decadal-scale sediment dynamics and...

Estuarine, Coastal and Shelf Science 71 (2007) 717e729www.elsevier.com/locate/ecss

Decadal-scale sediment dynamics and environmental change in theAlbemarle Estuarine System, North Carolina

D. Reide Corbett a,b,*, Dave Vance a, Erin Letrick a, David Mallinson a, Stephen Culver a

a Department of Geological Sciences, East Carolina University, Greenville, NC 27858, USAb Coastal Resources Management, East Carolina University, Greenville, NC 27858, USA

Received 30 March 2006; accepted 21 September 2006

Available online 29 November 2006

Abstract

During the summer of 2001, several short cores (<50 cm) were collected in the Albemarle Estuarine System (AES). Down-core measure-ments for radiochemical tracers (210Pb, 137Cs) and organic matter signatures (d13C, d15N, C:N ratio, and LOI) have been used to elucidatepotential temporal changes in fluxes and cycles of organic matter in the AES. Pb-210 geochronology indicates temporal and spatial variationsin sediment deposition rates (0.08e0.57 cm y�1) with highest rates near the AES western limit relative to the rest of the estuary. Low accumu-lation rates, deficient excess 210Pb inventories, and near linear 137Cs profiles throughout the AES suggest that sediments are resuspended bywind-generated waves and currents and flushed from the system by river discharge and wind-tides, probably to Pamlico Sound to the south.Sediments in the AES are accumulating at rates less than the current rate of relative sea-level rise for this region except in protected portionsof the estuary. Thus sediment accumulation in the AES is controlled in the short term by storm wave-base and in the long term by the creation ofaccommodation space by basin subsidence and sea-level rise. The geochemical and sedimentological data characterize the evolution of the Al-bemarle Sound and associated tributaries over the past 200e300 years. The majority of cores collected throughout the system show a significantdecrease in 13C and increase in 15N isotopic signatures up-core. Thus, the estuarine system of eastern North Carolina has changed from a marine-influenced, high brackish environment to the modern-day system, which is a highly variable, terrestrially influenced, low brackish environment.� 2006 Elsevier Ltd. All rights reserved.

Keywords: sediment accumulation; organic matter; stable isotopes; estuary; environmental change

1. Introduction

Estuaries are perhaps the most studied, but often least un-derstood of all coastal systems due to a mixture of numerouscomplex physiochemical processes (Nichols and Biggs, 1985).Because estuaries act as a filter between terrestrial and marineenvironments, estuarine sediments are valuable repositories ofinformation about the nature and history of coastal systems(Krank, 1984). Sedimentological (e.g., grain size) andgeochemical parameters (e.g., carbon and nitrogen stable

* Corresponding author. Department of Geological Sciences, East Carolina

University, Greenville, NC 27858, USA.

E-mail address: [email protected] (D.R. Corbett).

0272-7714/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.ecss.2006.09.024

isotopes) are known to vary within an estuarine system(Folger, 1972; Shultz and Calder, 1976; Peters et al., 1978;Riggs, 1996; Maksymowska et al., 2000; Yamamuro, 2000;Abril et al., 2002), and therefore can be used to understandand characterize different estuarine environments and localdynamic processes. By examining these sediment parametersin detail, it may be possible to determine the past, present,and likely future health of the estuary. However, it is importantto consider estuarine dynamics whenever interpreting sedi-mentary deposits as they are the product of multiple sedimentsources that have been modified/reworked by biological(bioturbation) and physical (i.e., resuspension, erosion, anddeposition) processes (Kniskern and Kuehl, 2003).

The AlbemarleePamlico estuaries form the second largest(after Chesapeake Bay) estuarine system and probably least

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718 D.R. Corbett et al. / Estuarine, Coastal and Shelf Science 71 (2007) 717e729

studied system of its type in the United States. The AES is anoligohaline estuary with virtually no astronomical tide, leavingthe physical hydrology and circulation governed predomi-nantly by freshwater inflow and winds. By virtue of its lackof astronomical tides, it is unique for a system of its size. Pre-vious research in the system has focused on descriptive ratherthan dynamic aspects, with virtually no reports on turbidity orsediment dynamics (Wells and Kim, 1989). The objective ofthis study is to assess the recent depositional history withinthe AES by evaluating: (1) the processes and mechanismsthat control the transport and accumulation of fine-grainedsediment using radiometric dating (210Pb and 137Cs); and (2)the possible change in nutrient and particulate organic sources,via down-core geochemical signatures (13C, 15N, C:N), asa function of barrier island inlet migration, sediment supply,and storm history. This study represents one of the firstattempts to evaluate the modern (decadal-scale) sedimentdynamics in the AES.

2. Albemarle Estuarine System

The AES is composed primarily by the Albemarle Soundand adjacent tributaries (Alligator River, North River, and Pas-quotank River), Currituck Sound, Croatan Sound, and Roa-noke Sound (Fig. 1; Giese et al., 1985). The AlbemarleSound (w1250 km2), the largest portion of the field area, isa drowned-river estuary that generally becomes deeper andwider to the east (Riggs, 1996; Sager and Riggs, 1998). Eightrivers and numerous smaller tributaries flow into the Albe-marle Sound making the total drainage area approximately46,000 km2 (Giese et al., 1985; Riggs et al., 1993). The

Albemarle Sound has a maximum depth of approximately7.3 m, but much of the central region of the Sound averages5.5 m (Giese et al., 1985). The AES is dominated by freshwa-ter due to protection from the Atlantic Ocean by a continuousbarrier island (Folger, 1972; Giese et al., 1985; Riggs, 1996);inflow of saline water occurs only through the Croatan andRoanoke Sounds via Oregon Inlet. Most of the freshwater in-flow comes from the Roanoke and Chowan Rivers (Gieseet al., 1985; Harned and Davenport, 1990). Wind is the mostimportant short-term force circulating the water within the es-tuarine system (Giese et al., 1985). The salinity of the Albe-marle Sound ranges from 0 to 7 with very little verticalstratification (Riggs et al., 1993). Salinity is at a minimum dur-ing March when runoff is greatest and at a maximum duringDecember due to low inflow of fresh water (Giese et al.,1985; Riggs et al., 1993).

Throughout the study area, Pleistocene quartz sand plat-forms (water depth 0e2 m) occur around the perimeter ofthe Sound. Sediments transition into silt, clay and abundant or-ganic matter as water depth increases towards the east (Riggset al., 1993). Mud fractions within the system are derived fromseveral different terrestrial sources as they are transporteddown the Roanoke and Chowan Rivers particularly duringhigh precipitation conditions (Riggs et al., 1993). The large in-organic sand fraction to the east is mainly derived from theOuter Banks (inlet and storm processes and aeolian transport)(Riggs et al., 1993). The percentage of organic matter through-out the modern AES ranges from 0.3% to 86% and is mainlyderived from eroding swamp forests in the west and saltmarshes in the east (Riggs et al., 1993; Riggs, 1996). Approx-imately 70% of the sediment in the central Albemarle Sound

Fig. 1. Bathymetric map showing location of 20 sampling sites throughout the Albemarle Estuarine System and the paleo-inlets along the northern Outer Banks.

Dates indicate when inlets opened and closed. Note that multiple inlets were open between w1600 and w1800.

719D.R. Corbett et al. / Estuarine, Coastal and Shelf Science 71 (2007) 717e729

can be described as silty clay with abundant organic rich muds(Folger, 1972; Riggs et al., 1993; Riggs, 1996).

Although the AES is currently dominated by river flow, thishas probably not been the case over the last several centuries.Since the late 1500s, the AES has seen a significant change inthe number and location of inlets (Fig. 1). This decrease innumber of inlets and their relative proximity to AlbemarleSound offers a good indication of a more tidally influencedsystem in the recent past (1500e1800), when the study areawas characterized as a productive, clear, saline estuary withabundant marine fauna (O’Connor et al., 1972). The geologicchanges associated with post-glacial rise of sea level that haveoccurred in the outer Albemarle and Currituck Sounds overmillennial timescales have been well documented by severalresearchers (Riggs et al., 1995; Sager and Riggs, 1998; Mallin-son et al., 2005). Their investigations have found that thenortheastern estuaries of NC contain a complex history of riv-erine incisement and backfilling sequences that include fresh,brackish, and marine sediment units. The result of these pro-cesses is a thick sequence of shallow water sediments that re-cord a complex evolutionary history with episodes of bothcoastal deposition and erosion. Our research is the first to doc-ument the details of environmental change in the AES over thepast few centuries using geochemical, sedimentological, andmicropaleontological evidence.

3. Sampling and analytical methods

Between June and July 2001, 20 short push-cores were col-lected throughout the AES (Fig. 1). Cores from the AlbemarleSound and adjacent tributaries were primarily collected alongtheir central axis. Cores were extruded and sectioned into 2-cm intervals for 0e4 cm and 3-cm intervals from 4 cm to thebase of the cores. Upon returning from the field, approximatelyhalf of each interval was oven-dried at 60 �C, ground with a mor-tar and pestle, and then stored for geochemical analysis. The re-maining wet sediment was retained for grain size analysis.

Sediments for carbon and nitrogen bulk and isotopic com-positions were determined using continuous flow-isotope ratiomass spectrometry (Europa Scientific Hydra IRMS; UC DavisStable Isotope Facility) following a simple HCl fumigationprocedure to remove inorganic carbon (Harris et al., 2001).Isotopic results are reported in the d-notation, as per mil(&) relative to international standards (V-PDB [Vienna-PeeDee Belemnite] for carbon and atmospheric N2 for nitrogen)with a precision estimated at �0.2&.

Water content was determined from weight loss by dryingat 60 �C. Sediment porosity and bulk density were calculatedfrom water content and an assumed grain density of2.4 g cm�3 (Benninger and Wells, 1993). Percent organic mat-ter (%OM) was determined by loss on ignition. Grain size dis-tribution of subsamples was determined from disaggregatedsediments after removal of organics by repeated treatmentsof 30% H2O2. Subsamples were analyzed on a Beckman Coul-ter LS230 Particle-size Analyzer.

210Pb, 226Ra, and 137Cs activities were determined bygamma spectroscopy. Samples were initially dried at 60 �C,

homogenized by grinding, packed into standardized vessels,and sealed before counting for at least 24 h. Sample sizeranged between approximately 2 and 40 g, depending oncounting geometry (vial or tin, respectively). Gamma countingwas conducted on one of two low-background, high-efficiency,high-purity Germanium detectors (Coaxial- and Well-type)coupled with a multi-channel analyzer. Detectors were cali-brated using a natural matrix standard (IAEA-300) at each en-ergy of interest in the standard counting geometry for theassociated detector. Activities were corrected for self adsorp-tion using a direct transmission method (Cutshall et al.,1983; Cable et al., 2001).

137Cs activities were measured using the net counts at the661.7 keV photopeak. Excess 210Pb activities were determinedby subtracting total 210Pb (46.5 keV) from that supported by226Ra. 226Ra activities were determined by allowing samplesto equilibrate for greater than three weeks before recounting.226Ra was then determined indirectly by counting the gammaemissions of its granddaughters, 214Pb (295 and 351 keV) and214Bi (609 keV).

4. Results

4.1. Decadal-scale sediment accumulation

Radionuclide tracer data (210Pb, 137Cs, and 226Ra) wereemployed in this study to aid in the understanding of recentsediment dynamics in the AES, spatially and temporally(down-core). Radionuclide tracers were analyzed in 17 shortcores (Tables 1, 2 and Fig. 1). Surface activities of excess210Pb and 137Cs vary considerably throughout the study area.The highest surface activities were found near the heads of es-tuaries, whereas the lowest activities were prominent in theeastern sounds.

Inventories of excess 210Pb and 137Cs in all cores (Table 1)were calculated to help assess sediment focusing or removalwithin the estuary and localized sediment mixing. Sedimentand mass accumulation rates (Table 2) were calculated usingthe constant flux-constant sedimentation (CF-CS) model (Ap-pleby and Oldfield, 1992) and represent a maximum rate forthe length of detectable excess 210Pb in the core. Down-core137Cs activities were used to substantiate the 210Pb-determinedaccumulation rates. Wherever possible, the first appearance(1952) 137Cs horizon or peak (1963) of 137Cs activity wasused to estimate a sediment accumulation rate.

Expected inventories from atmospheric sources of excess210Pb and 137Cs for eastern North Carolina were 26.5 and18.0 dpm cm�2, respectively (Benninger and Wells, 1993). In-ventories for excess 210Pb were above or near expected at fourstations, S1, S3, S8, and S9 (Table 1). The 137Cs down-corepeak corresponding with the 1963 atmospheric maximum wasonly present in four cores; S1, S7, S9, and S20 (Table 2, Figs.2e4). Highest activities of 137Cs in the remaining cores weretypically found at the surface. In these cores, excess 210Pb and137Cs were typically below detectable activities at approxi-mately the same depth, indicating the importance of physicaland biologic sediment mixing (Christiansen and Emelyanov,


Table 1

Radionuclide data for cores collected (see Fig. 1 for location). Surface activity is presented to demonstrate the variability in the system. Inventories represent down-

core integrated activities. Cores not analyzed for radionuclides due to coarse-grained material are indicated by dashes

Site Area Core

length

cm

Excess 210Pb 137Cs

Surface activity Inventory Surface activity Inventory

dpm g�1 dpm cm�2 dpm g�1 dpm cm�2

S1 Albemarle 40 8.8 52.4 1.5 32.5

S2 Albemarle 42 11.2 17.5 1.1 7.4

S3 Albemarle 42 11.4 21.3 1.2 4.6

S4 Albemarle 28 8.1 13.6 1.1 5.5

S5 Albemarle 16 0.6 7.5 0.2 1.8

S6a Alligator 34 6.0 4.9 0.6 0.5

S7 Alligator 22 6.6 15.5 0.6 6.2

S8 Pasquatank 28 14.7 29.1 3.1 8.9

S9 Pasquatank 40 11.4 24.1 1.6 19.3

S10 Pasquatank 61 4.1 6.8 1.3 4.0

S11 North 34 7.3 19.1 0.8 4.7

S12 North 31 5.0 17.2 0.4 2.8

S13 Currituck 74 2.5 4.4 0.4 3.4

S14a Currituck 37 1.4 3.7 0.4 8.1

S15a Croatan 15 0.9 1.8 0.07 0.1

S16 Croatan 25 e e e eS17 Croatan 15 e e e e

S18 Kitty Hawk Bay 25 e e e e

S19 Roanoke 31 2.1 8.2 0.3 2.4

S20 Roanoke 51 1.6 18.0 0.06 6.6

a Measurable excess 210Pb and 137Cs only present in the surface sediments.

1995; Dellapenna et al., 2003) or the migration of 137Cs down-core with time (Johnson-Pyrtle and Scott, 2001). Regardlessof the process, calculated accumulation rates based on the137Cs horizon must be considered maximum values.

For simplicity, the sedimentological data have been sepa-rated into three major regions: (1) Albemarle Sound (Fig. 2);(2) Lateral Rivers (Fig. 3); and (3) Eastern Sounds (Fig. 4).Grain size varied widely among cores (the <63 mm fraction

Table 2

Sediment accumulation rates for 12 of the 20 sites occupied throughout the

Albemarle Sound and adjacent water bodies

Site Area 210Pb accumulation 137Cs accumulation

cm y�1

cm y�1 g cm�2 y�1

Albemarle Sound

S1 Albemarle 0.57 � 0.07 0.21 � 0.03 0.54a

S2 Albemarle 0.18 � 0.03 0.07 � 0.01 0.17

S3 Albemarle 0.13 � 0.02 0.04 � 0.01 0.11

S4 Albemarle 0.08 � 0.02 0.04 � 0.01 0.11

Lateral Rivers

S7 Alligator 0.21 � 0.07 0.07 � 0.03 0.32a

S8 Pasquatank 0.25 � 0.04 0.09 � 0.01 0.17

S9 Pasquatank 0.14 � 0.02 0.04 � 0.01 0.15a

S10 Pasquatank 0.16 � 0.02 0.05 � 0.01 0.11

S11 North 0.13 � 0.03 0.08 � 0.01 0.17

S12 North 0.11 � 0.01 0.07 � 0.02 0.17

Eastern Sounds

S13 Currituck 0.06b 0.04b 0.08

S20 Roanoke 0.33 � 0.04 0.12 � 0.03 0.26a

a Accumulation rate was calculated using the depth of the down-core 137Cs

peak activity.b Accumulation rate is based on two-point regression.

ranged from 0% to 98%), but generally increased to the eastand down lateral rivers (Wells and Kim, 1989). Most sitesshowed little variation in down-core grain size (S1e3, S6,S 8e10, S12, S14; Figs. 2e4). The significant down-corevariation in grain size at some sites can be attributed to prox-imity to shallow shoals (S7, S11, S15) or varying sources ofmaterial (marsh, dunes, etc.; S13, S20). Accumulation ratesshowed a general decreasing trend from west to east, fromfresh to estuarine waters. The major exception is S20(0.33 cm y�1), located in a protected embayment of RoanokeSound and surrounded by eroding salt marsh.

Cores taken in the central basin of Albemarle Soundshowed a gradational west to east radionuclide trend. The lin-ear accumulation rate at S1 (Fig. 2) ranged between 0.54 and0.57 cm y�1 with an excess 210Pb inventory two and a half tothree times higher than the other cores (52.4 dpm cm�2,Table 1). The remaining Albemarle cores, S2, S3, and S4(Fig. 2), showed excess 210Pb and 137Cs detectable to similardepths in each core (8.5 to 11.5 cm) with increasing activitylevels of each tracer to the surface. These mid- and outer-Al-bemarle Sound stations have only 10 to 13 cm of sedimentyounger than 120 years, suggesting that sediment accumula-tion at these stations is fundamentally different than at thehead of the estuary. Accumulation rates for stations S2eS4ranged from 0.08 to 0.18 cm y�1 with 137Cs increasing line-arly up-core.

The Lateral Rivers show little variation in accumulationrates with the exception of the two stations at the head ofthe Alligator and Pasquatank (S6 and S8; Fig. 3, Table 2).At the head of the Alligator (S6), only the top 2 cm containedactive excess 210Pb, with grain size remaining fairly uniform


S1

0.1 10

Dep

th

(cm

)

0

10

20

30

40

0 20 40 60 80 100 0 20 40 60 80 100 0 20 40 60 80 100 0 20 40 60 80 100

S2

Excess 210

Pb and 137

Cs Activity (dpm g-1

)

Grain Size (% <63µm)

S3 S4

1 0.1 101 0.1 101 0.1 101

Fig. 2. Down-core profiles of 210Pb (C), 137Cs (,), and percent grain size less than 63 mm (6) from cores collected in the Albemarle Sound.

down-core. The 2 cm of laminated recent sediment probablyrepresent an ephemeral layer underlain by an erosional sur-face; therefore, unlike S1, very little sediment is transportedin from the low-lying drainage basin of the Alligator River.The site at the head of the Pasquatank had one of the highestaverage accumulation rates in the AES at 0.25 cm y�1 (Table2). The accumulation rates of the remaining stations in theLateral Rivers ranged from 0.11 to 0.21 cm y�1. These stationsare very similar (rates and processes) to those found in the cen-tral basin of mid- and outer-Albemarle Sound.

The stations in the Eastern Sounds (Fig. 4, Table 2) weregenerally very sandy with the exception of S20. Becausesand offers no adsorption for the radionuclides, activitieswere very low, causing erratic profiles for excess 210Pb, and137Cs activities were at or below background levels. Thus, in-ventories of the radionuclides were also very low. The down-core radionuclide trend at S20 was fairly high (0.33 cm y�1).The accumulation rate calculated from the 137Cs peak (a mid-point of 10 cm in the broad peak was used) was 0.26 cm y�1

and agreed well with the 210Pb, thus indicating moderatelyuniform accumulation.

4.2. Spatial and temporal variations of 13C and 15N

In order to simplify the temporal variation in stable isotopicsignatures of organic matter, two time slices have been pre-sented for all sample locations: (1) modern/surficial sediments(Figs. 5 and 6, top symbol); and (2) the deepest sampleanalyzed within each core, which is approximately 200e300 years ago (AD 1700e1800) assuming steady-state accu-mulation throughout the core at the measured 210Pb rates(Table 2, Figs. 5 and 6, bottom symbol). In addition, spatialand down core trends are combined in two cross sections ofsedimentological characteristics and organic matter signatures(Figs. 7 and 8). Surficial d13C data showed a regional trend ofincreasing carbon isotopic values from west to east (Fig. 5, topsymbol). More negative values were found in the AlbemarleSound and adjacent Pasquotank, North, and Alligator Rivers.Surface values for d13C in the Albemarle Sound ranged from�28.3& at S1 to �26.7& at S4. Less negative values were

found in Kitty Hawk Bay and Currituck, Croatan, and Roa-noke Sounds.

Temporal variations in d13C were also seen throughout thestudy area (Fig. 5, top versus bottom symbol, and Figs. 7 and8). Most sites showed decreasing d13C (more negative) fromthe past to modern times suggesting a change in the sourceof organic matter within the estuarine environment duringthe last 200e300 years. The d13C gradient with depth wasgreater in the eastern tributaries than the west (there is a greaterdifference in values between the base and surface of the core).

Regional d15N trends revealed high values to the west thatdecrease to the east (Fig. 6). Surface values were highest in theAlbemarle Sound and Pasquatank River. Surface values ford15N in Alligator River were much lower than for other bodiesof water in the western portion of the study area. Water bodiesto the east showed much lower surface values and many sitesshowed negative values.

Regional vertical trends of d15N varied throughout thestudy area, especially in the west. In the western region ofthe study area (Albemarle Sound and Lateral Rivers), valuesof d15N were low at the base of cores and increased up-core(Figs. 6e8). Although most cores in the Eastern Soundsshowed a general trend toward increasing d15N from thebase to the surface, some reveal a decrease in nitrogen isotopevalues (Fig. 6). Those cores that showed a decrease in d15Nwere in open areas of the Albemarle Sound with fairly coarsematerial and low organic matter (<2%).

5. Discussion

5.1. Fate of sediments in the AES

Grain-size and percent organic matter data (Figs. 7 and 8) fromthe surface and down-core sediments in the AES showed thatorganic-rich muds are the dominant sediment type in the centralbasin portion of the AES and the embayed tributary channels. Sed-iments transition to fine and medium sands on the perimeter plat-form and the back-barrier shoals at the eastern end of AlbemarleSound. Sediments in the Eastern Sounds consist of mostly siltyto clayey fine sands along their central axes and coarsen toward


S8 S9 S10

S11 S12

S6

0.1 10

Dep

th

(cm

)D

ep

th

(cm

)D

ep

th

(cm

)

0

10

20

30

40

0

10

20

30

40

0

10

20

30

40

0 20 40 60 80 100 0 20 40 60 80 100

0 20 40 60 80 100 0 20 40 60 80 100 20 40 60 80 100

0 20 40 60 80 100 0 20 40 60 80 100

0

S7

Excess 210

Pb and 137


)

1 0.1 101

0.1 101 0.1 101

0.1 101

0.1 1010 10.1 1

Fig. 3. Down-core profiles of 210Pb (C), 137Cs (,), and percent grain size less than 63 mm (6) from cores collected in the Lateral Rivers.

their perimeters, back-barrier shoals and in regions of strong cur-rents (Wells and Kim, 1989; Letrick, 2003).

Effects of environmental variables on radionuclides (210Pb,137Cs, and 226Ra) vary, but are consistent with other findings(He and Walling, 1996; Ligero et al., 2001). Generally, sandierenvironments have very low to no activities of 210Pb and 137Cs,which is consistent with the findings of He and Walling(1996). The low surface activities in the Eastern Sounds aredue to the increased percentage of sand in this region (Wellsand Kim, 1989). Near linear up-core increase of 137Cs activityis evident in many cores (S2e4, S8, S10e13; Figs. 2e4) andmay be a function of physical processes. Studies by Christian-sen and Emelyanov (1995) and Christiansen et al. (2002) have

also shown that increasing up-core activity of 137Cs can be at-tributed to a mixed layer settled from resuspended sediments.The impact of salinity on adsorption of 137Cs is very slight dueto the low, broad salinity gradient in Albemarle Sound, butdecreased activities from S1 to S4 could also result fromresuspension-related desorption of 137Cs by NHþ4 in pore water(Johnson-Pyrtle and Scott, 2001).

Sediment accumulation rates in the AES (Table 2) averaged0.15 cm y�1 (excluding stations which were above predictedatmospheric inventory; S1 and S8; Table 1). Stations abovepredicted atmospheric inventory represent depositional centerswith greater accumulation rates. For example, the high accu-mulation rate (0.54e0.57 cm y�1) and excess 210Pb inventory


(52.4 dpm cm�2) at S1 is likely associated with the junction ofthe two major river systems (Roanoke and Chowan). Usinga series of vibracores collected along northesouth transectsin Albemarle Sound and seismic data collected in the samearea, Sager and Riggs (1998) concluded that the organic-richmud averages approximately 0.5 m in thickness in the centralbasin of Albemarle Sound and represents <400 years of post-colonial accumulation. Immediately underneath the organic-rich mud and separated by a 1100-year hiatus are laminatedmuds and fine sands which were deposited in open-estuarineconditions (Sager and Riggs, 1998). At the eastern end ofthe AES, the organic rich muds overlie open shelf baymouthsands (Mallinson et al., 2005). Using the average thicknessof the organic-rich mud and the approximate age constraintof the organic-rich mud thickness, a post-colonial averageaccumulation rate for sediments in Albemarle Sound is0.13 cm y�1, about 60% less than rates recorded in Chesa-peake Bay tributaries (Brush, 1984). This average accumula-tion rate is in agreement with the average decadal rate forthis study, suggesting that the accumulation rate measuredfor the top 10e15 cm of sediment in the AES has been steadyfor the interval during which the entire 0.5-m thickness of theorganic-rich mud accumulated. Riggs (1996) further con-cluded that accumulation of organic-rich mud is chiefly con-trolled by wave-base, which is a function of fetch and water

S13 S14

S15 S20

0.1 10

Dep

th

(cm

)D

ep

th

(cm

)

10

0

20

30

40

10

0

20

30

40

0 20 40 60 80 100 0 20 40 60 80 100

0 20 40 60 80 100 0 20 40 60 80 100

Excess 210

Pb and 137


)

1 0.1 101

0.1 101 0.1 101

Fig. 4. Down-core profiles of 210Pb (C), 137Cs (,), and percent grain size

less than 63 mm (6) from cores collected in the Eastern Sounds.

depth. Thus, sediments within range of wave-base have the po-tential of being removed via resuspension during high-energystorm events (Riggs and Ames, 2003). Due to the constraintsof wave-base, accommodation space in the AES is limitedby relative sea-level rise and basin subsidence. Regional ratesof relative sea-level rise from tidal gages in Charleston, SouthCarolina (1921e2000), Hampton, Virginia (1927e2000), andlimited data from Duck, North Carolina (1980e2000) are0.31 cm y�1, 0.32 cm y�1, and 0.46 cm y�1, respectively(Riggs and Ames, 2003). Micropaleontological data from theOuter Banks indicate a similar rate of relative sea-level riseof 0.5 cm y�1 after AD 1800 to present (Kemp et al., 2005).The relationship between the average accumulation rate forthe AES, regional rates of relative sea-level rise, and the be-low-predicted atmospheric inventory of 210Pb and 137Cs indi-cate that accumulation of sediments is responding to relativesea-level rise in the long term, but short-term rates of accumu-lation are influenced more by wave-base. Therefore, the aver-age rate of sediment accumulation in the AES appears to beinfluenced by both short-term storm events and long-termrelative sea-level rise.

Oldfield et al. (1989) have shown that inventories of excess210Pb in cores can be used to assess net sediment flux in estu-aries when the appropriate conditions are met (i.e., coreswhere grain-size or salinity are not causing desorption or anoverall decrease in activities). Excess 210Pb in cores fromthe AES were largely below predicted flux and indicate thatthere is a net loss of sediments from the AES probably to Pam-lico Sound to the south. The stations above predicted atmo-spheric excess 210Pb inventory are in the protected reachesof the AES (S1, S8, S20) where accumulation rates were high-est, between 0.25 and 0.57 cm y�1 (Tables 1 and 2). Wells andKim (1989) synthesized three decades of research in the Albe-marleePamlico estuarine system and hypothesized that littlesediment escaped the AES and any transport of sedimentwas restricted to local redistribution by currents and stormwaves. Data from this study and work by Corbett et al.(2004) in the Pamlico basin to the south provide more infor-mation to broaden the hypothesis of Wells and Kim (1989).Tully (2004) provided evidence of 210Pb inventories in excessof that predicted for atmospheric deposition in Pamlico Sound,suggesting that a secondary source is contributing to the accu-mulation of sediments in the basin. Thus, as indicated by in-ventories in the AES and in the central basin of PamlicoSound (Tully, 2004), it is proposed that sediments are being re-moved from the AES and transported through Croatan Soundinto Pamlico Sound where they are deposited in the deep cen-tral basin. Wells and Kim (1989) and Benninger and Wells(1993) concluded that relatively little sediment is lost throughthe inlets to the open-ocean. They supported a unidirectionaltransport mechanism for sediments to the Pamlico Sound byperiodic storm events (e.g., nor’easters and hurricanes) andhigh freshwater discharge from the rivers.

Sediment accumulation rates in the AES were similar tomany of the regional radionuclide studies from the NeuseRiver in North Carolina (Benninger, 1990; Giffin and Corbett,2003), and Chesapeake Bay and its tributaries (Dellapenna


Fig. 5. Spatial and temporal variations in organic 13C. The variations in signature were divided into marine, estuarine and terrestrial. The top symbol at each core

location indicates the surficial sediment, while the bottom symbol is indicative of the deepest sample analyzed (approximately 200e300 ybp).

et al., 1998, 2003). Studies in the Neuse River estuary (a trib-utary of the Pamlico Sound estuarine system) agree well withthe results of this study, showing that sediments are trans-ported down-river unidirectionally towards Pamlico Soundand deposited at rates between 0.14 and 0.55 cm y�1 (Ben-ninger, 1990; Giffin and Corbett, 2003). Dellapenna et al.(1998, 2003) showed that sediments are accumulating at rates<0.2 cm y�1 in the physically dominated tributary estuaries ofthe Chesapeake Bay such as the York River; these rates aresimilar to results from this study. Our results, along with thosefrom regional estuaries, suggest that sediments are accumulat-ing at average rates generally slower than relative sea-levelrise. Accumulation rate keeps pace with relative sea-levelrise and exceeds it where accommodation space allows orwhere storm events are less frequent and so resuspensionand mobilization is less prevalent.

5.2. Historical record: paleoenvironmental change

Geochemical and sedimentological data collected from sur-face sediments in Albemarle Sound exhibit a transition down-stream from a terrestrial-dominated environment to anincreasingly marine-influenced environment. In the modernenvironment, grain size increases while %OM decreasesfrom west to east (Fig. 7) and toward the Albemarle Soundwithin the Lateral Rivers (Fig. 8). Grain size increases and

%OM decreases substantially at S4, as a sandy platform isreached (Riggs, 1996) at the wide eastern end of AlbemarleSound. Coarse, organic-poor sediments found from S4 toS18 are likely transported from the barrier islands by wind,waves, and storm (overwash) events.

Combining all of the down-core data from the AlbemarleSound provides a better representation of the historic changesthat have occurred in this region and allows for comparisonwith the current environmental patterns. Down-core variationsin grain-size and %OM are limited to the middle reaches ofthe Albemarle Sound (S2e4). Generally, the amount of OMhas increased up-core whereas grain size has decreased up-core and westward over time. A large NeS fetch is created atS4 as a result of the combined width of the Albemarle Soundand length of the Pasquotank and Alligator Rivers. This longfetch may result in higher waves and sediment resuspension dur-ing periods of intense northerly or southerly winds. Organic-richfine-grained sediment is primarily transported from west to east.Due to the nature of the source, the Chowan and Roanoke Rivers,and the increased potential of flocculation (either biologically orelectrochemically) as salinity increases above 2 (Nichols andBiggs, 1985), typically west of S3 (Williams et al., 1973; Gieseet al., 1985), more fine-grained sediment is deposited in thewestern region of Albemarle Sound.

d13C and d15N in the Albemarle trunk estuaries exhibita gradational change up-core, with little change in the C:N


Fig. 6. Spatial and temporal variations in organic 15N. The top symbol at each core location indicates the surficial sediment, while the bottom symbol is indicative

of the deepest sample analyzed (approximately 200e300 ybp).

ratio, reflecting a change from a marine environment in thepast to a more terrestrial-dominated environment at present,with potential anthropogenic influences (Figs. 7 and 8). d13Cvalues become isotopically ‘‘heavier’’ as marine influences in-crease (Sackett, 1964; Shultz and Calder, 1976; Sherr, 1982;Matson et al., 1983; Matson and Brinson, 1990; Thorntonand McManus, 1994; Middelburg and Nieuwenhuize, 1998).A change in the d13C isotopic gradient occurs at a depthgreater than 120 years (dashed line) suggesting that therewas a change in the estuarine environment before 120 yearsB.P. An age estimate of approximately 200e300 years is cal-culated for the deepest sample of each core assuming steadystate accumulation at the measured rate (Table 2). The isotopicsignatures suggest a transition from a more marine-influencedestuarine environment to a primarily terrestrially dominatedestuarine environment, since d13C signatures typically do notchange over time despite burial and decomposition (Sherr,1982; Nordt et al., 1994; Freudenthal et al., 2001).

The d13C composition of the organic matter in the LateralRivers (Fig. 8) seems to follow the same trend as the maintrunk of the Albemarle Sound. In the modern environment,d13C values are generally uniform across much of the regionsuggesting that the rivers are primarily terrestrially influenced(< �26&). However, d13C values become increasinglyvariable with depth. Sediments approximately 200e300 years old transition from isotopically ‘‘heavier’’ valuesto isotopically ‘‘lighter’’ values toward the Albemarle Sound,

an up-core pattern similar to that of the main Albemarle trunk.Again, this change may be a function of greater marine influ-ence in the past or it may be due to variations in the abundanceof C3 versus C4 plants that once existed in the region; such var-iations would slightly alter the d13C composition of organicmatter delivered to the system.

d15N values decrease downstream in the Pasquotank Riverbut upstream in the Alligator River (Fig. 8). The PasquotankRiver is a well-populated area whereas the Alligator River issparsely populated and surrounded by swamp forest and marshenvironments. According to numerous studies, anthropogenicinput of nitrate to a water body results in discernable changesin nitrogen isotope values of organic matter (Heaton, 1986;Chapelle, 1993; Thornton and McManus, 1994). d15N valuessuggest that anthropogenic influences (fertilizer, waste, etc.)in the Pasquotank River region may be affecting the modernestuarine system. In the generally undeveloped AlligatorRiver, nitrogen isotope values are near 0&, close to the valuefor marsh peats (Cheng et al., 1964); this trend continues withdepth but with lower d15N values reaching farther north.

The Currituck, Croatan, and Roanoke Sounds along theeastern portion of the study area are environments with highvariation in grain size, %OM, stable carbon and nitrogen iso-topes, and C:N ratios. These sounds are located parallel to thebarrier islands and have irregular coastlines, including smallislands, combining salt marsh and sediment bank shorelinetypes. Episodic deposition and erosion of sediment occurs in


Fig. 7. Cross section of sedimentological and organic geochemical data from sites occupied down the truck of the Albemarle Sound from S1 to S18 (see Fig. 1 for

site locations). Dashed line represents the depth at which excess 210Pb was no longer detectable (approximately 200e300 ybp).

these sounds due to variable winds and tides. Many areas arecharacterized by small, secluded coves, fringed by saltmarshes, which tend to collect fine-grained, organic-rich sed-iment. Therefore, it is the variability of dynamic processes

interacting with sediment sources and depositional environ-ments (marshes, estuaries, Pleistocene platforms, etc.) thatprovides the very high spatial variability in this region (incomparison to the main trunk of Albemarle Sound) and


Fig. 8. Cross section of sedimentological and organic geochemical data from sites occupied from the Pasquotank to the Alligator Rivers (see Fig. 1 for site

locations). Dashed line represents the depth at which excess 210Pb was no longer detectable (approximately 200e300 ybp).

accounts for the irregular depositional and erosional patternsof sediment.

The sedimentological and sedimentary geochemical signa-tures provide evidence of significant environmental change inthe AES over the last few centuries. Down-core variations indead benthic foraminiferal assemblages confirm these findings(Vance et al., 2006). In a core from site S1, a down-core shift

in assemblage occurs at 6 cm depth from a brackish estuarinebasin assemblage, dominated by the agglutinated foraminiferAmmotium salsum, to a low brackish, inner estuarine basinassemblage co-dominated by the agglutinated taxa Ammoba-culites subcatenulatus and Miliammina fusca with A. salsumas a subsidiary species. At sites S3 and S4, a subtle but re-versed trend of increasing salinity occurs down-core. Brackish


estuarine basin assemblages, dominated by A. salsum, charac-terize the entire cores. However, the presence of organic lin-ings of the calcareous foraminifer Ammonia sp. at depth inS3 and S4 and a single dead specimen of the calcareous taxonElphidium excavatum at the bottom (40e42 cm) of S3 suggesta higher salinity environment in the past at these sites. Ammo-nia sp. is found living today in back-barrier estuarine environ-ments near Oregon Inlet (Fig. 1) where salinities are greaterthan 8 (Vance et al., 2006). Elphidium excavatum is found to-day along the Outer Banks barrier island foreshore, shorefaceand within the ebb- and flood-tidal delta complex of OregonInlet (Vance et al., 2006). These calcareous foraminiferaltaxa were not recorded anywhere in the modern surficial sed-iments of the estuaries of the AES by Vance et al. (2006).

In the early nineteenth century the last two inlets adjacentto the AES closed; Roanoke Inlet in 1811 and New CurrituckInlet in 1828 (Fisher, 1962; Fig. 1). This general time periodcorrelates with the major sedimentological and geochemicalchanges described in this paper as well as with the changein foraminiferal assemblages reported by Vance et al. (2006).Stable isotope data indicate that isotopically ‘‘heavier’’ d13Cvalues occurred farther west than today; similar values arenow restricted to the eastern sounds. Together, our data andthose from Vance et al. (2006) show that the closing of theinlets resulted in a significant shift from a marine-influencedto a terrestrial-dominated estuarine environment.

A modern analogue for the pre-1800 AES is Pamlico Sound(Abbene et al., 2006). The present day southern portion of Pam-lico Sound has salinities averaging 22 with fine-grained to sandysediments, d13C values averaging near�22&, and is dominatedby a foraminiferal assemblage characterized by the calcareousforaminiferal taxa Elphidium excavatum and Ammonia parkin-soniana (Abbene et al., 2006). This portion of Pamlico Soundhas two large inlets, Hatteras and Ocracoke, that allow salinewaters to penetrate well into the Sound. With the presence ofseveral inlets adjacent to the AES pre-1828 (Fisher, 1962), itis likely that similar high brackish conditions persisted behindthe northern Outer Banks at that time. The data suggest an earlynineteenth century transition from a high brackish estuarine sys-tem to a restricted estuarine system dominated by oligohalineconditions; these latter conditions persist to the present day.

6. Summary

It is apparent from several lines of evidence that sedimentsources and depositional processes within the AlbemarleSound estuarine system have changed in the last 200e300 years. Data reveal that sedimentologic parameters (grainsize and %OM) and geochemical parameters (stable carbonand nitrogen isotopes and C:N values) are interrelated withinthe estuarine system. These parameters vary spatially andvertically in response to the degree of terrestrial and marineinfluences and the variability in dynamic processes and depo-sitional environments.

Radionuclide analysis of sediment dynamics in the AES in-dicate an average sediment accumulation rate of 0.13 cm y�1

(for stations which were not above predicted atmospheric excess

210Pb inventory). Maximum rates of accumulation occur in theprotected reaches of the estuary where inventories are above pre-dicted atmospheric deposition and accumulation rates rangefrom 0.25 to 0.57 cm y�1. Excess 210Pb inventory of sedimentcores throughout the AES indicates that, with the exception ofcores in protected reaches of the estuary, there is a net deficitin predicted inventories. This suggests that sediments are resus-pended by wind-generated waves and currents and flushed fromthe system by river discharge and wind-tides, probably to Pam-lico Sound to the south. Sediments in the AES are accumulatingat rates less than the current rate of relative sea-level rise for thisregion, except in protected portions of the estuary. This supportsthe conclusion that sediment accumulation in the AES is con-trolled in the short-term by wave-base in relation to storm eventsand the creation of accommodation space by basin subsidenceand sea-level rise in the long term.

Overall, these geochemical and sedimentological data char-acterize the evolution of the Albemarle Sound and associatedtributaries over the past 200e300 years. This estuarine systemin eastern North Carolina has changed from a marine-influ-enced, high brackish environment to the modern-day system,which is a highly variable, terrestrially influenced, low brack-ish environment.

Acknowledgments

We thank S.R. Riggs, J. Watson, C. Smith and L. Gainsfor their assistance and support. The research is part of theNorth Carolina Coastal Geology Cooperative Program(NCCGC). Funding for the USGS cooperative agreementaward 02ERAG0044 is gratefully acknowledged. Commentsfrom two anonymous reviewers significantly improved themanuscript.

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