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Seasonal variations in 7Be activity in the sediments of Cape Lookout Bight, North Carolina
Vol.
Gaxhimica ef Cowwchimica Acla 54, pp. 237-245 Copyright 0 1990 Pergamon Press pk. Printed in U.S.A.
0016-7037/90/$3.00
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Seasonal variations in ‘Be activity in the sediments of Cape Lookout Bight, North Carolina E. A. CANUEL,’ C. S. MARTENS,’and L. K. BENNINGER’ ‘Curriculum in Marine Sciences, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA ‘Department of Geology, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA (Received May 23, 1989; accepted in revised formOctober 3 1, 1989)
Abstract-Short-term sediment accumulation rates were determined using activity distributions of 7Be in the surlicial sediments of a station (A-l) in Cape Lookout Bight, North Carolina (USA). This semienclosed coastal marine basin is characterized by high annual accumulation rates. Protected from high energy conditions by its geomorphology, the relatively quiescent waters of the bight’s interior cause it to act as an efficient settling trap for fine-~ned sediment. Lack ofbioturbation in the surface sediments allowed for variations in depth-integrated activity profiles of ‘Be to be interpreted as short-term accumulation events. Beryllium-7 inventories exhibited seasonal cyclicity with maxima occurring during spring (February through June). The inventory of ‘Be ranged from 5.8 to 32.6 dpm cm-’ and was always well in excess of the atmospherically supported value (X = 3.8 dpm cm-*; n = 15). Accumulation rates calculated from ‘Be activity profiles indicate that the delivery of particulate matter to the sediments of Cape Lookout Bight is not constant throu~out an annual cycle. The highest monthly accumulation rates appeared to be associated with north/northeast storm activity. Inputs were generally highest during late winter/early spring when storm frequency is greatest. Short-term accumulation rates derived from this study agree with *“Pb rates calculated for a ten-year period of accumulation. The annual sediment accumulation rates for each of the two years over which the study was conducted was found to be 4.8 + 0.8 g cm-* y-’ and 3.7 + 1.2 g cme2 y-‘. I~ODW~ION COSMIC
mulation in nearshore systems (~ISHNASWAMI et al., 1980; OLSEN et al., 1985,
SPALLATIONreactions occurring within the Earth’s atmosphere produce a suite of natural radionuclides. Knowledge of the production and delivery rates of cosmic ray-produced isotopes allows for the quantification of a variety of geochemical processes over a range of time scales. One of the most useful applications of natural radionuclides has been as chronometers (TUREKIAN and COCHRAN, 1978; GOLDBERG and BRULAND, 1974). Naturally produced radionuclides have been used to determine sediment accumulation rates both in coastal and oceanic sediments. Although mdiochronomet~c tracers have been widely used in the measurement of sediment a~umulation rates, until recently most applications have been concerned with time scales greater than a year. Beryllium-7 (t,,:! = 53.3 days) is produced by cosmic ray spallation reactions with nitrogen and oxygen in the Earth’s atmosphere. Though produced predominancy in the stratosphere, most of the 7Be delivered to the Earth’s surface is derived from the troposphere (DUTKIEWICZ and HUSAIN, 1985). This is due to the long residence time of stratospheric air (- 1 y) relative to the mean life (l/X = 77 days) of this nuclide. In the atmosphere, 7Be associates with aerosols and is delivered to the Earth’s surface by p~cipitation. gallium-7 remains soluble as its 7Be+2species in “acidic” rainfall. However, once in the marine environment 7Be is adsorbed onto inorganic particulate matter by cationic adsorption processes (BLOOM and CRECELIUS, 1983). The unique properties of ‘Be (i.e., a short half-life, its reactivity with particulate matter, and a measurable rate of delivery to the surface of the Earth) have been utilized in recent years to study sediment accuRAY
1986; DIBB and RICE, 1989).
In
this paper we report the results of a study investigating the distribution of ‘Be activity in the surface sediments of Station A-l located in Cape Lookout Bight, NC. This back barrier island lagoon, located on North Carolina’s Outer Banks (34’37W, 76*33W) is chamcterized by rapid sediment accumulation and insignificant bioturbation. This paper examines the use of’Be in quantifying sediment accumulation in the bight’s interior on a monthly time scale. Owing to the lack of biological mixing, we have been able to interpret the depth distribution of ‘Be in these sediments as resulting from short-term accumu~tion. Inventories of ‘Be have been used to quantify seasonal variations as well as episodic pulses in sediment delivery to this coastal site. Also reported are total (wet and dry) atmospheric delivery data for 7Be over a sixteen month period at a coastal site (Morehead City, NC) in the southeastern United States. STUDY SITE Cape Lookout Bight is a 2 km* body of water located approximately 115 km southwest of Cape Hatteras on the Outer Banks of North Carolina, USA (Fig. 1). This shallow, semienclosed basin reaches a m~imum depth of 8 m (mean low tide) at Station A-l, the site of this study. Located behind a barrier island cuspate foreland, the bight waters are influenced by the back barrier island lagoon system as well as offshore waters. Two barrier islands (Shackleford Banks and Core Banks) oriented at approximately 90” to one another, an inlet (Barden Inlet), and a no~hw~~y migrating spit define the system (see Fig. 1). Although the water column of Cape 237
238
E. A. Canuel, C. S. Martens, and L. K. Benninger
f
cur
FIG. 1. The Cape Lookout Bight, NC, study site. Station A- 1 (see CHANTONet al., 1983, for more detail) was the location at which our sampling took place. Rain samples were collected from the roof of the Institute of Marine Science,MoreheadCity, NC, approximately 20 km from Station A- 1.
Lookout Bight remains oxygenated throughout the year, sediments at Station A-l are anoxic below the upper few millimeters except during winter months when dissolved NO; and labile Fe and Mn oxides are present in the upper 5-7 cm (“brown-oxidized” layer). Winter sediments are oxidized because of a coupling between high rates of accumulation and decreased rates of metabolic activity in response to lower temperatures. Despite the fact that winter sediments do become oxidized, microelectrode profiles taken during early spring indicate that dissolved oxygen does not penetrate deeper than the upper few millimeters (CANUEL, unpubl. data). Throughout most of the bight (- 1.6 km2) sediments are predominantly high-porosity mud. Particles in the upper 1.5 m of the sediment column are mostly (90%) silts and clays (primarily illite, kaolinite, and silt-sized quartz grains) with mean diameters of 5-10 pm (WELLS, 1988). Mechanical interlocking of very small quartz grains may be an important factor in resisting resuspension. Work by other investigators has demonstrated that these cohesive sediments are resistant to resuspension at bottom currents in the 25-50 cm set-’ range (WELLS, 1988). The organic carbon content of sediments in the upper 15 cm range from 3.3 to 4.0% (dry weight) and do not exhibit any pronounced seasonal variation (CANUEL, unpubl. data). During spring (February through May) a small, patchily distributed population of spionid polychaetes (~2 cm in length) colonizes the upper 3-4 cm of the sediment (BARTLETT,198 1). These organisms are surface deposit feeders (FAUCHALDand JUMARS, 1979) believed to feed discriminantly on sediment particles of particular size and content. Despite the seasonal occurrence of polychaetes, bioturbation is thought to be unimportant in Cape Lookout Bight sediments. This is supported by the absence of burrows and the presence of fine-scale (- 1 cm) laminations in the upper 4 cm of box cores taken during spring (MARTENSand KLUMP, 1984). Box core X-radiographs taken during summer months indicate that there may be some reworking in the upper 10
cm associated with bubble tubes. These tubes, approximately 2-20 mm in diameter, are produced by vertically migrating, methane-rich gas bubbles. Cape Lookout Bight acts as a settling basin for particulate matter in transit between the back barrier lagoon system and offshore waters. During ebb tidal flows, waters high in suspended matter from Back and Core Sounds enter the bight via Barden Inlet. After an unknown residence time in the water column of Cape Lookout Bight, a portion of this particulate matter is removed to the sediment. The remaining fraction is either transported offshore or is returned to the back barrier sounds on flood tide through Barden Inlet. CHANTON et al. (1983) determined the sediment accumulation rate at Station A-l to be 4.1 f 0.7 g cmm2 y-l based on “‘Pb geochronology and burial of hurricane sand layers. They found that accumulation of excess 210Pbwas 30 times above its estimated atmospheric input, thereby demonstrating the efficiency with which the bight traps tidally transported particles. This high rate of sediment accumulation and the absence of physical mixing by bioturbation at station A-l allowed us to use the depth distribution of ‘Be to quantify accumulation processes on a monthly time scale. METHODS Atmospheric inputs Total deposition (wet and dry) was collected on a monthly basis from June 1987 through September 1988 from the roof (-30 feet above ground level) of the Institute of Marine Sciences, Morehead City, NC, approximately 20 km west of Cape Lookout Bight. The sampling was conducted using a polyethylene SEAREX (Sea-Air Exchange Program) rain collector (see figure in PELTZERet al., 1984)
with an effectivecollectingarea of 0.5 m2.The amount of precipitation was monitored throughout each sampling period At the conclusion of each month, the volume of rain collected was measured. The collector was acid washed and the rinses added to the sample. A beryllium standard (1 ml at 1 mg Be per ml in 1% HCI; Sigma Chemical Co.) was partitioned equally between each of the carboys containing a given month’s sample and each carboy was homogenized. An iron solution (100 mg Fe as FeC&) was then added to each carboy. The carboys were mixed and the pH increased to 9 by the gradual addition of approximately 1 liter of dilute N&OH ( 1:1 reagent NH,,0H:H20) in order to precipitate oxides or hydroxides of iron and beryllium. The carboys sat for at least overnight, and the precipitate from each was collected and combined to make a single sample representing the total atmospheric input of ‘Be for that month. The combined precipitate was transferred to 25 ml plastic centrifuge tubes and concentrated by centrifugation. The N&OH solution was decanted and the precipitate dissolved in 6 N HCl. Samples were filtered through Nuclepore filters (0.4 pm) and a known volume of the 6 N HCl solution sealed in 95 cm3 aluminum cans. The activity of a suite of gamma emitters was determined using an intrinsic germanium detector (detector resolution was 1.9 keV at 1.3 MeV) and a multichannel analyzer system. Samples were counted for approximately 24 h, and the activity of ‘Be as well as other gamma-emitting radionuclides was determined. Counting efficiencies were determined by calibration over a range of can height geometries with a ‘Be standard of known activity (Isotope Products Laboratories, Burbank, CA). Recovery of the beryllium standard added to the rain samples was determined by flame (nitrous oxide-acetylene) atomic absorption spectrophotometry. All data have been corrected for beryllium standard recovery (55-85%), and all errors are statistical counting errors expressed as + 1 standard deviation (FRIEDLANDER et al., 1981).The
‘Be activity for each sampling period was decay-correctedto the middle of each sampling period using the C = C,,e-” relationship where “t” equals one-half the number of days in each sampling period.
Seasonal variation of ‘Be in sediment
Spatial variability in surface sediments
Table
In order to assess spatial variability in the surface sediments at the field site, a study was undertaken to measure differences in the inventory of ‘Be and other gamma emitters (“K and “‘Cs), percent organic carbon, and total nitrogen content of the surface (upper 3 cm) sediments of Cape Lookout Bight. Ten cores (4.5 cm i.d.; lo15 cm length) were collected in September 1987 near Station A-l. A single core was collected at a central site, and three cores spaced approximately 1 m apart were then collected along each of three transects moving away from the location at which the first core was collected. The upper 3 cm of each core was removed and dried at 60-70°C. The dried sediment was ground with a mortar and pestle and sealed in 95 cm3 aluminum cans. Samples were counted for approximately 24 h and the activity of a suite of gamma emitters determined by gamma ray spectroscopy (LARSENand CUTSHALL, 1981). The 3 cm increment was chosen because a large fraction (-30%) of the ‘Be inventory was found to occur over this region. A second variability study was conducted in July 1988 to examine variation in the distribution of 7Beactivity at a sample depth resolution comparable to that used in processing sample cores. In this study, five small cores (4.5 cm id.; 8-10 cm in length) were collected. The cores were extruded and the upper 3 cm sectioned into three 1 cm intervals. Sediment sections were treated according to the same methodology as the above-mentioned samples.
Sediment sampling Sediment cores were collected from Cape Lookout Bight over a two-year period on approximately a monthly basis. All samples were obtained by divers using SCUBA so as to minimize disturbance to the surficial sediment. Cores were taken using lucite tubes (8 cm i.d.) approximately 30 cm in length. Samples were returned to the laboratory under quasi-ambient conditions (dark and at approximate
Month
239
Beryllium-7 Atmospheric Inputs Morehead City, NC
1.
Precipitatfo”
June ‘07 July August September October November December January ‘88 February March April nay JUn.? July Augus f September
Activity
(cm)
(dpm 1-l)
7.9 6.4 23.8 19.4 0.4 14.9 4.9 16.3 9.8 6.2 8.3 19.7 8.0 9.9 31.8 4.5
39.12 154.35 60.65 63.39 26.17 65.58 101.79 51.61 88.28 91.97 102.40 95.21 142.92 91.69 41.01 62.94
FlUX (dpm cm-’
moel)
0.37 1.84 2.49 1.66 0.23 1.33 0.94 0.94 1.31 1.07 1.63 2.63 1.52 1.30 1.45 0.36
Inventory (dpm cm“)
3.91 4.31 3.15 3.46 3.20 3.16 3.45 3.41 3.94 5.30 5.11 4.76 4.67 3.52
ambient temperature) where they were extruded and sectioned. Sediment sections were dried at 60-70°C and ground into a fine powder using a mortar and pestle. The dry weight of each sediment section was determined and samples were sealed in 95 cm3 aluminum cans. Samples were counted for 24-48 h and the ‘Be activity determined directly by gamma ray spectroscopy. Activity values were corrected for counting efficiency in the same way as rain samples. Beryllium7 activities are reported on a dpm g-’ dry weight sediment basis and statistical counting erron expressed as + 1 standard deviation. All ‘Be activity values have been decay-corrected to the middle of the interval between sample collections in the same manner as were the rain samples described above.
RESULTS
Morehead Citv. N.C. Atmospheric input of ‘Be
The mean (n = 16) monthly atmospheric deposition of 7Be was 1.3 + 0.7 dpm cmm2 (Fig. 2) at the Morehead City, NC, site. This value is consistent with average flux measurements at geographically comparable sites (1 .O and 1.1 dpm cmm2 at Oak Ridge, TN, and Norfolk, VA, respectively, OLSEN et al., 1985; and 1.1 dpm cm-’ at Solomons, MD, DIBB, J JASONDJ
FMAMJ
1997
JAS
1909
Morehead City, N.C. 8~rqllium.l AtmosphericInputs 3-
JJASONDJFMAMJJAS 1997
1999
2. Precipitation (cm mo-‘) and flux of ‘Be via precipitation (dpm cm-* mo-‘) at Morehead City, NC. Data are presented on a monthly basis for June 1987 through September 1988. FIG.
1989). The average daily flux was 0.04 dpm cme2, which is within the range of values (0.02 to 0.1 dpm cme2) reported for daily ‘Be fluxes by other investigators (WALTON and FRIED, 1962; PIERSON, 1963; LAL and PETERS, 1967). Using
the mean ‘Be lifetime for radioactive decay (T = l/X) of 77 days, this flux would support a steady-state inventory of 3.1 dpm cm-2. The measured monthly flux of 7Be ranged from 0.2 to 2.6 dpm cme2 (Table 1) at the Morehead City sampling station. In order to assess what component of the sediment inventory was attributable to direct atmospheric deposition, an inventory of ‘Be was calculated by decay-correcting the previous month’s atmospherically derived standing crop and adding it to the present month’s atmospheric flux. Prior to June 1987, ‘Be flux data were not obtained; therefore, inventory data, including decay-corrected inputs from previous months, could not be calculated for the initial part of the study. For this reason, inventory values are only reported from August 1987 through the end of the study. These inventory data ranged from 3.2 to 5.3 dpm cme2 (Table 1) with a mean (n = 15) value of 4.0 + 0.9 dpm cmm2.
E. A. Canuel, C. S. Martens, and L. K. Benninger
240
BERYLLIUM-7
Horizontal variability in surface sediments
Horizontal variability of organic carbon, total nitrogen, and a suite of gamma-emitting radionuclides (‘Be, 13’Csand 4oK) is summarized in Table 2. The coefficient of variation (CV = 25%) was well below that for the population of inventories obtained during the monthly sampling (see below). These data indicate that the seasonal patterns reported below cannot be explained by horizontal variability at the sampling site. The study of July 1988 showed that the ‘Be activity is most variable in the surface section and systematically decreases with depth. The coefficients of variation were 26, 2 1, and 8% for the O-l, l-2, and 2-3 cm sections, respectively (Table 2). All inventories in this paper are reported as +25% which is, for most samples, an overestimate of the error associated with their determination.
-I f s 5 0 -10 m
ACTIVITY
DISTRIBUTION
-5
.
.
-10
..*
-1, ASONDJFMAMJJASONDJFMAMJJASO 1986 1987
-IS ,988
FIG. 3. Beryllium-7 activity (dpm g-‘) distribution with depth in Cape Lookout Bight sediments over the study period. Asterisks represent the mid-point of each sample interval analyzed.
Sediment inventory of ‘Be
The distribution of ‘Be activity in the upper 12 cm of Cape Lookout Bight sediments is illustrated in Fig. 3 for the entire study period. Isopleths of constant activity exhibit seasonal cyclicity. Activity in the surface sediments of Cape Lookout Bight was highest in late winter-early spring, exceeding 10 dpm g-’ during February through May 1987 and between February and June 1988. Penetration of the activity distributions was enhanced during summer months. Activity in the surface sediments is several times higher than has been previously reported, and the depth range over which ‘Be activity is present is greater than other investigators have found (KRISHNASWAMI et al., 1980; OLSEN et al., 1986; DIBB and RICE, 1989). These data suggest that Cape Lookout Bight
Table
2.
Cape Lookout Variability
Bight. NC Studies
September, Sample
1987
0rg.C (X)
Tot.N (X)
1 2 3
2.42 2.59 2.62
0.30 0.31 0.32
7.19 6.94 6.36
1.32 0.91 1.89
12.63 13.49 14.83
: 6 7 8 9 10
2.64 2.48 2.74 2.65 2.82 2.73 2.47
0.32 0.30 0.34 0.31 0.34 0.36 0.31
5.92 3.91 1.35 4.25 8.26 4.14 5.23
0.95 1.53 1.26 2.02 1.?2 1.61 1.43
12.68 12.57 14.71 14.09 14.32 14.41 14.14
j;
78. (dpm cm-‘)
13’CS (dpm cm-‘)
40K (dpm cm-‘)
2.61
0.32
5.96
1.46
13.79
s.d.
0.13
0.02
1.52
0.37
0.88
C.V.(X)
4.98
6.25
25.5
July, Sample depth
n
25.2
6.4
1986 x
(dpm g-l)
(d;id;-‘,
C.V.(X)
o-1
cm
5
10.31
2.68
26.0
1-2
cm
5
12.72
2.67
21.0
2-3
cm
3
14.99
1.12
7.5
sediments trap ‘Be efficiently relative to both the local atmospheric input and to sediments elsewhere. The inventory of ‘Be (dpm ‘Be cm-*) was determined by integrating polynomials fit to the data plotted as activity of ‘Be (dpm g-‘) versus mass depth (g cmw2). Mass depths expressed for 1 cm intervals and corrected for salt content were calculated according to the relationship gem-* = (1 - $)p where 4 is porosity as determined from 4 = O.~Z-~.~*and density (p) is 2.5 g cme3 (CHANTON, 1985). Beryllium-7 inventories ranged from 5.8 to 32.6 dpm cm-* with the highest seasonal inventory occurring in spring (between February and June) during both years (Table 3). The mean (n = 20) inventory value was 17.3 + 8.7 dpm cm-*. The total inventory was separated into two components, as is illustrated in Fig. 4: RESIDUAL inventory = inventory of previous sampling period decay-corrected to the date of present sampling. NEW inventory = total inventory (present sampling) - residual inventory. These designations make the assumption that ‘Be delivery and removal processes only occur in association with particulate matter. NEW inventory for each sample period was estimated by subtracting the RESIDUAL inventory (calculated by correcting the inventory of ‘Be in the previous sample period for radioactive decay to the time of subsequent sampling) from the total ‘Be inventory. Results for each core are illustrated in Fig. 5. Occasionally, the total inventory was entirely RESIDUAL (i.e., no delivery), and in some cases the RESIDUAL inventory was greater than the total inventory, indicating removal had taken place. Determination
of sediment
accumulation
rates
Sediment accumulation rates were calculated using the distribution of ‘Be activity in the surficial sediments of Cape Lookout Bight. Rates were determined according to the lower schematic in Fig. 4. The mean ‘Be activity (dpm g-l) of newly deposited sediment was determined by averaging activity in
Seasonal variation of ‘Be in sediment
Table
3.
Beryllium-7 Inventory Cape Lookout Bight Sediments
Sample
Ill”elltMy
NEW Inventory’
(dpm cm-‘)
E/14/86 9/11/86 11/21/86 Z/24/87 3/20/87 4/21/87 5128187 6/28/07 B/13/87 9/25/87
Mean Activity
(dpm cm-‘)
5.75(1.44)2 9.86(2.47) 9.16(2.29) 32.56(8.14) 25.28(6.32) 13.18(3.30) 25.56(6.39) 17.26(4.32) 10.27(2.57) 23.22(5.81)
5.86(2.66) 5.24(2.49) 29.90(8.17)3 3.80(8.29)3 -5.32(5.68) 17.41(6.W3 0.18(6.07)3 0.78(3.50) 17.35(5.99)
Accumulation1
(dpm g-l)
(g cm-‘)
11.50(0.38) 10.33(0.54) 26.54(0.55)
0.X(0.23) 0.51(0.24) 1.13(0.X)
14.07(0.10)
1.24(0.48)
12.4UO.64)
1.39(0.49) L i 4.78(0~32)~
11122/87 12/10/87 Z/27/80 3/29/t% 4/30/88 6/19/88 7/01/M 8/17/88 9/02/88 10101/88
12.25(3.06) 11.86(2.97) 12.38(3.ioj 25.15(6.29) 29.74(7.44) 31.49(7.87) 20.39(5.10) 14.95(3.74) 6.57(1.64) 8.78(2.20)
1.33(4.10)3 2.17(3.83)3 8.13(3.28) 16.88(6.62) i3.i5&52j 15.96(8.77)3 -6.55(8.45)3 3.,X8(4.65) -5.57c3.45) 4.27(2.47)
14.4X0.63) 17.54(0.88) 15.44(0.63) 15.49(0.64)
0.56(0.23) 0.96(0.38) 0.85(0.55) 1.03(0.57)
7.83(0.42) 4.41(0.25)
-0.71(0.44) 0.97(0.56) I: - 3.66(1.15)5
’ NW Inventory 2
3
4 5
and Mass Accumulation
Values in parenthesis (2 25% as determined
represent in spatial
NEY inventory less than error assumed to be neither delivery
since
preceding
error associated vith variability study). associated with nor removal of
sample
date.
inventory
measurement inventory.
Mass accumulation
(g cm-’
yr-1)
betveen
9-11-86
and 9-25-87.
Mass accumulation
(g cm-’
yr-‘)
between
9-25-87
and 10-l-88.
therefore
the surface region for those periods characterized by delivery or removal processes (Table 3). Dividing the amount of NEW inventory by the mean activity in the surface region yielded mass accumulation or removal (g cm-‘) over the time between samplings (Table 3 and Fig. 6). Accumulation rates were calculated by this technique rather than from the slope of a line fit to the semi-log plot of ‘Be activity vs. depth used by other investigators for several reasons. This data set demonstrates that the assumption of steady state at time scales of a month is not valid as neither the flux of the tracer nor the rate of accumulation remain constant at this time scale. Another reason for the new approach was that using the slope of lines fit to the data can only yield long-term rates as this approach gave rates of accumulation even for periods where there was not NEW inventory. Many of the activity profiles are not exponential in shape, and accumulation is often so small over a one-month interval (sometimes a single point) that we were not able to draw a line through this region. Lastly, the method used in this paper has as its sole assumption that we are able to adequately distinguish between the NEW and RESIDUAL pools of the 7Be inventory, an assumption that we feel is justified given the extensive data set we have collected. DISCUSSION
Atmospheric ‘Be deposition The distribution of ‘Be in nearshore sediments is determined by the direct atmospheric deposition of this tracer
241
(i.e., precipitation), by horizontal redistribution, and by the rate of sediment accumulation. Differences in the amount of precipitation, as well as the rate of stratosphere-troposphere exchange, can result in seasonal trends in the atmospheric deposition of ‘Be. For example, OLSEN et al. (1985) found that 40-45% of the annual ‘Be deposition occurred during spring when 30-35% of the annual precipitation at sites in Norfolk, VA, and Oak Ridge, TN, occurred. Precipitation at Morehead City, NC, varied considerably from month to month (4.5-3 1.8 cm mo-I). Other than the fact that August rainfall was consistently higher in both 1987 and 1988, there does not appear to be any seasonal pattern in rainfall for this site (Table 1 and Fig. 2). The relationship between atmospheric delivery of ‘Be and the amount of precipitation was found to be statistically significant (P = 0.04) despite there not being a strong correlation (r’ = 0.3) between the two. This weak correlation agrees with observations that the concentration of ‘Be in rain is not directly correlated with the amount of precipitation. MAHONEY and BONDIETTI(1985) demonstrated that the concentration of 7Be in rain decreased by a factor of 3 during a storm event which deposited approximately 6 cm of rain. Similarly, DIBB (1989) found that the specific activity of ‘Be in serial samples taken during a single precipitation event decreased over time. These data suggest that the concentration of ‘Be in rain may be higher during drier months and periods characterized by short-duration precipitation events. Precipitation was highest during the month of August in both 1987 and 1988; however, the atmospheric flux of ‘Be was not highest for either of these sample periods. This is possibly due to the fact that rainfall was not partitioned equally throughout the month. Rather, a few events characterized by prolonged precipitation account for most of the rainfall. During both August 1987 and 1988, more than 75% of the monthly rainfall resulted from four “events.” In fact, 42% of the total monthly rainfall (3 1.8 cm) in August 1988 occurred on a single day. Conversely, months with less total precipitation but whose rainfall was distributed over several short-duration events (see July 1987 and 1988 data in Fig. 2) exhibited an enhanced 7Be flux relative to the relationship determined for the atmospheric flux of 7Be vs. the amount of precipitation. An alternative explanation for the lack of a clear relationship between the amount of rainfall and the ‘Be flux is that dry deposition was an important mechanism in the delivery of 7Be. Other investigators, however, have not found this to be the case (OLSENet al., 1985). The highest flux of ‘Be occurred during spring, possibly in response to the mid-latitude folding of the tropopause. This phenomenon is thought to occur during spring (late April to early June) and results in increased exchange between the stratosphere and troposphere. Enhancement of the stratospheric component results in an increase in the tropospheric inventory of ‘Be. Both the May and June flux measurements of ‘Be were higher than the mean flux over the entire study period and support a similar finding by DIBB (1989). There does not appear to be a systematic relationship between the ‘Be atmospheric delivery data and NEW ‘Be inventory in the surface sediments of Cape Lookout Bight. In other words, sample periods characterized by higher than
242
E. A. Canuel, C. S. Martens, and L. K. Benninger ‘Be
ACTIVKV .( dpm
- !$‘I I
0
0
10
20
MEAN NEW ACTIVITV
MASS ACCUMULATION
( 0 cm-2 ) = l
NEW ~NVENXFW ohm
MEAN NEW ACTIVITV
l
( dpm
cm’* 1 l
g’l )
Em. 4. Schematic illustrating how inventory was classified and accumulation rates were calculated. In the first step the ‘Be inventory was separated into its NEW and RESIDUAL components. The NEW inventory (dpm cmV2) was then divided by the mean activity (dpm g-‘) of newly deposited sediment in order to determine mass accumulation (g cme2).
average delivery of 7Be from precipitation do not reflect an enhanced ‘Be activity signal (see Tables 1 and 3). This may be due to the lag between the time required for particles to scavenge the 7Be signal from the water column (OLSENet al., 1989, estimated 7Be removal rates to be - 15% day-’ in the Susquehanna-Chesapeake Bay system) and the time it takes for this particulate matter to be delivered via horizontal transport to the bight sediments. Alternatively, because the variation in atmospheric delivery is small relative to the monthly variation in the distribution of 7Be in the sediments, small differences in the atmospheric signal may not be discernible using our methods. A period of the atmospheric study conducted at the Morehead City, NC, station overlapped that of DIBB (1989) at Solomons, MD. Comparison of these two studies over June through October 1987, when they overlapped, demonstrates that local atmospheric deposition can be quite variable over short time scales despite the similarity in long-term averages. Over this period, the Solomons, MD, site received 38.3 cm of precipitation and the flux of 7Be was 4.4 dpm cm-*. At
Morehead City, the precipitation over the same time period was 65.9 cm and the ‘Be flux was 6.6 dpm cm-*. This comparison suggests that when working with the inventory of 7Be in a given sediment system it may be necessary to measure local atmospheric deposition.
Cape Lookout Bight sediment accumulation The ‘Be activity distributions for each sample period are presented in Fig. 5. The total inventory (area under the polynomials fit to the data) has been separated into NEW and RESIDUAL components. Viewing these profiles in sequence, we are able to make month-by-month comparisons of the data on an annual basis over the study period. These data suggest that seasonal patterns exist in the delivery of ‘Be to the sediments of the Cape Lookout Bight system. Spring months (February through June) have the highest total inventory of ‘Be as well as the largest fraction of NEW inventory. Beryllium-7 inventories during the summer season ap-
Seasonal variation of ‘Be in sediment
243
BERYLLIUM-7 ACTIVITY ( dpm - g ‘l)
‘88
‘87
JAN
FEB
MAY
JUN
JUL
AUG
SEP
OCT
NOV
DEC
FIG. 5. Depth distribution of ‘Be activity for each sample. The darkened region of each month’s inventory reflects the RESIDUAL inventory. The area representing each month’s RESIDUAL inventory was placed so as to illustrate the amount of mass a~umuIatio~.
pear to be largely RESIDUAL, reflecting radioactive decay of inputs during spring. There appears to be a deepening in the distribution of ‘Be activity for summer sample periods despite the lack of evidence for new inputs. This feature of the summer data may be explained by sediment disturbance caused by methane bubble tubes. Throughout the rest of the year, ‘Be inventories are low, with the exception of samples appearing to reflect episodic delivery events such as September 1987. Assuming that ‘Be is neither added to nor lost from the sediments except in association with particulate matter, we can attribute changes in the NEW and RESIDUAL pools of the ‘Be inventory in the sediments of Cape Lookout Bight to variations in sediment delivery. This of course assumes that evidence demonstrating that biological reworking in the sediments of Cape Lookout Bight is an unim~~nt process on an annual basis is also true at shorter time-scales. We do know that macrofauna are virtually absent from Station Al during warm summer and fall months (BARTLETT, 1981) when sediments are anoxic. Rates of sediment accumulation in Cape Lookout Bight were variable over the course of this study (Fig. 6). Accumulation rates calculated for sample periods characterized by input ranged from 0.4 to 1.4 g cm-’ mo-’ (5 = 0.8; n = 11). These accumulation rates sum to an annual rate of 4.8 f 0.8 g cm-’ y-’ for the period between 11 September 1986 and 25 September 1987 and 3.7 -t 1.2 g crns2 y-l between 25 September 1987 and 1 October 1988 (Table 3). Both ‘Be accumulation rates are within the lu uncertainty of the 4.1 + 0.7 g cm-’ y-’ rate determined for the same station using *“Pb geochronology (CHANTON et al., 1983) and major storm sand layers. The close agreement between the 7Be and *‘OPbstudies is somewhat surprising considering that the 2’0Pb rate has been averaged over a ten-year period of accum~ation (1971-198 1) while the 7Be rate was determined by summing the short-term rates of accumulation over the 1986- 1988 period of this study. The agreement between these rates of accumulation, determined independently and over very different time scales, supports previous assumptions
regarding the steady-state nature of accumulation in Cape Lookout Bight sediments at time scales of years to decades. Storm ~~~uen~eson sediment delivery
Monthly sediment accumulation at Station A-l during spring is in excess of the mean annual accumulation rate for both years of the two-year study. High rates of accumulation were also measured during spring in preliminary ‘Be work conducted in Cape Lookout Bight (CHANTON, 1985). Northeast winds predominate along the North Carolina coast during winter and spring (PORTER,unpubl. data). It is plausible that under such conditions, delivery of particulate matter to the sediments of Cape Lookout Bight may be enhanced due to increased turbidity in the back barrier lagoons (Fig. I). During a station occupied during spring tide, while winds were blowing from the northeast, increased bottom currents were measured near Station A-l (WELLS, 1988). These northerly winds force water southward through Core Sound
Sediment Accumulation Rates
DG. 6. Calculated sediment a~~rn~tion/~rnov~ rates on a monthly basis for each sample period. During h&arch,April, June, August, November, and December of 1987 and in July and August 1988 there was no significant change in the NEW inventory (see Table 3), indicating addition or removal of new sediment was unimportant.
244
E. A. Can&l, C. S. Martens, and L. K. Benninger
enhancing ebb tidal flows. Under such conditions, extended ebb flows (i.e., missed good tides) have been recorded. It is probable that delivery from the back barrier lagoon system is increased under such circumstances in response to currents through Barden Inlet which persist for longer than the typical tidal cycle. Such conditions could result in an increased inventory of ‘Be activity in the surface sediments. Evidence supporting the relationship between northeast storm activity and enhanced delivery of p~iculate matter is demonstrate by the high ‘Be inventory for the September 1987 sample. This sampling took place during a period when winds were from the north and in excess of 20 mph (average wind speeds are 10 mph for this time of year) over a period of several days. Our hypothesis that these meteorological conditions resulted in the increased delivery of particulate matter to the bight is supported by a high NEW inventory of ‘Be (17.4 dpm cm-‘) and the highest calculated sediment accumulation rate (1.4 g cmm2 mo-‘) observed during the study (Table 3). SUMMARY AND CONCLUSIONS Delivery of ‘Be from its atmospheric sources was highest in spring, as has been observed in recent comparable studies (OLSENet al., 1985; DIBB, 1989). This seasonality is thought to result from the mid-latitude folding of the tropopause. The atmospheric flux of ‘Be was not strongly correlated with precipitation patterns despite rainfall being the most important delivery mechanism for this radionuchde. Our data demonstrate that ‘Be can be a useful tracer of sediment accumulation in shallow coastal systems una&cted by bioturbation. By examining short-term changes in the sedimentary inventory of ‘Be, this technique is able to distinguish between NEW and RESIDUAL components of the inventory and to derive sediment accumulation rates on time scales of a month. Cores obtained before and after major storm events demonstrate that changes in the ‘Be inventory can be used to quantify the impact of individual storm events, which may result in the delivery of pulses of new sediment by horizontal transport processes. Variations in sediment ‘Be inventories were not closely coupled with variations in its atmospheric input. This is probably due to a lag in the time it takes for particles in the water column to scavenge ‘Be and be subsequently delivered to the sediments. Both the accumulation and inventory data appear to be associated with north-northeast storm activity with maxima occurring in late winter-early spring (i.e., February through June). Ac~~o~le$~~e~fs-The authors would like to thank Dan Albert, Jeff Chanton, Emmett Duffy, George Pauly, and Frank Wilson for diving and field assistance. Frank Wilson and Hugh Porter assisted in thicollection of rain samples and precipitationdata when one of the authors (E.A.C.) could not be at the collection site. Emmett DuI@ and Marc Alperin provided helpful discussions and comments in the early stages of the preparation of this manuscript. Reviews by R. Carpenter, D. Hammond, and an anon~ous reviewer cont~buted significantly to the final preparation of this paper. The faculty and staff of the UNC Institute of Marine Science in Morehead City, NC, generously provided laboratory space and boating facilities. Financial support was provided by NSF grants CKX8416963 and OCE-8716528 (CSM) NASA grants NAGW-1455 and NAGW-834 (CSM), and a UNC Limited Service Traineeship (EAC). Editorial handling: K. W. Bruland
REFERENCES BARTLETTK. ( 1981) M~rofauna distribution and seasonal influences on interstitial water chemistry of Cape Lookout Bight, N.C. MS. thesis, Univ. of North Carolina, Chapel Hill. BLOOMN. and CRECEUUSE. A. (1983) Solubility behavior of atmospheric ‘Be in the marine environment. Mar. Chemistry 12,
323-331. J. P. (1985) Sulfur mass balance and isotopic fractionation
CHANTON
in an anoxic marine sediment. Ph.D. dissertation, Univ. of North Carolina, Chapel Hill. CHANTONJ. P., MARTENSC. S., and KFPHU~ G. W. (1983) Lead2 10 sediment geochronology in a changing coastal environment. Geochim. Cosmochim. Acta 47, 119 l-l 804. DIBBJ. E. ( 1989) Atmospheric deposition of beryllium-7 in the Chesapeake Bay region. J. Geophys.Rex 94,226 l-2265. DIBBJ. E. and RICE D. L. (1989) Temporal and spatial distribution of beryllium-7 in the sediments of Chesapeake Bay. Est. Coast. SherfSci. 28,395-406. DUTK~EWICZ V. A. and HUSAINL. (1985) Stratospheric and tropospheric components of ‘Be in surface air. J. Geophys. Res. 90, 5783-5788. FAUCHALDK. and JUMARSP. A. (1979) The diet of worms: A study of polychaete feeding guilds. In Oceanography& Marine Biology AnnualReview(eds. M. BARNES). ,. Vol. 17. DD.193-284. Aberdeen Univ. Press. . FRIEDLANDERG., KENNEDY J. W., MACIAS E. S., and MILLER J. M. (1981) Statistical considerations in radioactivity measure ments. In Nuclear and Radiochemistry.Chap. 9, pp. 339-365. J. Wiley & Sons. GOLDBERG E. D. and BRULANDK. ( 1974) Radioactive geochronologies In The Sea (ed. E. D. GOLDBERG),Vol. 5, Chap. 14, pp. 45 l-489. J. Wiley & Sons. KRISHNASWAMI S., BENMNGERL. K., ALLERR. C., and VON DAM~W K. L. (1980) Atmospherically derived radionuclides as tracers of sediment mixing and accumulation in near-shore marine and lake Earth Planet. sediments: Evidence from ‘Be, ““Pb and 239*240Pu. Sci. Lett. 47, 307-3 18. I..
KRISHNASWAMI S., MONAGHANM. C., WESTRICHJ. T., BENNETT J. T., and TUREKIANK. K. (1984) Chronologies of sedimentary processes in sediments of the FOAM site, Long Island Sound, Connecticut. Amer. J. Sci. 284, 706-733. LAL D. and PETERSB. (1967) Cosmic-ray-produced radioactivity on the earth. Handbk. Phys. 46,551-612. LARSENI. L. and CUTSHALLN. H. (198 1) Direct determination of ‘Be in sediments. Earth Planet. Sci. Lett. 54, 379-384. MAHONEYL. A. and FSONDIE’EIE. A. (I 985) Beryllium-7 deposition to terrestrial v~e~tion in Tennessee. Tech. Memo. 96 11, Oak Ridge Nat’]. Lab., Oak Ridge, TN. MARTENSC. S. and KLUMP J. V. (1984) Biogeochemical cycling in an organic-rich marine basin-4. An organic carbon budget for sediments dominated by sulfate reduction and methanogenesis. Geochim. Cosmochim. Acta 48, 1987-2004. OLSEN C. R., LARSEN I. L., LOWRYP. D., CUTSHALLN. H., TODD J. F., WONG T. F., and CASEYW. H. (1985) Atmosphe~c fluxes and marsh-soil inventories for 7Be and “@Pb.J. Geovhvs. _ _ Rex 90. 10,487-10,495. OLSEN C. R., LARSENI. L., LOWRY P. D., CUTSHALLN. H., and NICHOLSM. M. (1986) Geochemistry and deposition of ‘Be in river-estuarine and coastal waters. J. Geophys.Res. 91,896-908. OLSEN C. R., LARSENI. L., LO~XY P. D., MCLEANR. I., and DoMOTORS. L. (1989) Ra~onuclide di~butions and sorption behavior in the Su~uehanna-Che~peake Bay System. Power Plant Environ. Rev. Div., Dept. Nat. Resources, State of Maryland. PEIRSOND. H. (1963) Beryllium-7 in air and rain. J. Geophys.Res. 68,3831-3832. PELTZERE. T., ALFORDJ. B., and GAGOSIANR. B. (1984) Methodology for sampling and analysis of lipids in aerosols from the remote marine atmosphere. Woods Hole Oceanog. Inst. Tech. Rept. WHOI-84-9.
Seasonal variation of ‘Be in sediment TUREKIANK. K. and COCHRANJ. K. (1978) Determination of marine chronologies using natural radionuclides. In Chemical Oceanogruphy (eds. J. P. RILEY and R. CHESTER),Vol. 7, Chap. 40, pp. 3 13-360. Academic Press. WALTON A. and FRIED R. E. (1962) The deposition of beryllium-7
Beryllium-7
2-4 4-6 6-6 6-10 to-12 12.14
6.OQ(O.27) 2.64(0.20) 1.97(0.20) 0.29(0.11) 0.07(0.07)
and phosphorus 32 in precipitation at north temperate latitudes. J. Geophys.Res. 67, 5335-5340. WELLS J. T. (1988) Accumulation of fine-Brained sediments in a periodically energetic elastic environment, Cape Lookout Bight, North Carolina. J. Sediment. Petrol. 58, 596-606.
Appendix Activity Distributions
(dpm g”)
aa35
llaB.6
.i&2uz
10.33(0.54) 3.26(0.44) 1.79(0.30) 1.29(0.24) 0.62(0.21) 0.62(0.17)
36.55(0.72) 14.52(0.37) 6.69(0.37) 1.76(0.24) 1.02(0.13) 0.72(0.17)
11.50(0.36) 4.16(0.29) 0.56(0.16) l.lO(O.16) 0.54(0.13) 0.74(0.20) 0.13(0.07)
245
3aaz
AaBz
16.60(0.36) 16.22(0.36) 7.41(0.31) 3.39(0.15) 0.66(0.17)
15.52(0.46) 6.01(0.30) 3.36(0.25) 0.46(0.13) 0.20(0.11)
Qsllul O-l cm 1-2 2-3 3-4 4-6 6-6 6-10 IO-12
s&Q2 14.75(0.76) 13.75(0.47) 13.70(0.62) 10.23(0.57) 7.40(0.42) 6.27(0.39) 3.35(0.31) 1.46(0.26)
-Rwlh O-lcm 1-2 2-3 3-4 4-5 5-7 7-9 9.11 11.13
§z2&32 6.97(0.70) 7.95(0.49) 6.57(0.49) 6.37(0.42) 7.35(0.39) 5.92(0.32) 4.13(0.23) 1.26(0.13)
&l3s! 0.16(0.06) 2.67(0.51) 6.51(0.57) 6.04(0.59) 4.10(0.39) 2.75(0.32) 4.90(0.47) O.SS(O.16) 0.45(0.20)
12.46(0.65) 12.23(0.50) 12.62(0.56) 9.79(0.53) 4.60(0.44) 3.76(0.26) 4KqO.44) 4.97(0.37) 3.97(0.44)
LE&E 10.92(0.74) 6.76(0.45) 6.02(0.42) 4.90(0.42) 2.36(0.35) 2.42(0.26) 2.23(0.26) O.QQ(O.16) 0.69(0.13)
x&&a2 14.60(0.53) 9.50(0.31) 5.61(0.34) 4.76(0.29) 4.40(0.31) 2.11(0.17) 0.96(0.15) 1.52(0.17) 0.71(0.11)
zws8 16.13(0.70) 10.70(0.55) 4.60(0.36) 3.01(0.30) 2.16(0.30) 1.67(0.25) 1.17(0.22) 0.74(0.16) 0.37(0.15)
QQ@h O-lcm l-2 2-3 3-4 4-5 5-7 7-9 9.11 II-13
ikz9s 15.27(1.31) 17.53(0.76) 16.66(0.71) 16.50(0.73) 9.74(0.43) 3.19(0.26) 1.46(0.11) 1.39(0.22) 0.63(0.20)
MQI8.8 15.46(0.79) 16.31(0.66) 17.95(0.74) 16.96(0.64) 11.75(0.47) Q.ll(0.26) 3.66(0.22) 1.10(0.17) 0.59(0.17)
SDB 16.66(0.66) 21.56(0.72) 16.63(0.76) 13.20(0.56) 10.20(0.55) 10.60(0.40) 7.65(0%X) 6.45(0.16) 4.33(0.12)
ZIEM 15.06(0.60) 15.05(0.76) 11.20(0.72) 8.69(0.56) 6.96(0.45) 4.99(0.30) 3.50(0.27) 0.36(0.15) 0.16(0.10)
81Ld8 6.20(0.69) 6.25(0.39) 7.67(0.50) 6.54(0.39) 5.27(0.22) 4.25(0.24) 2.52(0.24) 2.77(0.20) 0.77(0.05)
E&al 6.69(0.47) 5.47(0X) 2.92(0.29) 2.62(0.29) 2.17(0.22) 1.01(0.14) 0.39(0.12)
J2Quh O-lcm l-2 2-3 3-4 4-5 5-7 7-9 9.11 II-13
xd#a 3.44(0.29) 5.30(0.31) 5.26(0.36) 3.47(0.29) 3.14(0.20) 1.96(0.16) 2.14(0.13) 1.01(0.13) 0.62(0.11)
Gaxhimica ef Cowwchimica Acla 54, pp. 237-245 Copyright 0 1990 Pergamon Press pk. Printed in U.S.A.
0016-7037/90/$3.00
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.oo
Seasonal variations in ‘Be activity in the sediments of Cape Lookout Bight, North Carolina E. A. CANUEL,’ C. S. MARTENS,’and L. K. BENNINGER’ ‘Curriculum in Marine Sciences, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA ‘Department of Geology, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA (Received May 23, 1989; accepted in revised formOctober 3 1, 1989)
Abstract-Short-term sediment accumulation rates were determined using activity distributions of 7Be in the surlicial sediments of a station (A-l) in Cape Lookout Bight, North Carolina (USA). This semienclosed coastal marine basin is characterized by high annual accumulation rates. Protected from high energy conditions by its geomorphology, the relatively quiescent waters of the bight’s interior cause it to act as an efficient settling trap for fine-~ned sediment. Lack ofbioturbation in the surface sediments allowed for variations in depth-integrated activity profiles of ‘Be to be interpreted as short-term accumulation events. Beryllium-7 inventories exhibited seasonal cyclicity with maxima occurring during spring (February through June). The inventory of ‘Be ranged from 5.8 to 32.6 dpm cm-’ and was always well in excess of the atmospherically supported value (X = 3.8 dpm cm-*; n = 15). Accumulation rates calculated from ‘Be activity profiles indicate that the delivery of particulate matter to the sediments of Cape Lookout Bight is not constant throu~out an annual cycle. The highest monthly accumulation rates appeared to be associated with north/northeast storm activity. Inputs were generally highest during late winter/early spring when storm frequency is greatest. Short-term accumulation rates derived from this study agree with *“Pb rates calculated for a ten-year period of accumulation. The annual sediment accumulation rates for each of the two years over which the study was conducted was found to be 4.8 + 0.8 g cm-* y-’ and 3.7 + 1.2 g cme2 y-‘. I~ODW~ION COSMIC
mulation in nearshore systems (~ISHNASWAMI et al., 1980; OLSEN et al., 1985,
SPALLATIONreactions occurring within the Earth’s atmosphere produce a suite of natural radionuclides. Knowledge of the production and delivery rates of cosmic ray-produced isotopes allows for the quantification of a variety of geochemical processes over a range of time scales. One of the most useful applications of natural radionuclides has been as chronometers (TUREKIAN and COCHRAN, 1978; GOLDBERG and BRULAND, 1974). Naturally produced radionuclides have been used to determine sediment accumulation rates both in coastal and oceanic sediments. Although mdiochronomet~c tracers have been widely used in the measurement of sediment a~umulation rates, until recently most applications have been concerned with time scales greater than a year. Beryllium-7 (t,,:! = 53.3 days) is produced by cosmic ray spallation reactions with nitrogen and oxygen in the Earth’s atmosphere. Though produced predominancy in the stratosphere, most of the 7Be delivered to the Earth’s surface is derived from the troposphere (DUTKIEWICZ and HUSAIN, 1985). This is due to the long residence time of stratospheric air (- 1 y) relative to the mean life (l/X = 77 days) of this nuclide. In the atmosphere, 7Be associates with aerosols and is delivered to the Earth’s surface by p~cipitation. gallium-7 remains soluble as its 7Be+2species in “acidic” rainfall. However, once in the marine environment 7Be is adsorbed onto inorganic particulate matter by cationic adsorption processes (BLOOM and CRECELIUS, 1983). The unique properties of ‘Be (i.e., a short half-life, its reactivity with particulate matter, and a measurable rate of delivery to the surface of the Earth) have been utilized in recent years to study sediment accuRAY
1986; DIBB and RICE, 1989).
In
this paper we report the results of a study investigating the distribution of ‘Be activity in the surface sediments of Station A-l located in Cape Lookout Bight, NC. This back barrier island lagoon, located on North Carolina’s Outer Banks (34’37W, 76*33W) is chamcterized by rapid sediment accumulation and insignificant bioturbation. This paper examines the use of’Be in quantifying sediment accumulation in the bight’s interior on a monthly time scale. Owing to the lack of biological mixing, we have been able to interpret the depth distribution of ‘Be in these sediments as resulting from short-term accumu~tion. Inventories of ‘Be have been used to quantify seasonal variations as well as episodic pulses in sediment delivery to this coastal site. Also reported are total (wet and dry) atmospheric delivery data for 7Be over a sixteen month period at a coastal site (Morehead City, NC) in the southeastern United States. STUDY SITE Cape Lookout Bight is a 2 km* body of water located approximately 115 km southwest of Cape Hatteras on the Outer Banks of North Carolina, USA (Fig. 1). This shallow, semienclosed basin reaches a m~imum depth of 8 m (mean low tide) at Station A-l, the site of this study. Located behind a barrier island cuspate foreland, the bight waters are influenced by the back barrier island lagoon system as well as offshore waters. Two barrier islands (Shackleford Banks and Core Banks) oriented at approximately 90” to one another, an inlet (Barden Inlet), and a no~hw~~y migrating spit define the system (see Fig. 1). Although the water column of Cape 237
238
E. A. Canuel, C. S. Martens, and L. K. Benninger
f
cur
FIG. 1. The Cape Lookout Bight, NC, study site. Station A- 1 (see CHANTONet al., 1983, for more detail) was the location at which our sampling took place. Rain samples were collected from the roof of the Institute of Marine Science,MoreheadCity, NC, approximately 20 km from Station A- 1.
Lookout Bight remains oxygenated throughout the year, sediments at Station A-l are anoxic below the upper few millimeters except during winter months when dissolved NO; and labile Fe and Mn oxides are present in the upper 5-7 cm (“brown-oxidized” layer). Winter sediments are oxidized because of a coupling between high rates of accumulation and decreased rates of metabolic activity in response to lower temperatures. Despite the fact that winter sediments do become oxidized, microelectrode profiles taken during early spring indicate that dissolved oxygen does not penetrate deeper than the upper few millimeters (CANUEL, unpubl. data). Throughout most of the bight (- 1.6 km2) sediments are predominantly high-porosity mud. Particles in the upper 1.5 m of the sediment column are mostly (90%) silts and clays (primarily illite, kaolinite, and silt-sized quartz grains) with mean diameters of 5-10 pm (WELLS, 1988). Mechanical interlocking of very small quartz grains may be an important factor in resisting resuspension. Work by other investigators has demonstrated that these cohesive sediments are resistant to resuspension at bottom currents in the 25-50 cm set-’ range (WELLS, 1988). The organic carbon content of sediments in the upper 15 cm range from 3.3 to 4.0% (dry weight) and do not exhibit any pronounced seasonal variation (CANUEL, unpubl. data). During spring (February through May) a small, patchily distributed population of spionid polychaetes (~2 cm in length) colonizes the upper 3-4 cm of the sediment (BARTLETT,198 1). These organisms are surface deposit feeders (FAUCHALDand JUMARS, 1979) believed to feed discriminantly on sediment particles of particular size and content. Despite the seasonal occurrence of polychaetes, bioturbation is thought to be unimportant in Cape Lookout Bight sediments. This is supported by the absence of burrows and the presence of fine-scale (- 1 cm) laminations in the upper 4 cm of box cores taken during spring (MARTENSand KLUMP, 1984). Box core X-radiographs taken during summer months indicate that there may be some reworking in the upper 10
cm associated with bubble tubes. These tubes, approximately 2-20 mm in diameter, are produced by vertically migrating, methane-rich gas bubbles. Cape Lookout Bight acts as a settling basin for particulate matter in transit between the back barrier lagoon system and offshore waters. During ebb tidal flows, waters high in suspended matter from Back and Core Sounds enter the bight via Barden Inlet. After an unknown residence time in the water column of Cape Lookout Bight, a portion of this particulate matter is removed to the sediment. The remaining fraction is either transported offshore or is returned to the back barrier sounds on flood tide through Barden Inlet. CHANTON et al. (1983) determined the sediment accumulation rate at Station A-l to be 4.1 f 0.7 g cmm2 y-l based on “‘Pb geochronology and burial of hurricane sand layers. They found that accumulation of excess 210Pbwas 30 times above its estimated atmospheric input, thereby demonstrating the efficiency with which the bight traps tidally transported particles. This high rate of sediment accumulation and the absence of physical mixing by bioturbation at station A-l allowed us to use the depth distribution of ‘Be to quantify accumulation processes on a monthly time scale. METHODS Atmospheric inputs Total deposition (wet and dry) was collected on a monthly basis from June 1987 through September 1988 from the roof (-30 feet above ground level) of the Institute of Marine Sciences, Morehead City, NC, approximately 20 km west of Cape Lookout Bight. The sampling was conducted using a polyethylene SEAREX (Sea-Air Exchange Program) rain collector (see figure in PELTZERet al., 1984)
with an effectivecollectingarea of 0.5 m2.The amount of precipitation was monitored throughout each sampling period At the conclusion of each month, the volume of rain collected was measured. The collector was acid washed and the rinses added to the sample. A beryllium standard (1 ml at 1 mg Be per ml in 1% HCI; Sigma Chemical Co.) was partitioned equally between each of the carboys containing a given month’s sample and each carboy was homogenized. An iron solution (100 mg Fe as FeC&) was then added to each carboy. The carboys were mixed and the pH increased to 9 by the gradual addition of approximately 1 liter of dilute N&OH ( 1:1 reagent NH,,0H:H20) in order to precipitate oxides or hydroxides of iron and beryllium. The carboys sat for at least overnight, and the precipitate from each was collected and combined to make a single sample representing the total atmospheric input of ‘Be for that month. The combined precipitate was transferred to 25 ml plastic centrifuge tubes and concentrated by centrifugation. The N&OH solution was decanted and the precipitate dissolved in 6 N HCl. Samples were filtered through Nuclepore filters (0.4 pm) and a known volume of the 6 N HCl solution sealed in 95 cm3 aluminum cans. The activity of a suite of gamma emitters was determined using an intrinsic germanium detector (detector resolution was 1.9 keV at 1.3 MeV) and a multichannel analyzer system. Samples were counted for approximately 24 h, and the activity of ‘Be as well as other gamma-emitting radionuclides was determined. Counting efficiencies were determined by calibration over a range of can height geometries with a ‘Be standard of known activity (Isotope Products Laboratories, Burbank, CA). Recovery of the beryllium standard added to the rain samples was determined by flame (nitrous oxide-acetylene) atomic absorption spectrophotometry. All data have been corrected for beryllium standard recovery (55-85%), and all errors are statistical counting errors expressed as + 1 standard deviation (FRIEDLANDER et al., 1981).The
‘Be activity for each sampling period was decay-correctedto the middle of each sampling period using the C = C,,e-” relationship where “t” equals one-half the number of days in each sampling period.
Seasonal variation of ‘Be in sediment
Spatial variability in surface sediments
Table
In order to assess spatial variability in the surface sediments at the field site, a study was undertaken to measure differences in the inventory of ‘Be and other gamma emitters (“K and “‘Cs), percent organic carbon, and total nitrogen content of the surface (upper 3 cm) sediments of Cape Lookout Bight. Ten cores (4.5 cm i.d.; lo15 cm length) were collected in September 1987 near Station A-l. A single core was collected at a central site, and three cores spaced approximately 1 m apart were then collected along each of three transects moving away from the location at which the first core was collected. The upper 3 cm of each core was removed and dried at 60-70°C. The dried sediment was ground with a mortar and pestle and sealed in 95 cm3 aluminum cans. Samples were counted for approximately 24 h and the activity of a suite of gamma emitters determined by gamma ray spectroscopy (LARSENand CUTSHALL, 1981). The 3 cm increment was chosen because a large fraction (-30%) of the ‘Be inventory was found to occur over this region. A second variability study was conducted in July 1988 to examine variation in the distribution of 7Beactivity at a sample depth resolution comparable to that used in processing sample cores. In this study, five small cores (4.5 cm id.; 8-10 cm in length) were collected. The cores were extruded and the upper 3 cm sectioned into three 1 cm intervals. Sediment sections were treated according to the same methodology as the above-mentioned samples.
Sediment sampling Sediment cores were collected from Cape Lookout Bight over a two-year period on approximately a monthly basis. All samples were obtained by divers using SCUBA so as to minimize disturbance to the surficial sediment. Cores were taken using lucite tubes (8 cm i.d.) approximately 30 cm in length. Samples were returned to the laboratory under quasi-ambient conditions (dark and at approximate
Month
239
Beryllium-7 Atmospheric Inputs Morehead City, NC
1.
Precipitatfo”
June ‘07 July August September October November December January ‘88 February March April nay JUn.? July Augus f September
Activity
(cm)
(dpm 1-l)
7.9 6.4 23.8 19.4 0.4 14.9 4.9 16.3 9.8 6.2 8.3 19.7 8.0 9.9 31.8 4.5
39.12 154.35 60.65 63.39 26.17 65.58 101.79 51.61 88.28 91.97 102.40 95.21 142.92 91.69 41.01 62.94
FlUX (dpm cm-’
moel)
0.37 1.84 2.49 1.66 0.23 1.33 0.94 0.94 1.31 1.07 1.63 2.63 1.52 1.30 1.45 0.36
Inventory (dpm cm“)
3.91 4.31 3.15 3.46 3.20 3.16 3.45 3.41 3.94 5.30 5.11 4.76 4.67 3.52
ambient temperature) where they were extruded and sectioned. Sediment sections were dried at 60-70°C and ground into a fine powder using a mortar and pestle. The dry weight of each sediment section was determined and samples were sealed in 95 cm3 aluminum cans. Samples were counted for 24-48 h and the ‘Be activity determined directly by gamma ray spectroscopy. Activity values were corrected for counting efficiency in the same way as rain samples. Beryllium7 activities are reported on a dpm g-’ dry weight sediment basis and statistical counting erron expressed as + 1 standard deviation. All ‘Be activity values have been decay-corrected to the middle of the interval between sample collections in the same manner as were the rain samples described above.
RESULTS
Morehead Citv. N.C. Atmospheric input of ‘Be
The mean (n = 16) monthly atmospheric deposition of 7Be was 1.3 + 0.7 dpm cmm2 (Fig. 2) at the Morehead City, NC, site. This value is consistent with average flux measurements at geographically comparable sites (1 .O and 1.1 dpm cmm2 at Oak Ridge, TN, and Norfolk, VA, respectively, OLSEN et al., 1985; and 1.1 dpm cm-’ at Solomons, MD, DIBB, J JASONDJ
FMAMJ
1997
JAS
1909
Morehead City, N.C. 8~rqllium.l AtmosphericInputs 3-
JJASONDJFMAMJJAS 1997
1999
2. Precipitation (cm mo-‘) and flux of ‘Be via precipitation (dpm cm-* mo-‘) at Morehead City, NC. Data are presented on a monthly basis for June 1987 through September 1988. FIG.
1989). The average daily flux was 0.04 dpm cme2, which is within the range of values (0.02 to 0.1 dpm cme2) reported for daily ‘Be fluxes by other investigators (WALTON and FRIED, 1962; PIERSON, 1963; LAL and PETERS, 1967). Using
the mean ‘Be lifetime for radioactive decay (T = l/X) of 77 days, this flux would support a steady-state inventory of 3.1 dpm cm-2. The measured monthly flux of 7Be ranged from 0.2 to 2.6 dpm cme2 (Table 1) at the Morehead City sampling station. In order to assess what component of the sediment inventory was attributable to direct atmospheric deposition, an inventory of ‘Be was calculated by decay-correcting the previous month’s atmospherically derived standing crop and adding it to the present month’s atmospheric flux. Prior to June 1987, ‘Be flux data were not obtained; therefore, inventory data, including decay-corrected inputs from previous months, could not be calculated for the initial part of the study. For this reason, inventory values are only reported from August 1987 through the end of the study. These inventory data ranged from 3.2 to 5.3 dpm cme2 (Table 1) with a mean (n = 15) value of 4.0 + 0.9 dpm cmm2.
E. A. Canuel, C. S. Martens, and L. K. Benninger
240
BERYLLIUM-7
Horizontal variability in surface sediments
Horizontal variability of organic carbon, total nitrogen, and a suite of gamma-emitting radionuclides (‘Be, 13’Csand 4oK) is summarized in Table 2. The coefficient of variation (CV = 25%) was well below that for the population of inventories obtained during the monthly sampling (see below). These data indicate that the seasonal patterns reported below cannot be explained by horizontal variability at the sampling site. The study of July 1988 showed that the ‘Be activity is most variable in the surface section and systematically decreases with depth. The coefficients of variation were 26, 2 1, and 8% for the O-l, l-2, and 2-3 cm sections, respectively (Table 2). All inventories in this paper are reported as +25% which is, for most samples, an overestimate of the error associated with their determination.
-I f s 5 0 -10 m
ACTIVITY
DISTRIBUTION
-5
.
.
-10
..*
-1, ASONDJFMAMJJASONDJFMAMJJASO 1986 1987
-IS ,988
FIG. 3. Beryllium-7 activity (dpm g-‘) distribution with depth in Cape Lookout Bight sediments over the study period. Asterisks represent the mid-point of each sample interval analyzed.
Sediment inventory of ‘Be
The distribution of ‘Be activity in the upper 12 cm of Cape Lookout Bight sediments is illustrated in Fig. 3 for the entire study period. Isopleths of constant activity exhibit seasonal cyclicity. Activity in the surface sediments of Cape Lookout Bight was highest in late winter-early spring, exceeding 10 dpm g-’ during February through May 1987 and between February and June 1988. Penetration of the activity distributions was enhanced during summer months. Activity in the surface sediments is several times higher than has been previously reported, and the depth range over which ‘Be activity is present is greater than other investigators have found (KRISHNASWAMI et al., 1980; OLSEN et al., 1986; DIBB and RICE, 1989). These data suggest that Cape Lookout Bight
Table
2.
Cape Lookout Variability
Bight. NC Studies
September, Sample
1987
0rg.C (X)
Tot.N (X)
1 2 3
2.42 2.59 2.62
0.30 0.31 0.32
7.19 6.94 6.36
1.32 0.91 1.89
12.63 13.49 14.83
: 6 7 8 9 10
2.64 2.48 2.74 2.65 2.82 2.73 2.47
0.32 0.30 0.34 0.31 0.34 0.36 0.31
5.92 3.91 1.35 4.25 8.26 4.14 5.23
0.95 1.53 1.26 2.02 1.?2 1.61 1.43
12.68 12.57 14.71 14.09 14.32 14.41 14.14
j;
78. (dpm cm-‘)
13’CS (dpm cm-‘)
40K (dpm cm-‘)
2.61
0.32
5.96
1.46
13.79
s.d.
0.13
0.02
1.52
0.37
0.88
C.V.(X)
4.98
6.25
25.5
July, Sample depth
n
25.2
6.4
1986 x
(dpm g-l)
(d;id;-‘,
C.V.(X)
o-1
cm
5
10.31
2.68
26.0
1-2
cm
5
12.72
2.67
21.0
2-3
cm
3
14.99
1.12
7.5
sediments trap ‘Be efficiently relative to both the local atmospheric input and to sediments elsewhere. The inventory of ‘Be (dpm ‘Be cm-*) was determined by integrating polynomials fit to the data plotted as activity of ‘Be (dpm g-‘) versus mass depth (g cmw2). Mass depths expressed for 1 cm intervals and corrected for salt content were calculated according to the relationship gem-* = (1 - $)p where 4 is porosity as determined from 4 = O.~Z-~.~*and density (p) is 2.5 g cme3 (CHANTON, 1985). Beryllium-7 inventories ranged from 5.8 to 32.6 dpm cm-* with the highest seasonal inventory occurring in spring (between February and June) during both years (Table 3). The mean (n = 20) inventory value was 17.3 + 8.7 dpm cm-*. The total inventory was separated into two components, as is illustrated in Fig. 4: RESIDUAL inventory = inventory of previous sampling period decay-corrected to the date of present sampling. NEW inventory = total inventory (present sampling) - residual inventory. These designations make the assumption that ‘Be delivery and removal processes only occur in association with particulate matter. NEW inventory for each sample period was estimated by subtracting the RESIDUAL inventory (calculated by correcting the inventory of ‘Be in the previous sample period for radioactive decay to the time of subsequent sampling) from the total ‘Be inventory. Results for each core are illustrated in Fig. 5. Occasionally, the total inventory was entirely RESIDUAL (i.e., no delivery), and in some cases the RESIDUAL inventory was greater than the total inventory, indicating removal had taken place. Determination
of sediment
accumulation
rates
Sediment accumulation rates were calculated using the distribution of ‘Be activity in the surficial sediments of Cape Lookout Bight. Rates were determined according to the lower schematic in Fig. 4. The mean ‘Be activity (dpm g-l) of newly deposited sediment was determined by averaging activity in
Seasonal variation of ‘Be in sediment
Table
3.
Beryllium-7 Inventory Cape Lookout Bight Sediments
Sample
Ill”elltMy
NEW Inventory’
(dpm cm-‘)
E/14/86 9/11/86 11/21/86 Z/24/87 3/20/87 4/21/87 5128187 6/28/07 B/13/87 9/25/87
Mean Activity
(dpm cm-‘)
5.75(1.44)2 9.86(2.47) 9.16(2.29) 32.56(8.14) 25.28(6.32) 13.18(3.30) 25.56(6.39) 17.26(4.32) 10.27(2.57) 23.22(5.81)
5.86(2.66) 5.24(2.49) 29.90(8.17)3 3.80(8.29)3 -5.32(5.68) 17.41(6.W3 0.18(6.07)3 0.78(3.50) 17.35(5.99)
Accumulation1
(dpm g-l)
(g cm-‘)
11.50(0.38) 10.33(0.54) 26.54(0.55)
0.X(0.23) 0.51(0.24) 1.13(0.X)
14.07(0.10)
1.24(0.48)
12.4UO.64)
1.39(0.49) L i 4.78(0~32)~
11122/87 12/10/87 Z/27/80 3/29/t% 4/30/88 6/19/88 7/01/M 8/17/88 9/02/88 10101/88
12.25(3.06) 11.86(2.97) 12.38(3.ioj 25.15(6.29) 29.74(7.44) 31.49(7.87) 20.39(5.10) 14.95(3.74) 6.57(1.64) 8.78(2.20)
1.33(4.10)3 2.17(3.83)3 8.13(3.28) 16.88(6.62) i3.i5&52j 15.96(8.77)3 -6.55(8.45)3 3.,X8(4.65) -5.57c3.45) 4.27(2.47)
14.4X0.63) 17.54(0.88) 15.44(0.63) 15.49(0.64)
0.56(0.23) 0.96(0.38) 0.85(0.55) 1.03(0.57)
7.83(0.42) 4.41(0.25)
-0.71(0.44) 0.97(0.56) I: - 3.66(1.15)5
’ NW Inventory 2
3
4 5
and Mass Accumulation
Values in parenthesis (2 25% as determined
represent in spatial
NEY inventory less than error assumed to be neither delivery
since
preceding
error associated vith variability study). associated with nor removal of
sample
date.
inventory
measurement inventory.
Mass accumulation
(g cm-’
yr-1)
betveen
9-11-86
and 9-25-87.
Mass accumulation
(g cm-’
yr-‘)
between
9-25-87
and 10-l-88.
therefore
the surface region for those periods characterized by delivery or removal processes (Table 3). Dividing the amount of NEW inventory by the mean activity in the surface region yielded mass accumulation or removal (g cm-‘) over the time between samplings (Table 3 and Fig. 6). Accumulation rates were calculated by this technique rather than from the slope of a line fit to the semi-log plot of ‘Be activity vs. depth used by other investigators for several reasons. This data set demonstrates that the assumption of steady state at time scales of a month is not valid as neither the flux of the tracer nor the rate of accumulation remain constant at this time scale. Another reason for the new approach was that using the slope of lines fit to the data can only yield long-term rates as this approach gave rates of accumulation even for periods where there was not NEW inventory. Many of the activity profiles are not exponential in shape, and accumulation is often so small over a one-month interval (sometimes a single point) that we were not able to draw a line through this region. Lastly, the method used in this paper has as its sole assumption that we are able to adequately distinguish between the NEW and RESIDUAL pools of the 7Be inventory, an assumption that we feel is justified given the extensive data set we have collected. DISCUSSION
Atmospheric ‘Be deposition The distribution of ‘Be in nearshore sediments is determined by the direct atmospheric deposition of this tracer
241
(i.e., precipitation), by horizontal redistribution, and by the rate of sediment accumulation. Differences in the amount of precipitation, as well as the rate of stratosphere-troposphere exchange, can result in seasonal trends in the atmospheric deposition of ‘Be. For example, OLSEN et al. (1985) found that 40-45% of the annual ‘Be deposition occurred during spring when 30-35% of the annual precipitation at sites in Norfolk, VA, and Oak Ridge, TN, occurred. Precipitation at Morehead City, NC, varied considerably from month to month (4.5-3 1.8 cm mo-I). Other than the fact that August rainfall was consistently higher in both 1987 and 1988, there does not appear to be any seasonal pattern in rainfall for this site (Table 1 and Fig. 2). The relationship between atmospheric delivery of ‘Be and the amount of precipitation was found to be statistically significant (P = 0.04) despite there not being a strong correlation (r’ = 0.3) between the two. This weak correlation agrees with observations that the concentration of ‘Be in rain is not directly correlated with the amount of precipitation. MAHONEY and BONDIETTI(1985) demonstrated that the concentration of 7Be in rain decreased by a factor of 3 during a storm event which deposited approximately 6 cm of rain. Similarly, DIBB (1989) found that the specific activity of ‘Be in serial samples taken during a single precipitation event decreased over time. These data suggest that the concentration of ‘Be in rain may be higher during drier months and periods characterized by short-duration precipitation events. Precipitation was highest during the month of August in both 1987 and 1988; however, the atmospheric flux of ‘Be was not highest for either of these sample periods. This is possibly due to the fact that rainfall was not partitioned equally throughout the month. Rather, a few events characterized by prolonged precipitation account for most of the rainfall. During both August 1987 and 1988, more than 75% of the monthly rainfall resulted from four “events.” In fact, 42% of the total monthly rainfall (3 1.8 cm) in August 1988 occurred on a single day. Conversely, months with less total precipitation but whose rainfall was distributed over several short-duration events (see July 1987 and 1988 data in Fig. 2) exhibited an enhanced 7Be flux relative to the relationship determined for the atmospheric flux of 7Be vs. the amount of precipitation. An alternative explanation for the lack of a clear relationship between the amount of rainfall and the ‘Be flux is that dry deposition was an important mechanism in the delivery of 7Be. Other investigators, however, have not found this to be the case (OLSENet al., 1985). The highest flux of ‘Be occurred during spring, possibly in response to the mid-latitude folding of the tropopause. This phenomenon is thought to occur during spring (late April to early June) and results in increased exchange between the stratosphere and troposphere. Enhancement of the stratospheric component results in an increase in the tropospheric inventory of ‘Be. Both the May and June flux measurements of ‘Be were higher than the mean flux over the entire study period and support a similar finding by DIBB (1989). There does not appear to be a systematic relationship between the ‘Be atmospheric delivery data and NEW ‘Be inventory in the surface sediments of Cape Lookout Bight. In other words, sample periods characterized by higher than
242
E. A. Canuel, C. S. Martens, and L. K. Benninger ‘Be
ACTIVKV .( dpm
- !$‘I I
0
0
10
20
MEAN NEW ACTIVITV
MASS ACCUMULATION
( 0 cm-2 ) = l
NEW ~NVENXFW ohm
MEAN NEW ACTIVITV
l
( dpm
cm’* 1 l
g’l )
Em. 4. Schematic illustrating how inventory was classified and accumulation rates were calculated. In the first step the ‘Be inventory was separated into its NEW and RESIDUAL components. The NEW inventory (dpm cmV2) was then divided by the mean activity (dpm g-‘) of newly deposited sediment in order to determine mass accumulation (g cme2).
average delivery of 7Be from precipitation do not reflect an enhanced ‘Be activity signal (see Tables 1 and 3). This may be due to the lag between the time required for particles to scavenge the 7Be signal from the water column (OLSENet al., 1989, estimated 7Be removal rates to be - 15% day-’ in the Susquehanna-Chesapeake Bay system) and the time it takes for this particulate matter to be delivered via horizontal transport to the bight sediments. Alternatively, because the variation in atmospheric delivery is small relative to the monthly variation in the distribution of 7Be in the sediments, small differences in the atmospheric signal may not be discernible using our methods. A period of the atmospheric study conducted at the Morehead City, NC, station overlapped that of DIBB (1989) at Solomons, MD. Comparison of these two studies over June through October 1987, when they overlapped, demonstrates that local atmospheric deposition can be quite variable over short time scales despite the similarity in long-term averages. Over this period, the Solomons, MD, site received 38.3 cm of precipitation and the flux of 7Be was 4.4 dpm cm-*. At
Morehead City, the precipitation over the same time period was 65.9 cm and the ‘Be flux was 6.6 dpm cm-*. This comparison suggests that when working with the inventory of 7Be in a given sediment system it may be necessary to measure local atmospheric deposition.
Cape Lookout Bight sediment accumulation The ‘Be activity distributions for each sample period are presented in Fig. 5. The total inventory (area under the polynomials fit to the data) has been separated into NEW and RESIDUAL components. Viewing these profiles in sequence, we are able to make month-by-month comparisons of the data on an annual basis over the study period. These data suggest that seasonal patterns exist in the delivery of ‘Be to the sediments of the Cape Lookout Bight system. Spring months (February through June) have the highest total inventory of ‘Be as well as the largest fraction of NEW inventory. Beryllium-7 inventories during the summer season ap-
Seasonal variation of ‘Be in sediment
243
BERYLLIUM-7 ACTIVITY ( dpm - g ‘l)
‘88
‘87
JAN
FEB
MAY
JUN
JUL
AUG
SEP
OCT
NOV
DEC
FIG. 5. Depth distribution of ‘Be activity for each sample. The darkened region of each month’s inventory reflects the RESIDUAL inventory. The area representing each month’s RESIDUAL inventory was placed so as to illustrate the amount of mass a~umuIatio~.
pear to be largely RESIDUAL, reflecting radioactive decay of inputs during spring. There appears to be a deepening in the distribution of ‘Be activity for summer sample periods despite the lack of evidence for new inputs. This feature of the summer data may be explained by sediment disturbance caused by methane bubble tubes. Throughout the rest of the year, ‘Be inventories are low, with the exception of samples appearing to reflect episodic delivery events such as September 1987. Assuming that ‘Be is neither added to nor lost from the sediments except in association with particulate matter, we can attribute changes in the NEW and RESIDUAL pools of the ‘Be inventory in the sediments of Cape Lookout Bight to variations in sediment delivery. This of course assumes that evidence demonstrating that biological reworking in the sediments of Cape Lookout Bight is an unim~~nt process on an annual basis is also true at shorter time-scales. We do know that macrofauna are virtually absent from Station Al during warm summer and fall months (BARTLETT, 1981) when sediments are anoxic. Rates of sediment accumulation in Cape Lookout Bight were variable over the course of this study (Fig. 6). Accumulation rates calculated for sample periods characterized by input ranged from 0.4 to 1.4 g cm-’ mo-’ (5 = 0.8; n = 11). These accumulation rates sum to an annual rate of 4.8 f 0.8 g cm-’ y-’ for the period between 11 September 1986 and 25 September 1987 and 3.7 -t 1.2 g crns2 y-l between 25 September 1987 and 1 October 1988 (Table 3). Both ‘Be accumulation rates are within the lu uncertainty of the 4.1 + 0.7 g cm-’ y-’ rate determined for the same station using *“Pb geochronology (CHANTON et al., 1983) and major storm sand layers. The close agreement between the 7Be and *‘OPbstudies is somewhat surprising considering that the 2’0Pb rate has been averaged over a ten-year period of accum~ation (1971-198 1) while the 7Be rate was determined by summing the short-term rates of accumulation over the 1986- 1988 period of this study. The agreement between these rates of accumulation, determined independently and over very different time scales, supports previous assumptions
regarding the steady-state nature of accumulation in Cape Lookout Bight sediments at time scales of years to decades. Storm ~~~uen~eson sediment delivery
Monthly sediment accumulation at Station A-l during spring is in excess of the mean annual accumulation rate for both years of the two-year study. High rates of accumulation were also measured during spring in preliminary ‘Be work conducted in Cape Lookout Bight (CHANTON, 1985). Northeast winds predominate along the North Carolina coast during winter and spring (PORTER,unpubl. data). It is plausible that under such conditions, delivery of particulate matter to the sediments of Cape Lookout Bight may be enhanced due to increased turbidity in the back barrier lagoons (Fig. I). During a station occupied during spring tide, while winds were blowing from the northeast, increased bottom currents were measured near Station A-l (WELLS, 1988). These northerly winds force water southward through Core Sound
Sediment Accumulation Rates
DG. 6. Calculated sediment a~~rn~tion/~rnov~ rates on a monthly basis for each sample period. During h&arch,April, June, August, November, and December of 1987 and in July and August 1988 there was no significant change in the NEW inventory (see Table 3), indicating addition or removal of new sediment was unimportant.
244
E. A. Can&l, C. S. Martens, and L. K. Benninger
enhancing ebb tidal flows. Under such conditions, extended ebb flows (i.e., missed good tides) have been recorded. It is probable that delivery from the back barrier lagoon system is increased under such circumstances in response to currents through Barden Inlet which persist for longer than the typical tidal cycle. Such conditions could result in an increased inventory of ‘Be activity in the surface sediments. Evidence supporting the relationship between northeast storm activity and enhanced delivery of p~iculate matter is demonstrate by the high ‘Be inventory for the September 1987 sample. This sampling took place during a period when winds were from the north and in excess of 20 mph (average wind speeds are 10 mph for this time of year) over a period of several days. Our hypothesis that these meteorological conditions resulted in the increased delivery of particulate matter to the bight is supported by a high NEW inventory of ‘Be (17.4 dpm cm-‘) and the highest calculated sediment accumulation rate (1.4 g cmm2 mo-‘) observed during the study (Table 3). SUMMARY AND CONCLUSIONS Delivery of ‘Be from its atmospheric sources was highest in spring, as has been observed in recent comparable studies (OLSENet al., 1985; DIBB, 1989). This seasonality is thought to result from the mid-latitude folding of the tropopause. The atmospheric flux of ‘Be was not strongly correlated with precipitation patterns despite rainfall being the most important delivery mechanism for this radionuchde. Our data demonstrate that ‘Be can be a useful tracer of sediment accumulation in shallow coastal systems una&cted by bioturbation. By examining short-term changes in the sedimentary inventory of ‘Be, this technique is able to distinguish between NEW and RESIDUAL components of the inventory and to derive sediment accumulation rates on time scales of a month. Cores obtained before and after major storm events demonstrate that changes in the ‘Be inventory can be used to quantify the impact of individual storm events, which may result in the delivery of pulses of new sediment by horizontal transport processes. Variations in sediment ‘Be inventories were not closely coupled with variations in its atmospheric input. This is probably due to a lag in the time it takes for particles in the water column to scavenge ‘Be and be subsequently delivered to the sediments. Both the accumulation and inventory data appear to be associated with north-northeast storm activity with maxima occurring in late winter-early spring (i.e., February through June). Ac~~o~le$~~e~fs-The authors would like to thank Dan Albert, Jeff Chanton, Emmett Duffy, George Pauly, and Frank Wilson for diving and field assistance. Frank Wilson and Hugh Porter assisted in thicollection of rain samples and precipitationdata when one of the authors (E.A.C.) could not be at the collection site. Emmett DuI@ and Marc Alperin provided helpful discussions and comments in the early stages of the preparation of this manuscript. Reviews by R. Carpenter, D. Hammond, and an anon~ous reviewer cont~buted significantly to the final preparation of this paper. The faculty and staff of the UNC Institute of Marine Science in Morehead City, NC, generously provided laboratory space and boating facilities. Financial support was provided by NSF grants CKX8416963 and OCE-8716528 (CSM) NASA grants NAGW-1455 and NAGW-834 (CSM), and a UNC Limited Service Traineeship (EAC). Editorial handling: K. W. Bruland
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323-331. J. P. (1985) Sulfur mass balance and isotopic fractionation
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Beryllium-7
2-4 4-6 6-6 6-10 to-12 12.14
6.OQ(O.27) 2.64(0.20) 1.97(0.20) 0.29(0.11) 0.07(0.07)
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Appendix Activity Distributions
(dpm g”)
aa35
llaB.6
.i&2uz
10.33(0.54) 3.26(0.44) 1.79(0.30) 1.29(0.24) 0.62(0.21) 0.62(0.17)
36.55(0.72) 14.52(0.37) 6.69(0.37) 1.76(0.24) 1.02(0.13) 0.72(0.17)
11.50(0.36) 4.16(0.29) 0.56(0.16) l.lO(O.16) 0.54(0.13) 0.74(0.20) 0.13(0.07)
245
3aaz
AaBz
16.60(0.36) 16.22(0.36) 7.41(0.31) 3.39(0.15) 0.66(0.17)
15.52(0.46) 6.01(0.30) 3.36(0.25) 0.46(0.13) 0.20(0.11)
Qsllul O-l cm 1-2 2-3 3-4 4-6 6-6 6-10 IO-12
s&Q2 14.75(0.76) 13.75(0.47) 13.70(0.62) 10.23(0.57) 7.40(0.42) 6.27(0.39) 3.35(0.31) 1.46(0.26)
-Rwlh O-lcm 1-2 2-3 3-4 4-5 5-7 7-9 9.11 11.13
§z2&32 6.97(0.70) 7.95(0.49) 6.57(0.49) 6.37(0.42) 7.35(0.39) 5.92(0.32) 4.13(0.23) 1.26(0.13)
&l3s! 0.16(0.06) 2.67(0.51) 6.51(0.57) 6.04(0.59) 4.10(0.39) 2.75(0.32) 4.90(0.47) O.SS(O.16) 0.45(0.20)
12.46(0.65) 12.23(0.50) 12.62(0.56) 9.79(0.53) 4.60(0.44) 3.76(0.26) 4KqO.44) 4.97(0.37) 3.97(0.44)
LE&E 10.92(0.74) 6.76(0.45) 6.02(0.42) 4.90(0.42) 2.36(0.35) 2.42(0.26) 2.23(0.26) O.QQ(O.16) 0.69(0.13)
x&&a2 14.60(0.53) 9.50(0.31) 5.61(0.34) 4.76(0.29) 4.40(0.31) 2.11(0.17) 0.96(0.15) 1.52(0.17) 0.71(0.11)
zws8 16.13(0.70) 10.70(0.55) 4.60(0.36) 3.01(0.30) 2.16(0.30) 1.67(0.25) 1.17(0.22) 0.74(0.16) 0.37(0.15)
QQ@h O-lcm l-2 2-3 3-4 4-5 5-7 7-9 9.11 II-13
ikz9s 15.27(1.31) 17.53(0.76) 16.66(0.71) 16.50(0.73) 9.74(0.43) 3.19(0.26) 1.46(0.11) 1.39(0.22) 0.63(0.20)
MQI8.8 15.46(0.79) 16.31(0.66) 17.95(0.74) 16.96(0.64) 11.75(0.47) Q.ll(0.26) 3.66(0.22) 1.10(0.17) 0.59(0.17)
SDB 16.66(0.66) 21.56(0.72) 16.63(0.76) 13.20(0.56) 10.20(0.55) 10.60(0.40) 7.65(0%X) 6.45(0.16) 4.33(0.12)
ZIEM 15.06(0.60) 15.05(0.76) 11.20(0.72) 8.69(0.56) 6.96(0.45) 4.99(0.30) 3.50(0.27) 0.36(0.15) 0.16(0.10)
81Ld8 6.20(0.69) 6.25(0.39) 7.67(0.50) 6.54(0.39) 5.27(0.22) 4.25(0.24) 2.52(0.24) 2.77(0.20) 0.77(0.05)
E&al 6.69(0.47) 5.47(0X) 2.92(0.29) 2.62(0.29) 2.17(0.22) 1.01(0.14) 0.39(0.12)
J2Quh O-lcm l-2 2-3 3-4 4-5 5-7 7-9 9.11 II-13
xd#a 3.44(0.29) 5.30(0.31) 5.26(0.36) 3.47(0.29) 3.14(0.20) 1.96(0.16) 2.14(0.13) 1.01(0.13) 0.62(0.11)