Using Ra isotopes to examine transport processes controlling benthic fluxes into a shallow estuarine lagoon

Geochimica et Cosmochimica Acta, Vol. 64, No. 21, pp. 3685–3699, 2000 Copyright © 2000 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/00 $20.00 ⫹ .00

Pergamon

PII S0016-7037(00)00469-5

Using Ra isotopes to examine transport processes controlling benthic fluxes into a shallow estuarine lagoon G. J. HANCOCK,1,* I. T. WEBSTER,1 P. W. FORD,1 and W. S. MOORE2 1 CSIRO Land and Water, Canberra, ACT, Australia Department of Geological and Marine Sciences, University of South Carolina, Columbia, South Carolina USA

2

(Received May 24, 1999; accepted in revised form June 9, 2000)

Abstract—Measurements of the benthic flux of four naturally occurring radium isotopes in a shallow lagoon in the Bega River estuary has provided information on the types and rates of transport processes operating in the lagoon sediments. The measurement techniques included Ra mass budgets of the lagoon, Ra fluxes into benthic chambers, and modelling of the pore water and solid phase Ra profiles in a sediment core. The sediment profile of 210Pb, and the solid phase and pore-water profiles of the longer-lived Ra isotopes, 228Ra (half-life 5.7 years) and 226Ra (half-life 1600 years), indicate bioturbation to a depth of 10 cm. A diffusionbioturbation model has been used to assess the relative importance of molecular diffusion and bioturbation as transport processes controlling the benthic flux of Ra. The flux of the shortest-lived isotope, 224Ra (half-life 3.7 days), is not significantly enhanced by bioturbation, and its flux is consistent with diffusion-controlled release. However bioturbation enhances the 228Ra flux by a factor of more than two over the flux due to molecular diffusion alone. Modelled pore-water profiles and flux calculations are consistent with a bioturbation time scale between 0.5 and 2 years. The measured benthic flux of 226Ra is much greater than can be accounted for by the modelled profile, and may be due to slow 226Ra desorption from the sediment, variable sediment accumulation rates, or groundwater flow. Based on 226Ra pore-water and flux measurements at the time of this study, groundwater flow has an upper limit of 0.3 cm d⫺1. Copyright © 2000 Elsevier Science Ltd flow lagoons that are deposition zones for most of the finegrained sediment in the estuary. Since this fine-grained sediment carries most of the pollutant load of the river, biogeochemical processes operating in the lagoon sediments may have considerable influence on the mobilization, transport, and fates of these pollutants. Previous work in this estuary (Webster et al., 1994; Hancock and Murray, 1996) focussed on the main river channel of the estuary, which contains sediment made up mainly of coarse-grained sand. In these studies, the distribution of Ra in the main estuary channel was used to determine the extent of sediment-water column exchange occurring as a result of tidal pumping. In the lagoons the sediment is made up of fine-grained silt and clay minerals, and the fluxes of Ra isotopes from sediment into the lagoon water are expected to be controlled by different processes. The main aim of this work is to quantify the fluxes of Ra isotopes into an estuarine lagoon, and determine the rates and mechanisms of Ra release from bottom sediments. By using a range of methods to measure the Ra flux, and by examining the behavior of the four Ra isotopes with greatly differing halflives, we are able to close the Ra budgets of the lagoon, and gain information on the relative importance of some of the processes controlling benthic fluxes in the lagoon.

1. INTRODUCTION

Physical, chemical, and biological processes operating at the sediment-water interface have a profound effect on the cycling of elements between water and sediments (Sanstchi et al., 1990). In coastal waters and oceans, Ra isotopes have been used to provide important information on these processes, including diffusion (Kadko et al., 1987), pore-water and surface water exchanges (Bollinger and Moore, 1993; Webster et al., 1994), water transport rates (Torgensen et al., 1996; Turekian et al., 1996), and groundwater outflow (Rama and Moore, 1996). Four Ra isotopes occur in nature, with half-lives ranging from 3.7 days to 1600 years (Fig. 1). They are continually produced in sediments by the decay of insoluble Th parents. In freshwater, Ra is tightly bound to the sediment grains, but in saline water Ra bound to the surface of sediment grains readily undergoes ion exchange with other dissolved cations. This leads to nonconservative increases in dissolved Ra activities in estuaries and coastal waters (Li et al., 1977; Moore, 1992; Rama and Moore, 1996). The sources of this additional Ra include Ra desorbed from suspended riverine sediment, Ra generated and released from bottom sediments, and Ra advected into the estuary by groundwater. The importance of each of these sources depends on the physical, chemical, and biological processes operating in the estuary (Moore, 1992). In this paper we examine the behavior of Ra in a shallow lagoon in the Bega River estuary. This estuary is typical of many on the southeast coast of Australia, and contains back-

* Author to whom correspondence ([email protected]).

should

be

2. METHODS 2.1. Study Site Zecks Lagoon is part of the Bega River estuary, located on the south coast of New South Wales, Australia (Fig. 2). The estuary is shallow, with an average depth of 1.5 m. Zecks Lagoon, and the much larger Blackfellows Lagoon are in the mid-estuary region, about 6 km from the mouth of the estuary. Both these lagoons are tidally influenced, and are connected to the main estuary by narrow sinuous channels. Zecks

addressed 3685

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G. J. Hancock et al.

Fig. 1. Relevant radionuclides in the uranium and thorium three decay series, showing Ra isotopes and their parents.

Lagoon is ⬃1 m deep, covers an area of 400 m2, and is confined by steep hill slopes. The bottom sediments in the main river channel are mainly coarse-grained sand and gravel, but in the lagoons fine-grained silt and clay minerals dominate. Tidal heights in the nearby ocean were typically 1–1.5 m, but these were strongly damped in the estuary due to its constricted entrance.

2.2. Sample Collection Sampling occurred from 25 to 29 November 1996. The sampling sites are shown in Fig. 2. Salinities and water temperatures were measured in situ. The vertical tidal range was monitored at the entrance of the channel connecting Zecks Lagoon to the main estuary. Water samples were collected from two sites in the lagoon (sites 1 and 2), from one site in the channel connecting the lagoon to the main estuary (site 4), and from two sites in the main estuary channel (sites 5 and 6). The most intensive sampling occurred at the central lagoon site (site 1) and at the lagoon channel entrance (site 4). At these sites sampling occurred at regular intervals over one complete tidal cycle. Water samples were pumped through a 1 ␮m cartridge filter into 20 L polypropylene containers. A known volume of this water was then gravity fed through a column containing Mn-impregnated acrylic fiber (Mn fiber; Moore, 1976) to quantitatively remove Ra. A sediment core, 37 cm long, was collected from the center of the lagoon (site 2) using a 15 cm diameter PVC tube. This core was sectioned immediately after collection. The sections were put into bags, excess air extruded, and the bag refrigerated. Two benthic chambers (diameter 0.30 m, height 0.165 m, volume 11.5 L) were deployed for a period of 48 hours at sites 1 and 2. The benthic chambers were mixed by an internally mounted battery-operated bilge pump, driven at low speed. At the end of the deployment period, a 1.6 L water sample was slowly pumped from each benthic chamber. At the time of deployment, and immediately before recovery of the chambers, bottom water samples were collected in close proximity to the chambers.

2.3. Laboratory Analysis Measurements of short-lived Ra isotopes (223Ra and 224Ra) were made by partially drying the Mn-fiber, and placing it in an air circulation system, described by Moore and Arnold (1996). 219Rn and 220Rn, formed by the decay of 223Ra and 224Ra, were flushed from the Mn-fiber into a scintillation cell where alpha particles from the decay of Rn and its daughters were detected by a photomultiplier tube, and identified using a delayed coincidence system. After the completion of 223Ra and 224Ra measurements, Ra was leached from the Mn-fiber with HCl, and the longer-lived isotopes, 228 Ra and 226Ra, determined by alpha-particle spectrometry (Hancock and Martin, 1991). This method utilized a calibrated (⫾1%) yield tracer (225Ra in equilibrium with 229Th, supplied by Amersham), and involved radiochemical separation of Ra, electrodeposition of Ra onto a stainless steel disc, and alpha-particle spectrometry using silicon surface barrier detectors. This method was also used to determine the activities of all four Ra isotopes in benthic chamber and pore water samples. The agreement between measurements of 224Ra and 223Ra by delayed coincidence counting and alpha-particle spectrometry was checked using selected lagoon water samples, and calibrated 227Ac and 228 Th solutions in which 223Ra and 224Ra were in secular equilibrium. The techniques agreed within ⫾5%. The quantitative removal of Ra by Mn-fiber was checked and verified by the laboratory analysis of selected lagoon water samples that had been subsampled prior to treatment with Mn-fiber. Thorium isotopes (232Th, 230Th, and 228Th) were measured by alpha-particle spectrometry, using the method of Martin and Hancock (1992). Thorium was also measured in selected water samples, and all sediment core sections. The 227Ac activity of the sediment was determined from the 223Ra activity, measured using alpha-particle spectrometry more than 2 months after sample collection. Gamma spectrometry (Murray et al., 1987) was used to determine 226 Ra and 228Ra in sediment samples. X-ray fluorescence was used to determine iron and sulphur in dry sediment. Sediment pore water was obtained by centrifuging the wet sediment within 48 hours of collection. Between 150 and 300 mL of wet sediment was centrifuged to yield 80 –150 mL of pore water. To

Using Ra isotopes to examine benthic fluxes

3687

Fig. 2. Location of sample sites (●). minimize the effects of sediment oxidation during storage and handling, transferal and centrifuging was done under nitrogen, and only the central portion of the sediment mass was extracted and transferred to the centrifuge bottles. The supernatant pore water was filtered through a 0.45 ␮m membrane filter. Separate sediment samples were dried at 50°C, and the porosity determined from the weight loss. Desorption experiments (Webster et al., 1995) were used to determine the amount of sediment-bound Ra available for ion-exchange between sediment and pore water. Duplicate amounts of wet sediment, equivalent to 10 g dry weight, were equilibrated for 2 h with 800 mL seawater diluted to a salinity of 20 with distilled water. One duplicate was spiked with a known activity of 223Ra. After equilibration, the solution was centrifuged, the supernatant filtered through a 0.45 ␮m filter membrane, and the Ra activity of the filtrate measured. After correcting for the presence of natural 223Ra using the unspiked duplicate, the fraction of 223Ra tracer adsorbed by the sediment was determined from its loss from solution. This fraction was used in combination with the filtrate activities of 224Ra, 228Ra, and 226Ra to determine their ion-exchangeable activities. The exchangeable activity of 223Ra was inferred from the solution activity of 223Ra in the unspiked duplicate.

3. RESULTS AND DISCUSSION

3.1. Surface Water River and lagoon water data are presented in Table 1. The uncertainties in Ra isotope activities correspond to 1␴ counting errors. Water temperature varied between 20°C and 26°C. Salinity was most variable in the river channel, ranging between 8 –26. In the lagoon, the salinity range was much narrower (12–16). Suspended sediment concentrations in the water column ranged between 1 and 2 mg L⫺1, and the 228Th activity of the water contributed less than 0.2% to the total 224Ra activity. Thus 228Th support for surface water 224Ra activity is ignored. The shortest-lived Ra isotope, 224Ra, has the highest activities in surface water, and shows the greatest activity range. Estuary water, which nominally includes samples collected at the channel entrance during flood tide, shows a positive corre-

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G. J. Hancock et al.

Table 1. Ra isotope activities (mBq L⫺1) in surface water samples from the lagoon, river and connecting channel. Uncertainties are ⫾5% or less for 226Ra, 228Ra, and 224Ra, and ⫾10% or less for 223Ra. Site

Deptha

Salinity

Lagoon 1 1 1 1 1 1 2 1

Surf Surf Surf Mid Bot Surf Surf Surf

14.4 16.0 11.9 13.1 15.0 14.8 12.3 15.8

Fall Fall Fall Fall Fall Fall Fall Fall

95 93 83 83 83 76 76 59

cm cm cm cm cm cm cm cm

41.2 40.8 32.8 32.0 35.3 34.7 31.3 39.8

2.74 2.27 1.25 1.95 2.80 2.08 1.68 2.53

23.5 25.2 17.4 23.3 21.1 24.9 22.4 23.2

3.80 3.66 3.77 3.63 3.31 3.45 3.85 3.55

1 1 1 1 1 2 Channel 4 4 4 4 4

Surf Surf Surf Surf Surf Surf

13.5 15.9 12.3 16.2 13.2 14.0

Rise Rise Rise Rise Rise Rise

65 67 78 92 95 97

cm cm cm cm cm cm

35.0 39.6 33.5 41.5 39.8 39.8

1.92 1.83 2.07 1.98 2.00 2.39

23.2 24.2 24.5 24.3 23.8 23.8

3.63 3.83 3.77 4.08 3.40 3.50

Surf Surf Surf Surf Surf

14.3 15.1 15.0 12.9 15.1

Fall Fall Fall Fall Fall

96 89 77 68 59

cm cm cm cm cm

34.3 34.3 36.6 30.5 30.4

2.26 1.97 2.24 2.22 1.49

26.9 22.3 24.0 19.3 20.4

3.66 3.42 3.64 3.82 3.56

4 4 4 4 4 4 River 5 5 5 6

Surf Surf Surf Surf Bot Surf

9.9 12.0 16.7 24.0 25.7 20.7

Rise Rise Rise Rise Rise Rise

65 66 74 84 84 95

cm cm cm cm cm cm

23.3 27.1 34.5 42.5 38.4 51.9

1.02 1.27 2.03 2.20 2.27 2.42

17.5 19.0 24.4 26.9 24.2 29.8

3.32 3.63 3.88 3.72 3.46 4.68

Surf Surf Surf Surf

8.1 14.0 21.8 11.8

Fall 66 cm Rise 75 cm Rise 95 cm Rise 76 cm

17.2 31.2 44.0 27.8

0.97 1.48 2.17 1.08

14.0 21.6 22.6 18.9

3.20 3.26 3.75 3.26

a

224

Tide

Ra

223

Ra

228

Ra

226

Ra

Surf: surface water. Mid: mid-depth (⬃0.40 m). Bot: bottom (⬃0.70 m).

lation between 224Ra and salinity up to a salinity of about 20 (Fig. 3). At higher salinities 224Ra activities decrease. A similar relationship exists for the other isotopes, although the trend for 226 Ra is weak. In our previous study of the Bega Estuary (Webster et al., 1994), we noted significant long-estuary variations in radium activities. In particular, Ra activities were highest in mid-estuary. In the down-estuary direction, Ra ac-

tivities first increase as a result of increased desorption from sediments as the salinity increases, but as the mouth of the estuary is approached activities are reduced by dilution with low activity seawater. Variations in salinity and Ra activity at the entrance of Zecks Lagoon occur due to the back and forward motion of estuarine water with the tides. By contrast, the retention and mixing of water in Zecks Lagoon has apparently produced water characterized by its relatively constant salinity and Ra isotope activity. This is illustrated for 224Ra in Fig. 3. There is no significant difference between the Ra activity of water at the two lagoon sites, and the Ra content of water collected at the channel entrance during the falling tide was not significantly different from the mean of all lagoon water samples. 3.1.1. Flux of radium from the lagoon

Fig. 3. (䡩).

224

Ra in water from the river channel (●) and Zecks Lagoon

We first estimate the flux of Ra from the sediments using a mass budget of Zecks Lagoon. Exchange between the lagoon and the main channel of the estuary occurred through a narrow, shallow channel about 6 m wide, 1 m deep, and 75 m long. Flow through the channel occurred as a consequence of the rise and fall of the tide in the estuary. If the average water depth within the lagoon is h(t), and the area of the lagoon is A, then the water flow through the channel is given by

Using Ra isotopes to examine benthic fluxes

Fig. 4. Measured and fitted water levels in Zecks Lagoon. Also shown are the times of radium data collection on the rising and falling tides.

dh Q⫽A . dt

(1)

We assume that the activity of the Ra isotope within the lagoon, c L , is a function of time, t, but is uniform throughout the lagoon. The mass balance of Ra within the lagoon is described by d 共 Ahc L兲 ⫽ AF ⫺ ␭ Ahc L ⫹ Qc w dt

(2)

where F is the flux of Ra from the sediments into the water column and ␭ is the decay rate of the isotope. The last term on the right-hand side of this equation represents the mass of Ra exchanging between the lagoon and the estuary through the connecting channel. Thus, c w (t) is the activity of the Ra in the estuary when the tide is rising (Q ⬎ 0) and it is the activity within the lagoon (c w ⫽ c L ) when Q ⬍ 0. The relatively steep sides of the lagoon basin allows the assumption that the lagoon area is independent of water level, so Eqn. 2 can be reduced to d dh 共hc L兲 ⫽ F ⫺ ␭ hc L ⫹ c w . dt dt

(3)

We estimate F using the budget method by adjusting its value until the model predicts the measured activities within the lagoon. Application of the method requires estimation of dh/dt, c L and c w . Tidal heights at the entrance of the channel were damped by a factor of 3 and phase shifted from those predicted in the nearby ocean. To estimate the tides within the estuary, we assumed the tide to be comprised of two tidal constituents, the M 2 of period 12.4 h and the S 1 of period 24 h. The amplitudes and phases of the two constituents were estimated by leastsquares fitting to the measured water levels at the lagoon channel entrance. Figure 4 compares the fitted water levels to the measurements. Activities of Ra were obtained in the connecting channel (site 4) on six occasions on a rising tide and five occasions on a falling tide during the study (Fig. 5, Table 1). Figure 5 shows the measurements for 224Ra which exemplify the measure-

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Fig. 5. Measured and fitted 224Ra activities on the rising tide (●) and on the falling tide (䡩).

ments for the other three isotopes. Due to variations in Ra activities along the estuary and the incoming tide, the activity of all Ra isotopes in the connecting channel increased with water level. To estimate the Ra activity of water flowing into the lagoon for the model, we assume that the activity is linearly related to water level and obtain the regression coefficients using a least-squares-fitting procedure. Combining our fitted relationship between water level and Ra activities with our estimated time series of water levels within the estuary, we obtain a continuous time series of activities for the water flowing into the lagoon. The outflow activities are assumed to be representative of those within the lagoon during the three outflow periods for which we have measurements. Although the model predicts a small decrease (⬃1%) in 224Ra during each outflow period due to radioactive decay (compensated in part by the bottom flux), we neglect it. The average lagoon activity during the three outflow periods is taken to be the average of the five outflow measurements and is compared to the average model-predicted activity during the same three periods. We adjust the benthic Ra flux to obtain coincidence between the measured and predicted outflow activities (Table 2, column 2). The uncertainties correspond to one standard error. 3.2. Benthic Chambers The flux of Ra into benthic chambers is estimated by considering the change in the interior chamber water over the deployment period. The initial Ra activity of chamber water is estimated from the activity of exterior water collected at the time of deployment. The measured Ra activities of water extracted from the chamber are not identical to Ra activities within the chamber at the end of the deployment, as the sample’s contents are diluted with exterior water which enters the chamber while the sample is being withdrawn. Making the reasonable assumption that the chamber is well mixed during sample extraction, it can be shown that the actual activity of chamber water (c c ) is related to the external activity of lagoon water (c L ), the measured chamber activity at the conclusion of deployment (c m ), and the sample and chamber volumes (v and V, respectively) by Eqn. 4,

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G. J. Hancock et al. Table 2. Estimates of benthic Ra fluxes by various methods. Uncertainties correspond to ⫾1 SD. Flux from sediment (Bq m⫺2 d⫺1) Benthic chambers Lagoon budget

#1

#2

Diffusion only

Diffusion and bioturbationa

3.4 ⫾ 0.7 0.32 ⫾ 0.07 ⫺0.2 ⫾ 0.6 ⫺0.1 ⫾ 0.6

3.7 ⫾ 0.8 0.17 ⫾ 0.06 0.53 ⫾ 0.36 0.06 ⫾ 0.03

3.9 ⫾ 0.7 0.14 ⫾ 0.06 1.18 ⫾ 0.20 0.07 ⫾ 0.02

3.5 0.12 0.32 ⫾ 0.05b 2.5 ⫾ 0.5 (⫻10⫺3)b

4.9 ⫾ 0.3b 0.27 ⫾ 0.03b 0.87 ⫾ 0.07b 3.9 ⫾ 0.1 (⫻10⫺3)b

224

Ra Ra 228 Ra 226 Ra

223

a b

Sediment profile

Bioturbation time scale of ␶ ⫽ 0.5 y used. Uncertainties correspond to a flux range derived from the limiting values of ⌰ (0.021– 0.032).

c c ⫽ 关c m ⫺ c L兴

1 v ⫹ c L. V 共1 ⫺ e ⫺v/V兲

(4)

The initial and final (corrected) Ra activities of the benthic chamber water are given in Table 3. 3.2.1. Benthic flux calculations In the benthic chambers the activities of all Ra isotopes increased significantly over the 2 day deployment period, with 224 Ra and 223Ra showing the largest relative increase. The flux of Ra into the chambers is calculated assuming the flux (F) is constant over the time of deployment. For a chamber of height H, the variation in the chamber activity (c c ) of Ra is dc c F ⫽ ⫺ ␭ c c. dt H

(5)

With the boundary condition c c ⫽ c L,

t⫽0

the solution of Eqn. 5 is F ⫺ ␭ Hc c ⫽ e ⫺␭t F ⫺ ␭ Hc L

(6)

which may be rearranged to F⫽

H␭ 共c ⫺ c Le ⫺␭t兲. 1 ⫺ e ⫺␭t c

(7)

The calculated fluxes are shown in Table 2 (columns 3 and 4). The errors in the calculated fluxes are based on ⫾1 standard deviation in the Ra activity choosing the most unfavorable

combination of errors in the initial exterior activity and the measured interior activity. 3.3. Comparison of Direct Flux Measurements The two direct methods of Ra flux measurement (lagoon budget and benthic chambers) are relevant to just the sampling period (2–3 days), but reflect very different spatial scales. The lagoon budget method gives the average Ra flux across the basin, whereas benthic chambers measure flux over a small, localized area. Because the sensitivity of the lagoon budget method depends on the extent of changes in the Ra activity of input and output lagoon water, and the half-life of the isotope, the uncertainties of flux measurement of the longer-lived 226Ra and 228Ra by this method are relatively high. By contrast, flux estimates of short-lived 223Ra and 224Ra are well constrained, with relative uncertainties being around 20%. Benthic chambers have the advantages of a low effective volume/area ratio, and the ability to accumulate Ra activity, leading to large changes in the Ra activity of the entrained surface water, and greater sensitivity of flux estimates. This is reflected by a much lower relative error of flux estimation for 226 Ra and 228Ra. The two benthic chambers are not significantly different in their flux measurements of 224Ra, 223Ra, and 226 Ra, but the 228Ra flux estimates differ by about a factor of 2, and are significantly different at the 1␴ level. The two methods of direct flux measurement give good agreement for 224Ra, and benthic chamber measurements of the 226 Ra flux fall within the large uncertainties associated with the lagoon budget measurement. However, the lagoon-wide 223Ra flux is about double the benthic chamber estimates, and the lagoon-wide 228Ra flux, although consistent with benthic

Table 3. The Ra content of initial (exterior) and final benthic chamber water. Units are mBq L⫺1. The final activities have been corrected for dilution by exterior water during sampling. Chamber #1

224

Ra Ra 228 Ra 226 Ra 223

Chamber #2

Initial

Final

Initial

Final

39.8 ⫾ 2.0 2.00 ⫾ 0.15 23.8 ⫾ 1.4 3.40 ⫾ 0.13

64.2 ⫾ 6.8 3.67 ⫾ 0.52 30.2 ⫾ 2.0 4.14 ⫾ 0.31

39.0 ⫾ 1.8 2.45 ⫾ 0.23 23.0 ⫾ 1.1 3.50 ⫾ 0.11

68.3 ⫾ 5.7 3.79 ⫾ 0.43 38.1 ⫾ 2.3 4.41 ⫾ 0.25

Using Ra isotopes to examine benthic fluxes Table 4. Sediment core data. Radionuclide uncertainties are approximately ⫾5% except Depth (cm) 0–2 2–4 4–6.5 6.5–9 9–11.5 11.5–14 14–16.5 16.5–22 22–27 27–32 32–37

Porosity 0.86 0.85 0.79 0.76 0.78 0.80 0.79 0.81 0.80 0.80 0.76

227

Ac

4.1 4.0 4.4 4.7 4.0 3.4 4.4 3.8 4.2 3.7 4.8

232

Th

94.2 103.6 94.0 98.9 96.9 96.0 105.7 94.1 91.4 95.5 104.7

230

Th

76.7 74.1 71.8 72.4 66.1 71.8 81.3 73.5 73.4 67.3 70.8

227

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Ac (⫾12%). Units are in Bq kg⫺1 dry wt. 228

Th

64.3 59.0 58.4 59.5 60.9 74.6 85.2 88.1 95.4 91.1 105.7

228

Ra

70.0 50.0 53.5 57.2 61.0 69.7 82.9 83.7 96.9 94.0 104.8

226

Ra

42.0 36.4 38.1 39.0 38.0 38.0 41.9 37.1 37.2 37.9 42.2

Solid phase sediment core data are summarized in Table 4. The sediment is fine-grained, made up mainly of silt and clay minerals. Burrowing worms were evident in the upper 9 cm. The porosity was highest in the upper 4 cm (0.86), and relatively constant below this depth (0.79). The sediment dry weight activities of 232Th, 230Th, and 227Ac are approximately constant with depth, indicating constant particle size and composition. The excess 210Pb activity (210Pbex) of the sediment, given by the difference between the 210Pb and 226Ra activities, is constant to a depth of about 9 cm (Fig. 6). Below, the reduction in 210 Pbex activity is consistent with an exponential decrease with

depth. Such a profile indicates that sediment mixing is occurring in the upper 9 cm on a time scale which is short compared with the 210Pb half-life (22 y). This mixing depth is close to the world-wide mean mixing depth for marine sediment (9.8 cm; Boudreau, 1998). The rate of decrease in 210Pbex below 9 cm, in conjunction with 137Cs data, has been used to determine the sedimentation rate in Zecks Lagoon (0.34 cm y⫺1; Hancock, 2000). Below a depth of 2 cm, the sediment activity of the longestlived isotope, 226Ra (half-life 1600 y), is constant, and shows constant deficiency with respect to its parent, 230Th. This is illustrated by the 226Ra/230Th activity ratio (AR) (Fig. 7), which remains close to value of about 0.54. The sediment activity of the next longest-lived isotope, 228Ra (half-life 5.7 y) is also deficient with respect to its parent 232Th, with the 228 Ra/232Th AR lying close to the 226Ra/230Th AR in the 2–9 cm depth interval (Fig. 7). However, below 9 cm the 228Ra activity increases until the 228Ra/232Th reaches a value consis-

Fig. 6. Log-linear plot of excess 210Pb(210Pbex) and depth. The Pbex profile is consistent with mixing in the upper 9 cm, and an exponential decrease below.

Fig. 7. Bottom sediment depth profile of Ra/Th activity ratios.

chamber #1, is significantly lower than benthic chamber #2. Some of these differences may reflect the greater variability of “spot” measurements, compared to lagoon-wide estimates. 3.4. Sediment Profile

210

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G. J. Hancock et al. Table 5. Ra pore-water activities (mBq L⫺1). Uncertainties are less than ⫾10% except Depth (cm) 0–2 2–4 4–6.5 6.5–9 9–11.5 11.5–14 14–16.5 16.5–22 22–27 27–32 32–37

224

Ra

251 248 205 199 207 567 614 683 602 575 582

223

Ra

15.2 10.7 10.0 14.3 9.8 19.9 20.3 21.1 17.2 16.3 16.2

tent with unity at 25 cm, indicating production-decay equilibrium. The activity of the daughter nuclide, 228Th (half-life 1.9 y), is close to radioactive equilibrium with its parent, 228Ra, throughout the core (Table 4). The sediment 228Ra activity is enhanced by about 40% in the 0 –2 cm section relative to sediment immediately below. Other studies have noted high surficial sediment activities of 226Ra (Kadko et al., 1987) and 228Ra (Hammond et al., 1990) in ocean sediment. The likely explanation for the excess Ra is that the surficial sediments are enriched in Mn oxides which scavenge Ra from pore water as it diffuses upward (Hammond et al., 1990). An estimate of the extent of 228Ra scavenging in Zecks lagoon sediment can be calculated by considering the inventory of “scavenged” 228Ra in the upper 9 cm of mixed sediment. This inventory is given by the enhancement of 228Ra activity above a value of 51 Bq kg⫺1, an activity corresponding to the activity of 228Ra in internal lattice sites of sediment grains (see below). Assuming steady state with respect to 228Ra scavenging and decay in the mixed layer, the scavenging rate is found by multiplying the enhanced 228Ra inventory (118 Bq m⫺2) by the 228Ra decay constant (0.12 y⫺1). This rate of scavenging (0.04 Bq m⫺2 d⫺1) is only about 5% of the observed 228Ra benthic flux (data presented later in this paper). We conclude that scavenging of Ra by surficial MnO2 plays only a minor role in controlling the benthic Ra flux, and does not significantly affect transport of Ra in the sediment profile. Ra pore-water profiles are shown in Table 5. The activities of all isotopes are higher than surface water activities. In the 2–11 cm depth interval the activity of each isotope is approximately constant, but below 11 cm the average activity of all isotopes increase by factors of between 1.5 and 5. In the upper 2 cm the pore-water activity of 226Ra and 228Ra is 20 –30% higher than pore water immediately below it. Overall, the trends in porewater 228Ra and 224Ra correlate well with sediment 228Ra and 228 Th. Uniform deficiencies of 228Ra and its daughter 228Th in the upper layer of marine sediment have been observed elsewhere (e.g., Carpenter et al., 1984; Hammond et al., 1990), and have been attributed to enhanced transport of 228Ra across the sediment-water interface due to bioturbation and bioirrigation. Given the observed presence of burrowing worms and the 210 Pbex profile, biological activity is likely to be directly responsible for the sediment deficiency of 228Ra, and low porewater activities of 226Ra and 228Ra in the upper 10 cm of sediment, by enhancing their transport across the sediment-

228

Ra

109 83 66 70 131 306 500 408 395 500 421

226

Ra

14.5 10.1 8.3 10.1 8.7 20.0 26.8 24.4 23.8 25.9 29.5

223

Ra (⫾15%). 224

Ra/228Ra 2.30 3.00 3.11 2.84 1.58 1.85 1.23 1.67 1.52 1.15 1.38

water interface. Biological activity is also likely to be indirectly responsible for the low 224Ra pore-water activities in the upper 10 cm via the parent– daughter relationship 228Ra 3 228Th 3 224 Ra; i.e., loss of 228Ra leading to lower sediment activities of its daughter 228Th, and hence a reduced production capacity for 224 Ra. The low pore-water activity of 223Ra in the upper 10 cm of sediment is also likely to be related to biological activity, although this does not appear to be due to loss of parent activity; there is no evidence for the extent of change in the 227 Ac sediment activity required to bring about a 30 – 40% change in the 223Ra pore-water activity. Mechanisms which could lead to reduced pore-water activity include adsorption of 223 Ra by a surficial MnO2 layer, and extensive and rapid flushing of 223Ra from the upper 10 cm by bioturbation and bioirrigation. Based on the 228Ra uptake analysis (above), uptake of 223Ra by a surficial MnO2 layer is an insignificant removal mechanism, and results presented later in this paper show that neither the measured 223Ra benthic flux or the model-derived time scale of bioturbation can explain the 223Ra pore-water profile. An alternative explanation is that the pore-water activity of 223 Ra in deep sediment is elevated relative to the upper 10 cm due to an increase in the rate of release of 223Ra produced in deep sediment into pore water. This increase could arise from the spatial redistribution of 227Ac activity in the individual sediment grains of deep sediment. Alpha recoil can preferentially locate decay products onto the surface of wet sediment grains (Fleischer, 1982). Since 227Ac is formed by the alpha decay of 231Pa, water-saturated sediment grains in the lagoon might have a significantly higher activity of surface-bound 227 Ac compared to soil grains delivered to the lagoon from recently eroded dry hill-slopes. Evidence for relocation of parent activity is shown by the 224Ra/228Ra AR of pore water. Despite virtual equilibrium of the 232Th series in sediment below 22 cm, the 224Ra/228Ra AR is significantly greater than 1, having a mean of about 1.5 (Table 2). This observation is consistent with 228Th activity being preferentially sited on the surfaces of sediment grains, compared to 232Th. Relocation of 227 Ac activity would be significant over a time scale equal to or longer than the 227Ac half-life (22 y). Based on a sediment accumulation rate of 0.34 cm y⫺1, the age difference between young sediment in the mixed bioturbated layer and older sediment below could easily amount to 20 years or more.

Using Ra isotopes to examine benthic fluxes

3.4.1. The production of radium in sediments The flux of Ra from bottom sediments into the water column, and the shape of the pore-water Ra profile depend on the interplay between the rate of production of exchangeable Ra by Th isotopes and the processes that can remove Ra (Cochran 1980; Kadko et al., 1987). Exchangeable Ra is defined as Ra dissolved in pore water and Ra adsorbed to the surface of sediment grains, this adsorbed fraction being available for desorption into pore water by ion-exchange processes. Exchangeable Ra is produced by the decay of sediment-bound Th and Ac parent isotopes. These parents are either bound to the surface of the sediment grains directly producing surfacebound Ra, or they are within the mineral structure of the sediment grain, and produce exchangeable Ra by their decay and recoil ejection of the resultant Ra daughter into pore water. The ejected Ra atoms are then partitioned between the sediment surface and pore water by adsorption processes. To determine the production rate of exchangeable Ra we need to estimate the amount of sediment-bound Th producing this exchangeable Ra. For the shorter-lived isotopes, 224Ra and 223Ra, we use the results of our desorption experiments to determine the activity of exchangeable Ra in the sediment, and hence infer their rate of production by their parents, 228Th and 227Ac, (Webster et al., 1994; 1995). The desorption experiments were carried out 60 days after the collection of the sediment core, which is more than 5 half-lives of 224Ra and 223Ra. Consequently, production/ decay equilibrium will have been established for these isotopes, and their rate of production per unit volume of dry sediment (␥) is determined from

␥⫽

␭ce 1⫺␸

3693

throughout the sediment grains. Assuming this is the case, the production of exchangeable 226Ra and 228Ra is estimated from

␥ ⫽ f ␭ a Th␳

where f is the fraction of Ra produced by the sediment available for ion-exchange (0.48), a Th is the total Th activity of the sediment, and ␳ is the sediment dry density. Because ␥ is based on the volume occupied by dry sediment grains, it is unaffected by sediment porosity. This fact, together with the relatively constant activities of 232Th and 230Th through the sediment column, means that ␥ for 228Ra and 226Ra can be assumed constant with depth. A value corresponding to the mean of all sediment profile measurements is used for a Th in Eqn. 9 (97.7 Bq kg⫺1 for 232Th and 72.7 Bq kg⫺1 and 230Th). For 224Ra and 223 Ra, ␥ is assumed constant in the 10 cm upper mixed layer, where their parent activities are approximately constant. 3.4.2. The desorption function To model diffusion of Ra from bottom sediments we use a desorption function (⌰) to describe ion-exchange equilibrium of Ra between pore water and sediment, such that ⌰⫽

cp ce

(10)

where c p is the pore-water activity of radium (Webster et al., 1994; 1995). The desorption function is related to the commonly used distribution coefficient (K d , defined as the ratio of the surface-bound activity of Ra per unit mass of dry sediment to the dissolved activity of Ra in water) by the expression

(8)

where ␭ is the Ra decay constant, ␸ is the sediment porosity, and c e is the activity of exchangeable radium per unit volume of saturated sediment, which includes Ra sorbed to the surfaces of the sediment grains, as well as Ra within the pore water. For the longer-lived 228Ra and 226Ra, production/decay equilibrium is not established in the upper sediment profile (Fig. 7), and we infer their production rate from the excess Th parent activity of the sediment above that of Ra, after all the exchangeable activity of the Ra has been removed. Bottom sediments in Zecks Lagoon have an average 226Ra/230Th AR of 0.54, indicating 46% of the 226Ra in the sediment has been desorbed. In addition, our desorption experiments show that bottom sediments contain a residual 1.3 Bq kg⫺1 of desorbable 226Ra, an activity corresponding to about 2% of the total 230Th activity of the sediment. Thus the total desorbable (exchangeable) 226Ra activity amounts to 48% of the 230Th activity of the sediment, and the fraction of exchangeable 226Ra produced by 230Th decay is estimated to be 0.48. This fraction is close to the corresponding value used by Rama and Moore (1996) for fluvial sediments (0.40), but is just outside than the range of values used by Kadko et al. (1987) to estimate the release of Ra from deep sea sediments (0.5– 0.7). Our lower value probably reflects the fluvial origin of Zecks Lagoon sediment, with a higher fraction of Th being locked inside crystalline minerals. Apart from the upper 2 cm of sediment, the 228Ra/232Th AR in the upper mixed layer of sediment is similar to the 226Ra/ 230 Th AR, suggesting 230Th and 232Th are similarly distributed

(9)

⌰⫽

冋

Kd ␳s ⫹␸ ␳w

册

⫺1

where ␳ s is the in situ wet density of the sediment, ␳ w is the density of water and ␸ is sediment porosity. From our desorption experiments using surficial (0 –2 cm) sediment, ⌰ was found to be similar for all Ra isotopes, with a mean value of 0.032 ⫾ 0.003 (Table 6). In deeper sediment (14 –16.5 cm), the mean value of ⌰ was 0.042. This deep value of ⌰ was checked by examining the 228Ra activity profile in Table 2. The difference between the total activity of sediment-bound 228Ra (a t ) and the activity in internal lattice sites (a i ) must represent exchangeable 228Ra bound to the sediment, i.e., a e ⫽ a t ⫺ a i. 228

The internal activity of Ra is estimated by assuming equilibrium with the internal activity of its parent 232Th, i.e., a i ⫽ (1 ⫺ f )a Th, where (1 ⫺ f ) represents the internal fraction of 232 Th (0.52). Thus c e is estimated from the relationship c e ⫽ a e␳ d共1 ⫺ ␸ 兲 where ␳ d is the dry sediment density. Because there is not much difference between a t and a i in the upper 10 cm of the sediment profile, the relative uncertainty associated with c e (and ⌰ using Eqn. 10) in this depth region is high. The best estimates of ⌰ are inferred from sediment deeper than 14 cm. In this region ⌰ has a mean of 0.021 ⫾ 0.003, approximately half the experimentally determined values (Table 6). A possible explanation

3694

G. J. Hancock et al. Table 6. Values of the desorption function (⌰) and production (␥) determined for lagoon sediment.

␥ (Bq m⫺3 d⫺1 dry sediment) Depth (cm) Mixed layer 0–2 Deep sediment 14–16.5 14–37

224

Ra

11 200a

223

Ra

220a

226

Ra

⌰ 228

224

Ra

0.10c

38c

0.10c

38c

223

Ra

Ra

226

Ra

228

Ra

0.032a

0.031a

0.034a

0.028a

0.048b

0.040b

0.039b

0.050b 0.021d

a

Derived from desorption experiments using 0 –2 cm sediment. Derived from desorption experiments using 14 –16.5 cm sediment. These values were rejected in favor of the in situ 228Ra value.d c ␥ is derived from Eqn. 9, with f ⫽ 0.48, and is assumed constant with depth. d Derived from in situ sediment activities of 228Ra. The value given corresponds to a mean of ⌰ estimates for all sediment sections below 14 cm. b

for this difference may lie with the oxidation state of the sediment. Figure 8 shows that the S content of the sediment increases to a depth of 25 cm, the rate of increase accelerating below 9 cm. Fe is relatively constant. Such a relationship is consistent with the formation of FeS2 in the sediment by the reduction of dissolved SO4 in pore water, and indicates that the sediment has become increasingly reducing with depth. Because our desorption experiments result in the aeration of the sediment, we postulate that ⌰ derived from the 228Ra profile (0.021) better represents the in situ desorption properties of Ra in deeper anaerobic sediment. In the next section of this paper we formulate a diffusion model, and the lower value of ⌰ is used in this model to represent the adsorption/desorption properties of sediment below 10 cm. The surface 2 cm of sediment is certainly oxidized to some extent, and our value of ⌰ determined by aerobic desorption experiments (0.032) is prob-

ably a good representation of this sediment. In the 2–10 cm depth region, where the redox conditions may vary, we assume ⌰ lies between the two limiting values determined under anaerobic and aerobic conditions (0.021– 0.032). 3.4.3. Diffusion model In modelling the distribution of exchangeable Ra in the sediment column, we assume that exchangeable Ra produced by sediment grains is in continual ion-exchange equilibrium with dissolved Ra in pore water, and that diffusion and bioturbation control the vertical transport of Ra. The vertical distribution of exchangeable Ra can be described by dc e d ⫽ dt dz

冉

␸Ds

冊

dc p dc e dB ⫹w ⫹ ⫺ ␭ c e ⫹ ␥ 共1 ⫺ ␸ 兲 dz dz dz (11)

where z is the vertical coordinate (positive upwards), D s is the diffusivity of Ra within the sediment, and ␥ is the production rate of Ra per unit volume of dry sediment. In this expression, the first term on the right-hand side represents the contribution of diffusion of Ra through the pores on the exchangeable Ra profile. The diffusivity, D s , is lower than the value in free water, and is estimated by dividing the free-water value (6.9 ⫻ 10⫺5 m2 d⫺1; Li and Gregory, 1974) by the tortuosity squared (␪2). The tortuosity was estimated from the porosity using (Boudreau, 1996)

␪ 2 ⬇ 1 ⫺ 2 ln 共 ␸ 兲.

(12)

The second term on the right-hand side of Eqn. 11 describes the effects of sediment accumulation at a constant rate w. The coordinate system is such that the sediment surface is always z ⫽ 0. The third term represents transport of exchangeable radium by bioturbation which we will describe later. The fourth and fifth terms on the right hand side of Eqn. 11 represent decay and production of exchangeable Ra per unit volume of wet sediment. By substituting the desorption function, ⌰, (Eqn. 10) into Eqn. 11, we obtain an equation for the steady-state, porewater profile within the sediment, Fig. 8. Bottom sediment depth profile of sulphur (●) and iron (E).

d dz

冉

␸Ds

冊

dc p w dc p dB ␭ ⫹ ⫹ ⫺ cp ⫹ ␥共1 ⫺ ␸兲 ⫽ 0. dz ⌰ dz dz ⌰

(13)

Using Ra isotopes to examine benthic fluxes

3695

For solution, Eqn. 13 requires two boundary conditions. At the sediment surface, we specify that the pore-water activity matches that in the water column; that is, c p ⫽ c w at z ⫽ 0

(14)

and with increasing depth in the sediment we assume that dc p /dz 3 0, or equivalently that cp 3

⌰ ␥ 共1 ⫺ ␸ 兲 as z 3 ⫺⬁. ␭

(15)

To assess the relative importance of diffusion and bioturbation in controlling Ra fluxes, we initially assume bioturbation is negligible, and calculate theoretical diffusive fluxes. Setting B ⫽ 0, and assuming constant ⌰, ␸, and ␥, the solution to Eqn. 13 is expressed analytically as

冉

cp ⫽ cw ⫺

⌰ ␥ 共1 ⫺ ␸ 兲 ␭

冊

exp

z ⌰ ␥ 共1 ⫺ ␸ 兲 ⫹ ␰ ␭

(16)

Fig. 9. Measured 228Ra activities compared to theoretical profiles obtained assuming molecular diffusive transport to sediment surface, and a surface layer of enhanced mixing.

where

␰⫽

2⌰ ␸ D s

⫺w ⫹

(17)

冑w 2 ⫹ 4⌰ ␸ D s␭

is a length scale for Ra activity variation within the sediment. This can be interpreted as the depth over which a concentration drawdown can be expected as a result of diffusion. The diffusive flux across the sediment surface into the water column is F⫽⫺

冉

冊

␸Ds ⌰ ␥ 共1 ⫺ ␸ 兲 cw ⫺ . ␰ ␭

(18)

3.4.4. Application We calculate ␰ for each Ra isotope from the core data. The average porosity over the 37 cm core is measured to be 0.80; a nominal value of ⌰ is 0.030, and w (from 210Pb and 137Cs data) is 0.34 ⫾ 0.06 cm y⫺1. Taking D s to be 4.8 ⫻ 10⫺5 m2 d⫺1, we calculate representative length scales of 0.0025, 0.0044, 0.074, and 8.0 m, respectively for 224Ra, 223Ra, 228Ra, and 226 Ra. If sedimentation is neglected (w ⫽ 0), there is negligible effect on the calculated length scales for 223Ra and 224Ra, but the length scale for 228Ra reduces by 22% and that for 226 Ra, the longest-lived isotope, reduces by a factor of 8. Table 2 shows the theoretical “Diffusion only” fluxes of the Ra isotopes calculated using Eqn. 18. For this calculation, we assume that for 223Ra, 224Ra, which have decay length scales of less than 1 cm, ⌰ ⫽ 0.032, and ␸ ⫽ 0.86, values measured for the surficial sediment. For 228Ra and 226Ra we use a porosity corresponding to the average porosity of the core (␸ ⫽ 0.82). Due to the potentially variable nature of ⌰ in the upper 10 cm, we constrain the 228Ra and 226Ra fluxes within a range determined by using values of ⌰ pertinent to both reducing and oxidizing conditions (⌰ ⫽ 0.021 and 0.032). The value given in Table 2 corresponds to a mid-point of this range, with the range expressed as the limits of the associated uncertainties. 3.4.5. Pore-water profiles Because ␰ is much less than the thickness of the uppermost sediment section used to determine the pore-water profile (2

cm), the effects of diffusion on the pore-water activities of 224 Ra and 223Ra are not seen. The much longer-lived 228Ra and 226 Ra have much larger values of ␰, and the pore-water profiles of these isotopes are expected to show variations reflecting diffusion, as well as other transport processes occurring on a time scale equal to or shorter than their half-lives. Figure 9 compares the measured profile of 228Ra (black circles) with that calculated using Eqn. 16 (solid line). Average values of porosity and the desorption factor are used (␸ ⫽ 0.82, ⌰ ⫽ 0.026), and the Ra water column activity is assumed to be that measured during our study. Figure 9 shows that above 10 cm the measured profile of 228Ra is markedly deficient compared to the theoretical diffusive profile, but below this depth the profiles are similar. This deficiency is consistent with our proposal that transport of dissolved 228Ra is enhanced near the sediment surface by biological activity. 3.4.6. Bioturbation We adapt the model to include bioturbation in a surficial sediment layer (Fig. 10). The bioturbation model is nonlocal mixing (Boudreau and Imboden, 1987) that describes mixing caused by infaunal activities that transport material between widely separated points. Boudreau and Imboden suggest that animals such as polychaetes, shrimp, and bivalves can move material on a scale comparable to the thickness of the bioactive zone. In this layer of thickness ⌬, we specify that dB ⫽ 共c s ⫺ c៮ s兲/ ␶ dz

(19)

where c s is the activity (concentration) of radium adsorbed to the sediment grains, c៮ s is the average activity of adsorbed radium between z ⫽ ⫺⌬, 0; ␶ is a sediment mixing time scale. Here the bioturbation flux, B (units of Bq m⫺2 d⫺1), describes transport of Ra adsorbed to sediment within the surficial layer due to the feeding, excremental, and burrowing activities of macrobenthos. Beneath this layer, and at the sediment surface, B ⫽ 0. Transport of Ra across the sediment-water boundary is assumed to be controlled by molecular diffusion only, and

3696

G. J. Hancock et al.

Fig. 10. A schematic of the sediment transport model. Vertical arrows indicate transport of dissolved Ra by molecular diffusion, and curved arrows indicate Ra transport in association with sediment particles by bioturbation.

advective irrigational exchange of pore water and surface water is not considered. The particular formulation for bioturbation, Eqn. 19, is described by Boudreau (1997) as the case when each layer in the mixed layer exchanges equally with every other layer (Boudreau, 1997, Eqn. 3.123). If we take c s to be the bulk activity of Ra adsorbed to the sediment, then c s , c p , and c e are related by c e ⫽ ␸ c p ⫹ c s . With Eqn. 10, we obtain cs ⫽ ␺cp

(20)

⫺1

where ␺ ⫽ (1 ⫺ ⌰␸)⌰

. Equations 19 and 20 give

dB ⫽ ␺ 共c p ⫺ c៮ p兲/ ␶ dz

(21)

where c៮ p is the average pore-water activity of the bioturbated layer. We obtain numerical solutions to Eqn. 13 with dB/dz specified by Eqn. 21 for z ⬎ ⫺⌬ and dB/dz ⫽ 0 for z ⫽ 0 and z ⱕ ⫺⌬ where ⌬ ⫽ 10 cm. The boundary conditions are that the pore water at the sediment surface equals the measured water column activity as in Eqn. 12, and that the pore-water activity equals the activity calculated using the analytical solution (Eqn. 16) at a depth of at least 1 m within the sediment. The equations were represented in finite-difference form and solved as a time-dependant system to steady state using a standard forward-time, centered-space solution method (Roache, 1982). Figure 9 also shows the theoretical 228Ra profiles with bioturbation computed with ␶ ⫽ 0.5 y and ␶ ⫽ 2 y. These profiles assume average values of ⌰ and ␸ (0.026 and 0.82), and w ⫽ 0.34 cm y⫺1. Although there is a likely variation in the value of ⌰ in the bioturbated layer from the value used, this variation causes less than a 10% change in the profile shown. The profile computed with ␶ ⫽ 0.5 y predicts about the correct pore-water activity in the top 10 cm of the sediment column; with ␶ ⫽ 2 y, the surficial activity is predicted to be about 50% higher than what is measured. Below 10 cm depth, the two predictions are almost identical. For the case of ␶ ⫽ 0.5 y, the predicted surface flux (using the two limits of ⌰) is in the range F ⫽ 0.80 ⫺ 0.94 Bq m⫺2 d⫺1. This flux range is more than twice the flux

Fig. 11. Measured 226Ra activities in pore water, compared to theoretical profiles with three rates of sediment accumulation (w).

range calculated assuming that molecular diffusion is the dominant transport mechanism (F ⫽ 0.28–0.37 Bq m⫺2 d⫺1), and shows the extent to which mixing has enhanced the benthic flux of 228Ra. Hammond et al. (1990) took a somewhat simpler approach to the estimation of 228Ra flux from marine sediments by integrating the deficiency of 228Ra relative to its 232Th parent over the depth of the sediment column. The apparent flux was determined from ␭ D, where D was the integrated 228Ra deficiency. However, this scheme does not include the effects of sediment accumulation. Our bioturbation model includes a sediment accumulation term, and we show in Appendix A that this term reduces the apparent 228Ra flux determined by direct integration of the sediment profile by 22%, from 1.05 ⫾ 0.09 Bq m⫺2 d⫺1 to 0.82 ⫾ 0.10 Bq m⫺2 d⫺1. Theoretical and measured pore-water activities of 226Ra are shown in Fig. 10. The theoretical values are derived as for 228 Ra, by utilizing the diffusion model in conjunction with a 10 cm bioturbation zone (␶ ⫽ 0.5 y). Figure 11 shows that below 10 cm the theoretical 226Ra activities determined for w ⫽ 0.34 cm y⫺1 are much too low through the sediment column by a factor which is mostly greater than 2. Explanations for this discrepancy are discussed in the next section. 3.5. Processes Influencing the Benthic Ra Flux 3.5.1.

224

Ra

The agreement between the methods of direct 224Ra flux measurement and the flux calculated assuming diffusion is the only transport process indicates that molecular diffusion is the most important process influencing the benthic flux of 224Ra. For this isotope, the benthic flux increases from 3.5 to 5.2 Bq m⫺2 d⫺1 when bioturbation is invoked with ␶ ⫽ 0.5 y. With bioturbation, the predicted flux falls outside the range of measured fluxes and suggests that the bioturbation rate estimated from the 228Ra profile may be too high. Increasing the bioturbation time scale to ␶ ⫽ 2 y reduces the calculated benthic flux to 4.3 Bq m⫺2 d⫺1, which is closer to the measurements. The difference between the 224Ra and 228Ra estimates of ␶ could

Using Ra isotopes to examine benthic fluxes

reflect the different half-lives of the two isotopes, and seasonal variations in the rate of bioturbation. The 224Ra estimate of ␶ represents bioturbation at the time of sample collection, whereas the 228Ra estimate is derived from 228Ra activities of the sediment profile, which retains a memory of previous rates of bioturbation over a much longer period (months to years). 3.5.2.

223

Ra

From Eqn. 18 the 223Ra flux obtained without bioturbation is F ⫽ 0.12 Bq m⫺2 d⫺1. This is similar to the fluxes measured by the two benthic chambers, but less than half the flux calculated from the lagoon budget. Including bioturbation increases the estimated flux to 0.24 Bq m⫺2 d⫺1 with ␶ ⫽ 2 y, and 0.30 Bq m⫺2 d⫺1 with ␶ ⫽ 0.5 y. These fluxes fall between the direct flux measurements made by benthic chambers and the lagoon budget. Because of the wide flux range spanned by the direct 223Ra measurements, the influence of bioturbation on the 223 Ra flux is not as easily discerned as for 224Ra, with our analysis suggesting that bioturbation in the central part of the lagoon has increased the 223Ra flux by a factor of between 1 and 2.5. Although bioturbation may have significantly enhanced the benthic flux of 223Ra relative to molecular diffusion, it cannot fully explain the low 223Ra pore-water activities observed in the bioturbated zone, relative to pore water below. Assuming constant production of 223Ra by 227Ac throughout the sediment profile, and average values of 0.80 and 0.026 for porosity and ⌰ in the upper 10 cm of sediment, a benthic flux of 1.0 Bq m⫺2 d⫺1 is required to reduce the exchangeable inventory (and hence the pore-water activity) of 223Ra in the bioturbated layer by the observed amount (30 – 40%). This is more than 3 times the highest measured flux (0.32 Bq m⫺2 d⫺1 given by the lagoon budget). Our model indicates that a bioturbation time scale of days–weeks is necessary to produce the required flux, a time scale not consistent with 228Ra and 224Ra fluxes. We conclude that increased production of 223Ra in deep sediment resulting from redistribution of 227Ac induced by alpha-recoil, described earlier in this paper, is the most likely mechanism leading to low 223Ra pore-water activities in the upper 10 cm of sediment. 3.5.3.

228

Ra

The diffusion model with bioturbation gives a flux 228Ra of 0.87 Bq m⫺2 d⫺1, compared to benthic chamber measurements of 0.53 ⫾ 0.36 and 1.18 ⫾ 0.20 Bq m⫺2 d⫺1. These values are probably not inconsistent with one another. Given that the sediment profile reflects the 228Ra flux averaged over a much longer time period than benthic chamber measurements, its value is probably a good representation of the spatially and temporally averaged flux of 228Ra in the central part of the lagoon. The general agreement between the modelled and measured 228Ra pore-water profiles shows that the benthic flux of 228 Ra is strongly influenced by bioturbation, which is likely to be less spatially and temporally homogeneous of a process than molecular diffusion. The lower lagoon budget flux estimate (⫺0.2 ⫾ 0.6 Bq m⫺2 d⫺1) suggests lower 228Ra fluxes, and possibly bioturbation, in other regions of the lagoon.

3.5.4.

3697

226

Ra

The pore-water activities and measured benthic flux of 226Ra are much higher than the theoretical values derived using the combined diffusion-bioturbation model; the average benthic chamber 226Ra flux measurement (0.07 Bq m⫺2 d⫺1) is 16 times higher than the theoretical bioturbation-enhanced flux (3.9 ⫻ 10⫺3 Bq m⫺2 d⫺1). The high measured value could have several possible explanations, two of which involve sedimentation processes. The first, which involves slow desorption of 226Ra from recently deposited freshwater sediment, has been used to explain 226Ra anomalies in Winyah Bay (Elsinger and Moore, 1980). Most of the fine-grained sediment delivered to Zecks Lagoon is likely to have been delivered during periods of high river flow and low salinity. Because this sediment has been transported directly from the freshwater region of the Bega River catchment, it will, at the time of deposition in the lagoon, contain a large complement of exchangeable Ra. Burial by biological activity would expose the newly deposited sediment to different chemical conditions, such as increased salinity and redox changes. Slow release of 226Ra by gradual changes in the salinity of pore water, and/or by diagenesis of crystalline minerals due to increasingly anaerobic conditions at depth would provide an additional source of exchangeable 226 Ra. The rate of release could be quite variable, depending on the salinity response of the lagoon water, and the period of time since sediment deposition. Consequently, a sediment delivery event could, within the time frame of bioturbation (0.5–2 y), lead to benthic Ra fluxes above that predicted by our model. The effect of this process would be most pronounced for the longest-lived isotope, 226Ra, due to its low production rate and its low exchangeable inventory in the bioturbated layer. Based on our 210Pb-derived sediment accumulation rate (0.34 cm y⫺1 or 1.7 kg m⫺2 y⫺1), the annual release of just 20% of 226Ra delivered to the lagoon in association with freshwater sediment would be needed to sustain the measured benthic flux. Variations in the sediment accumulation rate (w) through the 100 years or so of sediment buildup represented in the 37 cm long core could also affect 226Ra flux and pore-water profiles. Below 10 cm depth the theoretical 226Ra pore-water profile (dotted line, Fig. 9), which is based on a sediment accumulation rate of 0.34 cm y⫺1, is not a good representation of the measurements. Figure 11 shows the effect that lower sediment accumulation rates have on the modelled 226Ra pore-water profile. Decreasing w to 0.1 cm y⫺1 substantially increases the degree of agreement between the measurements and the theoretical profile, and all measured values fall between the curves defined by w ⫽ 0.1 and w ⫽ 0 cm y⫺1. Another process capable of enhancing Ra flux from sediments is transport by groundwater flow (Rama and Moore, 1996). Because of its long half-life, this mode of transport is most significant for 226Ra, particularly if transport is slow. Based on the 226Ra pore-water activity below the bioturbated layer (about 25 mBq L⫺1) and the sediment porosity (0.79), an upwelling flow of 0.3 cm d⫺1 would be required to advect sufficient 226Ra into the bioturbated layer to sustain the observed benthic flux (0.07 Bq m⫺2 d⫺1). Due to the possible effects of slow 226Ra release and variable sedimentation rates, described above, 0.3 cm d⫺1 is effectively an upper limit on groundwater flow, with the true flow possibly being much less.

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As such, this calculation limits groundwater flow in Zecks Lagoon to less than 3% of the average groundwater flow observed in a salt marsh on the east coast of the USA (Rama and Moore, 1996). The difference probably reflects the different geology, hydrology, and sediment permeability of the two systems. 4. SUMMARY AND CONCLUSIONS

The fluxes of the four isotopes of Ra from the sediment to the overlying water column were estimated using three different methods: directly using a lagoon mass budget and benthic chamber measurements, and indirectly from the Ra sediment profile. The indirect approach used solid phase and pore-water profiles of Ra in lagoon sediment. These profiles were interpreted with the aid of a mathematical model which combined diffusive transport, mixing within a bioturbated layer, sedimentation, production, decay and desorption of Ra. Sediment in Zecks Lagoon is a source of dissolved Ra to the lagoon water and the Bega River estuary, with positive benthic fluxes being measured for all isotopes. The benthic chamber method was most sensitive for flux measurements. In general, there was good agreement between the directly measured fluxes of 224Ra, and within the limits of uncertainty, 226Ra. The direct methods show differences in the measurements of 223Ra and 228Ra fluxes, and may reflect lagoon-wide flux variability. The transport processes controlling the benthic flux of Ra include diffusion, bioturbation, and possibly groundwater flow. The development of a mathematical model describing the porewater and solid phase Ra distribution in the sediment profile has provided a means of determining the relative significance of these processes. On a time scale of days the fluxes of the two shortest-lived isotopes (224Ra and 223Ra) are determined primarily by diffusive processes. The flux of 228Ra is enhanced by bioturbation in the upper 10 cm of the sediment, with bioturbation more than doubling the 228Ra flux relative to diffusion. The sediment profile and measured fluxes of 224Ra, 223Ra, and 228 Ra are consistent with a model-generated bioturbation time scale of between 0.5 y and 2 y. The measured 226Ra benthic flux is enhanced relative to diffusion-bioturbation model calculations. This higher flux has several possible explanations, including the slow release of 226 Ra from recently deposited sediment, variable sedimentation rates, and/or upward advection of Ra in association with pore water driven by groundwater flow. Based on 226Ra flux measurements, the upward movement of groundwater is constrained to be ⬍0.3 cm d⫺1. This work has shown that the measurement of benthic fluxes and the sediment profile of multiple Ra isotopes with quite different half-lives offers a means to differentiate between the relative significance of transport processes operating in marine sediments, and the time scales over which they operate. In particular, Ra isotopes can yield information on the time scales of bioturbation not provided by commonly used tracers such as 210 Pb and 234Th. Acknowledgments—We thank G. Caitcheon, N. Grigg, and three anonymous reviewers for helpful comments and suggestions on an earlier version of this paper.

REFERENCES Bollinger M. S. and Moore W. S. (1993) Evaluation of salt marsh hydrology using radium as a tracer. Geochim. Cosmochim. Acta 57, 2203–2212. Boudreau B. P. and Imboden D. M. (1987) Mathematics of tracer mixing in sediments: III. The theory of nonlocal mixing within sediments. Am. J. Sci. 287, 693–791. Boudreau B. P. (1996) The diffusive tortuosity of fine-grained unlithified sediments. Geochim. Cosmochim. Acta 60, 3139 –3142. Boudreau B. P. (1997) Diagenetic Models and Their Implementation, Springer-Verlag. Boudreau B. P. (1998) Mean mixed depth of sediments: The wherefore and the why. Limnology Oceanogr. 43, 524 –526. Carpenter R., Peterson M. L., Bennett J. T., and Somayajulu B. L. K. (1984) Mixing and cycling of uranium, thorium, and Pb-210 in Puget Sound sediments. Geochim. Cosmochim. Acta 48, 1946 –1964. Cochran J. K. (1980) The flux of 226Ra from deep-sea sediments. Earth Planet. Sci. Lett. 49, 381–392. Elsinger R. J. and Moore W. S. (1980) Ra-226 behaviour in the Pee Dee River–Winyah Bay Estuary. Earth Planet. Sci. Lett. 48, 239 – 249. Fleischer R. L. (1982) Alpha-recoil damage and solution effects in minerals: Uranium isotopic disequilibrium and radon release. Geochim. Cosmochim. Acta 46, 2191–2201. Hammond D. E., Marton R. A., Berelson W. M., and Ku T.-L. (1990) Ra-228 distribution and mixing in San Nicolas and San Pedro Basins, Southern California Borderland. J. Geophys. Res. 95, 3321– 3335. Hancock G. J. and Martin P. (1991) The determination of radium in environmental samples by alpha-particle spectrometry. Appl. Radiat. Isotopes 42, 63– 69. Hancock G. J. and Murray A. S. (1996) The source and distribution of dissolved radium in the Bega River estuary, southeastern Australia. Earth Planet. Sci. Lett. 138, 145–155. Hancock G. J. (2000) Identifying the source of resuspended sediment in an estuary using the 228Th/232Th activity ratio: The fate of lagoon sediment in the Bega River estuary, Australia. Marine Freshwater Res. 51 (in press). Kadko D., Cochran J. K., and Mitchell L. (1987) The effect of bioturbation and adsorption gradients on solid and dissolved radium profiles in sediments from the eastern equatorial Pacific. Geochim. Cosmochim. Acta 51, 1613–1623. Li Y.-H. and Gregory S. (1974) Diffusion of ions in sea water and in deep-sea sediments. Geochim. Cosmochim. Acta 38, 703–714. Li Y.-H, Mathieu G. G., Biscaye P., and Simpson H. J. (1977) The flux of Ra-226 from estuarine and continental shelf sediments. Earth Planet. Sci. Lett. 37, 237–241. Martin P. and Hancock G. J. (1992) Routine analysis of naturally occurring radionuclides in environmental samples by alpha-particle spectrometry. Research Report 7, Supervising Scientist for the Alligator Rivers Region, AGPS, Canberra, Australia. Moore W. S. (1976) Sampling 228Ra in the deep ocean. Deep Sea Res. 23, 647– 651. Moore W. S. (1992) Radionuclides of the uranium and thorium decay series in the estuarine environment. In Uranium-series Disequilibrium: Applications to Earth, Marine and Environmental Sciences (eds. M. Ivanovich and R. S. Harmon), pp. 396 – 422. Clarendon. Moore W. S. and Arnold R. (1996) Measurement of 223Ra and 224Ra in coastal waters using a delayed coincidence counter. J. Geophys. Res. 101, 1321–1329. Murray A. S., Marten R., Johnston A., and Martin P. (1987) Analysis for naturally occurring radionuclides at environmental levels by gamma spectrometry. J. Radioactive Nucl. Chem. 115, 263–288. Rama and Moore W. S. (1996) Using the Ra quartet for evaluating groundwater input and water exchange in salt marshes. Geochim. Cosmochim. Acta 60, 4645– 4652. Roache P. J. (1982) Computational Fluid Dynamics. 5th ed. Hermosa. Sanstchi P., Hohener P., Benoit G., and Buchholtz-ten Brink M. (1990) Chemical processes at the sediment-water interface. Marine Chem. 30, 269 –315. Torgensen T. E., Deangelo E. C., O’Donnell J., Turekian K. K., Tanaka N., and Turekian V. C. (1996) 224Ra distribution in surface and deep

Using Ra isotopes to examine benthic fluxes water of Long Island Sound, summer 1991. Cont. Shelf Res. 16, 1545–1559. Turekian K. K., Tanaka N., Turekian V. C., Torgensen T. E., and Deangelo E. C. (1996) Transfer rates of dissolved tracers through estuaries based on 228Ra: A study of Long Island Sound. Cont. Shelf Res. 16, 863– 873. Webster I. T., Hancock G. J., and Murray A. S. (1994) On the use of radium isotopes to examine pore water exchange in an estuary. Limnol. Oceanogr. 39, 1917–1927. Webster I. T., Hancock G. J., and Murray A. S. (1995) Modelling the effect of salinity on the desorption of radium from sediments. Geochim. Cosmochim. Acta 59, 2469 –2476.

librium is attained at depth d, Eqn. 8 holds, and the term ␥(1 ⫺ ␸) in Eqn. A1 is equal to ␭ c ed . The first term of Eqn. A1 represents the diffusive 228Ra flux, and can be rewritten as F 0 ⫺ F d , where F 0 is the diffusive flux at the sediment surface, and F d is the flux at depth d. Since exchangeable 228Ra at depth d has reached a constant activity, F d ⫽ 0. From our definition of B, bioturbation only transports Ra in association with sediment, and does not directly result in any transport of Ra across the sediment-water interface, or at depths below the mixed layer. Thus the two bioturbation terms are zero. Consequently, Eqn. A1 reduces to F0 ⫽ ␭

APPENDIX A

冏

dc p dz

0

⫺ w共c ed ⫺ c e0兲 ⫹ B 0 ⫺ B d ⫹ ␭ ⫺d

冕

0

共c ed ⫺ c e兲 dz ⫽ 0

⫺d

(A1) 0 e

228

冕

0

共c ed ⫺ c e兲 dz ⫺ w共c ed ⫺ c e0兲.

(A2)

⫺d

The effect of sediment accumulation on the apparent 228Ra flux derived by integration of the 228Ra deficiency profile can be quantified by assuming the 228Ra deficiency of the sediment column is in steady state, and integrating Eqn. 13 between the sediment surface ( z ⫽ 0) and a depth d, ( z ⫽ ⫺d) where d is greater than the depth at which equilibrium between 228Ra and 232Th is attained (d ⬎ 27 cm). This gives

␸Ds

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where c is exchangeable Ra activity at the sediment surface, and c ed is exchangeable 228Ra activity at depth d. Because 228Ra–232Th equi-

The difference term c ed ⫺ c e is the 228Ra deficiency at a specific depth z, and so the first term on the right-hand side is equivalent to ␭ D, corresponding to the flux derived by direct integration of the sediment profile. The second term on the right-hand side corrects for sedimentation. Applying Eqn. 23 with w ⫽ 0.34 cm y⫺1 results in a reduction of 22% of the flux derived by direct integration, from 1.05 ⫾ 0.09 Bq m⫺2 d⫺1 to F 0 ⫽ 0.82 ⫾ 0.10 Bq m⫺2 d⫺1. The corrected flux falls within the flux range derived from our bioturbation/diffusion model. Since the model uses sediment activities of 228Ra to derive bioturbation rates, its flux estimate in not entirely independent of the direct deficiency-integration method. Nevertheless, our model uses an experimentally derived parameter (⌰), and contains assumptions in the estimation of ␥. Thus, F 0 provides a consistency check on the accuracy of the parameter values, and the overall performance of the model.

APPENDIX B Notation a t, a e, a i a Th A B c L, c w, c c, c m, c p c ad, c e c៮ e , c៮ p Ds f F h H Kd Q t v V w z ␥ ⌬ ␪ ⌰ ␭ ␸ ␳ s, ␳ w ␳d ␶

Ra activity of total sediment, exchangeable sites, and internal lattice sites of sediment (Bq kg⫺1 dry weight) Total Th activity of sediment (Bq kg⫺1 dry weight) Area of lagoon (m2) Bioturbation of Ra (Bq m⫺3 d⫺1) Ra activity of lagoon water, channel water, actual and measured benthic chamber water, and pore water (Bq m⫺3) Adsorbed and exchangeable Ra activity of saturated sediment (Bq m⫺3) Average activity of exchangeable Ra and pore-water Ra in bioturbated layer (Bq m⫺3) Diffusivity of Ra (m2 d⫺1) Fraction of Th decays producing exchangeable Ra Flux of Ra from sediment (Bq m⫺2 d⫺1) Depth of lagoon water (m) Height of benthic chamber (m) Ra distribution coefficient (kg m⫺3) Water flow through lagoon channel (m3 d⫺1) Time (d) Volume of sample extracted from benthic chamber (m3) Volume of water contained in benthic chamber (m3) Sediment accumulation rate (m y⫺1) Sediment depth (m) Exchangeable Ra activity production rate (Bq m⫺3 d⫺1) Thickness of bioturbation layer (m) Tortuosity Desorption function Ra decay constant (d⫺1) Sediment porosity In situ density of wet sediment, and density of water (kg m⫺3) Density of dry sediment (kg m⫺3) Sediment mixing time scale (d)