www.elsevier.com/locate/margeo

Marine Geology 209 (2004) 91–112

An evaluation of mobile mud dynamics in the Mississippi River

deltaic region

D. Reide Corbetta,*, Brent McKeeb, Dan Duncanb

aDepartment of Geology, Coastal Resources Management, East Carolina University, 101 Graham Building, Greenville, NC 27858, USAbDepartment of Earth and Environmental Sciences, Tulane University, New Orleans, LA 70118, USA

Received 15 January 2003; received in revised form 28 May 2004; accepted 28 May 2004

Abstract

Large rivers are the primary interface between terrestrial and ocean environments. A relatively small number of rivers

account for a disproportionate amount of the freshwater and suspended materials that are delivered to the coastal ocean.

Sediment delivery to these coastal systems plays a key role in the global carbon cycle since deltas and continental shelves are

considered to be the main repositories of organic carbon in marine sediments. Particulate material in these environments are

typically deposited and resuspended several times before permanent accumulation or transport off the shelf. This sediment

cycling is an important component influencing biogeochemical processes that occur in coastal environments. During two

cruises in April and October 2000 on the shelf adjacent to the Mississippi River, water and sediment samples were collected for

analysis of suspended solids and particle reactive radionuclides (210Pb, 137Cs, 7Be and 234Th) to evaluate the transport and fate

of terrestrial and marine material. A comparison of the distribution of these tracers provides insight about the pathways and

residence times of particulate materials on the shelf. Inventories of these short-lived radiotracers showed variations of more than

two orders of magnitude, indicating dramatic variations in sediment deposition between sampling events.

Short-lived radiotracers indicate that river-borne materials are transported less than f 30 km from the river mouth before

initial deposition. However, seasonal variations in 7Be and 137Cs indicate significant remobilization of sediment and potential

export of sediment out of the study area during the high energy (e.g., wind/wave) winter months. In addition, depth profiles of7Be and excess 234Th indicate sediment deposition rates between 0.8 and 3.9 cm month� 1 (0.4–2.1 g cm� 2 month� 1) at two

shelf locations (near river and open shelf environments). These rates are much greater than those observed on decadal time

scales (1.3–2.0 cm year� 1 or 0.6–1.5 g cm� 2 year� 1) via 210Pb at the same sites. This further substantiates active sediment

reworking and potential export off the shelf as has been observed in other river-dominated ocean margins.

D 2004 Elsevier B.V. All rights reserved.

Keywords: radioisotope; thorium; beryllium; lead; cesium; sedimentation; resuspension; Mississippi River; sediment transport

0025-3227/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.margeo.2004.05.028

* Corresponding author. Tel.: +1-252-328-1367; fax: +1-252-

328-4391.

E-mail address: corbettd@mail.ecu.edu (D. Reide Corbett).

1. Introduction

Rivers deliver approximately 15� 109 tons of

sediment to coastal margins each year (Milliman,

1991). This sediment delivery to coastal systems plays

a key role in the global carbon and nutrient cycles as

D. Reide Corbett et al. / Marine Geology 209 (2004) 91–11292

deltas and continental shelves are considered to be the

main repositories of organic matter in marine sedi-

ments (Berner, 1982, 1989; Hedges and Keil, 1995;

Mayer et al., 1998). However, sediments are not

simply delivered and deposited along coastal margins.

As Wright and Nittrouer (1995) demonstrated, river-

derived sediments delivered to coastal systems typi-

cally undergo several cycles of transport, deposition,

and reactivation before inevitable long-term net accu-

mulation. Due to physical and biological forcing,

organic and inorganic particulate matter are both

continuously involved in these deposition/resuspen-

sion cycles along dispersal environments. An extreme

example of this reworking can be found on the

Amazon River Delta, where the upper f 0.5–2 m

of the deltaic deposits are remobilized on daily and

seasonal time scales (Kuehl et al., 1986, 1996; Mackin

et al., 1988; Jaeger and Nittrouer, 1995; Kineke and

Sternberg, 1995, Moore et al., 1996; Aller et al., 1996;

Aller and Todorov, 1997, Aller, 1998). This same non-

steady state condition is prevalent in many coastal

environments (McKee et al., 1983, 1984; Kuehl et al.,

1986, 1996; Aller, 1998; Sommerfield et al., 1999;

Allison et al., 2000).

Temporary vs. permanent storage of sediment that

occurs along these dispersal environments, due to

multiple deposition/resuspension cycles, and the time

scales of these events are important parameters in

understanding biogeochemical processes in the ben-

thos as well as the water column. Sediment reworking

processes can have a direct influence on diagenetic

reactions that occur in the sediments and on the

composition of the buried material, as well as nutrient

and oxygen availability in overlying waters (Rosa et

al., 1983; de Jonge and van Beusekom, 1995; Nishri,

1996; Giffin and Corbett, 2003; Tengberg et al.,

2003). The multiple deposition/transport cycles that

occur in large river systems lead to primarily suboxic

sediments with relatively efficient remineralization

(Aller, 1998). A better understanding of the dynamics

and history of particles within these environments is

needed before a quantitative understanding of the

biogeochemical processes in the seabed can be prop-

erly evaluated and suitable diagenetic models can be

applied.

As part of a large research initiative to understand

sediment and carbon dynamics on the shelf adjacent

to the Mississippi River (MiRIR, Mississippi River

Interdisciplinary Research), two research cruises were

conducted aboard the R/V Pelican. The objectives of

this study were to: (1) evaluate the inventories of

particle-reactive tracers (234Th, 7Be, and 137Cs) in

bottom sediments in the Mississippi River deltaic

region; and (2) to use these tracers to understand the

processes and mechanisms that control the transport

and fate of mobile muds on the continental shelf

associated with a major freshwater river. This study

represents the first attempt to evaluate the short-term

(seasonal) sediment dynamics in the Mississippi River

deltaic region.

1.1. Natural particle-reactive tracers

The particle-laden waters on the Louisiana shelf

are a characteristic signature of the Mississippi River

and its seaward plume. Many elements are adsorbed

by settling particles and, therefore, follow the path-

ways of the particles. Several naturally occurring and

anthropogenically introduced radionuclides (e.g.,234Th, 7Be, 210Pb and 137Cs) associate strongly with

particles and are therefore useful tracers of particle

transport and fate. Many investigators have used these

tracers in both coastal and open ocean environments

(McKee et al., 1984, 1986; DeMaster et al., 1986;

Moore et al., 1996; Smoak et al., 1996; Baskaran et

al., 1997; Feng et al., 1999).

Th-234 is continuously produced in seawater by

the alpha decay of its direct parent, 238U. Uranium

concentrations are typically small in river water and

increases with salinity to a maximum value in open-

ocean waters. Uranium is not readily scavenged from

the water column by particles in marine waters and

has shown both conservative and nonconservative

behavior in estuarine mixing zones. However, previ-

ous studies have demonstrated the removal of 234Th

on a variety of time scales. Removal rates as short as

0.5–1.0 days were measured in coastal environments

with a high sediment source (McKee et al., 1984) and

as long as 10–100 days in coastal waters with

relatively low suspended sediment concentrations

(Santchi et al., 1979; Coale and Bruland, 1985). In

any event, the rapid removal of 234Th relative to its

half-life, 24.1 days, makes it useful for examining

rates of geochemical and sedimentological processes.

Researchers have utilized 234Th in estuaries, the

coastal ocean, and the deep sea as an indicator of

D. Reide Corbett et al. / Marine Geology 209 (2004) 91–112 93

particle mixing in recently deposited sediments (Aller

and Cochran, 1976; McKee et al., 1986, 1995; Santchi

et al., 2001), short-term deposition in high deposition

environments (Day et al., 1995; Schmidt et al., 2001)

and seasonal variations in the processes controlling

the fate of particle-reactive species (Smoak et al.,

1996).

Similarly, 7Be (t1/2 = 53.3 days) has been used

extensively in shallow-marine environments to eval-

uate particle cycling (Olsen et al., 1986; Baskaran and

Santchi, 1993; Feng et al., 1999; Sommerfield et al.,

1999) and deposition rates (Dibb and Rice, 1989;

Canuel et al., 1990). 7Be, produced in the atmosphere

by cosmic-ray spallation of oxygen and nitrogen, is

rapidly associated with aerosols and supplied to the

earth’s surface by precipitation and dry deposition.

Once in the terrestrial and marine environments, 7Be

quickly adsorbs onto particles (Bloom and Crecelius,

1983; Olsen et al., 1986). Like 7Be, 137Cs (30.2 years)

was supplied to the earth’s surface by precipitation

and dry deposition. Cs-137 is an anthropogenic tracer

introduced into the environment beginning in the mid-

1950s associated with atmospheric nuclear testing.

The current fallout levels of 137Cs are near zero, due

to the cessation of nuclear weapons testing and a

relatively short residence time in the atmosphere.

Additional inputs of this tracer in coastal systems

are derived from the resuspension and delivery of fine

soil particles. Recent studies have demonstrated the

utility of these nuclides in identifying river-borne

sediments within an estuary (Baskaran et al., 1997)

and the continental margin (Sommerfield et al., 1999;

Allison et al., 2000). Together, these nuclides were

used in this study to evaluate spatial and temporal

trends in sediment deposition associated with the

Mississippi River plume.

2. Study area

The Mississippi River is the dominant stimulus for

coastal processes in the northern Gulf of Mexico,

boasting the third largest drainage basin area and the

seventh largest water discharge and suspended load

among world rivers (Milliman and Meade, 1983;

Meade 1996). Approximately 60% of the total sus-

pended matter and 66% of the total dissolved materi-

als transported from the conterminous U.S. to the

ocean are carried by the Mississippi River (Presley

et al., 1980). Draining approximately 47% of the

conterminous U.S., the Mississippi delivers approxi-

mately 2� 1014 g of suspended matter to the northern

Gulf shelf each year (Meade and Parker, 1985;

Meade, 1996).

The Mississippi River has an average freshwater

discharge of 380 km3 year� 1 through the distributar-

ies of its birdfoot delta, directing freshwater input

primarily to the west onto the Louisiana continental

shelf (Meade and Parker, 1985). In nearly all years,

along the length of the Mississippi River, mean

discharges during the high-water months can be

expected to be about three times the discharges during

the low-water months (Meade, 1995). Sediment de-

livered to the Mississippi Delta has been significantly

reduced in the last century due to the construction of

dams, diversions, and levees. The suspended sedi-

ment load carried to the Gulf of Mexico by the

Mississippi has decreased by approximately one half

since the 1800s, majority of that occurring since

1950. In typical years, the maximum monthly sedi-

ment discharge (March) in the lower river averages

five times greater than the minimum monthly dis-

charge (September).

As part of MiRIR, two cruises were planned and

designed to collect samples associated with the large

variations in river discharge (Fig. 1A). Although these

cruises do correspond with the maximum and mini-

mum river flow for water year 2000, this was a

relatively dry year resulting in a relatively low–high

flow period, peaking in late April at 19,150 m3 s� 1

(measured at Tarbert Landing, f 500 km upriver of

Southwest Pass). In comparison, the previous year had

a peak flow of 33,000 m3 s� 1 in early February. The

October cruise corresponded to low flow conditions of

approximately 4600 m3 s� 1. The transit time of river

water between Tarbert Landing and Southwest Pass

ranges from 2 to 5 days depending on river discharge

(Pereira and Hostettler, 1993).

During both cruise tracts, stations were selected in

order to gather representative samples from different

water masses present on the shelf: turbid, high pro-

ductivity and open Gulf (Fig. 2). Initial sample

locations were chosen based on sites occupied during

previous sedimentological studies, providing some

background information (McKee, unpublished data).

Additional sights were chosen to provide sufficient

Fig. 1. The important seasonal controls of sediment delivery and transport include (A) river flow as measured at Talbert Landing, (B) wave

energy as measured at Station 42040 east of Southwest Pass (29.21N, 88.20W), and (C) wind velocity measured at NOAA Station BURL1 at

Southwest Pass. Cruise dates are indicated by the shaded region. River flow measured at Talbert Landing for a 3-year period including the year

prior and following the cruises. Note the significantly lower discharge during our study period relative to other years. Annual patterns of wave

energy and wind velocity are similar during the 3-year period shown. Winds are typically from the southeast between April and October.

Between October and April, dominant winds are from the northeast with dramatic wind reversals associated with continental weather systems.

D. Reide Corbett et al. / Marine Geology 209 (2004) 91–11294

Fig. 2. Location of sites occupied during both spring (� ) and fall (open diamond) cruise tracts. Bathymetry (m) of the inner Louisiana shelf is

also shown. Closed triangles refer to near river and open shelf sites of downcore profiles (Figs. 8 and 9).

D. Reide Corbett et al. / Marine Geology 209 (2004) 91–112 95

coverage of the 4800 km2 area necessary to evaluate

the various water masses.

3. Methods

Water-column and seabed samples were collected

from approximately 50 sites on the shelf during each

cruise. At each site, a CTD equipped with 12–20

l Nisken-bottles was used to collect surface and

bottom water for total suspended matter (TSM), as

well as other constituents not reported here. TSM was

measured by filtering a measured quantity of water

(250–500 ml) through a preweighed 0.2 Am Neu-

cleopore membrane that was then immediately rinsed

with distilled-deionized water after filtration to re-

move salts. Filters were returned to Tulane University,

dried (60 jC for 48 h) and reweighed to determine the

weight of material retained on the filter. Total sus-

pended solid concentrations were estimated by the

difference between the filters’ weight before and after

filtration.

Sediment samples were collected via box core at

each site. The box core was subsampled with a 10 cm

(diameter) plexiglass core tube and extruded at 2 cm

intervals. Extruded subsamples for radiochemical

D. Reide Corbett et al. / Marine Geology 209 (2004) 91–11296

analysis of 234Th, 7Be, 210Pb, 137Cs, and 238U were

stored and returned to Tulane University. Samples for

water content and porosity were collected onboard

during extrusion. A measured volume of wet sediment

from each interval, typically 10 ml, was transferred to

a preweighed container that was sealed and brought

back to the laboratory. These samples were oven dried

at 60 jC for several days and the gravimetrically

determined water content, correcting for salt residue,

was used to calculate the sediment dry-bulk density.

Samples from all sediment cores were analyzed for234Th (t1/2 = 24.1 days), 7Be (t1/2 = 53.3 days), 226Ra

(t1/2 = 1600 years), and 137Cs (t1/2 = 30.2 years) by

direct gamma counting. Samples were initially dried

homogenized and packed into standardized vessels

before counting for approximately 24 h. Sample size

ranged between approximately 2 and 40 g, depending

on counting geometry (vial or tin, respectively).

Gamma counting was conducted on one of three

low-background, high-efficiency, high-purity Germa-

nium detectors (Coaxial-, BEGe-, and Well-type)

coupled with a multichannel analyzer. Detectors were

calibrated using a natural matrix standard (IAEA-300)

at each energy of interest (except 7Be) in the standard

counting geometry for the associated detector. The

counting efficiency of 7Be (477 keV) was determined

by linear regression of calculated efficiencies for

energies beyond 200 keV.137Cs activities were measured using the net counts

at the 661.7 keV photopeak. 226Ra activities were

determined by allowing samples to equilibrate for

greater than 3 weeks and recounting. 226Ra is then

determined indirectly by counting the gamma emis-

sions of its granddaughters, 214Pb (295 and 351 keV)

and 214Bi (609 keV). Total 234Th (63.3 keV) was

determined by direct gamma counting within 3

months of collection. Approximately 30% of the

samples were then recounted on the germanium

detectors after 6 months (f eight half-lives), allow-

ing time for decay of excess 234Th and quantification

of supported 234Th. Due to the large number of

samples and the high demand on the germanium

detectors, all the samples could not be recounted

using this method. Supported 234Th was also deter-

mined for all the samples by alpha spectroscopy of238U. Differences between the two methods averaged

15% and were within the counting uncertainty for

many of the samples. For consistency, we have

reported all the excess 234Th results as the difference

between total 234Th activity from that supported by238U (via alpha spectroscopy). Reported data for

short-lived nuclides (234Th and 7Be) are corrected

for radioactive decay that had occurred between

sample collection and analysis.

Total 210Pb was measured by alpha spectroscopy

following the methodology of Nittrouer et al. (1979).

Approximately 1.5 g of sediment is spiked with 209Po,

as a yield determinant, and partially digested with 8 N

HNO3 by microwave heating. Polonium-210,209

from the solution was then electrodeposited onto

nickel planchets in a dilute acid solution (modified

from Flynn, 1968). Excess 210Pb activities were

determined by subtracting the total 210Pb from that

supported by 226Ra.

4. Results and discussion

4.1. River discharge and suspended solids

Delivery of sediments through the delta and into

the Gulf is greatly influenced by temporary storage

and remobilization of sediments in the lower river

(McKee and Baskaran, 1999). As Demas and Curwick

(1988) demonstrated, periods of sediment storage in

the lower river are typically 4–8 months, after dis-

charge drops below 14,000 m3 s� 1 (early summer)

and before it exceeds 21,000 m3 s� 1 (mid-winter) as

measured at Tarbert Landing, located downstream of

the diversion channel to the Atchafalaya River. In fact,

they observed as much as an 80% decrease in sus-

pended sediment concentrations during flows between

7300 and 7600 m3 s� 1 and 30% increase at flows

between 25,000 and 28,000 m3 s� 1. Therefore, de-

livery of sediments from the upper river to the shelf

may be augmented or diminished depending on stor-

age/remobilization processes in the lower river. These

observations demonstrate that the delivery of sus-

pended sediment into our study area is dependent on

the river flow prior to MiRIR cruises (i.e., the history

of particles).

The 7 months prior to the first MiRIR cruise had

very little (15 days total) discharge above 14,000 m3

s� 1 (Fig. 1). This cruise was planned in early April in

an attempt to sample during or just after the typical

high flow period associated with spring melt. How-

D. Reide Corbett et al. / Marine Geology 209 (2004) 91–112 97

ever, below average discharge was prevalent through-

out the spring. The peak river discharge during 2000

(19,150 m3 s� 1, nearly half the maximum flow of the

previous year) occurred in late April, after the end of

our cruise. River discharge fell below 14,000 m3 s� 1

in early July and remained below this throughout the

second cruise. Assuming the correlation between river

discharge and storage/mobilization suggested by

Demas and Curwick (1988) is accurate and applica-

ble, we can infer that river discharge did not reach or

exceed a magnitude to produce significant sediment

remobilization in the lower river before or during

either cruise. Therefore, we would expect to have

significantly less sediment delivery during both

MiRIR cruises than that of a typical year.

Total suspended matter data collected during the

two cruises reiterate the dependence of TSM concen-

tration on river discharge. Average and standard error

(range in values) TSM concentrations for the spring

MiRIR cruise were 6.5F 0.8 mg l� 1 (Below Detec-

tion—20.5 mg l� 1) and 6.2F 0.9 mg l� 1 (BD—31

mg l� 1) for surface and bottom waters, respectively.

Similarly, average (and range) TSM concentrations for

the fall MiRIR cruise were 1.59F 0.2 mg l� 1 (BD—

6.0 mg l� 1) and 4.2F 0.8 mg l� 1 (0.9–38.3 mg l� 1)

for surface and bottom waters, respectively. These

TSM values are extremely low in comparison to the

yearly river average of 360 mg l� 1, compiled for a 29-

year period ending in the mid-1990s (Trefrey et al.,

1994; Meade, 1995). Although neither the average nor

the range of TSM concentrations varied considerably

between sampling periods, as would be expected from

high flow and low flow periods, the spatial patterns in

both surface and bottom waters are certainly different

between the two seasons (Fig. 3A–D).

TSM concentrations in both surface and bottom

waters of the spring cruise do not reveal a significant

plume associated with the mouth of the Mississippi

(Fig. 3A and B) as has been described in previous

studies (Trefrey et al., 1994; Wright and Nittrouer,

1995). Patterns of TSM in the water column may have

also been influenced by resuspension (due to wind

and waves, Fig. 1B and C) or the movement of the

river plume over time associated with change in

weather patterns. Also, the pulse event, rapid rise in

discharge, experienced by the river just prior to the

cruise followed by relatively low flow then increased

flow during the cruise may have led to the fragmented

spatial pattern of both the surface and bottom waters.

The influence of the increased flow was just becoming

evident near the river mouth at Southwest Pass during

the April cruise.

Low flow conditions during the fall cruise led to

very low TSM concentrations throughout the shelf

waters. TSM concentrations dropped dramatically (>5

fold) once waters moved from Southwest Pass onto

the shelf in both surface and bottom waters (Fig. 3C

and D). Concentrations of TSM in bottom waters were

typically three to four times than those of surface

waters. Again, increased bottom water TSM concen-

trations may be attributed to resuspension of shelf

sediments and/or redistribution within the benthic

boundary layer.

Both spring and fall cruises had low flow condi-

tions and associated low TSM in river and the

adjacent shelf water. It is important to note that during

this period the storage of sediment in the lower

Mississippi may have been an important component

in the overall sediment dynamics. Therefore, the

following discussions of short-term sediment deposi-

tion and remobilization are based on a period of

limited sediment delivery to the shelf.

4.2. Depositional environment

4.2.1. Mobile muds

Estimates of the depth of the mobile mud layer,

those sediments that were recently deposited/dis-

turbed, are based on the physical characteristics docu-

mented in the field and laboratory (color, porosity,

texture, X-radiography) and short-lived radiochemical

tracers (7Be, 234Th, see below). Spatial patterns during

both cruise periods show a definite river influence

(Fig. 4). In contrast, cores collected away from the

major influence of the river typically had a mobile

mud layer less than 3 cm. Cores collected closer to

Southwest Pass had thicker mobile mud layers,

extending as deep as 12 cm (MiRIR II). Interestingly,

cores collected during MiRIR II (fall cruise) had

thicker mobile mud layers at most locations near the

river plume. These data suggest that the spatial dis-

tributions of mobile muds adjacent to the river vary

greatly seasonally. This may be due to the delivery of

sediments to the study area throughout the summer,

relative to the short period of ‘‘high’’ flow that

occurred before the first cruise. In addition, any

Fig. 3. Total suspended matter (mg l� 1) from water samples collected during MiRIR I (surface water—A; bottom water—B) and MiRIR II

(surface water—C; bottom water—D).

D. Reide Corbett et al. / Marine Geology 209 (2004) 91–11298

disturbance to the seabed via resuspension/deposition

or increased biological mixing would also increase the

apparent mobile mud depth, rather than the delivery of

new sediments.

4.2.2. Distribution of radionuclides: 234Th, 7Be, and137Cs

The surficial sediments reflect processes that have

occurred during the recent past, as defined by the time

scale of interest. The period of time over which the

tracers integrate (e.g., four to five half-lives) is de-

pendent on the corresponding half-life. The tracers

utilized in this study have varying integration times

(234Th: f 100 days; 7Be: f 250 days; 137Cs/210Pb:

decades) that can provide insight into the time scale of

sediment deposition/remobilization. The distributions

of excess 234Th, 7Be, and 137Cs during the two cruises

are shown in Figs. 5–7, respectively. All data are

expressed as total inventories, which are obtained by

integrating the tracer activity with depth in the seabed

Fig. 4. Depth of recently reworked sediments (mobile muds, cm) within the confines of our study area during MiRIR I (A) and II (B). Contours

are in 2 and 3 cm for MiRIR I and II, respectively.

D. Reide Corbett et al. / Marine Geology 209 (2004) 91–112 99

and accounting for bulk density and porosity. Inven-

tories were typically summed over continuous sedi-

ment intervals, between 1 and 3 cm thick. For those

cores that were not continuous, activities were inter-

polated between measured intervals. Excess Th-234

inventories calculated during MiRIR II are considered

a minimum estimate, since at least one interval from

most cores was counted beyond three half-lives of the

nuclide. These data were not included in the inventory

calculations in an attempt to reduce uncertainty. Sim-

ilarly, 137Cs inventories are considered minimum

since many of the cores did not reach a depth of zero

activity (78% and 88% of the cores still had activity at

the bottom interval for samples collected during

MiRIR I and II, respectively). In order to account

for this anomaly, we have only integrated the cores to

the deepest common depth (11 cm), which includes

the mobile mud layer in all but one station. Majority

of the cores analyzed (>95%) had a decreasing 137Cs

activity downcore without a discernable peak, which

D. Reide Corbett et al. / Marine Geology 209 (2004) 91–112100

includes several cores (f 15% of total collected)

analyzed to depths below 30 cm. Integrating the137Cs over at least the depth of the mud layer should

provide an observable increase or decrease in new

material. An increase in inventory (even integrated

over just the surface sediments) must be due to a new

source (sediments from the river or remobilized sedi-

ments with a greater activity) while a decrease in

inventory would indicate a removal of sediments from

the area. Although this shallow integration may pre-

vent quantitative interpretations, relative seasonal var-

iations should be discernable.

The range in 234Th inventories during both MiRIR

I and MiRIR II were not significantly different from

each other and varied by as much as two orders of

magnitude, 1.2–129 and 1.0–200 dpm cm� 2, respec-

tively. However, the average inventory (standard er-

ror) during the fall was twice that of the spring cruise

(44.2F 11.4 vs. 22.0F 3.3 dpm cm� 2). Similarly, the

ranges in 7Be inventories during the two cruises were

not significantly different (0.0–24.5 and 0.6–15.0

dpm cm� 2, respectively) and the average inventory

was much higher during the fall cruise (4.7F 0.4 vs.

2.1F 0.5 dpm cm� 2). There was only one site that

had an inventory greater than 4 dpm cm� 2 during the

spring cruise, which skews the data slightly. Neglect-

ing this anomalous site, the average 7Be inventory

during the spring was 1.6F 0.2 dpm cm� 2. The range

and average of the normalized 137Cs inventories were

slightly greater in the fall cruise (0.1–4.9; 2.2F 0.2

dpm cm� 2) than that of the spring (0.1–3.1; 1.6F 0.1

dpm cm� 2). Collectively, the fluctuation in tracer

inventories between sampling episodes suggests an

increased sediment delivery to/remobilization and

deposition on the shelf (i.e., higher inventories) during

the period prior to the latter cruise (integrating over a

period of higher flow and increasing wave energy and

wind velocities). In addition, the change in 137Cs

inventory may reflect an export of sediments off the

shelf during the winter months due to the longer

integration time (decades, see below). The spatial

distribution of the tracers provides additional infor-

mation to further support increased sediment delivery

in late spring/early summer and export of sediments

during the winter.

In order to evaluate the spatial distribution of the

excess 234Th inventory, it is important to consider its

source. Again, 234Th is continuously produced in

seawater by the alpha decay of its direct parent,238U, whose activity generally increases with salinity.

Therefore, particles deposited in deep offshore waters

have a higher supply rate than those deposited near

the river mouth. This increased supply rate is a

function of both the particle residence times and

concentration of 238U in the water column (e.g.,

extended settling time), thus a greater exposure of

particles to newly produced 234Th and elevated salin-

ities beyond the river’s influence. It is apparent that

the sediment inventory is partially influenced by the

depth of the water column (compare inventory con-

tours, Fig. 5A,B, to isobaths, Fig. 2). Due to the fairly

low flow conditions, salinities did not fall below 18,

except within the confines of Southwest Pass. Even

these lower salinities were restricted to stations adja-

cent to Southwest Pass. Surface salinities increased

to >25 within 15 km of Southwest Pass during the

higher flow conditions of MiRIR I (spring cruise)

and within f 8 km during the fall cruise. Surface

and bottom water salinities on the shelf ranged from

approximately 18 to 35 and 32 to 36, respectively.

An estimate of the sediment 234Th inventory sup-

ported by production in the water column, followed

by complete scavenging and one dimensional verti-

cal flux can be calculated using the water depth and

an average salinity for the area (based on CTD casts

at each site). The measured inventory at each station

can then be normalized to the depth/salinity of the

environment. Therefore, using the ratio of measured

excess 234Th:water column supported 234Th invento-

ry, we have essentially removed the inventory’s de-

pendence on water column depth and salinity and can

evaluate spatial trends independently (Fig. 5C,D).

Using the normalized values, inventories greater than

one indicate an area of sediment focusing or increased

mass flux. Highest inventory ratios are in close

proximity to the river mouth during both cruises with

obvious sediment focusing associated with the river

plume during the MiRIR II (234Th ratio>18).

Unlike 234Th, 7Be and 137Cs do not have an in situ

production source; rather atmospheric deposition and

riverine input primarily control the supply of 7Be to

shelf sediments, while 137Cs is derived primarily from

the watershed. Daily atmospheric deposition of 7Be

has been estimated from a year of monthly samples

collected from Barataria Bay. The daily flux ranged

from 0.02 to 0.16 dpm cm� 2 with a mean of 0.07 dpm

Fig. 5. Excess 234Th inventories during MiRIR I (A) and II (B) in dpm cm� 2. Accounting for in situ production in the water column as a

function of depth and salinity, the 234Th ratio, which refers to the ratio of measured excess 234Th to that of the water column supported 234Th

inventory (234Th ratio) in the sediments, is presented for MiRIR I (C) and II (D). All stations were used to produce contours during MiRIR I (A

and C). Contours are based on closed diamonds for MiRIR II (B and D) due to short half-life of the tracer and lack of detector time.

D. Reide Corbett et al. / Marine Geology 209 (2004) 91–112 101

cm� 2 (n = 20; McKee, unpublished data). This mean

daily flux is within the range of values reported by

others in this region (Baskaran and Santchi, 1993;

Canuel et al., 1990). Assuming the mean is a good

representation of the atmospheric deposition (0.07

dpm cm� 2 day� 1), the flux would support a steady

state inventory of 5.4 dpm cm� 2, if the atmospheric

input were quantitatively removed to the sediments.

Atmospheric flux has been the primary source of 7Be

in many estuarine and shelf environments (Baskaran

and Santchi, 1993; Canuel et al., 1990; Dibb and Rice,

1989). However, riverine input may be quantitatively

more important source in systems with a high drain-

age-basin to estuarine surface area (Baskaran et al.,

D. Reide Corbett et al. / Marine Geology 209 (2004) 91–112102

1997). The Mississippi River basin (3.4� 106 km2)

dwarfs the aerial extent of river-borne sediment de-

position within the study area (4800 km2). This

extremely high drainage-basin to deposition area ratio

(f700) suggests that the river is likely a significant

source of 7Be and 137Cs to the shelf relative to direct

atmospheric input. This has also been demonstrated

on the California continental margin associated with

coastal river flooding of the Eel River (Sommerfield et

al., 1999). The Mississippi riverine 7Be source is

reflected in the dramatic differences of these tracer

inventories between the two sampling periods and the

spatial distribution of the inventories.

Measured 7Be inventories during the spring cruise

were all well below the steady state inventory supplied

by the atmosphere, except for one site near the river

mouth that had an inventory of 24.5 dpm cm� 2. This

anomalous site is also in the vicinity of the high excess234Th inventories during this cruise. 137Cs inventories

show a similar trend to that of 7Be, reiterating the

importance of the river for delivery of these nuclides.

However, the presence of 7Be in the bottom sediments

suggests net deposition of new material during the

previous few months. The spatial distribution of the7Be and 137Cs inventories had a discernible river

influence during both cruises (Figs. 6 and 7). Although

inventories measured during the fall cruise were typi-

cally two to three times higher than the spring and

mostly above that of atmospheric support (7Be), with

highest inventories observed near the river mouth

following the influence of the river plume.

4.2.3. Collective interpretations of the depositional

environment

River flow and wind/waves have the dominant

influence on the deposition/remobilization of mobile

muds in coastal Louisiana (Allison et al., 2000;

Walker and Hammack, 2000; Huh et al., 2001). In

addition, these forces have large seasonal variations

and dominate the shelf at different times of the year.

Long-term averages of freshwater input, wind energy

and wind direction in the northern Gulf of Mexico

show three characteristic sets of conditions: (1)

winter–spring, represented by January–April, is a

period of increasing and high river discharge, high

wind energy, and periodic wind reversals; (2) sum-

mer, represented by June–August, is a period of

decreasing and medium discharge, low wind energy,

and uniform wind direction; and (3) autumn, repre-

sented by September–November, is a period of low

discharge, rising wind energy and periodic wind

reversals. Between April and October, winds are

typically from the southeast but from October to

April, continental weather systems impact the north-

ern gulf, bringing dramatic wind reversals to the

region on time scales of 3–10 days (Henry, 1979).

During this 6-month period of time, winds are

typically stronger and from the northeast. In fact

during the 3 years of wind velocity presented (Fig.

1C), northeast winds blew >45% of the time between

October and April. Thus, during the summer, the

Mississippi River plume typically moves westward

along the shelf because of the combined effects of

the earth’s rotation and the prevailing winds. In

contrast, during the winter, plumes are periodically

pushed offshore in an S, SE, or even E direction for

several days associated with the passage of each

continental weather system, before returning again

to the more typical westward direction, influenced by

northeast winds. Therefore, the prevailing wind di-

rection and/or the frequency of frontal systems

(especially during the winter) is important in the

delivery and deposition/remobilization of river-borne

particles and the associated tracers. In addition,

biological mixing can alter the distribution of the

short-lived tracers and must be considered. Based on

this qualitative representation and the fact that the

MiRIR cruises were conducted under relatively low

flow conditions (Fig. 1), it may be assumed that our

spring cruise integrated over a period of relatively

little sediment delivery to the shelf (low discharge)

and decreasing sediment remobilization. In contrast,

the fall cruise would represent a period of variable

sediment delivery and increased opportunity for

disturbance.

By utilizing the three tracers (234Th, 7Be, and137Cs), time scales of depositional events may be

interpreted based on the varying mean lives of the

tracers (weeks, months, years). It is evident that the

dominant depositional area on the shelf for river-borne

material occurs just west of Southwest Pass. During

the cruises, the highest inventories of both short-lived

tracers occurred at this location (Figs. 5 and 6).

However, the aerial extent of this depositional zone

appears to increase between the two cruises. Little

flow over the winter (1999) followed by a short

Fig. 6. Spatial distribution of the 7Be inventory demonstrates the importance of the river as its source and the potential of sediment

remobilization during the winter. Inventories are shown for MiRIR I (A) and II (B) in dpm cm� 2. The apparent bull’s eye in the near river

contour during both cruises is due to exceedingly high inventories (>10 dpm cm� 2) in several near river stations.

D. Reide Corbett et al. / Marine Geology 209 (2004) 91–112 103

duration of higher flow, a pulse event in early March

and just prior to the cruise, probably accounts for the

relatively high excess 234Th (>30 dpm cm� 2) and 7Be

(>24 dpm cm� 2) inventories near the river mouth

compared to the rest of the study area during the April

cruise. The reduced wave energy and wind velocities

in conjunction with increased river flow, thus sedi-

ment delivery, following the spring cruise probably

resulted in increased deposition of sediments and

elevated tracer inventories (7Be and 137Cs) during

the late summer and early fall months (Figs. 6B and

7). Due to the shorter integration time of 234Th, the

high inventories during the fall cruise (Fig. 5D)

cannot simply be attributed to the increased deposition

during the summer months, since most of the 234Th

would have decayed by the time of sampling; but

Fig. 7. Spatial distribution of the 137Cs inventory in dpm cm� 2 for MiRIR I (A) and II (B). Differences in the two periods indicates deposition

prior to the fall cruise and potential export of sediment during the winter.

D. Reide Corbett et al. / Marine Geology 209 (2004) 91–112104

these high inventories are to be expected due an

increased mobile mud layer from newly deposited

material throughout the summer months followed by

a rapid increase in wind velocity and wave energy just

prior to the cruise (Fig. 1), thus remobilizing this

ephemeral layer. Based on these observations, it

seems apparent that much of the river-borne sediment

does not initially move beyond f 30 km from the

river mouth. This rapid deposition agrees with the

rapid reduction in water column TSM observed in this

and earlier studies (Trefrey et al., 1994). Initial sedi-

ment resuspension/remobilization has the potential to

increase tracer inventories, but does not transport

these mobile muds off-shelf on the time scale of

weeks (Allison et al., 2000), rather off-shelf transport

is most likely a function of multiple remobilization

D. Reide Corbett et al. / Marine Geology 209 (2004) 91–112 105

events throughout the winter months (Walker and

Hammack, 2000).

An interesting spatial pattern observed in the 7Be

and 137Cs data, not present in the shorter-lived 234Th,

is a path of relatively higher inventories (compared to

the remaining study area) moving away from South-

west Pass (resembling a plume) in the spring com-

pared to much higher more focused inventories in the

fall. These spatial differences suggest a seasonal

change in the sediment dynamics. The relatively

higher inventories moving away from the delta could

be associated with new sediment deposition near the

river followed by multiple resuspension/remobiliza-

tion events transporting higher activity sediments

away from the area of initial deposition and in the

direction of the dominant longshore current (west-

ward). Based on the above discussion, it seems

unlikely that sediments will be transported >30 km

away from Southwest Pass before settling to the

bottom during the observed increased river discharge.

Further, any mobile sediment that could be trans-

ported beyond that distance would certainly have an

increased 234Th signature above that measured due to

the prolonged exposure to the dominant source (dis-

solved 238U). Although decay of excess 234Th is a

possibility, the river flow during the spring cruise had

only increased a month prior to the cruise, e.g., the

short-lived tracer should still have been present.

Recent resuspension of these sediments would tend

to increase the 234Th inventory more than that of 7Be

due to the difference in primary sources (in situ vs.

riverine), as seen in the fall cruise. Therefore, the

likely source of this 7Be and 137Cs-enriched pathway

is the resuspension/remobilization of sediments dur-

ing the winter months (high wave energy/wind veloc-

ity) from the high depositional area near the river

mouth. The low excess 234Th inventories during the

spring cruise and the focused high inventories ob-

served during the fall (attributed to resuspension)

suggest the residence time in the water column during

resuspension is short and may provide some evidence

for fluid muds on the shelf. On the inner shelf of the

nearby Atchafalaya, thin layers ( < 10 cm) of fluid

muds form rapidly with the passage of cold fronts that

cause resuspension events followed by rapid settling

of particles as wind and wave energy subside (Allison

et al., 2000). These wind/wave-driven resuspension

events can occur 20–30 times each year between

October and April (Walker and Hammack, 2000).

Again, it is important to note that this study was

conducted during a fairly low flow year. The high 7Be

and 137Cs inventories observed during the fall may

actually be small compared to a ‘‘typical’’ year.

Regardless, high inventories near the river mouth in

the late fall are present even at these low flow

conditions, a function of deposition during the rela-

tively quiescent summer months. Increased weather

forcing during the winter could easily remobilize the

recently deposited sediments near the river mouth and

move them in the primary direction of longshore

transport, thus producing the path of higher invento-

ries along the shelf observed in April (Figs. 6A and 7).

This resuspension/remobilization is most evident

with the 137Cs inventories, since 137Cs lacks an active

atmospheric source and has a much longer half-life

than 7Be. The increase in 137Cs inventory in the fall is

certainly associated with the increase in sediment

delivery to the shelf throughout the summer months.

Assuming the river delivered a similar sediment load

(e.g., 137Cs inventory) to the shelf during the previous

summer as that observed during MiRIR II, it would

appear that the inventory decreased from the previous

fall (1999) to the spring sampling. This is probably a

conservative assumption as the river flow during the

study period was much lower than the previous year.

It is more likely that the fall Cs inventory following a

typical high flow period would be higher than that

observed during MiRIR II. Due to the long half-life of137Cs (30.2 years) and the continuous delivery of

riverine Cs-laden material, it would seem more likely

that the inventory would increase on such a short time

scale or at least remain the same in the surface

sediments. The observed decrease in 137Cs inventory

between seasons suggests an export of sediments

beyond the study area during the winter months. This

off-shelf transport has also been suggested by others

(Goni et al., 1997, 1998; Huh et al., 2001).

4.3. Sediment deposition: seasonal vs. decadal

Profiles of excess 234Th, 7Be, and 210Pb in the

seabed were determined for the two cruises at two

sites within the study area (Tables 1 and 2). The

stations represent near-river (108/15) and open shelf

(62/93) environments. Surface sediment activities and

seabed inventories of the short-lived nuclides were

D. Reide Corbett et al. / Marine Geology 209 (2004) 91–112106

seasonally and spatially variable. Depth profiles of

these two nuclides (234Th, 7Be; Fig. 8) were used to

estimate short-term sediment deposition rates (S)

assuming that the downcore profile is solely a product

of deposition. X-radiograph cores were collected

during both cruises to interpret sediment-mixing pro-

cesses (Fig. 9). Based on these X-rays and the noted

lack of macrofauna during core collection, bioturba-

tion does not appear to be an important mixing force,

Fig. 8. Excess 234Th, 7Be, and excess 210Pb activity in sediment cores col

stations: near river (A); and open shelf (B). Only surface (0–1 cm) data wa

counting delay. Lines represent regressions used in calculating deposition

although physical mixing cannot be ruled out. These

findings are supported by previous work on the

Atchafalaya River. Allison et al. (2000) found little

disturbance by burrows in high sedimentation areas

(short-term deposition based on 7Be; f 0.7 cm

month� 1). In contrast, they found near complete

homogenization of primary structures from bioturba-

tion caused by soft-bodied infauna in stations with

low sedimentation on the shelf (f 0.2 cm month� 1).

lected from MiRIR I (closed symbols) and II (open symbols) at two

s available for 234Th during MiRIR II at the near river station due to

/accumulation rates.

Table 1

Summary of excess 234Th and 7Be deposition data during both cruises at two reoccupied sites: near river (108/15); open shelf (62/93)

Cruise/station 234Th activitya

(dpm g� 1)

7Be activitya

(dpm g� 1)

234Th depthb

(cm)

7Be depthb

(cm)

234Thc maximum S

in cm month� 1

(g cm� 2 month� 1)

7Bec maximum S

in cm month� 1

(g cm� 2 month� 1)

MiRIR I

108 55.9F 25.2 1.0F 0.3 5 9 1.2F 0.2 (0.7) 1.5F 0.5 (0.9)

62 50.7F 19.2 3.8F 1.0 5 5 2.0F 0.2 (0.9) 0.8F 0.1 (0.4)

MiRIR II

15 77.6F 32.7 5.0F 1.2 1d 5 NM 3.0F 1.2 (2.1)

93 32.9F 16.7 2.4F 1.4 15 15 3.9F 1.2 (1.6) 2.2F 0.4 (0.9)

a Sediment activity measured in the 0–2 cm interval.b Sediment depth over which the inventory was determined.c Steady-state sediment deposition (S) is governed by: S= kz/(ln(C0/Cz)) where k is the corresponding decay constant, z is the sediment

depth, C0 is the activity at zero depth, and Cz is the activity at the z depth (McKee et al., 1983).d Minimum depth of penetration due to extended hold time before counting on deeper samples; NM= not enough data to calculate (n< 2).

Table 2

The 100-year average sediment accumulation rates for near river

and open shelf environments are significantly lower than the short-

term deposition estimated by excess 234Th and 7Be

Shelf region Pb-210 accumulation,

cm year� 1

(g cm� 2 year� 1)a

Deposition to

accumulation ratiob

Near river 2.0F 1.0 (1.5) 7–18 (6–17)

Open shelf 1.3F 0.4 (0.6) 7–36 (8–32)

a Accumulation rates are reported from cores collected in the

spring. However, rates are within error of those calculated for cores

collected in the fall. Rates calculated assuming constant flux and

constant sedimentation rate (simple model; S = kz/(ln(C0/Cz))

(Appleby and Oldfield, 1992).b Range is based on varying deposition rates (Table 1) from

spring and fall cruises when converted to cm yea� 1 (g cm� 2

year� 1). Accumulation rates are based on 210Pb rates in the

corresponding location.

D. Reide Corbett et al. / Marine Geology 209 (2004) 91–112 107

However, the two stations used here for comparison

both have relatively high deposition rates (Table 1).

X-rays from several cores collected near the river

show mass depositional events, indicated by very

little structure (Fig. 9). Again, intensive bioturbation

can have the same net effect, but does not appear to be

important in this area. Since physical mixing cannot

be neglected, these rates must be considered maxi-

mum deposition rates. Sedimentation rates were cal-

culated for both nuclides, assuming steady state, and

ranged between 0.8 and 3.9 cm month� 1 (0.4–2.1 g

cm� 2 month� 1). Unfortunately, only the surface

activities for 234Th were obtained for the near river

site during the fall cruise due to counting delays on

deeper samples.

Decadal scale measurements of sediment accumu-

lation in this same region have been calculated using210Pb (Table 2). Again, due to physical reworking of

the sediments, these rates should be considered max-

imum accumulation rates. However, the calculated

rates are similar to those previously documented

(Eadie et al., 1994, McKee and Baskaran, 1999;

Bianchi et al., 2002). Comparing the short-term de-

position rates with the decadal estimates of accumu-

lation provides insight into the degree of sediment

reworking on the shelf. The deposition to accumula-

tion ratio observed in this study (7–36) is much

higher than that observed off the Atchafalaya (0.38–

12; Allison et al., 2000), but similar to the Eel (2–24;

Sommerfield et al., 1999), Changjiang (10; McKee et

al., 1983, 1984; DeMaster et al., 1985), and near the

lower limits of the Amazon (25–1500; Kuehl et al.,

1986, 1995). The depositional environment of the

Mississippi River (water discharge, annual sediment

discharge, depth of deposition) is similar to these

rivers. Thus like the Eel, Changjiang, and the Ama-

zon, significant sediment reworking and export is an

important component of the annual sediment dynam-

ics in the vicinity of the river mouth.

Combining the excess 234Th inventories and de-

position rates of the near river environment observed

in this study (sites 108 and 15) with previous work

(McKee et al., 2004; McKee, unpublished data)

reiterates the importance of sediment remobilization

(Fig. 10). Short-term deposition rates are typically

Fig. 9. Porosity values and X-rays collected from the Near River (Site 108) and Open Shelf (Site 62) environments during the spring cruise

(MiRIR I). The Near River station shows very little structure and no indication of macrofauna. The Open Shelf station has some structure at

depth (>10 cm) with potential reworking in the surface sediments and no indication of macrofauna.

D. Reide Corbett et al. / Marine Geology 209 (2004) 91–112108

much greater than the longer accumulation rates

observed in the same area, indicating off-shelf (and

possible along-shelf) transport. In addition, this lon-

ger temporal data set demonstrates a similar pattern

observed during the two MiRIR cruises: increased

deposition and higher inventories in the summer/fall

with lower values in the corresponding spring

(April). This relationship may be a function of

sampling period, i.e., spring sampling precedes peri-

od of high deposition. However, there does appear to

be a qualitative relationship to the annual river

discharge. Although it is difficult to pinpoint the

exact connection, it may simply be related to the

bulk net supply of material delivered to the shelf

yearly. In any event, this long-term data set further

substantiates the possibility of off-shelf transport

Fig. 10. Excess 234Th inventories and deposition rates were determined from cores collected from a near river location (see Fig. 2 for location)

during a 12-year period (1988–2000). Inventories and deposition rates are roughly proportional to Mississippi River discharge. Short-term

deposition rates are much greater than the 100-year average derived using 210Pb results from a 2-m kasten core collected in the same area

(McKee, unpublished data) and represented by the dashed line.

D. Reide Corbett et al. / Marine Geology 209 (2004) 91–112 109

during the high energy (weather forcing) winter

months.

5. Summary

Sediment delivery to coastal systems plays a key

role in global biogeochemical cycles since deltas

and continental shelves are considered to be the

main repositories of organic matter in marine sedi-

ments. However, these sediment depocenters typi-

cally deviate from predictions of carbon storage/

remineralization made by diagenetic models. These

discrepancies can be attributed to the importance of

spatial and temporal sediment deposition/remobiliza-

tion in these environments. This study was con-

ducted to provide the framework for on-going

research toward understanding the dynamics of

mobile muds on the inner Louisiana shelf influenced

by the Mississippi River.

Seasonal variations in short-lived tracers were ev-

ident between the two cruises and can be explained by

the river flow and weather conditions. The spring

cruise represented a period of low flow and decreasing

weather forcing. The influence of the river was only

evident near the mouth. Increased inventories of the

tracers in the fall suggested the increased deposition

during the late summer months and increasing weather

forcing. Spatial variations in 7Be and excess 234Th

inventories indicate little transport of river-borne ma-

terial beyond f 30 km from Southwest Pass. Winter

frontal passages appear to remobilize sediments from

this high deposition area further west along the shelf.

In addition, seasonal and spatial variations in the 137Cs

inventories indicate significant sediment mobilization

and export beyond our study area.

Estimated short-term sediment deposition rates

relative to decadal-scale accumulation further demon-

strate the importance of sediment reworking near the

river mouth and in the open shelf environment. We

D. Reide Corbett et al. / Marine Geology 209 (2004) 91–112110

hypothesize that most sediment reworking and export

occurs during the winter months, typically a period of

low/increasing river discharge and increased weather

forcing. However, additional high frequency time-

series studies are required to further quantify the exact

mechanisms and extent of remobilization.

Acknowledgements

Financial support for this study was provided by

the Mississippi River Interdisciplinary Research

(MiRIR) program under ONR Grant # N00014-99-

1-0763 to BAM. We gratefully acknowledge the

captain and crew of the R/V Pelican for their

assistance in field sampling. We would also like to

thank Rodney Powell, Ryan Clark, and Mike Stewart

for their help in core collection and sampling. This

manuscript was significantly improved by the thor-

ough reviews of John Jaeger and Mark Baskaran.

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