Temporal and spatial distribution of beryllium-7 in the sediments of Chesapeake Bay

Estuarine,

Coastal

and

Shelf

Science

(1989) 28,395-406

Temporal and Spatial Distribution Beryllium-7 in the Sediments of Chesapeake Bay

Jack E. Dibb” University Chesapeake Received

of Maryland Biological 14 February

Keywords:

and Donald Center for Laboratory, 1988

of

L. Rice Environmental and Estuarine Solomons, Maryland 20688,

and in revised

form

19 September

Studies, U.S.A.

1988

beryllium; radionuclides; sedimentation; ChesapeakeBay

The sediment inventory of 7Bewas determined at six stations in the main stem of Chesapeake Bay nine times between April, 1986, and September, 1987. The inventories ranged from < 0.20-7.32 dpm cm-*. Comparison to the atmospherically supported 7Beinventory (range 2-4 dpm cm-‘) showed significant focusing of 7Bein the sediments in the zone of the turbidity maximum during the summer, and suggested that the spatial distribution of ‘Be in the sediments of the lower Bay was determined largely by horizontal redistribution of bottom sediments. The processescausing this redistributionof’Be in the lower Bayapparently had a recurrence frequency greater than the sampling frequency in this investigation. The temporal pattern of 7Beaccumulation at the six stations over the first year of this investigation allowed estimation of sedimentation rates, which suggested that the processesgoverning the distribution of’Be in ChesapeakeBay sediments were similar to the processesdetermining sedimentation patterns over about the past 100 years. Introduction

The cosmogenic radionuclide 7Be (t; = 53.3 d) has attracted considerable attention as a tracer of short-term processes in aquatic systems. The earliest use of 7Be in oceanography was as a tracer of water masses and mixing in the surface layers of the open ocean (Silker, 1972; Young & Silker, 1974). Recent work has demonstrated its applicability in coastal and estuarine waters (Aaboe et al., 1981; Martin et al., 1986; Olsen et aZ., 1986). 7Be has also been useful in the study of mixing in the top few centimeters of sediment (Krishnaswami et al., 1980; Robbins & Eadie, 1982; Rice, 1986; Casey et al., 1986). Olsen et al. (1986) developed a mass budget for 7Be to assessthe retention of particle active substances in the James River estuary. However, they were only able to develop a ‘ snap shot ’ picture from a one-time sampling program. This study focuses on the distribution of 7Be in Chesapeake Bay sediments over a 17 month period. The purposes of the study were to: (1) determine the spatial distribution of “Presentaddress:

University Oceans and Space, Science Hampshire 03824, U.S.A.

0272-7714/89/040395

+ 12 $03.00/O

of New Hampshire, and Engineering

Institute Research G-1989

for the Building, Academic

Study of Earth, Durham, New Press Limited

396

J. E. Dibb t.3 D. L. Rice

I I 8 i

.

SCliLE NA”Tll

CAPE

en,

ATMOSPHERIC DEPOSITION

1

r

1;:

Figure 1. Station locations indicated by 1 were occupied the June-July, 1987 cruise.

and the six subregions on all cruises, samples

of the Chesapeake Bay. Stations were collected at all 16 stations on

7Be in Chesapeake Bay sediments, (2) examine changes in this distribution over time and (3) attempt to elucidate the processes responsible for the spatial and temporal patterns of 7Be distribution that were observed. Methods

and materials

Sediment samples for 7Be determinations were collected 2-4 April, 13-15 May, 16-18 June, 13-15 August, and l-3 October in 1986, and 24-26 February, l-3 April, 30 June-2 July, and 9-l 1 September in 1987, at six stations along the main stem of the Chesapeake Bay (Figure 1). Ten additional stations were sampled on the June-July, 1987, cruise. Sediment for *lOPb determinations was obtained at all of the main stations except CAPE during the April 1987, cruise. Sediment samples for 7Be determinations were obtained with a modified Bouma box corer fitted with clear plexiglass core liners (surface area= 140 cm*). The liners allowed examination of the core immediately upon retrieval. Only cores which showed an undisturbed sediment-water interface were used for 7Be determinations. Cores for *l’l?b

Temporal and spatial distribution

of ‘Be

397

measurements were taken with a gravity corer (core liner i.d. = 6.2 cm), which was not able to retrieve suitable cores from the sandy sediments at the CAPE station. The cores for 7Be analysis from the first seven cruises were sectioned at 1 cm intervals to depths generally 2 10 cm. Approximately 0.5 cm of sediment was trimmed from the outside of all slices deeper than 1 cm to minimize contamination of deeper intervals by more active surface material adhering to the walls of the core liners. To ensure sufficient mass of sediment for non-destructive gamma counting, sediment from corresponding depth intervals of two cores were combined. The top 6 cm of each core were combined into a single sample on the June-July 1987 cruise. The O-6 and 6-12 cm intervals were collected in September, 1987. Cores destined for 7Be counting were sectioned on board ship within 30 min of core retrieval. Subsamples from each depth interval were taken for the determination of water content; then the sediments were frozen for transport back to the lab. The 210Pbcores were returned to the lab in the core liners and then sectioned at 1 cm intervals to 50 cm depth (except for the BALT station where only 38 cm of core was obtained). Processing of sediment samples for non-destructive gamma spectrometric determination of 7Be consisted of drying at 70-80 “C, grinding to create relatively uniform grain size, and sealing in 120 ml tin cans for counting. The gamma spectrometry system included a high-resolution, reverse-coaxial, intrinsic germanium detector and a computerbased multichannel analyser. Calibration and calculation of detector efficiency and sample activities followed the procedure outlined by Larsen and Cutshall (1981). The National Bureau of Standards’ sediment standard 4350b was the primary standard before March, 1987. Counting efficiency at the 477.6 kev 7Be photopeak was calculated by linear interpolation between the 351.9 kev 2141?bphotopeak and the 661.7 kev 137Csphotopeak. After March 1987, a 7Be standard from the Environmental Protection Agency was used. All 7Be activities were corrected for decay between sampling and counting dates. Sediment samples for ‘lOPb determinations were dried and ground, and ‘l’l?b activities were estimated by the 2’oPo alpha-counting method (Flynn, 1968) using2”Po as a yield tracer. 208Po and 210Powere spontaneously plated onto polished silver planchets and counted for 12-24 hours in a 4-channel alpha spectrometer equipped with high resolution, passivated implanted planar silicon (PIPS) detectors.

Results The sediment inventory Z of 7Be (dpm cm-‘) at each station was calculated for the first seven cruises by summing the specific activity Ai (dpm g-l) of all depth intervals i with detectable 7Be: N

i=

1

where the average particle density p, was taken to be 2.5 g cme3 and the porosity ‘pi was calculated from water loss on drying (Berner, 1971) (Table 1). The 7Be inventory supported by atmospheric deposition (Dibb, 1989) is included in Table 1 for comparison. Sediment 7Be inventories are variable in both time and space. The largest difference between stations on a given cruise was 5.95 dpm cmP2, and the biggest change between cruises at a single station was 4.46 dpm cmP2. In contrast, the atmospherically supported inventory only ranged from 1.75-3.68 dpm cme2 over the entire 17 months (Dibb, 1989).

398

3. E. Dibb & D. L. Rice

1. ‘Be inventories (dpm cmm2) in the sediments of Chesapeake are the cumulative one sigma counting errors for each core

TABLE

Bay. Uncertainties

Cruise 1986

1987

Station April

May

June

* 0.20

0.92 kO.13

2.36 kO.32

2.66 kO.29

2.33 * 0.22

2.04 kO.20

2.56 kO.27

BALT

2.14 kO.27

1.75 io.13

2.90 _+ 0.27

7.32 kO.34

3.64 kO.28

2.72 _+ 0.24

2.02 kO.25

CALV

1.27 kO.17

0.18 LO.07

2.56 f0.16

1.49 kO.12

2.05 kO.19

1.74 kO.22

2.33 kO.17

POT0

1.98 kO.17

0.71 kO.12

2.40 kO.17

1.60 kO.13

2.09 kO.14

1.61 io.12

2.25 kO.19

RAPP

5.71 +0.28

1.25 kO.13

1.54 kO.16

1.37 kO.18

2.09 kO.21

0.71 kO.18

2.56 kO.31

CAPE

1.26 &- 0.24

1.54 kO.23

4.61 kO.36

1.89 kO.27

3.91 k 0.23

0.24 io.19

3.54 kO.35

1.90

2.65

2.18

1.98

2.63

SUSQ

Atmospherically Supported Inventory”

0.03

August

October

Calculated from an atmospheric deposition monitoring Biological Laboratory in Solomons, Maryland, which 1989) (uncertainty c. 6%).

February

April

program at the Chesapeake began 19 March, 1986 (Dibb,

The highest 7Be sediment inventory was seen at BALT in August, 1986 and inventories at BALT were generally high in the summer. Very little 7Be had accumulated at the head of the Bay (SUSQ) in the early spring of 1986, but between April and May, 1986, this region accumulated more 7Be per unit area than any other part of the Bay. During this same period half of the stations were losing 7Be from the sediments at rates faster than radioactive decay could explain. In the lower Bay, particularly at CAPE, the inventory fluctuated widely, being relatively high on one cruise and low on the next for most of the investigation. For the last two cruises 7Be inventories were calculated by converting activity from dpm g-’ to dpm cme3 and multiplying by the 6 cm depth of the sampling interval (Table 2). The inventories calculatd for the June-July cruise are minimum values, as any 7Be deeper than 6 cm was not included; however, the O-6 cm depth range contained over 90% of all the 7Be detected on the first seven cruises. Depth distributions of 210Pb activity are shown in Figure 2. Activities in the surface intervals ranged from 2.20-6.48 dpm g dry sediment-‘, but all the profiles approached similar activities of 055-O-65 dpm g dry sediment-’ at depth. The “‘Pb profile at BALT suggests either very rapid sedimentation or mixing to c. 25 cm depth at this station. Unfortunately, the 38 cm of core retrieved at this station did not allow calculation of a definitive sediment accumulation rate.

Temporal and spatial distribution

of ‘Be

399

TABLE 2. ‘Be inventories in Chesapeake Bay sediments derived from 6 cm depth intervals from box cores collected 30 June-2 July, and 9-l 1 September 1987

‘Be Inventory (dpm cm -“) Station

Northern latitude

Western longitude

June-July

September

SUSQ

39”26’30” 39”22’15”

76”02’00” 76”08’05”

2.57kO.78 5.67kO.79 3.47 * 0.99 3.53kO.67

2.02 * 1.03

n.d. + 0.42 2.11 kO.35 n.d. kO.56

1.92kO.34

2.08 kO.24 0.19$-0.57 4.64 &- 0.46

1.19f0.30

SUSQ2

BALT BALT2 CALV CALV2 CALV3

POT0 POT02 POT03

39”13’10”

76”15’20”

39” 10’40” 38”30’05” 38”30’05” 38”30’05” 38”00’05” 38”00’05” 38”OO’OO” 37”40’05” 37”40’00” 37”40’05”

76”21’55” 76”28’00” 76”25’30” 76”22’35” 76”20’00” 76”15’00” 76”10’40” 76”06’05” 76”13’10” 75”58’10”

RAPP RAPP2 RAPP3 CAPE 37”15’00” 76”06’10” CAPE2 37”15’05” 76”10’10” CAPE3 37”15’05” 76”16’10” Atmospherically supported inventory”

1.54+0,70

Lost

1.90+ 1.04

4.68 + 0.47 3.27 + 0.45 3,81+0.40 6.31 kO.91 3,04$-0.91

7.56 f 1.33

3.17

2.51

“From Dibb (1989) (uncertainty c. 69,).

Discussion Evidence

of sediment focusing from 7Be sediment inventories

Atmospheric deposition is generally the dominant source of 7Be for estuarine systems (Olsen et al., 1986) and was dominant during this investigation (Dibb & Rice, 1989). In an hypothetical water body with no horizontal transport of water or particles, the 7Be inventory in the sediments would reflect the atmospherically supported inventory on any spatial scale of sampling. In such a system, the only factors causing differences between the amount of 7Be delivered from the atmosphere per unit area of water surface and the amount residing in the underlying sediments would be the time required for 7Be to adsorb to suspended particles, and for the particles to settle to the bottom. These factors would combine to create sediment inventories lower than atmospheric deposition would support. In an estuary, horizontal transport can be important and provides a mechanism whereby 7Be sediment inventories can locally exceed the inventory supported by atmospheric deposition. Lateral transport of particles from a wide area to a restricted area of deposition (i.e. sediment focusing) would create such a situation, and seems the likely explanation of the elevated 7Be sediment inventories at BALT in June, August and October 1986, and at CAPE in June and October 1986, and April 1987 (Table 1). (The 7Be inventory

at RAPP

in April Spatial

1986, probably

and temporal

also reflects

sediment

focusing.)

trends in 7Be inventories

The spatial and temporal variability of 7Be inventories reflect the dominant short-term processes affecting sedimentation in the Chesapeake Bay. The Susquehanna River supplies roughly half of the fresh water input to the Bay and nearly 90% of that entering the Bay north of the Potomac River (Schubel & Pritchard, 1986). The Susquehanna is the

400

J. E. Dibb t3 D. L. Rice

210Pb ACtlvlty

(dpm

g dry sediment-‘I

I234567

I234567

SUSQ

Figure 2. *Tb specific activity-depth Horizontal bars represent one sigma depth interval of each sample.

+ +

profiles in cores counting uncertainties,

BALT

collected vertical

in April, 1987. bars depict the

only major river to discharge directly into the Bay. The other tributaries all have estuarine reaches upstream of their confluences with the Bay. As a result, the Susquehanna River is by far the dominant source of fluvial sediments in the Bay. Officer et al. (1984) estimated that the sediments delivered by the Susquehanna River represent c. 609/6 of the total sediment, from all sources, accumulating in the Bay. The influence of the Susquenanna River on water circulation and sedimentary dynamics in the upper Bay is shown by the sedimentary inventories of 7Be (Figure 3c, e, g). The very low inventory at SUSQ and the relatively high inventories at BALT and CALV in April 1986 appear to reflect the erosion of surficial (7Be rich) sediments from the head of the Bay and subsequent redeposition downbay. (Alternatively, the low inventory at SUSQ in April, 1986, may be due to the deposition of large amounts of ‘ old ’ fluvial sediments that had collected behind the dams upstream on the Susquehanna since the previous high flow.) After the freshet, the inventory at SUSQ increases while that at CALV decreases, reflecting the tendency of suspended materials to settle out of the water column closer to the head of the estuary during reduced Susquehanna River flow (Schubel, 1972; Schubel & Pritchard, 1986). Perhaps more importantly, this large influx

Temporal and spatial distribution

of 7Be

401

(d) POT0

’

i

I ,y ,

,

, /

!

1986

I

,, ,

*

,

AMJJASONOJFMAMJ

AMJJASONOJFMAMJJAS

1987

1986

J AS

1987

Figure 3. (a) Monthly average discharge of the Susquehanna River as measured by the United States Geological Survey below the Conowingo dam. (b) Calculated atmospherically supported ‘Be inventory at Solomons, Maryland (Dibb, 1989). The calculations assumed no ‘Be present prior to the start of the monitoring program in March 1986, hence underestimate the inventory prior to late June 1986. (c-h) Observed ‘Be sediment inventories at 6 stations in the Chesapeake Bay. The error bars represent one sigma counting uncertainties propagated in the summation process used to calculate inventories from specific activities in 1 cm depth intervals.

of fresh water sets up the density stratification that drives the two-layer circulation and creates a turbidity maximum in the region of the Bay around the BALT station (Schubel, 1972; Schubel & Pritchard, 1986). Summer ‘Be inventories at BALT rise to several times the level that could be directly supported by atmospheric deposition as the upstream flow of deep water transports settling fine particulates to the turbidity maximum. ‘Be inventories in the lower Bay show no obvious temporal trends (Figure 3d, f, h). The fairly wide oscillations about the atmospherically supported inventory suggests that the major short term process impacting the bottom sediments is redistribution by currents. The fact that these oscillations generally decrease in magnitude in a northward direction suggests that offshore processes or events may be driving them. Bay wide 7Be sediment inventory

A Bay-wide ‘Be sediment inventory may be estimated from the inventories at the six stations. However, the size of the Chesapeake Bay and the heterogeneity of its sediments demand consideration of the extent to which arbitrarily located sampling stations are

3. E. Dibb & D. L. Rice

402

TABLE 3. Estimation of a Bay wide ‘Be sediment 30 June-2 July, 1987. (See text for discussion)

Segmenta

Areab m-a

SUSQ BALT CALV POT0 RAPP CAPE

257.3 861.2 903.9 1077.1 1598.2 1797.1

Whole

Bay

S,Clay-silt’

Station@

47.3 84.1 51.3 33.7 12.8 2.4

inventory

Average Inventory’

%San&

567 3.50 1.06 1.14 4.68 -

52.7 15.9 487 66.3 87.2 97.6

2

12 12 12 12

from

Station&

the inventories

Average Inventory’

1

2.57

3 3 3 1,2,3

0 4.64 3.27 4.39

-

observed

Segment Average Inventory8 4.04 2.94 0.54 3.46 3.45 4.28 3.24”

6494.8

“See Figure 1 bFrom Cronin (1971) (uncertainty c. 3%) ‘Includes: clay, silty clay, clay silt, sandy silt (uncertainty c. 2.50/6) See Table 2 ‘Simple averages of the inventories at the stations indicated in d (uncertainty c. 1.5 dpm crne2) ‘Includes: clay sand, silty sand, clay-silt-sand, sand (uncertainty c. 2.5%) gAreally weighted average of sand and clay-silt inventories (uncertainty c. 2 dpm cm h(Uncertainty c. 5 dpm cm-*)

‘)

TABLE 4. Sediment depositional fluxes (g dry sediment cm-‘) derived from changes in ‘Be sediment inventories between successive cruises from April 1986 to April 1987. Uncertainties are c. 0.15 and 0.02 g sed cm-’ for positive and negative fluxes, respectively AprilMay Station SUSQ BALT CALV POT0 RAPP CAPE “Interval bNegative

41 d” 0.064 0.035 -0.276b -0.112 - 0.333 0.056

May-

June 34d 0.120 0.120 0.165 0,131 0.050 0.246

JuneAugust

AugustOctober

58d 0.121 0.464 0.023 0.037 0.050 -0.133

OctoberFebruary

FebruaryApril

49 d

145d

36 d

0.068 -0.084 0.094 0.092 0.101 0.216

0.205 0.263 0.173 0.156 0,047 - 0.206

0,088 0,021 0.085 0.086 0.146 0,233

between the first day of one cruise and the first sign denotes loss of sediment between cruises

day of the succeeding

cruise

representative, particularly when the number of stations is small. The ten additional stations sampled on the June-July 1986, cruise were selected to address this problem by sampling several different types of bottom sediments in the vicinity of the main stations. The surface sediments in each subregion of the Bay display a range of composition and grain size distributions. When the data on grain size distribution alone is presented in map form, eight of the ten Shepard sediment classes are present in the Bay, and no fewer than four of the classesare found in each of the six subregions (Byrne et al., 1982; Kerrhin et al.,

Temporal and spatial distribution

TABLE

*“Pb

5. Sediment accumulation depth distributions

of

7Be

403

rates (g crnmz y-‘)

This

study

from

‘Be sediment

Officer

inventories

and

et al. (1984)

Station

‘Be

*‘OPb

2’0Pb

Station

SUSQ BALT CALV POT0 RAFT CAPE

0.665 + 2.4 0.820+ 2.7 0.264If: 0.9 0,391+ 1.3 0.061+ 0.2 0.411+ 1.2

0.084 & 0.006 0.245+0.101 0.063 + 0.003 0.078 f 0.008 0.164f0.024 No Core

1-24 5.55 0.30 0.68 0.25 0.48

(52) (914s) (834G) (24) (85)

uw

. I.50

. I.00

; OtQ

A 0

0-I’

38N

39N

37N

Latitude Figure 4. Sedimentation rates in the main stem of the Chesapeake Bay from ‘Be sediment inventories (0) and *‘OPb activity-depth profiles (A). Vertical bars are the reported uncertainty in the Officer et al. (1984) Z’“Pb derived sedimentation rates (0). The curve is the best fit through the Officer et al. (1984) data, when the points are weighted by the inverse of their uncertainties. (Modified from Officer et al., 1984).

1983). In the present study, no more than three stations were sited in any subregion, so the sediment size distribution classes were collapsed into just two classes (sand and clay-silt). The per cent of bottom covered by each class in the different segments was determined planimetrically (Byrne et al., 1982; Kerrhin et al., 1983) (Table 3). On the basis of the 7Be inventories observed on the 30 June-2 July 1987 cruise (Table 2), estimates of the inventories in clay-silt and sand sediments in each subregion were derived and areally weighted average inventories for each subregion were calculated (Table 3). The whole Bay areally weighted average 7Be inventory of 3.24 dpm cm-* compares remarkably well with the atmospherically supported inventory of 3.17 dpm cm-‘. If the whole Bay average 7Be sedimentary inventory is calculated using the inventories found at the six main stations in June-July 1987 (replacing the lost RAPP sample with RAPP2), the value is 3.11 dpm crn2. In view of the uncertainties the agreement between the

404

3. E. Dibb Q D. L. Rice

two estimated sediment inventories encouraging. Sediment

accumulation

and the atmospherically rates from

supported

inventory

is

‘Be and ‘l”Pb

Comparison of the 7Be inventory at each station on successive cruises allows estimation of the flux of 7Be to or from the sediments during the time interval between cruises. Assuming that all of the 7Be in the sediments is associated with the particles, these 7Be fluxes yield estimates of short-term sediment accumulation rates. Because the time intervals between cruises are not negligible compared to the half-life of 7Be, radioactive decay must be taken into account. The net change in 7Be inventory AZ(t,), at each station: AZ(ti)=Z(ti)-Z(ti-J

e-’

“I

(2)

where Z(ti) is the inventory on a given cruise, Z(t,-,) is the inventory on the preceding cruise, h is the decay constant of 7Be, and t is time in days, may be calculated from the data in Table 1. If AZ(ti) >O, this is a minimum estimate of the 7Be added, because any 7Be added during the interval will also undergo decay. A better estimate of the flux to the sediments can be obtained by assuming a constant daily flux J over the interval and continuously decay correcting this newly added 7Be: ft Z(ti) = J e-“dt+Z(ti-l) e-hAti. (3) s f,- L The presumed constant daily flux is then: h(Z(ti) - I(t,-i)

J=

e-*.“‘l) 1 _ e-bA*,

(4)

and the total flux during the interval isJAti. When AZ(ti) < 0, the estimate of 7Be loss is also a minimum, since it is possible that 7Be was actually accumulating for most of the interval, then was removed in one event just before the time of sampling. However, this minimum estimate of 7Be loss will be used here for lack of any estimates of the timing of resuspension/erosion events. Estimates of particle flux Fp to the sediments during each interval are given by: Fp = 4,,IAP

(5)

where the flux of 7Be FBc is estimated as described above and A, is the 7Be activity on particles (Table 4). When the 7Be flux to the sediments is positive A, is taken to be 18 dpm g-i, the average 7Be activity on suspended particles in Chesapeake Bay during the course of this study (Dibb & Rice, 1989). When FBe is negative the 7Be activity in the surface layer of sediments on the first of each pair of cruises is used for A,. Summing the estimated sediment fluxes for the six intervals gives an estimate of sedimentation at each station over the year (April 1986 to April 1987 Table 5). The depth distributions of unsupported “‘I’b also yield estimates of sediment accumulation rates (Officer et al., 1984) (Table 5). These rates assumed mixed layer depths of 6 cm (SUSQ), 20 cm (BALT), 3 cm (CALV), 0 cm (POTO) and 5 cm (RAPP) and corrected for supported ‘i”Pb on the basis of the asymptote at depth in each core. The BALT calculations used the average of 0.62 dpm g-’ for supported “‘Pb. Because of the relatively short core obtained, only three depth intervals were used in the fit for sediment accumulation rate at BALT. Published “‘Pb derived sediment accumulation rates from cores approximately corresponding to the 6 stations (separations range c. 2-20 km) are also

Temporal and spatial distribution

oj’Be

405

listed in Table 5 (Officer et al., 1984). Interestingly, Officer and coworkers considered the high sediment accumulation rate at their station nearest to BALT to be anomalous and attributed it to ‘ slumping or rapid infilling of sediments related to dredging ’ (Officer et al., 1984). The reported sediment accumulation rate of 5.55g cme2 y-l at 914s (Officer et aE., 1984) is extremely high, but the location of this station in the region of the turbidity maximum suggests that the sediment accumulation rate would be higher than in most of the rest of the Bay. It is not surprising that the three sets of sediment accumulation rates differ in detail, since the 7Be derived rates are based on a single year and “‘l?b essentially gives an average over c. 100 years. However, the ranges are similar and the same trends are borne out by all three data sets: highest rates in the northern Bay (especially in the region of the turbidity maximum), low rates in the mid-Bay, and intermediate rates in the southern Bay. This trend is also seen in the complete data set from 23 cores described by Officer et al. (1984). In fact, if the sediment accumulation rates derived here are plotted with the rates calculated by Officer et al. (1984), the agreement is striking (Figure 4). Conclusions The distribution of 7Be in the sediments of the Chesapeake Bay is influenced by processes acting on a range of temporal and spatial scales. The sediment inventory of 7Be closely reflects the input of 7Be from the atmosphere over the whole Bay. 7Be sediment inventories that are elevated relative to the atmospherically supported inventory, particularly at BALT and CAPE, suggest the importance of sediment focusing in determining smaller scale aspects of the sedimentary distribution of 7Be. The seasonal variation in 7Be sediment inventory at BALT demonstrates the role of two-layer estuarine circulation and the turbidity maximum in the accumulation of 7Be in the sediments of this region of the Bay. Similarly, the inventory at SUSQ reflects the dominance of the Susquehanna River in the sedimentary dynamics of 7Be in the upper Bay, while the inventories in the lower Bay suggest that in this part of the Bay bottom currents redistribute 7Be on time scales shorter than the sampling frequency of this investigation. Interestingly, the temporal pattern of 7Be accumulation in the sediments of the Chesapeake Bay yields estimates of annual sedimentation rates that are very similar to rates derived from “‘Pb data, suggesting that the processes governing the distribution of 7Be in the sediments of the Bay during the year of this study are similar to the sedimentary processes operating in the Bay over about the last 100 years. Acknowledgements We would like to thank the officers and crew of the ORV/Ridgely Warfield and a long list of volunteers from CBL, who combined to make the field sampling program a success. W. Boynton, R. Demicco and T. Lowenstein reviewed this manuscript and offered many suggestions for its improvement. This research was supported by NSF grants OCE 8442759 and OCE 8511383 to D. L. Rice. References Aaboe, E., Dion, Geophysical

E. P. & Turekian, K. K. 1981 ‘Be in Sargasso Research, 86,3255-3257.

Sea and Long

Island

Sound

Waters.Journalof

406

J. E. Dibb 6’ D. L. Rice

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