Development and reworking of a seasonal flood deposit on the inner continental shelf off the Atchafalaya River

Continental Shelf Research 20 (2000) 2267–2294

Development and reworking of a seasonal flood deposit on the inner continental shelf off the Atchafalaya River Mead A. Allisona,*, Gail C. Kinekeb, Elizabeth S. Gordonc, Miguel A. Gon˜ic a Department of Geology, Tulane University, New Orleans, LA 70118, USA Department of Geology and Geophysics, Boston College, 140 Commonwealth Ave., Chestnut Hill, MA 02167, USA c Marine Science Program and Department of Geological Sciences, University of South Carolina, Columbia, SC 29208, USA b

Abstract Sediment cores and water column measurements of suspended sediment and flow conditions were taken on the continental shelf off the Atchafalaya River in Louisiana to examine the development and reworking of a seabed flood layer with seasonal variations in river discharge and hydrodynamics. Five stations in water depths of 5–23 m were occupied on the Atchafalaya inner shelf on four cruises from October 1997 to March 1999 representing a range of freshwater input and wave energy conditions. Downcore profiles of the short half-life (53 d) cosmogenic radiotracer 7Be showed a three to fivefold increase in seabed inventory and an increase in depth of penetration during the 1998 high Atchafalaya discharge period (April) at two inshore stations (5–7 m water depth). X-radiograph evidence of the absence of biological mixing at these sites suggests that the 7Be data is recording the deposition of a 1–3 cm thick annual flood deposit. The organic carbon contents and stable carbon isotopic compositions of this flood deposit are distinct and reflect the increased terrestrial influence of the riverine sediment flux. 210Pb and 137Cs sediment profiles indicate that this seasonal deposit is two to six times the long-term (e.g., decadal) accumulation at these sites. Passage of cold fronts on 3–7 d timescales interrupts the formation of these flood deposits, particularly during the rising to early high discharge period (December–March). The depth of sediment resuspension landward of 10 m water depth during these events may reach 1 cm and decreases offshore. Offshore stations (  20 m water depth) show only a small increase in deposition during the high Atchafalaya discharge period. Redistribution of sediment from shallower parts of the shelf during the remainder of the year is likely a major supplier to these areas. A station east of the *Corresponding author. E-mail address: [email protected] (M.A. Allison). 0278-4343/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 8 - 4 3 4 3 ( 0 0 ) 0 0 0 7 0 - 4

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Atchafalaya mouth exhibits no correlation with discharge and no long-term accumulation, indicating minimal influence from the Mississippi discharge 150 km to the east. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Mississippi River; Deltas; Sedimentation

1. Introduction The Atchafalaya River is a major distributary of the Mississippi River that leaves the main course 320 km upstream of the Gulf of Mexico near Simmesport, Louisiana. For several centuries until the construction of control works in 1963, the Atchafalaya captured an increasing percentage of the Mississippi discharge, a process that marks the initial stages of delta switching leading to the formation of a new delta lobe along the Louisiana coast (Penland and Boyd, 1981). Beginning in the 1950s, the lakes and swamps of the Atchafalaya basin, which prior to this time had served as a repository for much of the sediment discharge (Roberts et al., 1980), were infilled to the point where sediment was discharged directly into Atchafalaya Bay (Fig. 1) and the adjacent continental shelf. Several studies (Shlemon, 1972, 1975; Roberts et al., 1980; Van Heerden and Roberts, 1988) have documented the ensuing growth of coarse-grained subaerial deltas at the mouth of the Atchafalaya and Wax Lake outlets into Atchafalaya Bay. Suspended sediment discharged onto the adjacent shelf has also reinitiated mudflat accretion along the west Louisiana chenier plain (Fig. 1) as it is carried westward by prevailing coastal currents (Wells and Kemp, 1981).

Fig. 1. Map of the study area in the Gulf of Mexico. Arrows on the map indicate the location of major riverine discharge: (A) is the Atchafalaya River outlet, (B) is the Wax Lake outlet, and (C) is the Southwest Pass outlet of the Mississippi River Balize delta lobe.

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The new phase of delta growth associated with the Atchafalaya provides an opportunity to examine the mechanisms of sediment delivery to a shallow bay and low-gradient adjacent shelf dominated by a riverine plume undergoing expansion and deceleration with pronounced bed friction. This situation contrasts greatly with the bouyancy dominated, shelf-edge regime of the mature Balize delta lobe of the Mississippi (Wright and Coleman, 1974) just 150 km to the east. Of particular interest are the dynamics of sediment storage and remobilization on the incipient inner shelf prodelta lobe. These sediments are potentially an important site for the alteration and burial of organic carbon (Gon˜i et al., 1998) and other elements (e.g., Fe, Mn, S) involved in redox cycling. Studies by Crout and Hamiter (1981), Chuang and Wiseman (1983), Kemp (1986), and Moeller et al. (1993) suggest the delivery of Atchafalaya sediment to the seabed is controlled by seasonally variable meteorological forcing, particularly by cold front passage, as well as by the riverine discharge. The objective of the present study was to monitor seasonally the water column and seabed of the inner continental shelf adjacent to the mouth of the Atchafalaya River to determine (1) if a well-defined seabed flood deposit forms during periods of high Mississippi–Atchafalaya sediment discharge, (2) how water column processes associated with meteorological forcing trap and rework this deposit and (3) the relative quantities of fine-grained sediment that are incorporated into an inner shelf mud deposit or are dispersed to more distal parts of the system.

2. Study area The Atchafalaya River has been a distributary of the Mississippi since at least the 1500s (Fisk, 1952). Capture of Mississippi water by the higher gradient Atchafalaya was relatively limited until 1839, when log jams were cleared from the Atchafalaya channel and dredging was initiated (Morgan et al., 1953). Mississippi discharge through the Atchafalaya channel increased from about 13% in 1900 to 30% by 1952. To prevent total capture of the Mississippi drainage, flow into the Atchafalaya was regulated after 1963 at approximately 30% of total flow by the Old River control structure constructed by the US Army Corps of Engineers. Actual discharge varies (15–29% of total flow) from year-to-year below the targeted 30% figure (Mossa, 1996). Average discharge into the upper Atchafalaya Basin at Simmesport, Louisiana, immediately below the control structure, mirrors the main Mississippi annual hydrograph, and is relatively regular because of the large catchment (3.6  106 km2). Maximum discharge in the period from 1930 to the present has averaged about 9500 m3s ÿ 1, and occurs in January–June (peak in April) coincident with spring runoff from the catchment (Fig. 2). Minimum annual discharge in September–October averages about 2500 m3 s ÿ 1. Interannual variability in discharge since the construction of the control work in 1963 has resulted in extremes from about 600 to 19800 m3 s ÿ 1. Suspended sediment concentrations for the Atchafalaya have been measured sporadically since the late 1940s and continuously since 1973. Mean suspended sediment concentrations were 370 mg l ÿ 1 in the 1980s (Meade and Parker, 1985), with an average suspended sediment load at Simmesport for the

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Fig. 2. Atchafalaya water discharge at Simmesport, Louisiana, immediately below the divergence of the Atchafalaya from the Mississippi River. Roman numerals I–IV denote the cruise dates of the present study. Each study was conducted over a 7–10 d period centering on the plotted date.

period 1952–89 compiled by the US Army Corps of Engineers of 84  106 metric tons per year. The total suspended composition includes 17% sand. Roberts et al. (1980) suggest that much of the coarse fraction is now bypassing the Basin to Atchafalaya Bay, accelerated by confinement of the lower basin to a single navigation channel in the 1960s that scoured and remobilized previously deposited sand. The water and sediment discharge from the basin reaches Atchafalaya Bay through two outlets, the natural Atchafalaya and the artificial Wax Lake cut in 1942, that carry 70% and 30% of the discharge, respectively. Atchafalaya Bay is a shallow (2–3 m deep), mud-floored bay composed of several sub-bays } Atchafalaya, Cote Blanche, and Vermilion (Fig. 1) } whose exchange with the shelf is partially obstructed in the eastern sector by oyster shell reefs. In the large flood of 1973, subaerial accretion of lobate deltas began at the Atchafalaya and Wax Lake outlets into Atchafalaya Bay, by a process of mouth bar sand accretion and channel bifurcation (Shlemon, 1975; Roberts et al., 1980; Van Heerden and Roberts, 1988). By 1990, over 64 km2 of land accretion had taken place; the only significant accretion in the entire Mississippi lower delta plain. Navigation channel dredging has slowed the rate of growth, particularly in the Atchafalaya outlet, where the channel is maintained to a depth of about 6 m all the way across Atchafalaya Bay, allowing for bypassing of sediment directly to the Gulf of Mexico. The Atchafalaya shelf is low gradient } the 10 m isobath is more than 40 km offshore in the area of the Atchafalaya outlet channel } and contains many relict

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sand shoals and shell reefs. Away from these features, bottom sediments are finegrained, and resemble suspended sediments in the Bay and inner shelf (mean particle diameter of 2–6 m m; Wells and Kemp, 1981). The shelf gradient offshore of Atchafalaya Bay increases seaward of 10 m to 1 : 2000. Tides are mixed with a mean amplitude of 60 cm, producing tidal currents of 510 cm s ÿ 1 landward of the 10 m isobath (Kemp, 1986). Waves are also relatively low energy and typically from the southeast, the prevailing wind direction; mean wave heights are about 1.5 m and periods 4.5–6.0 s (Wells and Roberts, 1980). Near-surface current speeds of 10– 50 cm s ÿ 1 have been observed on the inner shelf and are strongly influenced by the passage of cold fronts on 3–7 d timescales from October–May (Crout and Hamiter, 1981: Adams et al., 1982; Chuang and Wiseman, 1983; Kemp, 1986; Moeller et al., 1993). The typical frontal sequence consists of pre-frontal onshore winds, relatively large waves, and coastal setup, followed by post-frontal northerly winds. Frictional coupling of the shallow bottom with wind processes leads to wave resuspension of shelf sediment during these events. Wells and Kemp (1981) refer to the turbid (10–280 mg l ÿ 1) plume exiting Atchafalaya Bay as the Atchafalaya mud stream. The mud stream is confined to landward of the 10 m isobath along the southwest Louisiana coast except during frontal passages, and at most times is moving westward entrained in a residual current flow of about 10 cm s ÿ 1. Wells and Kemp (1981) calculate that this mud stream carries 53  106 m3 yr ÿ 1 of suspended sediment, almost half the volume of sediment exiting Atchafalaya Bay. The reinitiation of mudflat accretion along the downdrift chenier coast west of Atchafalaya Bay (Fig. 1; Morgan et al., 1953; Morgan and Larimore, 1957; Wells and Kemp, 1981; Roberts et al., 1989; Huh et al., 1991) has resulted in shoreline mudflat accretion rates reaching 60–80 m yr ÿ 1 in some areas (Roberts et al., 1989). Although the forcing mechanisms are poorly understood, sediment supply to the chenier plain shoreface occurs mainly during the late winter and early spring, when high Atchafalaya discharge coincides with the main period of cold front passage (Mossa and Roberts, 1990).

3. Methods 3.1. Field Four sampling cruises to Atchafalaya Bay and the adjacent continental shelf of western Louisiana were conducted on the R/V Pelican in October 1997, March and April–May 1998, and March 1999. Cruises were timed to examine the effect of seasonal differences in Mississippi–Atchafalaya river discharge (Fig. 2) and wave energy in the study area. Five seabed stations were selected to characterize inshore water depths of 5–8 m (MI6, WH6), offshore water depths of 20–25 m (WL1, WH1), and updrift areas (T1). These stations were sampled on each cruise using differential GPS for site relocation. Large diameter boxcores (50  50 cm) were collected at these stations and subsampled using 15 cm diameter PVC pushcores for organic and radiochemical analyses. Sub-cores were extruded at 1 cm intervals onboard ship and

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samples were sealed for return to the laboratory. Sediment samples for elemental and isotopic analyses were collected from sub-cores on the first three cruises and refrigerated during transport back to the laboratory, then stored frozen until analysis. An additional plexiglas tray sub-core (2.5  10 cm cross-section) was taken from the box cores on the March 1999 cruise for X-radiography onboard ship using a portable Kramex Model PX-20N unit at 15 mA/70 keV. Suspended sediment samples from the mouth of the Atchafalaya River were collected on the first three field studies utilizing the seawater intake line aboard the R/V Pelican (3 m below water surface). The particulates were recovered via centrifugation and oven-dried at 558C. Water column data on fluid, flow, and suspended sediment conditions were collected at  40 stations in a series of shore-normal transects using a small profiling tripod (Sternberg et al., 1991). Instrumentation on the tripod included a MarshMcBirney electromagnetic current meter, KVH Industries model ROV103 fluxgate digital compass, D&A Instruments Optical Backscatterance Sensor (OBSTM), Ocean Sensors OS200 CTD, and a pump system capable of obtaining water/ suspended-sediment samples at discreet depths in the water column. Suspended sediment concentrations were measured directly by filtering the pumped samples through preweighed 8 m m Millipore filters which retain most particles larger than 1 m m as shown by Sheldon (1972) and substantiated by size analysis and comparison with gravimetric analysis using other filter types (Kranck and Milligan, 1979). These samples were used for calibration to convert OBS output to mass concentration of suspended sediment which varies for each environment with particle size and characteristics (Sternberg et al., 1991). The calibration was repeated after each cruise with over 200 water samples and resulted in a linear regression with uncertainty (root mean square value) of  24 mg l ÿ 1 (October 1997) to  66 mg l ÿ 1 (April 1998) in the range 0-2500 mg l ÿ 1 (results similar to those are plotted in Sternberg et al. (1991) and Kineke and Sternberg (1992)). Additional measurements included salinity, temperature, and light transmission from  3 m depth using a continuous flowthrough system on the ship. Shallow water surveys (58 m) were conducted from small boats and included water column profiles using a hand-deployed CTD/OBS system. 3.2. Radiochemical analyses A variety of particle-reactive radioisotopes have been applied to the study of seabed mixing and accumulation in coastal environments. The use of multiple tracers (210Pb, 137Cs, 234Th, and 7Be) allows for examination of seabed processes on several time scales, given that the characteristic time scale of each tracer is about 4–5 halflives. 210Pb (t1=2 ¼ 22:3 yr) and 234Th (t1=2 ¼ 24 d) are naturally occurring daughter products of the 238U decay series. 210Pb has four main sources in continental shelf environments: atmospheric deposition from 222Rn decay and scavenging by rainfall, riverine supply, in situ production from 226Ra in sediments, and transport from offshore waters. Because 234Th is not supplied from the atmosphere, riverine supply tends to be minor relative to in situ production, and inventories in the sediment

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above supported values are a reflection of offshore mixing and water column depth. 137 Cs (t1=2 ¼ 30 yr) is an anthropogenic tracer introduced beginning in 1956 with the onset of atmospheric testing of hydrogen bombs. 137Cs is supplied to the Earth’s surface through atmospheric deposition and has a non-steady-state source function, with decreasing supply since the 1972 ban on atmospheric nuclear testing. 7Be (t1=2 ¼ 53 d) is a cosmogenic tracer that is the atmosphere product of cosmic-ray spallation of nitrogen and other gases. The magnitude of local rainfall (e.g., atmospheric deposition rates) and riverine input primarily control the supply of both 7 Be and 137Cs to shelf sediments. Samples for radiochemical analysis were homogenized and sealed (wet) into preweighed 70 ml Petri dishes. Activities were measured by gamma decay using a Canberra low-energy intrinsic germanium detector with a 2000 mm2 planar surface area. Total activities of 234Th, 7Be, and 137Cs were measured using net counts of the 63.3, 477, and 661.7 keV photopeaks, respectively. Identical geometries were used for all samples and samples were counted for approximately 24 h each. Samples were recounted after  100 d to allow for measurement of 238U-supported levels of 234Th (24.5 d half-life) in the sediment, and to calculate total 210Pb activity from net counts of the 46.5 keV photopeak. Excess 234Th activities obtained by this method, and by measuring supported levels at the 352 keV photopeak of 214Pb, were found to be at or below detector resolution limits in all the Atchafalaya shelf sediments and will not be discussed further in the present paper. Total 210Pb activities were calculated from the second gamma count to allow ingrowth to secular equilibrium with the 226Ra parent. Excess 210Pb activities were calculated from independent measurement of the 352 keV 214Pb peak (Joshi, 1987), and from the average supported values (300– 400 dpm kg ÿ 1) obtained from relict station T1 and the base of cores. Samples were corrected for self-absorption using high-activity sealed standards following the methods of Cutshall et al. (1983). 7Be detection limits were determined with standards to be approximately 10 dpm kg ÿ 1. Following gamma counting, samples were oven dried at 608C and reweighed to calculate water content. 3.3. Elemental and isotopic analyses Sediment samples for elemental and isotopic analyses from the box cores were thawed, sub-sampled and oven-dried at 558C in the laboratory. The dried box core and suspended Atchafalaya River sediment samples were ground to pass a 250 m m sieve before analysis. Organic carbon and total nitrogen content were measured on vapor-phase-acidified samples following the method of Hedges and Stern (1984). Between 10 and 25 mg of sediment (and between 0.1 and 1.5 mg of cystine standard) were weighed into silver boats and exposed to concentrated HCl fumes for 24–36 h in an evacuated glass desiccator. Following acidification, the samples were analyzed by high-temperature combustion (9308C) on a Perkin-Elmer 2400 Elemental Analyzer. Analytical precision, determined by replicate analysis of the same sample, was better than  0.02 weight percent organic carbon and  0.01 weight percent nitrogen. Variance in C/N ratios was evaluated according to propagation of error associated with the measurement of organic carbon and total nitrogen (e.g. Gon˜i et al., 1998).

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The stable isotope analysis of organic carbon in sediments was performed by online combustion and isotope ratio mass spectrometry of pre-acidified samples using a VG OPTIMA stable isotope ratio mass spectrometers connected to a Carlo Erba CHN analyzer. The 13C/12C of the OC from each sample is expressed relative to the PDB standard by the convention ‘‘del’’ (per mil) notation: ! 13 12 13 12 ð C= CÞ ÿ ð C= CÞ sample standard 1000: ð1Þ d13 Cocð%Þ ¼ ð13 C=12 CÞstandard The analytical precision for this measurement is better than  0.3 % . We used twosample z-tests to statistically evaluate compositional differences between samples.

4. Results 4.1. Water column observations Measurements of salinity, temperature, and suspended sediment concentration were made over the inner shelf to document hydrography and the suspended sediment field during different conditions of river discharge and wave energy. October 1997 was characterized by low discharge, low wave energy conditions; March 1998 and 1999 by high discharge, high wave energy with passage of cold fronts; and April 1998 by high discharge, low to moderate wave energy (Table 1). The amount of freshwater on the inner shelf reflected the different river discharge conditions. For the March and April cruises with comparable discharge, surface salinities in the study area were generally less than 30 psu and less than 20 psu landward of the 5–10 m isobaths. In October 1997, when discharge was  25% that of March and April, surface salinities were 30–34 psu in the study area. For the March and April cruises, water exiting the main channel was fresh to the oyster reef,  18 km seaward of where the river enters Atchafalaya Bay. Suspended sediment concentrations in the river were highest in March (100–280 mg l ÿ 1) and April (100–150 mg l ÿ 1) and low in October 1997 (30–50 mg l ÿ 1). Cross-shore transects west of the river channel and Wax Lake outlets demonstrate strong temporal and spatial variability in the low salinity and turbid plumes (Fig. 3). These plumes can be distinct from each other and reflect conditions at the time of the sampling. October 1997 and March 1999 vary greatly in the amount of freshwater present, but both show a low salinity surface plume with strong stratification in the upper water column, and suspended sediment confined to water depths less than  10 m. March and April 1998 have a well-mixed water column close to shore with increasing stratification in salinity and suspended sediment seaward of  30 m water depth. Note that the sediment plume is confined close to shore even though the low salinity plume extends 50 km offshore. A time series of a cross-shore transect during the passage of a cold front further demonstrates that these transects are snapshots and the water column structure can evolve rapidly over short time periods, i.e., hours (Fig. 4). The water-column

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M.A. Allison et al. / Continental Shelf Research 20 (2000) 2267–2294 Table 1 Summary of conditions during the sampling cruises in 1997–1999a Cruise Name

P1097

P398

P498

P399

Dates Atchafalaya River discharge (m3 s ÿ 1) Offshore significant wave height Hs (m)

10/25–10/31/97 2600

3/5–3/11/98 10,200

4/27–5/7/98 10,200

2/27–3/11/99 9000

1.5

2.0

1.2

1.8

1.0–5.5 7.6 5.6–10 14 3–21 1015 998–1035 7 1–15 1808

0.4–2.8 6.5 3.9–11.4 22 19–25 1013 1009–1018 6 1–17 1708

0.6–3.1 7.1 3.7–10 18 11–23 1017 1009–1026 6 1–15 1808

0.5–3.3 6.3 3.0–8.3 air temp. (8C) 19 10–27 barometric pressure (hPa) 1015 1008–1023 wind speed (m s ÿ 1) 5 2–12 wind direction 1508

Offshore wave period (s) GDIL GDIL GDIL GDIL a

River Discharge from Army Corp of Engineers, New Orleans District. Offshore wave height and period from National Data Buoy Center buoy 42002. GDIL } Grand Isle C-Man station operated by National Oceanic and Atmospheric Administration.

distribution of salinity and suspended sediment evolves rapidly with changing wind conditions. Over a period of  6 h, air temperatures dropped 108C and sustained wind speed during that time was over 15 m s ÿ 1. During the period of increasing winds, the water column of the shallow shelf changed from stratified in salinity, density, and suspended sediment (first column) to well mixed and vertically homogeneous with respect to all properties (second column, approximately 12 h later). Suspended-sediment concentrations were approximately 1400 mg l ÿ 1 throughout the water column in depths of  5 m. Within two hours of the wind speed weakening, suspended sediments settled rapidly, forming a high-concentration suspension with maximum measured concentration of 25,000 mg l ÿ 1 at 20 cm above the bed (third column). The maximum total suspended load in the water column during this event represents a depth of resuspension of 9 mm (449 mg cm ÿ 2) and 1.4 mm (70 mg cm ÿ 2) at 5 and 8.5 m water depths, respectively. This estimate does not consider lateral transport and assumes a sediment porosity of 80%. Although direct measurements of nearbottom velocities are lacking, the observed change in suspended sediment level is likely due to the combined effect of waves and currents. The lack of correlation between suspended-sediment concentration and a water mass indicator like salinity suggests local resuspension. 4.2. Beryllium-7 Profiles of 7Be activity in the seabed were determined for the five time-series stations at low Atchafalaya discharge in October 1997 and during the subsequent early and late high discharge period in March 1998/1999 and April 1998, respectively

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Fig. 3. Cross-shelf profiles of salinity (psu) and suspended sediment concentration (mg l ÿ 1) along a line west of the Atchafalaya River mouth near Marsh Island on each of the four cruises. Location of the transect is plotted on Fig. 1. These transects demonstrate strong temporal and spatial plume variability with stratification in October 1997 and March 1999 and a well mixed water column in higher energy periods in March and April 1998. Note that the sediment plume is confined much closer to shore than the low salinity plume.

(Table 2). 7Be seabed penetration ranged from 2–8 cm at the sites during the four field studies. Surface sediment activities and seabed inventories of 7Be (Table 2) were seasonally variable, however, mean surface activities (1080 dpm kg ÿ 1) and inventories (up to 2.55 dpm cm2) were consistently highest at the inshore station (MI6) near the river mouth. All four Atchafalaya stations showed peak inventories during the April high discharge period, while the T1 station east of the mouth peaked in March (Table 2). Downcore profiles of 7Be activity (Fig. 5) at the inshore stations MI6 and WH6 show a similar logarithmic decrease with depth in March of 1998/ 1999 and October, with a significantly deeper profile in the April cores. Table 2 shows the results of calculations assuming that this penetration of the relatively short-lived isotope is the product solely of in situ (biological) mixing (Db ) or short-term sediment deposition (S). In the former, the mixing coefficient assuming

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Fig. 4. Transects of suspended sediment concentration (top panel), salinity (second panel), and sigma-T (third panel) on the inner shelf west of the Atchafalaya River mouth over a 19 h period in March 1998. The profiles at 1500 on 3/8/98 (column 1), 0300 (column 2), and 1000 (column 3) on 3/9/98 record the passage of a cold front across the study area. Note the breakdown of river plume stratification and an increase in total suspended sediment load on the inner shelf with time.

steady-state conditions is calculated using the equation (Aller and Cochran, 1976; Nittrouer et al., 1984): Db ¼ lðz=ðlnðC0 =Cz ÞÞÞ2

ð2Þ

in the latter, steady-state sediment deposition (S) is governed by S ¼ lz=ðlnðC0 =Cz ÞÞ

ð3Þ

where l is the decay constant of 7Be, z is sediment depth, C0 is activity at zero depth, and Cz is activity at the z depth. Both the mixing coefficient (cm2 yr ÿ 1) and sediment accumulation rates (cm yr ÿ 1) must be considered maximum values given the likelihood that both processes are active in upper few centimeters of shelf sediments. In the four Atchafalaya shelf stations, both Db and S are higher in the two inshore

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Table 2 Summary of 7Be data Surface 7Be activitya (dpm kg ÿ 1)

7

Be penetration depth (cm)

7

Be inventory (dpm cm ÿ 2)

Db (cm2 yr ÿ 1)b

Maximum S in cm yr ÿ 1 (g cm ÿ 2 yr ÿ 1)b

October 1997/ MI6 WL1 WH6 WH1 T1

710  66 962  68 245  52 342  37 n.d.

7 3 3 3 n.d.

1.04  0.15 0.78  0.20 0.21  0.06 0.34  0.10 n.d.

8.1  8.3 2.9 4.3  3.9 0.6 n.d.

6.3  2.6 (4.2) 3.7 (2.5) 5.9  4.5 (3.9) 1.7 (1.1) n.d. (n.d.)

March 1998/ MI6 WL1 WH6 WH1 T1

1245  88 492  49 614  66 151  28 884  72

4 5 3 2 7

0.88  0.14 0.52  0.12 0.34  0.09 0.13  0.04 2.18  0.74

4.6  0.9 0.7 3.1  1.9 0.2 19.6  13.9

4.7  0.4 (3.1) 1.8 (1.2) 4.6  0.3 (3.1) 1.0 (0.7) 9.7  3.4 (6.5)

April 1998/ MI6 WL1 WH6 WH1 T1

1192  59 544  49 989  100 388  35 1176  97

8 3 7 2 4

2.55  0.46 0.57  0.13 1.82  0.25 0.44  0.13 1.35  0.26

39.3  29.3 1.3  2.7 32.8  61.2 2.2 3.4  5.5

March 1999/ MI6 WL1 WH6 WH1 T1

1172  104 69  37 30  4 343  37 150  19

5 2 1 2 2

1.56  0.23 0.08  0.02 0.03  0.01 0.31  0.08 0.26  0.06

10.1  2.9 17.1 } 1.5 3.6

Cruise/Station

a b

13.9  4.2 2.5  1.9 11.5  9.7 3.2 (2.1) 3.7  2.8

(9.3) (1.7) (7.7) (2.5)

6.9  2.2 (4.6) } 2.6 4.2

Surface activity calculated from the 0–1 cm sediment interval; n.d.=no data collected. No errors reported when based on only two data points.

stations and all four stations show seasonally variable values with peak rates in April 1998 (Table 2). Although Db and S provide a method for intercomparing seabed behavior at each station, Fig. 5 demonstrates the limitations in interpretation in situations where 7Be penetration is limited to the upper 1–2 cm (WH6 in October 97, March 98, and March 99), or where deeper penetration (WH6 in April 98) may be either a product of downmixing (modeled as exponential decay) or a relatively instantaneous sedimentation event (vertical profile). 4.3. Lead-210 and Cesium-137 210

Pb and 137Cs were measured for all the 7Be sample intervals 510 cm deep in order to examine the surface sediment activities of particle-reactive radiotracers with origins and sediment delivery histories distinct from that of 7Be. Activities were also

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Fig. 5. 7Be activity in sediment cores from the four cruises for the two inshore stations (MI6 and WH6) on the Atchafalaya shelf. Maximum penetration and inventory of 7Be occurs in April 1998, coincident with the high discharge period of the Atchafalaya River. The April data from WH6 may be interpreted as a mixing profile calculated using a best-fit regression (a) or as a single rapid depositional event of up to 6 cm thickness (b).

measured to the base of the boxcores ( 30–35 cm) in the October 1997 cruise (Fig. 6) to allow calculation of a long-term ( 100 yr) average sediment accumulation rate for each of the sites. Results are summarized in Table 2. Total 210Pb activities for surface sediments range from 848–1970 dpm kg ÿ 1 at the four Atchafalaya stations, with a significant decrease in activity in three of the stations in April 1998. The inshore station WH6 has the widest range of 210Pb activities and exhibits minimum activity in March. 137Cs surface sediment activities show a more consistent seasonal signal, with values of 218–350 dpm kg ÿ 1 in the two offshore stations and 34–90 dpm kg ÿ 1 in the two inshore stations (Table 3). Downcore profiles of 210Pb activity (Fig. 6) generally exhibit a surface mixed layer (SML) of 3–14 cm thickness, with a logarithmic decrease in activity below this layer. Total activities in the offshore Atchafalaya cores and the T1 core decrease with depth to a minimum value (390–650 dpm kg ÿ 1) of 210Pb supported by 226Ra decay: the inshore cores were not deep enough to reach this point. 210Pb sediment accumulation rates (Table 3) of the inshore Atchafalaya cores exceed the offshore rates by a factor of 2–3. 137Cs accumulation rates can be calculated from the depth of penetration of this anthropogenic tracer introduced in significant amounts beginning in about 1956 (41 yr) below the SML. In the inshore cores, 137Cs was found to the base of the core and so only a minimum rate is reported in Table 3. The spatial trend in 137Cs accumulation is comparable to the 210Pb accumulation rates obtained from the same cores, but is more than 50% higher than 210Pb accumulation rates in the three cores that reached the limit of 137Cs penetration.

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Fig. 6. Downcore plots of total ( * ) and excess ( D ) 210Pb activity, 137Cs activity, and porosity for the four Atchafalaya shelf stations. The surface mixed layer (SML) is the interval of uniform excess 210Pb activity caused by biological and physical mixing. Deep penetration of 137Cs and 210Pb at the two inshore stations (MI6 and WH6) is interpreted as being the product of more rapid sediment accumulation than the offshore stations. Exponential decay of 210Pb activities was calculated using best-fit regression lines (plotted) to derive sediment accumulation rates. All data were obtained from cores collected in October 1997.

Surface activities and accumulation rates at the T1 site east of the river are lower than the Atchafalaya sites. Accumulation rates were calculated from the March 1998 core at this site. In April 1998 the site showed supported levels of 210Pb at 1–2 cm depth and no penetration of 137Cs, indicating that relict sediment (i.e., >100 yr age) was exposed. Hence, the March accumulation rates measured at T1 are ephemeral.

M.A. Allison et al. / Continental Shelf Research 20 (2000) 2267–2294 Table 3 Summary of

210

Station/Cruise

MI6/ 10/97 03/98 04/98 03/99 WL1/ 10/97 03/98 04/98 03/99 WH6/ 10/97 03/98 04/98 03/99 WH1/ 10/97 03/98 04/98 03/99 T1/ 10/97 03/98 04/98 03/99 a

Pb and

2281

137

Cs data

Surface 210Pb activitya (dpm kg ÿ 1)

210 Pb accumulation rate in cm yr ÿ 1 (g cm ÿ 2 yr ÿ 1)

Surface 137Cs activitya (dpm kg ÿ 1)

137 Cs accumulation rate (cm yr ÿ 1) (g cm ÿ 2 yr ÿ 1)

1548  56 1604  58 1444  41 1435  60

0.68  0.18 (0.45)

220  19 266  20 224  12 306  23

>0.49 (>0.33)

1630  51 1481  47 1313  41 1464  50

0.27  0.06 (0.18)

90  11 58  7 47  6 34  6

1624  48 886  43 1970  70 1552  60

0.55  0.10 (0.37)

350  24 259  18 303  23 218  19

940  36 974  36 848  30 1137  39

0.18  0.02 (0.12)

67  7 49  6 52  5 49  6

n.d. 1100  43 1263  47 402  26

0.09  0.01 (0.06)

n.d. 58  8 80  10 0

0.43  0.06 (0.29)

>0.63 (>0.42)

0.24  0.05 (0.16)

0.18  0.04 (0.12)

Total surface activity of radioisotopes calculated from the 0–1 cm sediment layer.

4.4. Chemical and isotope data Table 4 presents the chemical and isotopic compositions for the top five centimeters of sediment collected from the five shelf sites and for Atchafalaya River suspended sediments. The surface (0–1 cm) samples exhibit %OC values that vary consistently with distance from shore, ranging from 1.35 to 1.63% at the two inshore stations (WH6 and MI6) and from 0.51 to 0.83% at the offshore stations (WH1 and WL1). The stable carbon isotopic signatures (d13C) of OC in these samples (Table 4) show a similar relationship, with more negative values at the inshore stations ( ÿ 22.5 to ÿ 23.2 % ) than at the offshore stations ( ÿ 21.1 to ÿ 21.9 % ). In the surface sample, C/N ratios fail to show any clear trend with distance from land. There are marked changes in the %OC and d13C values of all surface samples between cruises (Table 4). In all five stations, %OC increases significantly from an average of 0.98 in March 1998 to 1.07 in April 1998. In the same time period, d13C values decrease from an average of ÿ 21.8 to ÿ 22.1 % . Again, the C/N ratios of these samples do not display similar trends.

a

n.m., not measured.

0.51 0.58 0.58

0.55 0.62 0.59

0.67 0.71 0.69

0.82 0.66 0.73

1.38 n.m. 1.25 1.43 0.83 0.59 0.70

1.63 n.m. 1.58 1.58

1.60 1.40 1.08

n.m.

0–1 2–3 4–5

WH1

0.75 0.62 0.70

n.m. 1.28 n.m. 1.17

1.47 1.25 1.00

Atchafalaya R. suspended sediments Mouth SS 1.84 1.43 1.88

0–1 2–3 4–5

WL1

1.48 n.m. 1.46 1.51

1.35 1.37 1.14

0.59 0.43 0.65

0–1 1–2 2–3 4–5

WH6

1.47 1.09 1.17

n.m. 0.77 0.79

0.56 0.59 0.60

0–1 2–3 4–5

MI6

1.06 0.72 0.70

1.14 0.26 0.72

n.m. 9.88  1.35 n.m. 7.76  0.90 11.5  3.5 8.38  1.76 8.80  2.02 8.44  2.29 9.34  2.48 10.1  3.1

9.94  1.24 n.m. 9.49  1.13 8.52  0.89 8.62  1.77 7.88  1.80 9.53  2.33 10.2  3.6 11.8  4.3 6.77  1.42 10.1  1.3

12.2  2.0 12.7  2.1 10.4  1.7

10.1  1.3 19.0  5.6 11.9  2.1

10.0  1.0

9.67  1.59 9.67  2.33 11.0  3.0

3/98

15.0  7.6 12.8  3.2 16.2  6.6

0.51 0.90 0.70

Shelf sediments T1 0–1 2–3 4–5

3/99

10/97

4/98

10/97

3/98

C/N (atomic)

%OC (salt-free)

Station Core depth (cm)

Table 4 Elemental and isotopic composition of Atchafalaya river and shelf sedimentsa

10.1  1.0

12.3  4.8 10.9  3.6 9.88  2.88

10.5  2.4 9.76  2.58 9.33  2.13

9.68  1.20 n.m. 9.59  1.30 9.49  1.11

10.8  1.4 10.5  1.6 9.66  1.65

9.05  1.34 8.79  5.21 13.8  4.6

4/98

n.m.

13.3  5.3 7.78  2.49 11.4  3.5

9.28  1.90 9.03  2.44 14.9  5.6

8.45  0.90 n.m. 8.88  0.90 11.2  1.4

10.7  1.3 9.01  1.04 13.0  2.8

n.m. 15.1  5.2 12.1  3.3

3/99

ÿ 24.8

ÿ 21.2 ÿ 21.2 n.m.

ÿ 21.9 ÿ 21.6 ÿ 21.4

ÿ 22.8 n.m. n.m. n.m.

ÿ 22.7 ÿ 22.6 ÿ 22.9

ÿ 22.6 ÿ 24.2 n.m.

10/97

d13C ( % )

ÿ 25.0

ÿ 21.3 ÿ 20.8 ÿ 21.5

ÿ 21.2 ÿ 21.1 ÿ 22.1

n.m. ÿ 22.6 n.m. ÿ 22.6

ÿ 22.5 ÿ 22.7 ÿ 22.5

ÿ 21.5 ÿ 21.6 ÿ 22.0

3/98

ÿ 24.0

ÿ 21.4 ÿ 21.4 ÿ 21.6

ÿ 21.4 ÿ 21.4 ÿ 21.6

ÿ 23.2 n.m. ÿ 22.6 ÿ 22.4

ÿ 22.9 ÿ 22.5 ÿ 22.6

ÿ 21.8 ÿ 21.8 ÿ 24.1

4/98

n.m.

ÿ 21.1 ÿ 21.3 ÿ 21.4

ÿ 21.2 ÿ 21.4 ÿ 21.3

ÿ 22.6 n.m. ÿ 22.6 n.m.

ÿ 22.5 ÿ 22.5 ÿ 22.4

n.m. ÿ 23.9 ÿ 24.4

3/99

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Statistically significant differences in the downcore trends of elemental and isotopic compositions are evident between cruises (Table 4). For example, while %OC and d13C values show little variability within the upper five centimeters of most cores, significant contrasts are observed in the April 1998 profile (Fig. 7). Organic carbon content is higher in the 0–1 cm interval than in the 2–3 cm interval at all stations except the most distal site (WH1). d13C values also vary significantly between the 0–1 cm and 2–3 cm intervals from the inshore stations, albeit no significant differences are detected in the offshore stations.

5. Discussion 5.1. 7Be and flood layer formation The behavior of 7Be in shelf sediments is an indicator of the relative influence of atmospheric flux, sediment supply from adjacent rivers, and lateral marine transport (Olsen et al., 1986). Atmospheric fluxes of 7Be have been shown to vary spatially

Fig. 7. Downcore profiles of organic carbon content (%OC) and stable carbon isotope composition (d13C) of the top five centimeters from sediments collected in March and April 1998. Open symbols ( * , %OC; & , d13C) represent data from March 1998 and closed symbols ( * , %OC; & , d13C) represent data from April, 1998. Average standard deviation for %OC is less than the symbol size and average standard deviation for d13C is shown as the error bar at the bottom of each profile. Organic carbon content is higher and d13C is more negative (terrestrial) in the 0–1 cm interval of the inshore stations (WH6, MI6) in April, supporting the radiochemical evidence that a flood depositional layer was forming at this time.

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according to latitude and temporally mainly according to rainfall amount (Canuel et al., 1990; Baskaran et al., 1993), with additional washout and dilution effects. Utilizing the Baskaran et al. (1993) mean values for bulk (wet+dry) atmospheric deposition for Galveston, Texas (106 dpm l ÿ 1) prorated to local rainfall over the mean 76 d lifetime of 7Be (1=l), derived by averaging daily NOAA precipitation records for the southwest coast of Louisiana, a first-order estimate of predicted atmospheric input to the seabed would have been 2.2 dpm cm ÿ 2 in October 1997, 5.7 and 3.0 dpm cm ÿ 2 in March and April of 1998, and 2.1 dpm cm ÿ 2 in March of 1999. For the four time-series data points from the five stations (19 total points), predicted atmospheric inventories range from 118–7000% of observed values, with no significant correlation. In part, values above 100% are a function of the storage of 7Be in suspended particulates. These values also suggest that lateral water column transport and the additional riverine flux of particulates plays a strong role in dictating 7Be seabed inventories. The predominance of meteorological forcing and the spatial variability of water column sediment inventory caused by intensity of resuspension and proximity to the turbid Atchafalaya River source, do not allow absolute fluxes of 7Be to the seabed to be calculated. Cross-shore transects of water column parameters during cold front passage (April–October) suggests surface sediment particles are reexposed to adsorption and desorption of dissolved 7Be in the water column on 3–7 d frequencies. Quantitative flux studies in this setting would require time-series information about 7Be activities and inventories of the riverine endmember and the suspended load for each station. However, a qualitative figure for comparison with predicted (atmospheric) input can be obtained by averaging the values for the two, high-energy inshore stations (5–8 m water depth) with the two, lower-energy offshore (  22 m water depth) stations. Utilizing the values in Table 2, a mean observed inventory of 0.58, 0.47, 1.35, and 0.50 dpm cm ÿ 2 for Cruise I through IV is obtained. Predicted versus observed inventory is high (379%) in the relatively low Atchafalaya discharge period (2900 m3 s ÿ 1) October 1997. The much lower (1213%) observed 7 Be inventory during the early high Atchafalaya discharge (10,500 m3 s ÿ 1) in March of 1998 (and to a lesser degree in March 1999) may reflect that this is the peak season for frontal passage and resuspension of 7Be from the seabed. The relatively higher observed inventory (222%) in April 1998 (11,600 m3 s ÿ 1) is interpreted to be produced by the buildup of a flood deposit on the seabed in the 52–53 d between March and April 1998 sampling. Total Atchafalaya sediment discharge over the 76 d mean lifetime of 7Be in the sediments was 28  106 tons prior to the April 1998 sampling, 35% higher than in March 1998 (21  106 tons) and a 12-fold increase over October 1997 (2.2  106 tons). Sediment discharge was slightly higher in March of 1999 (22  106 tons) than in 1998. In April 1998, the two inshore stations exhibited high 7Be inventories and greater depth of 7Be penetration than other seasons (Fig. 5). Logarithmically decreasing profiles observed in short-period radiotracers are typically interpreted as being the product of biological mixing (Aller and Cochran, 1976). Although calculated mixing rates (Db in Table 2) are moderately high and would appear to coincide with the

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period of spring population increase in benthic biology, mixing rates are an order of magnitude lower in the two offshore stations. X-radiographs of the two inshore stations show little biological disruption of primary sedimentary layering, while the two offshore stations are nearly homogenized by biological activity (Fig. 8). An increase in biological mixing cannot explain the three to fivefold increase in 7Be inventories between March and April 1998 at the inshore stations. 137 Cs surface activities (Table 3) show an even clearer pattern of relative riverine influence: the four Atchafalaya stations have relatively unvarying surface activities seasonally, with inshore stations activities of 218–350 dpm kg ÿ 1 and offshore activities of 34–90 dpm kg ÿ 1. As 137Cs is supplied to the ocean surface by atmospheric deposition, the increased activity at the inshore stations is likely the product of 137Cs fallout in the Mississippi drainage basin and scavenged by particles during surface runoff. These lines of evidence (7Be, 137Cs, X-radiographs) suggest that the combined effects of peak water and sediment discharge and the decrease in

Fig. 8. X-radiograph negative images (light=coarser sediment) of box cores from the study area. Contrasts have been increased from the original images to improve display of internal structures. Stations with high sediment accumulation (MI6) exhibit mm to cm-scale primary stratification with limited disturbance by burrows. Stations with low sedimentation (WH1) show a near complete homogenization of primary structures from bioturbation caused by soft-bodied infauna. Core T1 from updrift of Atchafalaya Bay shows mm-scale interlamination. This core is lower in porosity than the Atchafalaya stations and shows no penetration of 137Cs, indicating that this is relict sedimentary environment (e.g., no modern sediment accumulation).

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resuspension due to the passage of a winter cold front resulted in the deposition of a flood layer on the inner shelf between the March and April cruises. This flood layer was most evident at the inshore stations. Table 5 summarizes two methods for a first-order estimate of the thickness and mass of sediment in the flood deposit at the four Atchafalaya stations in April 1998 using the March data as a baseline. In Method A, the difference in 7Be deposition rate (S) yields a flood layer of  1 cm thickness ( 0.8 g cm ÿ 2) in the inshore stations and 1–3 mm ( 0.15 g cm ÿ 2) in the offshore stations. Given that some biological mixing is present, this calculation derives a maximum deposition rate. The second method (B) from Dibb and Rice (1989) is independent of biological mixing rate. Excess mass of sediment between March and April is calculated by assuming a constant daily sediment flux (J) in g cm ÿ 2 over the interval and continuously decay correcting this newly added 7Be: J¼

ðID ÿ ðID eÿlDt ÞÞ þ ID Ap

ð4Þ

where ID is the difference in 7Be seabed inventories in March and April divided by the number of days after the March sampling, l is the decay constant of 7Be, Dt is the time after the March sampling. Ap is the 7Be activity of the particles, which, in the absence of activities for suspended sediments on the Atchafalaya shelf is the average of the surface sediment activity (0–1 cm interval) in March and April. This method yields a sediment delivery to the inshore stations of about 2 g cm ÿ 2, 1.5 g cm ÿ 2 in the station offshore of the Chenier Coast, and 0.1g cm ÿ 2 offshore of the river mouth. Method B is subject to errors induced by the absence of information about absolute fluxes of 7Be from the riverine, atmospheric, and advective sources. By either method, the thickness of the flood deposit was probably not at maximum in late April 1998: river discharge values remained above the

Table 5 7 Be-derived thickness of the flood sediment layer developed between the March and April 1998 cruises Station

Water depth (m)

Collection dates

(A) Difference in maximum S (B) Excess 7 Be inventory (g cm ÿ 2) (cm)

MI6 WL1 WH6 WH1 T1

7 24 7 21 17

(3/7 (3/7 (3/7 (3/8 (3/6

& & & & &

4/29) 4/28) 4/29) 4/29) 4/27)

Ratio of 7Be deposition rate to 210Pb accumulation rate (A; B)

(g cm ÿ 2)

1.3 0.9 1.8 0.1 0.07 0.12 1.0 0.7 2.4 0.3 0.2 1.5 No net deposition between cores

(2.0, 4.0) (0.38, 0.69) (1.9, 6.4) (1.7, 12.3)

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10,500 m3 s ÿ 1 discharge present in early March for another 30–35 d after the April sampling (Fig. 2).

5.2. Elemental and isotopic evidence for flood layer formation There are significant differences between the average compositions of river suspended particles and those of surficial sediments in the Atchafalaya shelf (Table 4). One is the higher concentration of organic carbon (%OC) in river particles relative to those measured in shelf sediments. The other major difference is that river sediments are significantly more depleted in 13C than their shelf counterparts (i.e., more negative d13C values). Several factors contribute to these measured differences, including the higher carbon loading of river particles relative to shelf sediments (e.g., Hedges and Keil, 1995) and the higher concentration of organic matter derived from terrestrial vascular plants (e.g., Gon˜i et al., 1998). In contrast, the C/N ratios of most river and shelf sediments, all of which are relatively low (9 to 11), are statistically comparable. A major reason for the lack of a distinct river C/N composition is that these particles contain a significant fraction of soil organic matter from the Mississippi/Atchafalaya River drainage (Gon˜i et al., 1998), which displays a C/N ratio of 10–13 (e.g., Tiessen et al., 1984; Parton et al, 1987). Because the C/N ratios of soil organic matter are so similar to those of marine particles, which include phytoplankton-derived materials with C/N ratios of  7 (Redfield et al., 1963), this parameter cannot be used to further assess the occurrence of a flood layer. The differences in OC content and d13C values, on the other hand, have the potential to distinguish a recently deposited flood layer of a predominant riverine origin from those of other sources (e.g., resuspension of shelf sediments, biogenic sedimentation). Under this scenario, if river-derived materials contributed significantly to the deposition event that occurred between March and April, we would expect the 0–1 cm sediment horizons collected in April to yield higher %OC and more negative d13C values than their March counterparts. Indeed, all the stations display (Table 4) an increase in the %OC of surface sediments from March to April. Similarly, the d13C values of most shelf samples from April are more negative (i.e., more depleted in 13C) than those from March. The lack of a stronger contrast between the March and April samples can potentially be explained by the thickness of the sampling interval (1 cm), which may result in the incorporation of previously deposited materials at stations where the flood layer thickness was 51 cm. Furthermore, spatial heterogeneity in the distribution of the flood layer around each station and variability in the exact location of the ship at each site may also contribute to the lack of a clearer signal. Utilization of profiles from individual cores (Fig. 7) is likely to minimize this problem and provide stronger evidence for a deposition event. If a 1–3 cm flood layer was deposited between March and April 1998, as the radiochemical data suggest, the 0–1 cm interval should display higher %OC and more negative d13C values than the 2–3 cm interval from each box core collected in

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April 1998. Notably, the organic carbon content is significantly higher in the 0–1 cm interval than in the 2–3 cm interval for all but the most distal station (Fig. 7). Significant differences in d13C values are also observed between the 0–1 and 2–3 cm intervals from the inshore stations, but not the offshore stations. Overall, the data presented in Table 4 and Fig. 7 are consistent with the hypothesis of a carbon-rich, isotopically depleted layer being deposited between March and April. The compositional differences, in agreement with the radiochemical evidence, suggest that the largest addition of river materials occurred at inshore stations MI6 and WH6, close to the Atchafalaya River mouth. The offshore station WL1 shows lesser impact, while the most distal station, WH1, displays no recognizable contribution from the flood event.

5.3. Sediment accumulation on the Atchafalaya shelf 210

Pb and 137Cs geochronologies (Table 3) average out the effects of annual flood deposition and reworking to accumulation rates on a time scale of decades to  100 yr. 137Cs rates are often measured to evaluate the possibility of deep mixing (e.g., below the surface mixed layer) in 210Pb profiles, which would yield an anomalously high 210Pb accumulation rate. The relatively higher 137Cs in all the cores could be attributed to the presence of deep biological mixing, however, Xradiographs (Fig. 8) indicate that the inshore sites have been subjected to minimal bioturbation. A more likely explanation is the downward mobilization of 137Cs in sediment pore waters. Santschi et al. (1983) and Sholkovitz and Mann (1984) demonstrated that 137Cs is mobile in marine sediment pore waters, particularly when subjected to repeated pore water salinity and redox variations. Such conditions would be expected on the Atchafalaya inner shelf with variable riverine influence and physical mixing of the seabed. This is further suggested by the absence of a peak in 137 Cs activity in the downcore profiles that corresponds to the 1963 maximum in fallout rates. %OC, C/N, and d13Coc downcore distributions (Table 4) from boxcores collected at station MI6 show a high degree of variability with depth that may be associated with flood layers. For example, there are subsurface peaks (Fig. 7) in %OC (>1.4%), and minima in d13Coc ( ÿ 23 % ) that may related to the enhanced deposition of river-derived particles. Additional sampling at finer intervals may better constrain the historical occurrences of flood layers. 210 Pb accumulation rates from the five stations demonstrate that the inner shelf immediately adjacent to the Atchafalaya navigation channel is an area of relatively high sequestration of fine-grained sediment. The Atchafalaya inner shelf can be considered as an early shelf phase of prodelta accretion seaward of the sand-rich delta front lobes accreting in Atchafalaya Bay (Van Heerden and Roberts, 1988). Minimal to no accumulation (e.g., relict seabed) at the T1 station updrift of the Atchafalaya mouth suggests that the Mississippi discharge from the Balize delta plays an insignificant role on the pattern and quantity of sediment accumulation. 210 Pb sediment accumulation rates calculated for the Mississippi Bight by Shokes

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(1976), Rotter (1985) and indicate the region of highest Mississippi accumulation is confined to a 20 km radius of the river mouth passes. Relatively high accumulation at station WH6 along the downdrift chenier coast is indicative of the strong role that the westward coastal current plays in the dispersal of Atchafalaya sediment. This supports previous observations of rapid fluid mud deposition on the chenier plain mudflats during winter frontal events (Wells and Kemp, 1981; Kemp, 1986). Low accumulation at the offshore stations indicate much of the Atchafalaya sediment is confined to the inner shelf (510 m water depth) by the westward residual flow. A comparison of short-term (seasonal) deposition rates from the 7Be data with longer-term (decadal) accumulation rates from the 210Pb data provides a method for estimating the reworking rate at each of the shelf stations. In Table 5, 7Be sediment deposition rates associated with the formation of the flood deposit in March–April 1998 are >189% greater than the 210Pb accumulation rates at the inshore sites. Seasonal deposition rates exceed decadal accumulation rates by >166% at the offshore site adjacent to the Chenier Coast (WH1) as well, suggesting the annual sediment supply to this site is also primarily delivered via the coastal mud stream during the peak Atchafalaya discharge period. Long-term accumulation rates exceed seasonal deposition rates at the offshore site from the Atchafalaya mouth (WL1), indicating that redistribution from proximal shelf areas after the flood depositional event is of relatively greater importance here than the other sites in sediment delivery. The decreased in 7Be inventories in April 1998 relative to October 1997 may represent interannual differences, but more likely is a function of decay or partial removal of an annual flood layer, as suggested by the long-term accumulation rates at three of the four sites. Decreasing 7Be inventories between October and March, despite rising Atchafalaya discharge, indicate winter frontal passage is a contributor to this winnowing process. The passage of tropical storms across this shelf at intervals of every few years is also likely a major factor in sediment export to more distal areas.

5.4. Comparison with flood sedimentation on other major rivers Large fluxes of terrigenous suspended sediment, capable of depositing a seasonal flood layer on the inner-to-middle continental shelf, have been observed on a number of major rivers summarized in Table 6. In all these systems except the Amazon, the period of high sediment deposition rates on the shelf coincides with peak sediment discharge of the adjacent river in a range of bed stress (tides, shelf currents, waves) conditions. In the Amazon, a thick (up to 150 cm) surface layer of homogeneous excess 210Pb activity has been observed on the continental shelf to vary seasonally in thickness by an average of 17 cm (Kuehl et al., 1986, 1995). Maximum thickness of this layer occurred during periods of low wind stress and minimum riverine input, which led Kuehl et al. (1995) to suggest layer deposition is more a function of the location of the bottom salinity front on the open shelf caused by the enormous freshwater input of the Amazon.

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Table 6 Summary of Flood Deposits on Major River Shelves River

Peak water discharge (103 m3 s ÿ 1)

Annual sediment discharge (106 ton)

Water depth of deposition (m)

Maximum thickness of deposit (cm)

Deposition References to accumulation ratioa

Atchafalaya Eel

9.5 7–10

84 10–150

55–15 40–150

1–3 8

0.38–12 2–24

Changjiang

50–60

500

20–50

15+

10

Amazon

230

1100–1300

10–30

150 (17)b

25–1500

This study Sommerfield et al. (1999); Sommerfield and Nittrouer (1999) McKee et al. (1983, 1984); DeMaster et al. (1985) Kuehl et al. (1986, 1995)

a

Calculated as the ratio of the short-term (  100 d) 234Th or 7Be deposition rate to the  100 yr 210Pb accumulation rate (in g cm ÿ 2 d). b Figure in parentheses refers to the seasonal change in thickness observed in the 210Pb surface mixed layer.

These three examples of seasonal flood sedimentation summarized in Table 6 differ from the Atchafalaya in two major respects: water depth and magnitude. Sediment deposition is focused in water depths greater than the 510 m water depth observed on the Atchafalaya shelf. This is a function of the relatively low energy of the Atchafalaya margin, where limited tidal currents and generally small significant wave heights result in limited bed shear stress for reworking and advection offshore after the period of strong cold front passage. High water column turbidities were also observed in cross-shore transects (Fig. 3) to be limited to 510 m water depth during most conditions, suggesting flocculation and settling takes place rapidly and that particle resuspension is most prevalent in these water depths. Flood layer deposition thickness is reduced on the Atchafalaya shelf relative to other major rivers, in part because of the smaller magnitude of the peak riverine sediment flux. The Eel sediment flux, although similar to the Atchafalaya (Table 6), is deposited on a narrow margin below wave base (40–150 m) where resuspension effects are limited (Sommerfield et al., 1999). Atchafalaya deposition is also limited by the overlapping periods of peak cold front (resuspension) period and peak sediment discharge. The limits of the flood depositional layer were more easily recognizable in these thicker deposits than on the Atchafalaya shelf because of their distinct sedimentological character (X-radiograph), and their terrestrially dominated radiochemical (210Pb, 234Th, 137 Cs, or 7Be) and organic signatures (see Leithold and Hope (1999) on the Eel deposit).

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6. Conclusions Sediment cores and water-column data collected at five stations on the Atchafalaya shelf during four cruises in 1997–1999 support the following conclusions. 7

Be activities and inventories in surface sediments indicate a flood deposit forms on the inner shelf during the high Atchafalaya River discharge period (March–April). In water depths 510 m this sediment deposit can be several centimeters thick, with decreasing thicknesses at the offshore stations. (2) The timing of maximum flood deposition of sediment containing 7Be occurs coincident with a decline in the frequency and intensity of cold front passage between March and April. These events generate a breakdown of plume-derived, water column stratification on the Atchafalaya shelf and may cause resuspension of up to several centimeters of the seabed landward of the 10 m isobath. (3) Based on the organic carbon contents and d13Coc compositions of the samples analyzed, the flood deposit has a greater terrestrial influence than surface sediment deposited prior to the high discharge period (e.g., March). (4) 210Pb and 137Cs geochronology indicate that the annual flood deposition rate in 1998 (a near-average discharge year) was 2–6 times the long-term sediment accumulation rate at inshore sites (510 m water depth). Offshore sites (  20 m water depth) exhibit decreased short-term (7Be) deposition rates relative to long-term (210Pb) accumulation rates (WL1 ratio is 51), indicating that sediment supply to these sites is more balanced seasonally, and that these sites are the recipient of sediments resuspended from higher-energy, inshore areas. (1)

Acknowledgements Financial support for this study was provided by the Office of Naval Research (N00014-98-1-0083). The authors would like to acknowledge the assistance provided by the scientists and crew on the cruises of the R/V Pelican. Particular aid in sample collection and processing was provided by A. Simoneau, M. Teixeira, R. Potter, D. Perkey, D. Hartz, K. Peterson, B. Perlet, T. Dellapenna, E. Higgins, A. Carlson, and K. Hart. The following reviewers were instrumental in substantially improving the manuscript: R. Wheatcroft, J. Jaeger, and two anonymous colleagues.

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