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Using the radium quartet (228Ra, 226Ra, 224Ra, and 223Ra) toestimate water mixing and radium inputs in Loch Etive, Scotland

Citation for published version:Hsieh, Y-T, Geibert, W, Van-Beek, P, Stahl, H, Aleynik, D & Henderson, G 2013, 'Using the radium quartet(228Ra, 226Ra, 224Ra, and 223Ra) to estimate water mixing and radium inputs in Loch Etive, Scotland',Limnology and Oceanography, vol. 58, no. 3, pp. 1089-1102. https://doi.org/10.4319/lo.2013.58.3.1089

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Using the radium quartet (228Ra, 226Ra, 224Ra, and 223Ra) to estimate water mixing and

radium inputs in Loch Etive, Scotland

Yu-Te Hsieh,a,1,* Walter Geibert,b,c Pieter van-Beek,d Henrik Stahl,c Dmitry Aleynik,c andGideon M. Henderson a

a Department of Earth Sciences, University of Oxford, South Parks Road, Oxford, United KingdombSchool of Geosciences, University of Edinburgh, The King’s Buildings, West Mains Road, Edinburgh, United Kingdomc Scottish Association for Marine Sciences (SAMS), Scottish Marine Institute, Oban, Argyll, United KingdomdLaboratorie d’Etudes en Geophysique et Oceanographie Spatiales (LEGOS), Observatoire Midi- Pyrenees, Toulouse, France

Abstract

The radium (Ra) quartet (228Ra, 226Ra, 224Ra, and 223Ra) has been investigated in Loch Etive, a Scottish fjord,to provide new constraints on water mixing rates and on the inputs of Ra from sediments. Maximum watertransport rates for the inflowing estuarine layer at 5 m depth, determined from the excess 223Ra (223Raex), indicatethat this water travels at no more than 2.4 6 0.2 cm s21 net and that it takes 17 6 2 d for waters to travel from themouth to the head of the loch if no horizontal mixing is taken into account. Alternatively, neglecting advection,the short-lived Ra distribution could be explained by horizontal mixing rates of 6.1 3 106 cm2 s21 (223Raex) or 9.13 106 cm2 s21 (224Raex). Periodic overturning circulation plays an important role in resetting chemical cycles inthe isolated deep basin of the inner loch. Sediment in this deep basin provides the major input of 228Ra to theisolated deep water, and the accumulation of 228Ra in deep waters allows an assessment of sedimentary fluxes of228Ra, a poorly constrained aspect of the 228Ra input to the global ocean. The calculated sedimentary 228Ra fluxof 2.1 6 0.2 (3 109) atoms m22 yr21 in the inner deep basin is comparable with previous measurements ofsedimentary 228Ra inputs from shelf sediments, supporting existing global 228Ra budgets, which are used to assessglobal rates of groundwater discharge to the ocean.

Estuaries play an important role in the transformationof river water to seawater, showing strong physical andchemical gradients. Estuarine environments provide inputsof trace elements and nutrients to the ocean and can act asa significant filter between the land and the open ocean(Howe et al. 2010). Understanding the physical mixing andadvection of waters in these environments is an importantaspect of understanding the chemical cycles of estuariesand their role in controlling chemical fluxes to the openocean.

Since fish farming started in the 1970s, fjords have beenincreasingly used for aquaculture. The fish farm industryhas been found to have negative effects on the fjordecosystem, such as the spreading of infectious diseases,waste pollution, and marine eutrophication (Cloern 2001).Studies (Karakassis et al. 2000; Skogen et al. 2009) havefocused on assessing and modeling the environmentaleffects of fish farming in fjords. However, the fjordenvironment is a complex system in terms of watercirculation and chemical exchange (Howe et al. 2010).Thus, understanding the physical and chemical environ-ment in fjords would improve the assessment of theenvironmental effects of fish farming in fjords, which alsohas a societal and economic value.

Fjords often have glacially over-deepened and semi-enclosed marine basins that provide a particular style ofestuarine environment and generate distinct water circu-lation to control chemical and physical transitions fromterrestrial to marine environments (Howe et al. 2010).Fjord circulation systems have also been shown toinfluence global ocean circulation by regulating glacialrunoff and the water exchange between estuarine andcoastal environments (Straneo et al. 2011). Despite theinterest they generate, water mixing rates and chemicalfluxes are not well quantified in typical fjord systems.

Radium (Ra) has four naturally occurring isotopes(226Ra, T1/2 5 1600 yr; 228Ra, T1/2 5 5.75 yr; 223Ra, T1/2

5 11.4 d; and 224Ra, T1/2 5 3.66 d), the so-called radiumquartet (Rama and Moore 1996). These isotopes have beenused as powerful tracers for estimation of chemical fluxesto the ocean, including riverine inputs (Li and Chan 1979);sediment inputs (Hancock et al. 2000); and submarinegroundwater discharge (SGD) (Charette et al. 2001; Mooreet al. 2008). Ra isotopes are produced by the radioactivedecay of thorium isotopes, typically in sediments. They aredissolved in pore water and released from sediments intoseawater. Ra is therefore a useful tracer for studyingchemical interaction at the sediment–water interface. Inaddition, Ra behaves conservatively (except in the case ofits known radioactive decay) once released into seawater.Ra isotopes have therefore also been widely applied to thestudy of a variety of oceanographic processes and chemicalfluxes to the ocean, such as diffusion (Kadko et al. 1987),large-scale ocean mixing (Kaufman et al. 1973; Chung

* Corresponding author: yuteh@mit.edu

1 Present address: Department of Earth, Atmospheric andPlanetary Sciences, Massachusetts Institute of Technology, Cam-bridge, Massachusetts

Limnol. Oceanogr., 58(3), 2013, 1089–1102

E 2013, by the Association for the Sciences of Limnology and Oceanography, Inc.doi:10.4319/lo.2013.58.3.1089

1089

1980; Ku and Luo 1994), horizontal and vertical eddymixing (Broecker et al. 1967; Sarmiento et al. 1976; VanBeek et al. 2008), shelf-water and coastal ocean mixing(Rutgers van der Loeff et al. 1995; Moore 1998, 2000a),water-mass and particulate residence time (Broecker et al.1973), pore-water exchanges (Cochran and Krishnaswami1980; Webster et al. 1994), and sediment benthic fluxes(Bollinger and Moore 1984; Hammond et al. 1990; Han-cock et al. 2000).

An example of the power of Ra isotopes in seawater istheir use in assessing global SGD fluxes to the oceans. Theinventory of 228Ra in the surface Atlantic Ocean constrainsthe total SGD input to the ocean (Moore et al. 2008),provided that other important fluxes from rivers andsediments are known. This approach has demonstrated thatSGD provides significant input of nutrients and otherdissolved species to the ocean, potentially higher than thatassociated with riverine discharge. Confirming the use ofopen-ocean 228Ra to assess SGD fluxes requires firmknowledge of the flux of 228Ra from its other majorsource—marine sediments. Fjords with periodically isolat-ed deep water in contact with marine sediment, such asLoch Etive, provide natural ocean laboratories in which toassess this sedimentary 228Ra flux.

In this study we have provided new constraints on the Racycle in the fjord system with a set of analyses on Loch Etive,a Scottish fjord. Resulting surface-water data constrain therates of mixing in the loch. In the deep waters, the dataconstrain the sedimentary fluxes of 228Ra, with relevance toassessment of global budgets and SGD fluxes.

Methods

Study area—Loch Etive is a sea loch located on the westcoast of Scotland (Edwards and Sharples 1986) (Fig. 1).The loch hosts a number of mussel farms and is used foroccasional recreational boating. Details of the bathymetryand sedimentology of the loch have been investigated in ahigh-resolution seismic and gravity-coring survey (Howeet al. 2001, 2002). The loch is separated into an inner (inland;145 m maximum depth) and an outer (seaward; 70 mmaximum depth) basin by a sill of 13 m depth at Bonawe (seeFig. 1). The outer basin is separated from the coastal oceanby another sill, the Falls of Lora. The geological setting is amixture of igneous and metamorphic rocks, mostly granite,which restricts groundwater penetration through this area.Tide flow drives seawater in and out of the loch. Seawater ismixed with freshwater from River Awe and River Etive fromlarge catchment areas totaling 1400 km2 (Wood et al. 1973).The mean freshwater discharge is 3038 3 106 m3 yr21

(Edwards and Sharples 1986).The hydrology and circulation of Loch Etive are

reasonably well known (Edwards and Edelsten 1977)(Fig. 2a). In the inner basin, water masses are structuredin three different vertical units (Edwards and Grantham1986). The freshest waters are observed in a very shallowlens at the surface, with a vertical extent of only tens ofcentimeters. This layer is dominated by estuarine outflow.Below this surface layer seawater is mixed with smalleramounts of freshwater to varying degrees. While tidal flowslead to reversing flow patterns, in this second layer,

Fig. 1. Map of Loch Etive with location of sampling sites. The loch is divided into an innerloch and an outer loch by Bonawe Sill at LE5. LE6 is also the HYPOX station in the inner lochfor the time-series oxygen data shown in Fig. 3. River Etive and River Awe provide freshwaterinput into the loch, as indicated by arrows.

1090 Hsieh et al.

estuarine inflow dominates. This intermediate layer withreduced salinity prohibits flow of waters to depth in theinner deep basin, resulting in a usually stagnant deep thirdwater mass in the inner basin below the depth of a secondpycnocline (around 25 m). After a particularly dry periodor when temperatures are cold during winter mixing with

freshwater is reduced, and high-salinity dense seawater canflow over the Bonawe Sill (13 m) and sink into the deepinner basin to cause an overturn circulation. The averagerepeat period for overturn circulation is about 16 months(Edwards and Edelsten 1977), but there is no regular cycleto this overturning period.

Fig. 2. (a) Schematic of hydrography and Ra cycles in Loch Etive. The dashed line indicates the inner loch water stratificationbetween mixed surface layer and isolated deep water. In the surface layer, two horizontal arrows indicate water mixing from seawater andriver inputs. The arrow across the dashed line presents the inner loch overturning circulation, which happens periodically. Two upwardarrows indicate sedimentary Ra input. R is the radioactive decay of Ra. CTD data of (b) temperature (uC), (c) salinity, and (d) oxygen(mg L21) in Loch Etive on 03 and 04 August 2010. The locations of station are shown by black dots. Samples were collected from thesurface at all stations [black dots in (b)] and from three depth profiles at LE4 (54 m), LE6 (136 m), and LE8 (70 m), respectively [solidlines in (b)].

Fjord mixing based on radium isotopes 1091

Dissolved oxygen is constantly consumed in the deepwaters and therefore decreases, so that the level of oxygenin deep waters can be used as an index to monitoroverturning events. For example, dissolved oxygen sud-denly increased from 0.9 to 9.5 mg L21 during an overturnevent in May 2000 and then gradually decreased with timeto semi-anoxic levels in the deep inner basin (Austin andInall 2002). The change of dissolved oxygen also signifi-cantly controls other aspects of the chemistry of the loch,such as the manganese (Mn) cycle (Overnell et al. 2002;Statham et al. 2005).

The distribution of Ra isotopes in Loch Etive isdominated by seawater, river, and sediment inputs(Fig. 2a). The concentration of Ra isotopes in the isolateddeep water of the inner basin is reset by overturn events.Between such overturns, 228Ra, 224Ra, and 223Ra concen-trations can decrease as a result of radioactive decay orincrease as a result of addition from the sediment.Measurement of changing Ra concentrations in the deepwaters therefore has potential to be used to assess the ratesof sedimentary Ra input to seawater.

Sampling methods—Water samples were collectedaboard the R. V. Calanus during a 2-d cruise on 03 and04 August 2010 in Loch Etive. Samples from 5 m depthwere collected from seven stations from the mouth of theloch (LE1), the outer loch (LE4 and LE5), the inner loch(LE6, LE8, and LE9), and the head of the loch (LE10)(Fig. 1). Deep-water samples were collected from threestations at LE4, LE6, and LE8 (Fig. 2b). Sta. LE6 wasreoccupied 10 months later on 08 June 2011 to assess thetemporal evolution of Ra in the deep loch.

Each sample consisted of water from twelve 5 literNiskin bottles mounted on a conductivity–temperature–depth (CTD) rosette and fired at the same depth as oneanother. Resulting 60 liter samples were stored briefly incubitainers before shipboard pre-concentration of Ra.Samples were passed through a column of MnO2-coatedacrylic fiber by gravity at a flow rate , 0.5 L min21 toconcentrate the Ra (Moore et al. 1985). For a summary ofmethods and considerations of Ra sampling refer to Moore(2008). After sampling, Mn fibers were rinsed three timeswith Milli-Q water to remove adhering salt and squeezed toreduce liquid content. A subsample of 250 mL was alsocollected from each location and depth for a separatemeasurement of 226Ra concentration (Foster et al. 2004).

Measurements—Seawater 224Ra and 223Ra by Ra de-layed-coincidence counting system (RaDeCC): Mn -fibersamples were first counted using a four-channel RaDeCCfor 224Ra and 223Ra activities (Moore and Arnold 1996).Fibers were then stored for 2–3 weeks before a secondcount on the same device to determine how much of the224Ra was supported by thorium-228 (228Th). Whenrequired, as a result of still-elevated activities in the secondcount, a third and final count was performed to assess howmuch of the 223Ra was supported by actinium-227 (227Ac)(Geibert et al. 2008). Supported 223Ra and 224Ra activities,as measured on the second and third counts, weresubtracted to give the excesses of 223Ra and 224Ra (223Raex

and 224Raex, respectively) in each sample. The RaDeCCwas calibrated regularly with two standard samples fromthe parent nuclides of short-lived Ra, 227Ac, and 232Th,which had been obtained from the International AtomicEnergy Agency–Marine Environmental Laboratories,Monaco, in 2009 (Scholten et al. 2010). As a result of therelatively low activities of short-lived Ra for a coastalenvironment, the overall uncertainties of 224Ra and 223Raare 10% to 40% (2 standard errors [SE]).

Seawater 228Ra and 226Ra by multi-collector inductivelycoupled plasma mass spectrometry (MC-ICP-MS)—Thelong-lived Ra isotopes (226Ra and 228Ra) were measuredusing a newly developed technique, which makes use ofMC-ICP-MS (Hsieh and Henderson 2011). After RaDeCCcounting, Mn fibers were ashed in a furnace and thenleached with HCl for the purification of Ra. Ra wasprecipitated by Sr(Ra)SO4 in the leaching solution and thenconverted to Sr(Ra)CO3 for dissolution (Cohen andO’Nions 1991) before ion-exchange column chemistry(AG50-X8 and Sr-spec). The non-spiked Mn fiber sampleswere measured for the 228Ra : 226Ra activity ratios. Theratios were then multiplied by 226Ra concentrations tocalculate 228Ra concentrations. For measurement of 226Raconcentrations, water samples (250 mL) were spiked withan enriched 228Ra spike following the procedure describedin Foster et al. (2004). In short, Ra is precipitated withCaCO3 in seawater and then purified by ion exchangecolumns (AG1-X8, AG50-X8, and Sr-Spec) before analysis.The pure Ra aliquot was analyzed by MC-ICP-MS, with228Ra and 226Ra simultaneously measured in two ioncounters. Bracketing measurements of a uranium standard,Certified Reference Material–145, were used to assess andcorrect for mass bias and ion-counter gain between eachsample. Chemical blanks were monitored for the wholeprocedure and are less than 1% of 226Ra of the sample andnot detectable for 228Ra. The machine memory was alsomonitored and corrected between each measurement. Theoverall uncertainties of 226Ra and 228Ra are 3% to 4% (2 SE).The improved analytical precision allows us to moreprecisely calculate water mixing and Ra fluxes in Loch Etive.

Results

Temperature, salinity, and oxygen data—Temperature,salinity, and oxygen data are presented in Fig. 2. Whilesalinity in the surface layer varies between 2 and 18 with ahigh dissolved oxygen of , 9.5 mg L21 and temperaturesaround 15uC, samples at 5 m all have salinities of . 24(Fig. 3a). At depth, the inner loch had a homogeneouswater mass below 30 m, with temperatures of 11–12uC,salinity of 28–29, and dissolved oxygen of 7.5–8.5 mg L21

during the August 2010 cruise (Fig. 4f). A colder waterlayer with temperature , 10uC and lower dissolved oxygenof , 8 mg L21 covered the whole inner loch between 20 and30 m. In the outer loch, deep water showed oxygendepletion (, 7 mg L21) below 42 m.

Time-series oxygen data (Fig. 5) have been collected forthe Hypox mooring (Stahl 2011) from December 2009 toJanuary 2012 at 124 m at the inner-loch deep-basin

1092 Hsieh et al.

observatory (LE6 in Fig. 1). The Hypox time-series oxygendata generally show consistent results with the CTDmeasurements. The data indicate that deep water in theinner loch had just overturned between 22 and 28 June 2010,before our sampling. Oxygen concentrations in the deepinner loch decrease from 11 mg L21 to 3 mg L21 with timeafter this overturn circulation. Between the first and thesecond cruises (August 2010–June 2011), time-series oxygendata indicate that the deep water of the inner basin hadremained isolated since the overturn event in late June 2010.

Ra isotopes—The radioactivity of Ra isotopes ispresented in millibequerel (mBq) in this article. To convertfrom mBq to disintegration per minute (dpm), the unitcommonly used in the marine radioactive research com-munity, 1 mBq is equal to 0.06 dpm.

Concentrations of 223Raex and 224Raex range from belowthe detection limit (, 1.7 mBq 100 L21) to 33 mBq 100 L21

and 203 mBq 100 L21, respectively (Table 1). In the watercolumn of the inner basin (Fig. 4d,e), 223Raex and 224Raex

show higher activities in the subsurface layer at the 5 mdepth (6 and 40 mBq 100 L21, respectively), no measureableactivity at mid-depths, and then an increase toward the basinfloor (3.0 and 37 mBq 100 L21, respectively). However,223Raex and 224Raex from the second cruise at LE6 are bothbelow the detection limit in deep-water samples at all depths

in the inner basin. At 5 m depth (Fig. 3b,c), 223Raex and224Raex are both found to decrease with distance away fromthe mouth of the loch (from 10.8 to 4.3 mBq 100 L21 andfrom 100 to 30 mBq 100 L21, respectively).

The activities of 226Ra and 228Ra in the water samplesrange from 72 to 143 mBq 100 L21 and from 295 to 407 mBq100 L21, respectively. The activity ratios of 228Ra : 226Rashow a range from 2.5 to 4.4 (Table 1). In the inner basin(Fig. 4a), 226Ra activity shows a constant distribution (137 67 mBq 100 L21) below 30 m during the first and secondcruises. 228Ra activity is also constant with depth (363 623 mBq 100 L21) in the first cruise but significantly decreases,from 377 to 137 mBq 100 L21, at 90 m during the secondcruise (Fig. 4b). In the outer basin, the water column shows alayer of oxygen depletion below 42 m, with constant salinityand temperature. In this layer, 226Ra and 228Ra both showthe highest values of 138 and 392 mBq 100 L21, respectively,in the water column. At 5 m depth, 226Ra activities arerelatively constant between 118 and 125 mBq 100 L21

(Fig. 3d), but 228Ra activities increase from 307 to 407 mBq100 L21 (Fig. 3e) toward the head of the loch.

Discussion

Surface-water mixing in Loch Etive—The short-lived Raisotopes, 223Ra and 224Ra, reflect horizontal mixing and net

Fig. 3. Surface profiles of (a) salinity, (b) 224Raex, (c) 223Raex, (d) 226Ra, (e) 228Ra (mBq100 L21), and (f) 228Ra : 226Ra activity ratio at 5 m (except LE4 at 10 m) in Loch Etive plottedwith the distance from the mouth of the loch. Note the relatively slight change in salinity withinthe loch, relative to the larger and generally systematic changes of Ra concentration with distancefrom the mouth of the loch.

Fjord mixing based on radium isotopes 1093

advective transport. Key to their application here is theconsideration of the specific hydrographic features of LochEtive. The typical features of an estuary, the outwardtransport of fresh and brackish water at the surface, andthe inward transport of seawater at deeper layers arepresent in Loch Etive, as seen in the salinity data, but theyare forced through two shallow sills. The outer sill, theFalls of Lora, leads to turbulent mixing over the entiredepth, and the water produced here has a salinity . 26(LE1 and LE2). At Bonawe Sill (13 m), which separates theinner and outer basins, the net outward movement offresher water is more restricted to the very surface, whereasthe water at 5 m depth has a near-constant salinity of justabove 24. The latter is the water that will mainly enter the

inner basin during high tides. This combination of inflowand outflow at different depths at the same location haspreviously been described by Edwards and Grantham(1986). As our transect in surface waters is located at 5 mdepth, considerably below the brackish surface layer, ourdata are likely to reflect the net inflowing layer. The saltintroduced at 5 m depth will leave the inner basin as part ofthe brackish surface layer, with a net discharge that islarger than the inflow, as a result of the addition offreshwater from rivers. At 5 m depth, salinity decreasesfrom 26.9 to 24.3 (Fig. 3a) toward the head of the loch.Activities of 223Raex and 224Raex decrease in concert withthis salinity change (Fig. 3b,c). The decrease in activity of223Raex and 224Raex with distance (x) can most readily be

Fig. 4. Water-column profiles of (a) 226Ra, (b) 228Ra, (c) 228Ra : 226Ra, (d) 224Raex, (e) 223Raex (mBq 100 L21), and (f) salinity,temperature, and oxygen in the inner basin of Loch Etive. Data are from Sta. LE6, with the black line and filled squares representing datafrom 03 August 2010 and the gray line and open squares representing data from 08 June 2011. A. R. indicates activity ratio.

1094 Hsieh et al.

explained by the radioactive decay of Ra entering the lochin seawater as that water is transported toward the head ofthe loch by mixing or by advection.

The estuarine circulation in Loch Etive indicates thatadvective transport dominates Ra budgets and allowscalculation of a maximum inflow rate in the estuarineinflow layer at 5 m depth. Alternatively, Ra might becontrolled by mixing into the Loch, and we therefore assessa pure mixing scenario that assumes that the tidal in- andoutflow are not vertically separated. The horizontalmagnitude of salinity change is small at the 5 m depth(Fig. 3a) and is therefore consistent with both scenarios.

In an advection-only scenario, the distribution of 223Raex

and 224Raex in the subsurface layer at the 5 m depth can beexpressed by the following equation:

wLA=Lx~lA ð1Þ

where w is the mean water transport rate; x is the distancefrom the mouth of Loch Etive; A is 223Raex or 224Raex

activity; and l is the decay constant for 223Ra ( 0.0608 d21)or 224Ra (0.189 d21).

The solution for w is then given by

w~lx=ln(Ai=A)~X1=2=T1=2 ð2Þ

where Ai is the initial activity of 223Raex and 224Raex (i.e., atLE1); X1/2 is the distance at which A 5 0.5Ai; and T1/2 is thehalf-life of 223Ra or 224Ra. Based on the data in Fig. 3b andc, we obtain water transport rates of 6.1 and 2.4 cm s21

from 224Raex and 223Raex, respectively. For comparison,previous measurements made by current meters show thatthe water transport rates are usually below their detectionlimit of 5 cm s21 in the water 40 m below the surface inLoch Etive (Edwards and Edelsten 1977). Inall et al. (2004)found an average speed of 20 cm s21 using a current meteraround the Bonawe Sill during spring tides. In terms of

depth, distance, and timescale, the Ra-derived watertransport rates appear to reflect a long-term (days toweeks) average water transport throughout the surfacelayer, which is more suitable for assessing long-termaverage chemical fluxes but not for short-term (minutesto hours) variability.

The water transport rate calculated by 224Raex is < 2.5times higher than the rate calculated by 223Raex. Assumingthat seawater inflow is the only input for 223Raex and224Raex to the surface water, 223Raex should show less decaywith distance than does 224Raex as a result of its longer half-life. Put another way, if the 223Raex-derived mixing iscorrect, observed 224Raex values should decrease rapidly inthe loch to values significantly lower than were observed.This discrepancy can be explained if there is an additionalinput of 224Raex to the surface water (see discussion below).The slower transport rate, derived from 223Raex, is morelikely to be a true estimate of transport and indicates a rateof 2.4 6 0.2 cm s21, which is broadly consistent with apreviously estimated horizontal mixing transport rate of <3.5 cm s21 in the inner loch basin (Inall 2009). This resultsin a seawater transport time of 17 6 2 d from the mouth tothe head of Loch Etive in the layer at 5 m depth. If there isa source of 223Ra within the loch, in addition to 224Ra, thenthese transport rates would be lower, but such a sourceappears unlikely, as discussed below.

In the alternative mixing-only scenario, following theapproach in Moore (2000a), we obtain a horizontal mixingrate of 6.1 3 106 cm2 s21 (223Raex) or 9.1 3 106 cm2 s21

(224Raex) in the layer at 5 m depth (see Fig. 6). The valuefrom 224Ra is possibly slightly affected by some additionalinputs from the Rivers Awe and Etive, as discussed below.The mixing rates represent an upper limit for averagehorizontal mixing in the period observed, as they neglect thepossible net advection in this subsurface layer. The actualsituation in Loch Etive will include both advective terms (if

Fig. 5. Time-series oxygen data (HYPOX) at 124 m at the inner-loch deep-basin. The data(gray dots) are collected from December 2009 to January 2012 and are compared with the CTD(open squares) and Winkler (black triangles) data at 116 m and 100 m, respectively.

Fjord mixing based on radium isotopes 1095

Table

1.

Ra

diu

mis

oto

pic

da

tain

Lo

chE

tiv

e.

Sa

mp

lest

ati

on

Da

teD

epth

(m)

Sa

lin

ity

T(u

C)

Ox

yg

en(m

gL

21)

22

6R

a(m

Bq

10

0L

21)

22

8R

a(m

Bq

10

0L

21)

22

8R

a:2

26R

a(a

ctiv

ity

rati

o)

22

3R

aex

(mB

q1

00

L2

1)

22

4R

aex

(mB

q1

00

L2

1)

LE

10

4A

ug

20

10

52

6.9

13

.88

.91

226

53

076

12

2.5

60

.11

0.8

61

.71

006

7L

E4

03

Au

g2

01

01

18

.11

5.1

10

.11

026

33

156

12

3.1

60

.17

.36

2.3

606

81

02

5.9

13

.18

.41

186

33

186

12

2.7

60

.10

.56

1.5

106

72

32

6.8

13

.58

.51

276

53

276

12

2.6

60

.1b

db

d5

42

9.2

12

.27

.01

386

53

926

15

2.8

60

.15

.26

2.2

656

8L

E5

03

Au

g2

01

05

24

.51

3.3

8.5

11

86

53

256

12

2.7

60

.16

.36

2.2

576

7L

E6

03

Au

g2

01

05

24

.51

3.2

8.3

12

06

53

306

12

2.8

60

.16

.06

1.7

406

51

52

6.3

12

.48

.11

126

32

956

12

2.7

60

.14

.76

1.5

326

72

82

8.1

9.5

8.4

13

86

53

706

13

2.7

60

.1b

db

d4

52

8.8

11

.08

.31

426

53

776

13

2.7

60

.1b

db

d7

02

9.0

11

.68

.31

406

53

636

15

2.6

60

.1b

db

d9

02

9.0

11

.68

.21

326

53

776

13

2.6

60

.1b

db

d1

08

29

.01

1.6

8.2

13

56

53

536

13

2.6

60

.1b

db

d1

20

29

.01

1.6

8.0

13

36

53

456

13

2.6

60

.12

.36

1.2

236

51

30

29

.11

1.6

8.0

13

86

53

576

13

2.6

60

.14

.76

1.5

306

51

36

29

.11

1.6

8.0

13

86

53

556

13

2.6

60

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1096 Hsieh et al.

resolving tidal cycles or when including overturning events)as well as a mixing component (when looking at timescalesthat integrate tidal fluctuations or during slack tides).

Sedimentary inputs of 224Ra and 228Ra to the surfacewater—Based on the advection-only scenario discussedabove, 223Raex-derived water transport rate (Fig. 7a) and theradioactive decay of 224Ra can be used to calculate a theoreticalcurve of 224Raex (solid line) in the surface water with distancefrom the mouth to the head of loch (Fig. 7b). If there was noadditional input of 224Ra to the surface water, the decrease of224Raex activity should follow this theoretical curve, butobserved activities fall above this expected decay curve.

The source of 224Raex likely to explain these highervalues is the shelves of the loch, particularly aroundBonawe, where these shelves are pronounced and wherestrong cross-sill flow (Inall et al. 2004; Stashchuk et al.2007) may enhance the release of alpha-recoiled 224Ra fromsediments on the sill flat to surface water. 224Raex

concentrations continue to decrease further into the lochbeyond the Bonawe Sill, but this decrease cannot beexplained by simple exponential decay, and a further sourceof 224Ra is required at the head of the loch (see the grayarea highlighted in Fig. 7b). In contrast, the Bonawe Silldoes not increase 228Ra to the surface water. This is due tothe high flushing rate and the shorter contact time of waterand the host rock on the sill flat. To build up high Raactivity in water, 228Ra needs a much longer contact timethan does 224Ra (Porcelli and Swarzenski 2003).

The additional source of 224Raex at the head of the lochis associated with higher 228Ra and 228Ra : 226Ra ratios insurface waters (Fig. 3e,f). The increase of 228Ra seentoward the head of the loch (Fig. 3e) in the surface wateris not controlled by radioactive decay, as the half-life of228Ra is significantly longer than the surface-water trans-port rate, so an input of 228Ra is required. The decrease insalinity (from 24.5 to 24.3) in the 5 m depth waters at thehead of the loch is small (Fig. 3a), indicating that therequired inputs of 228Ra and 224Ra to this layer are notfrom freshwater or groundwater inputs. As at Bonawe inthe middle of the loch, shelf sediments in the inner loch arelikely to be the source of this 228Ra and 224Ra. Thebathymetry of Loch Etive shows a large area of the shelf(with water depths , 15 m) at the head of the loch (Howeet al. 2001) where riverine sediment depositions might bereleasing 228Ra and 224Ra to surface water.

Surface-water 228Ra and 224Raex concentrations at Sta.LE9 minus seawater values can be used to estimate theexcess 224Ra : 228Ra activity ratio from the sediment input.The resulting value is 0.15 6 0.02, which is lower than the

Fig. 6. Profiles of (a) 223Raex and (b) 224Raex at 5 m plottedwith the exponential regression line (solid line) and the calculatedhorizontal mixing coefficient (Kx) in the mixing-only scenario.

Fig. 7. (a) Surface-water transport time calculated from theradioactive decay of 223Raex with the distance from the mouth ofthe loch in the advection-only scenario. (b) Predicted 224Raex

decay curves based on the 223Raex-derived water transport rate(2.4 cm s21): the solid line represents the expected change in 224Raif it is provided only from seawater outside the loch. The dashedline demonstrates the need for additional input of 224Ra from thesill flat at LE5 and is curved to represent expected mixing if thereis no further source further into the loch. The gray shaded regionsindicate additional offset of 224Raex from the decay curve due tothe additional sediment input at the head of the loch.

Fjord mixing based on radium isotopes 1097

expected ratio of 1.0 based on the secular equilibrium228Th : 232Th ratio in sediments. This could be explained bythe radiogenic decay of 224Ra since the last time the waterwas in contact with sediments and the apparent water age(Moore 2000b) in the loch surface layer. Based on themodel in Moore (2000b), the apparent age could becalculated by comparing the estimated 224Raex : 228Raactivity ratio of 0.15 to the expected ratio of 1.0. Theestimate age is 10.1 d in the surface water at LE9, which iscomparable to the estimated water residence time (17 d) inthe surface Loch Etive.

Compared to 224Ra and 228Ra inputs, the input of 223Rafrom shelf sediments is insignificant in Loch Etive becausethe bedrock of this region is granite, which usually has a

lower content of uranium (U) (235U, grandparent of 223Ra)than of Th (232Th, parent and grandparent of 228Ra and224Ra, respectively). For example, the granite in the Isle ofSkye, Scotland, has an average 238U concentration of 1.1mg g21 and a 232Th concentration of 7.3 mg g21 (Tamme-magi 1976). Based on the atomic ratios of 232Th : 238U (6.6)and 238U : 235U (137.88) in granite, the estimated input (orproduction) of 223Ra activity is about 9100 times smallerthan that for 224Ra and 228Ra from the sediment orbedrock of Loch Etive. However, bedrock or shelfsediments may not be able to entirely explain the input of223Ra outside the mouth of Loch Etive. Outside the Loch,Ra has multiple sources from sediments, seawater, andrecirculated seawater.

Riverine inputs, seawater inflow, and shelf sediments inthe inner loch are the three major end members for Ra inLoch Etive. Deviations from the mixing line betweenfreshwater and seawater on the plots of 226Ra and 228Raagainst salinity (Fig. 8) allow sedimentary inputs to beclearly identified. For 226Ra (Fig. 8a), the trend shows twoend members mixing between seawater and river inputs.For 228Ra (Fig. 8b), the mixing between seawater and riverinputs is equal. However, the magnitude of these deviationsfrom the two end members mixing between the head andthe mouth of the loch indicates that the sediment inputcould contribute 10–23% of the 228Ra to the surface water.

Ra cycles and overturn circulation in the inner loch basin—Overturning circulation plays an important role in control-ling Ra cycles in the isolated deep waters of Loch Etive. Inthe inner basin, the vertical water profiles of 226Ra and228Ra show uniform distributions with depth in the watercolumn below 20 m. This can be interpreted as due to therecent overturning of loch waters, known to have occurred,based on O2 data, 1 month prior to our major sampling.This event reset the 226Ra and 228Ra activities in the watercolumn (Fig. 4a,b) and may have led to temporaryperturbation of deep-water 223Ra and 224Ra throughsediment disturbance or enhanced vertical mixing. Assum-ing that 224Ra is in steady state, the vertical profile of224Raex activity (Fig. 4d) indicates a strong vertical mixingcoefficient (Kz) of 18.8 cm2 s21, based on the commonlyused one-dimensional diffusion model summarized in Kuand Luo (2008). In contrast, a much lower value of Kz of, 2.5 cm2 s21 is obtained in the well-stratified deep waterin the inner loch (Inall 2009), which also explains theobservations of low 224Raex and 223Raex activities (belowdetection limit) during the second cruise.

The 228Ra activity in inner loch water below 20 m israther constant between 345 and 377 mBq 100 L21

(Fig. 4b), which is about 10–17% higher than the value of312 mBq 100 L21 in the 228Ra-salinity mixing relationshipbetween seawater and freshwater in Fig. 8b. This indicateseither that the mixing relationship was somewhat differentat the time of the last overturn or that the additional inputof 228Ra from the loch shelf sediment also contributes228Ra to the new deep waters during overturn. In this lattersituation, surface waters incorporate additional 228Ra fromshelf sediments before and as they overturn and fill the deeploch.

Fig. 8. The activities of 226Ra (a) and 228Ra (b) plottedagainst salinity in Loch Etive. For these longer-lived Ra isotopes,there is insufficient time for significant changes in activity due todecay in the loch waters. A mixing curve is therefore expectedbetween the waters of River Etive and the seawater outside of theLoch. 226Ra activities fall close to this mixing line, but 228Raactivities show somewhat higher 228Ra values as a result of theaddition of 228Ra from sediments to water within the loch.

1098 Hsieh et al.

Sedimentary 228Ra flux in the inner loch basin—Follow-ing overturn, the deep water in the inner basin is isolated,and addition of further 228Ra can happen only fromsediments below the loch. Addition of groundwater to thedeep loch is not expected, given the granite basement, andthere is no evidence for such flow from salinity measure-ments in this or other studies. The activity of 228Ra istherefore controlled only by the balance between radioac-tive decay and sedimentary inputs.

The sedimentary input flux of 228Ra in the deep basincan be calculated from data for Ra isotopes from the twocruises in August 2010 and June 2011. Oxygen data indicatethat deep waters of the inner loch remain isolated betweenthese dates and that there are no overturning events. The226Ra activity shows very consistent results between the twocruises in the deep inner basin, which supports suchisolation (Fig. 4a). Changes in 226Ra are not expectedbecause the half-life is long compared to the period betweenoverturning events, allowing insufficient time for decay andlittle chance for alpha-recoil from sediments. However, thedata for 228Ra activity and 228Ra : 226Ra ratio show asignificant decrease at 90 m in the inner basin from August2010 to June 2011 (Fig. 4b,c). This implies that the 228Rainput from the deep basin sediment is not high enough tobalance the output by radioactive decay in the watercolumn.

A simple box model is used to calculate the average228Ra sediment flux in the inner basin. Because of thesomewhat variable distribution of 228Ra activity in thewater column (Fig. 4b), the activity ratios of 228Ra : 226Rain Fig. 9 are used to normalize the 228Ra activities. Theactivities of 226Ra between these two cruises are relativelyconstant, with a mean value of 137 6 3 mBq 100 L21. Atthe time of the first cruise, the average activity ratio of228Ra : 226Ra in the deep water is constant (2.6 6 0.1)because an overturn event reset the value a month beforesampling. Therefore, the normalized 228Ra activity is 355 68 mBq 100 L21. Assuming no subsequent overturning andthat there is no sediment input, the activity of 228Ra shoulddecrease from 355 to 320 mBq 100 L21, and the activityratio should decrease from 2.60 to 2.34 as a result of theradioactive decay of 228Ra. At the time of the second cruise,228Ra activities higher than 320 mBq 100 L21 or activityratios higher than 2.34 demonstrate input of 228Ra fromdeep basin sediments.

The total additional 228Ra contributed to deep waters iscalculated from the observed increase in 228Ra above thatexpected from decay. The integrated increase of 228Rabetween 90 m (the depth of the top of deep waters) and136 m (the loch floor) during the period between the twocruises can be expressed by the following equations:

Ad~A1|2{t=T1=2 ð3Þ

Iex~

ðz1

z2

A2{Adð Þdz ð4Þ

of 228Ra (320 mBq 100 L21) estimated by the radioactivedecay; A1 is the normalized activity of 228Ra (355 mBq

100 L21) from the first cruise; A2 is the normalized activityof 228Ra (320, 340, and 345 mBq 100 L21 at 90 m, 120 m,and 136 m, respectively) from the second cruise; Iex is thedepth-averaged integral excess 228Ra inventory: A2 minusAd in the water column between z1 (90 m) and z2 (136 m)from the second cruse; t is the time between the two cruises(0.85 yr); and T1/2 is the half-life of 228Ra (5.75 yr). Theintegrated excess 228Ra inventory (Iex) is 6900 mBq m22.

Based on the above equations, the sedimentary 228Raflux [Fsed(228Ra)] can be further calculated in the followingequation:

Fsed228Ra� �

~Iex=t ð5Þ

where Fsed(228Ra) is the 228Ra sedimentary flux (atomsm22 yr21). The calculated 228Ra sedimentary flux is 2.1 60.2 (3 109) atoms m22 yr21. This flux is based on theassumption that vertical mixing cannot remove 228Ra fromthe deep waters, which is justified given the very slowvertical mixing (Kz , 2.5 cm2 s21) in the deep inner lochbasin (Inall 2009).

There are several factors controlling the sedimentary Raflux in estuarine environments: sediment grain size, pore-water diffusive transport (porosity and partition coefficientKd), bioturbation mixing, sedimentation rate, and Raproduction and decay (Cochran and Krishnaswami 1980;Kadko et al. 1987). The new calculated sedimentary flux of2.1 6 0.2 (3 109) atoms m22 yr21 in the inner basin is , 25times lower than the average global sediment flux of 50 6 25(3 109) atoms m22 yr21 from the fine-grained shelf

Fig. 9. The activity ratios of 228Ra : 226Ra in the inner deepbasin on 08 June 2011. The dashed line is the initial 228Ra : 226Raon 03 August 2010, calculated as the average of the four deepestsamples and shown with 2 SE uncertainty. The gray line is thecalculated 228Ra : 226Ra ratio assuming radioactive decay from 03August 2010 to 08 June 2011. Note that the 90 m sample is wellexplained by this decay, while deeper samples have a higher228Ra : 226Ra value, implying addition of 228Ra from sediments onthe loch floor.

Fjord mixing based on radium isotopes 1099

sediments seen in previous studies (Santschi et al. 1979;Cochran 1984; Hancock et al. 2000) but higher than the fluxof 1.0 6 0.5 (3 109) atoms m22 yr21 from the coarse-grainedshelf sediments (Hancock et al. 2006), as summarized inMoore et al. (2008). Sediments on the continental shelfgenerally have higher 228Ra flux values, in the range 0.1 to110 (3 109) atoms m22 yr21, than those on the continentalslope, which are in the range 5 to 11 (3 109) atoms m22 yr21

(Hammond et al. 1990), or on the deep seafloor, which are inthe range 1.4 to 4.3 (3 109) atoms m22 yr21 (Cochran andKrishnaswami 1980).

The grain size in the upper 0.60 m of sediment of theLoch Etive inner basin floor has a mean grain size range of40–70 mm from coarse silt to very fine sand (Howe et al.2002), which could explain the lower sedimentary 228Raflux in Loch Etive compared to the average flux from thefine-grained shelf sediments. The sedimentation rate in theinner basin of Loch Etive is relatively high, , 0.89 cm yr21

(Ridgway and Price 1987), which may also restrict Rasediment fluxes. Hancock et al. (2000) suggested thatgroundwater flow is important to enhance sedimentary Rafluxes in the estuarine environments, but groundwater flowis insignificant in Loch Etive. The low sedimentary 228Raflux observed in this study is therefore perhaps notsurprising and is consistent with sedimentary fluxescalculated in other settings. It certainly does not indicatean unusually high 228Ra sedimentary flux and thereforesupports the assertion that 228Ra inventories in the openocean require significant input from global groundwatersources (Moore et al. 2008).

The new estimated sedimentary 228Ra flux in Loch Etiveimproves our understanding of the 228Ra inventory in theocean, which is important in constraining global freshwaterand groundwater discharges to the ocean. Moore et al.(2008) have estimated a substantial discharge of submarinegroundwater to the Atlantic, based on the 228Ra inventoryin the surface ocean. The estimated total SGD is 2–4 (31016) L yr21, which is about 0.8 to 1.6 times the total riverflux to the Atlantic. In their inventory, the referred totalsedimentary 228Ra flux (1.3 3 1023 atoms yr21) is high,about 68% of the total inferred SGD 228Ra flux (1.9 3 1023

atoms yr21). Assuming that the total sedimentary flux of228Ra is 25 times lower, based on the new estimate in thisstudy, the inferred SGD 228Ra flux would become 3.2 31023 atoms yr21 and the SGD would become 3.4 to 6.7 (31016) L yr21. Thus, SGD may become 1.3 to 2.7 times theriver flux to the Atlantic. This rough calculation does notprovide any actual estimate of SGD. Nevertheless, ithighlights the importance of an improved understandingof sedimentary 228Ra flux to the global freshwater andgroundwater discharges in the ocean.

In summary, this study has used a suite of water-columnmeasurements of the Ra quartet to deduce seawateradvection rates and sedimentary inputs of Ra in LochEtive. In the advection-only scenario, the observed decreaseof 223Ra with distance from the mouth of the loch at 5 mdepth is used to derive a maximum seawater advection rateinto the loch of 2.4 6 0.2 cm s21. In the mixing-onlyscenario, the short-lived Ra distribution could be explainedby horizontal mixing rates of 6.1 3 106 cm2 s21 (223Raex) or

9.1 3 106 cm2 s21 (224Raex). Addition of both 224Ra and228Ra to this subsurface layer is required within the loch.These cannot come only from the river water of RiverEtive, at the head of the loch, but are instead likely to comelargely from shelf sediments of the loch. Sedimentary inputof 228Ra is also indicated in deep waters of the inner loch,and a flux of 2.1 6 0.2 (3 109) atoms m22 yr21 iscalculated. This value falls at the low end of previousestimate of sedimentary 228Ra fluxes and therefore supportsthe assertion that open-ocean 228Ra inventories are notexplained by sedimentary inputs and require significantinput in groundwaters around the globe.

AcknowledgmentsWe thank the crew of R/V Calanus for sampling assistance.

Sabine Cockenpot, Tristan Horner, Robyn Tuerena, and XinyuanZheng are also acknowledged for their assistance in samplecollections. We thank John Howe for providing the bathymetryand sedimentary background of Loch Etive and Mark Inall fordiscussion of ocean mixing in Loch Etive. Willard Moore andDon Porcelli are also thanked for helpful discussion. AlexThomas, Andrew Mason, and Steve Wyatt are acknowledgedfor assistance with mass spectrometry and laboratory support.W.G. and P.vB. were supported by the British Council ‘Alliance’grant 09.014. W.G. was supported by the Scottish Alliance forGeosciences, Environment and Society (SAGES). The cabledmooring installation in Loch Etive was supported by EU FP7project HYPOX (grant 226213). The Swire Educational Trust isthanked for Hsieh’s scholarship at University College, Universityof Oxford. We are grateful to Bo Thamdrup, the associate editor,Michiel Rutgers van der Loeff, and an anonymous referee for veryhelpful reviews and constructive comments.

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Associate editor: Bo Thamdrup

Received: 30 August 2012Accepted: 12 February 2013Amended: 25 February 2013

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