation resulting from strongonshore winds or changes in barometric pressure.

Water quality monitoring of all three sites usedan Ecolog� continuous water quality monitor thatwas deployed for periods ranging from 2 weeks to2 months to monitor conductivity, Cl� concentra-tion, and pH over a range of rising and falling tidalcycles. Additional samples for Na+, Cl� and con-ductivity measurement were collected manually.

Between 2003 and 2005, long sediment coreswere retrieved to depths of up to ca. 6 m below themud:water interface at all sites (Figure 2b), using aRussian corer deployed through the centre of asecurely anchored inflatable boat and sub-sampledimmediately in the field after detailed stratigraphiclogging and description following the method of

Troels-Smith (1955). In the laboratory, 3–5 g wetsediment was mixed with 50 ml deionised water,shaken mechanically for 24 h, filtered through a0.45 lm Millipore filter and made up to volumewith deionised water. Initial moisture content wasdetermined on separate 3–5 g sub-samples by ovendrying for 24 h at 105 �C. These data were used todetermine initial moisture content of the sedimentsand to calculate Na+ and Cl� concentrations andthe electrical conductivity of pore water in thesediment sequences.

Results

137Cs profiles and radionuclide inventories

In all cases, the lake sediment 137Cs profiles show adecrease in activity with depth (Figure 3) andgenerally have inventories that are much lowerthan the local reference inventories (Table 1),despite the fact that cores were retrieved from deepwater zones of the lakes, where sediment and137Cs focusing are likely to have occurred (seeAppleby 2001). Comparison of the 137Cs profilesof the Widdicombe Ley core collected in 2004,with that of the core collected in 1990 from

Figure 3. 137Cs profiles for three shallow coastal lagoons in Southwest England. Inventories relate to the core at the time of analysis

and counting errors are typically <±10%: see Table 1 for local reference inventories.

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approximately the same coring location, anddecay-corrected to 2004, reveals a significantchange in the 137Cs distribution (Figure 4). The2004 profile has a characteristic shape that couldbe accounted for in terms of the atmosphericfallout record (c.f. Figure 1), with 137Cs main-tained at a relatively high activity up-core of thepeak (marked by an arrow on Figure 4a) due to aninflux of 137Cs bearing topsoil to the lake. Theinventory, however, is considerably lower thanthat of the core collected in 1990 and is also lowerthan the local reference inventory, decay-correctedto 2004 (Table 1). The apparent peak in the 137Csprofile is at a depth that correlates remarkablyclosely with the predicted maximum tide height atthis site for the year 2004 (see below). The 137Csprofiles presented in Figures 3 and 4 thereforesuggest that 137Cs has been mobilised up-core and,at all three sites, lost from the sediments to theoverlying water column.

The 7Be and 210Pbun local reference inventoriesfor St Agnes are not significantly different fromthose of the lake sediment inventories (Table 1).There are also no significant losses of these twoatmospheric fallout nuclides from the lake sedi-ments of Slapton Ley (2005 core) and Widdicombe

Ley (2004 core). Assuming that the reference sitegives a reasonable estimate of atmospheric falloutfor the area, the loss of 137Cs from the lake sedi-ments of Big Pool amounts to some 28% of initialfallout decayed to 2004 (Table 1). Similar calcu-lations for Slapton and Widdicombe Ley (2004core) suggest significant 137Cs losses relative to thelocal reference inventories (Table 1).

210Pb chronologies

The 210Pb profile published for Slapton Ley(O’Sullivan et al. 1991) shows no evidence ofmajor disturbance within the sediment column.The 210Pb profile for Widdicombe Ley (Figure 5)places the basal date of the lake sediments, justabove the clay liner, at ca. 1875, in close agreementwith the documented creation of the lake in 1880(Anonymous 1882). The 210Pb profile for Big Pooldates the bottom of the organic gyttja at ca. 1865.This lies some 5 cm above the sand layer dated byFoster et al. (1991) and Banerjee et al. (2001) to atime closely corresponding to that of the Lisbontsunami of 1755. Sedimentation rates associatedwith the Big Pool core, estimated using the 210Pb

Table 1. Radionuclide inventories for all cores and reference sites for the Slapton and Big Pool areas (Counting errors are typically

<±10%).

Inventory

(mBq cm�2)

Lake sediment inventory as a

percentage of reference inventory

A. 137Cs (Decay corrected to December 2004)

Slapton

Reference Site (1987) 201

Reference Site (2005) 215

Slapton Ley (1987) 91 45

Slapton Ley (2005) 81 38

Widdicombe Ley (1990) 190 95

Widdicombe Ley (2004) 123 61

St Agnes

Reference site (2003) 237

Big Pool (2003) 171 72

Nuclide Reference inventory

(mBq cm�2)

Lake sediment inventory

(mBq cm�2)

B. 7Be and 210Pbun

Slapton Ley (2005) 7Be 23 27210Pbun 357 466

Widdicombe Ley (2004) 7Be 23 19210Pbun 357 304

Big Pool (2003) 7Be 17 23210Pbun 295 246

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crs model (Appleby 2001, 2004) (Figure 6), in-crease at times that coincide closely with docu-mented storm surges and coastal flooding in 1924,1925, 1962 and 1989, but also display evidence of asignificant decline in sedimentation after con-struction of new sea defences in 1996.

Unlike the 137Cs profiles, the 210Pb data and crsdating appear to produce chronologies that aresupported by other dating methods or by trends insedimentation that are consistent with documentedclimate events in the recent historical record.This suggests that the mechanism affecting thepreservation of the 137Cs profile does not applyto the preservation of the 210Pb record in thesediments.

Levelling surveys

Levelling of the three sites revealed that themud:water interface in each lake lies between 1.1and 3.1 m above UK Ordnance Datum (Newlyn)

(Slapton andWiddicombe) or UK Local OrdnanceDatum (Scilly) and that predicted maximum tidesfor the year 2004 (relative to Admiralty ChartDatum) would approach or even exceed these levelson several occasions throughout the year (Figure 7,Table 2). Indeed, at Slapton and Big Pool, maxi-mum tide levels are also predicted to exceedstanding water levels in the lagoons, especially inlate summer, when tidal amplitudes are at theirmaximum and water levels in the lagoons arelowest. Thus at certain times of the year, a positivepressure could be exerted on the freshwaterlagoons through or beneath the barriers; anobservation that could explain the observed waterlevel rise in Big Pool over a single tidal cycle in thelate summer of 2004. These observations are,however, likely to identify minima in elevationdifferences since no account has been taken in theanalysis of the tidal data of changes in sea surfaceelevation that could occur, for example, with stormsurges or marked reductions in barometric pressurecoinciding with a high tide (Thomas 1985).

Figure 4. 137Cs measurements for two cores collected at Widdicombe Ley in 2004 (a) and 1990 (b). Activities of the 1990 core are decay

corrected to 2004. (Counting errors <±10%). The arrow on 4a marks the apparent ‘peak’ in 137Cs activity (see text for explanation).

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Evidence that the barriers along the Devoncoastline may be permeable to water was alsoprovided by the work of Van Vlymen (1979). Hiswater balance study suggested that fresh water wasleaking in substantial quantities from Slapton Leyto the sea, but failed to recognise that water mightbe flowing in the opposite direction at high tide,when there were low water levels in the Ley.

Water quality monitoring

Monitoring of water quality, and spatial samplingof the water bodies over a range of tidal cycles, hasalso provided evidence of increases in conductivity,Na+ and Cl� concentrations, apparently uncon-nected with hydrological inputs. For example,continuous monitoring at Slapton Ley during a

Figure 5. Total and supported 210Pb profile and ‘crs’ determined sedimentation rates and age in Widdicombe Ley (2004 core).

Figure 6. 210Pb ‘crs’ determined sedimentation rates in the 19th, 20th and 21st centuries, Big Pool, St Agnes (2003 core).

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two month period in 2001 (Figure 8) showed thatconductivity rose over an alternating spring toneap and neap to spring tidal cycle by ca.20 ls cm�1 from the beginning to the end of themonitoring period. While small changes in con-ductivity are evident in Figure 8, the broad trendreflects the changing amplitude of the tidal cyclesand only a small contribution from freshwaterdraining the catchments diluting the salt water in-put. With the exception of a storm event in theearly part of the monitoring period, little rain fell inthe catchments. Similar results were obtained forWiddicombe Ley in the autumn of 2004, but hereconductivity rose by ca. 80 ls cm�1 over the

monitoring period. The greatest increase in con-ductivity was observed in Big Pool in late July/earlyAugust 2004 (Figure 9). Here, over a 7 day periodwithout rain, and when tidal amplitudes rose fromthe minimum to the maximum for the year (5.2 mrange), conductivity increased by ca. 300 ls cm�1.Spatial sampling surveys in the water body ofSlapton Ley have documented the existence ofconductivity, Na+ and Cl� ‘hot spots’ in samplescollected just above the mud:water interface at lowand high tide. While not spatially uniform, sam-pling at depth in the water column in close prox-imity to the location of Core SL1 (Figure 2) hasshown that Cl� and Na+ concentrations are as

Figure 7. Elevation of the mud:water interface and standing water level at each site, relative to Admiralty Chart Datum (see Table 2

for exact elevation differences), plotted with maximum tide heights for 2004.

Table 2. The elevation of the mud:water interface at the study sites above UK Ordnance (AOD), Local (ALD) and Admiralty Chart

(ACD) Datum. The tidal range at each site relative to ACD is given in the final column.*

Site AOD ALD ACD Tide range (ACD)

Slapton +1.1 m +3.7 m +5.6 m

Widdicombe +3.1 m +5.7 m +5.6 m

Big Pool +1.8 m +4.7 m +5.9 m

*UK Ordnance Datum is measured relative to mean sea level at Newlyn, Cornwall. Ordnance Survey Datum for the Scilly Isles,

however, is not correlated with the Newlyn datum and heights are given relative to mean sea level on St Mary’s, Isles of Scilly (Local

Datum).

Admiralty Chart Datum is measured relative to the lowest 20th century astronomical tide and has been correlated with UK Ordnance

Datum (Widdicombe and Slapton Ley) and with the Local Datum on the Scilly Isles (Big Pool).

Tidal predictions are given relative to ACD.

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much as 5–7 mg l�1 higher just above the mud:-water interface than they are at shallower depthswithin the water column or at the same samplinglocation at low tide.

The pore water quality data, and associatedstratigraphic records for the three sites, are shownin Figure 10. Conductivity, and Na+ and Cl�

concentrations in pore waters show a significantincrease downcore, and especially at depths wherelayers of permeable sand and/or wood peat areencountered in the profiles. Slapton Ley shows thestrongest gradient with conductivities exceeding12,000 ls cm�1 and Na+ and Cl� concentrationsexceeding 2000 mg l�1 close to the bottom of thecore. In all cases, the pore water concentrationsand conductivities, even close to the mud:waterinterface, exceed those of the overlying watercolumn as described above.

In all cases, highly permeable strata in all threelagoons lie just above the level of Admiralty ChartDatum (the level of the lowest astronomical tide),providing an opportunity for salt water to pene-trate the deeper lake sediments and be forcedupwards through the entire sediment sequence ona rising tidal cycle.

Discussion and conclusion

Atypical 137Cs profiles, and lower than expectedinventories for the cores collected from these threeshallow coastal lagoons, suggests that there is amechanism by which 137Cs is displaced up-coreand lost from the sediment column. Although themechanism appears to influence the 137Cs profiles,the 210Pb dates at all three sites are entirely

Figure 9. The rise in conductivity in Big Pool, St Agnes over a 7 day (14 tide) sequence rising from neap to spring tides (July/August

2004).

Figure 8. Trends in conductivity and tidal amplitude, Slapton Ley, May–June 2001.

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consistent with documented changes in the catch-ments and with other established chronologies.There is also no evidence from the lake cores that210Pb or 7Be are significantly depleted relative totheir local reference inventories. Several recentpublications have stressed the stability of 210Pbrelative to 137Cs in providing a robust chronology(e.g., Appleby 2001, 2004; Rose et al. 2004), al-though Brenner (2004) reports instances in Floridalakes where the influx of groundwater, rich in226Ra, causes disequilibrium between 210Pb and226Ra, a mechanism that does not appear to im-pact upon the 210Pb chronologies reported here.Bioturbation (He and Walling 1993) is unlikely tobe the responsible mechanism for 137Cs mobilisa-tion since this mechanism would affect all mea-sured radionuclides.

We suggest that salt water incursion into thebasal lake sediments occurs at all three sites and isdriven by positive pressure differences between sealevel and the lagoon water levels. Because seawater is highly charged with monovalent cations,especially Na+ and K+, it seems likely that these

have displaced 137Cs upcore over the last ca.50 years which has resulted in the re-mobilisationand loss of 137Cs to the water column. Thismechanism is analogous to those proposed foreither extremely saline lakes in arid/hyper-aridareas or extremely acid lakes (pH 4.0) by Long-more et al. (1983, 1986), although it appears torequire much lower salt concentrations in ourstudy than those measured in hyper-arid environ-ments. The critical control appears to be the fre-quency at which the salt water is forced upwardsthrough the sediment column, constantly replen-ishing a supply of monovalent cations that cancompete with 137Cs for available exchange sites.

Experiments on 137Cs mobility in soils andsediments have shown the importance of availableK+ in competing with 137Cs, and have emphasisedthe role of organic exchange sites in peat, and claymineral exchange sites in lake sediments, for 137Csadsorption (Comans et al. 1998). This supportsour explanation for 137Cs displacement in thesediments of these coastal lagoons. Davison et al.(1993) reported significantly greater losses of 137Cs

Figure 10. Pore water chemistry and stratigraphy of long cores taken from locations of 137Cs profiles in Slapton Ley (Core SL1 Figure

2b) (a), Widdicombe Ley (b) and Big Pool (c) (Coring locations are given in Figure 2).

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from re-suspended rather than undisturbed sedi-ment cores and concluded that 137Cs was unlikelyto be remobilised because of occasional acidifi-cation or eutrophication. They suggested thatinundation of coastal freshwater basins byseawater could reintroduce considerable burdensof formerly bound 137Cs into the aqueousenvironment.

Although extreme salinities may lead to thereplacement of 137Cs in sediment cores, success hasbeen achieved in using 137Cs to date sediment se-quences in fully marine or estuarine environments(e.g., Roman et al. 1997). This supports the con-tention that salinity levels in the overlying watercolumn may be less significant than salinity chan-ges in the interstitial waters of the 137Cs bearingsediments. Controlled laboratory experimentsusing seawater (Sholkovitz et al. 1983) showedthat the activity of 137Cs in sedimentary pore waterprofiles remained fairly constant (ca. 40 dpm/100 kg) and was twice that of the local seawater.137Cs did not appear to be involved in diageneticchemistry, but increased in pore waters as a resultof ion exchange reactions. Their flux estimates,based on the pore water data, showed that remo-bilisation and transport of 137Cs could be signifi-cant in a salt water environment.

The mechanism proposed here seems moreplausible than that suggested by Davison et al.(1993) who argued that overtopping of coastallagoons may lead to increased salinity and therelease of 137Cs from underlying lake sediments.Exchange of 137Cs in the pore waters of the sedi-ment column does not require frequent overtop-ping and is driven by normal variations in tidalamplitude. It seems likely, however, given predic-tions of future sea level rise, that coastal lagoonsthroughout the world may be under increasingthreat from increased salinity, not only because ofan increase in storm surges and overtopping, butbecause of the stronger pressure differentials thatare likely to develop between sea surface elevationsand standing water elevations. This has implica-tions not only for the re-mobilisation and releaseof 137Cs into the environment, but also forchanging the salinity and ecology of these coastallagoon systems.

Data reported here suggest that 137Cs is beingremobilised from coastal lagoon sediments. Theflux estimates, however, should be treated withsome caution since they are based on only a single

core retrieved from the deep water zone of eachlake. More intense sampling of the lakes based ona grid network would be required to refine theestimates of total 137Cs loss.

Acknowledgements

We thank Peter Ell, Libby Foster, Anker Laubel,Pete Marsh, Graham Paterson, Nathan Pittam,Steve Proffitt, Cliff Rowley and Matthew Winn forassistance in the field and Bob Hollyoak andElizabeth Turner for help with sample processingin the gamma spectrometry and particle size lab-oratories at Coventry University. Stuart Gill,Coventry University Cartographic Unit, producedthe high quality art-work. The Field StudiesCouncil field centre at Slapton Ley kindly pro-vided logistical support and we thank the localland-owners, the Duchy of Cornwall and EnglishNature, for site access at Widdicombe, Slaptonand Scilly.

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