Response of the Roanoke Delta to changes in the rate of sea-level rise and ..... for help with building the GIS data base, Ethan J. Theuerkauf for his amazing ...
Response of the Roanoke Delta to changes in the rate of sea-level rise and human-induced changes in sediment supply Anna M. Jalowska, Brent A. McKee, Antonio B. Rodriguez Marine Sciences Department, University of North Carolina ABSTRACT
RESULTS EM
EPS
AWc
B. Radiocarbon Results
Bay-head deltas are sensitive to changes in the rate of sea-level rise and sediment supply because they have a low elevation and are con ned to river valleys, the morphology of which strongly controls sediment accommodation.
A. Stratigraphic Cross Section
Modern sediment accumulation in bay-head deltas is dependent on the rate of sea-level rise, climate change, and human modi cations to river sediment discharge and load via changes in land use and placement
The transect across Roanoke River bayhead delta includes ve locations in distributary channel of the Roanoke. We chose the Eastmost River
of impoundments. Those factors make the response of bay-head deltas to accelerated sea-level rise difficult to predict, but important to understand because sea-level rise and reduction in sedimentation can lead to submergence
because main channel of the Roanoke was dredged in the past. Each location was cored twice with pneumatic drill and with vibracoring
and erosion of carbon-rich deltaic sediments and loss of important habitat. Extensive core descriptions along with uranium-series and radiocarbon geochronologies show that the Roanoke bay-head delta in North Carolina
method. EPN and EPS cores were taken at the point bar. The EPS core at the south of the point bar had very distinguished, orange stained legacy
radiocarbon dates of the sampled organic material re ect time of the sedimentation.
experienced two episodes of retreat during the past 6000 years. The rst event occurred around ca. 4500 cal. yr. BP. Across the 4500 cal. yr. BP ooding surface, the delta plain environment was replaced by pro-delta/upper bay
sediments. The EPN core from the northern part of the point bar was located more inland and had only 50 cm layer of legacy sediments.
The results show two events of rapid accretion and one event of delta retreat (Fig. 12).
and rates of sediment accumulation decreased from 2-3 mm/yr. to 0.6-0.7 mm/yr. This ooding event may have corresponded with the separation of the Chowan and Roanoke rivers, which coalesced in upper Albemarle Sound,
EM core was taken at the mouth of the Eastmost River. Two consecutive cores were taken in the Estuary AWa and AWc. The cross-section
and would have resulted in an increase in sediment accommodation and a decrease in sediment accumulation. The delta started to rapidly accrete and the pro-delta/upper bay was buried with > 2 m of delta plain sediment
represents modern and ancient ooding surfaces associated with back-stepping events. Leyers of legacy sediments reduce in depth towards
at AD 1600. This occurred in response to the initiation of agriculture in the drainage basin and stable sea level. The second episode of retreat is occurring today. The delta plain is eroding due to an increase in the rate of
to the Albemarle Sound.
126 Legacy sediment stained prodelta
3700 +/- 35 543 Cal. BP
815
Prodelta
Fourteen organic samples were recovered from the cores, and sent to NOSAMS for analyses. The data interpretation is based on the assumption that the
Radiocarbon results from the bottom of legacy sediments in EPS and EM cores correspond with the dates rst settlements in the Roanoke River watershed (Fig. 13). AWc
Radiocarbon results
sea-level rise that began at AD 1865 and a decrease in sediment supply from the construction of a dam at the fall line completed in AD 1955. Bay-head delta evolution is strongly in uenced by human-induced modi cations to the sediment budget, in addition to the rate of sea-level rise and the morphology of the river valley. High rates of accumulation of carbon-rich sediments in deltaic environments support carbon sequestration, while erosion
0
associated with delta retreat reintroduces older carbon into the system possibly making it bioavailable. Improving evolutionary models that describe bay-head delta progradation and retreat are needed to better understand
190 +/- 30 Cal. BP
the role of bay-head deltas in the global carbon cycle. EPS
EM
EPN
AWa
Fig. 1
1m
35 00 -4 500 BP
upstream reaches of the Lower Roanoke [Hupp et al., 2009]. After a major hurricane made landfall in August 1940, which resulted in excessive ooding,
280 +/ 30 (1506-1785 A D)
5m
a series of three dams were put in place. First dam, placed at the Fall Line in Roanoke Rapids, NC was nished in 1955. Completion of the Roanoke Rapids
Flooding Surface
dam was followed by a construction of two other dams above, by 1965.
threaded, meandering stream (Fig.2), which ows across Miocene sedimentary
Fluvial
material overlain by Quaternary alluvium [Brown et al., 1972]. The three dams
4110 3570
AWc EM
5120 5420
3700 4920
185 4530
1000 1100
Delta retreat, slow sed. rates
Fast accretion
(2-4 mm/yr)
(20-30 mm/yr)
6030 60
EPS
4920 +/- 35 632 Cal. BP
Upper bay 3570 +/- 30
5420 +/-40
EM
AWc
Fig.12 Radiocarbon dates for three cores EPS
EM
3700 +/- 40
190 +/- 30 (1650-1955AD)
99% of the sediment entering the Lower Roanoke. Subsequently, sediment
4000- 6000 BP
Old delta plain/ oodplain
4920 +/- 40
Fig.3 Change in sediment discharge in the Roanoke River from
and source of the materials delivered to the Albemarle Sound has changed (Fig. 3). The river oodplains just upriver from Albemarle Sound contains several natural
pre-dam (1910) and post-dam (1980) Meade et al. 1990)
4530 +/- 40
10 m
Fluvial
AWc
5420 +/- 35 676 Cal. BP
Old Delta Plain/ Old Floodplain
wetland communities of state and global signi cance, including the "globally
Fig.13 Calibrated dates for samples at the bottom of legacy sediments layers.
6030 +/- 40
endangered" Atlantic white cedar forest and approximately 20,000 acres of pristine cypress-gum AWa
swamp [Hupp et al., 2009]. The cypresses can be found stranded in the Sound, marking former
4 km
3 km
2 km
C. Pb-210 geochronology
1 km
position of the shoreline (Fig.4). Side-scan sonar imagery data (Fig. 6), acquired in 2008 along the
The EPN core was collected using vibracoring method. The core was split in halves and
shoreline at the mouth of the Roanoke River exposed many submerged tree stumps (Fig.7). The tree stumps and the stranded trees follow the isobaths of the submerged shoreline.
extruded in 2 cm intervals. Sediment sections were lyophilized and homogenized.
Fig. 11 Lithofacies analyses and depositional environments.
Albemarle Sound Albemarle Sound Albemarle Sound
Excess Pb-210 concentrations were measured via alpha spectrometry EM
6030 +/- 40 837 Cal. BP
CONCLUSIONS
AWc Albemarle Sound AWa
Time 1
Time 2
Time 3
[Nittrouer et al., 1978; McKee et al.1983; DeMaster et al. 1985}. To determine sedimentation rates the Constant Initial Concentration Model was applied
EPN
Fig.4 Stranded trees in the Albemarle Sound
EM
Time 4
[Appleby and Old eld, 1992; Appleby 2008]. Samples were also analyzed for Excess Pb-210 (dpm/g) 1971
Time 5
organic matter content (LOI) and particle size-class distribution (CILAS). 0.2 cm/yr 6.7 cm/yr
AWc EPS
7000
1200
5120 +/-40
?
Roanoke is 220 km long and drains an area of 3620 km2. The River is largely a single
900
Calibrated years BP
Prodelta
6000
2210
2210 +/-40 4110 +/-40
Now, the River below the dam is considered the Lower Roanoke River. The
Fig.2
580
Sand
High loads of sediments accumulated along the river’s oodplains and banks. This legacy sediment layer may be between 4 and 6 m in depth along
5000
280
600
800
Mouth Bar
Prodelta
and poor agriculture practices led to release of signi cant amounts of sediments to the river [Wolman, 1967; Jacobson and Coleman, 1986].
(>4cm/yr)
700
Fluvial (old cha nnel) Old De lta Plain/ Old Floodplain
Delta Plain
200
500
Prodelta
Prodelta U ppe r bay
The Roanoke River Valley and Albemarle Sound were impacted by the rst settlement in North America in the late 1600’s. Heavy deforestation
North Carolina Raleigh
Legacy staine d prode lta
10 00 -3 000 BP
Very rapid accretion
400
Delta plain
Flooding Surface
Roanoke River
100
300
3570 +/- 30 441 Cal. BP
Eroding delta plain
The Roanoke River connects the Blue Ridge Mountains of Northwestern Virginia with Albemarle- Pamlico Sound in Eastern North Carolina (Fig.1). Fig.2
Legacy sediment stained delta plain
AWc
Legacy staine d delta plain
Delta Plain
Virginia
years BP 3000 4000
2000
835
River channel/ Es tuary
BACKGROUND
1000
0
EPN
1957 AWa
EM
Pb-210 measurements were corrected for sand content with an assumption that sand fraction has Pb-210 activity at 0 dpm/g. Layers with sand content above 25% were excluded from the pro le (Fig.14). Sand layers are associated with ooding events and river overbanking. Before 1957 we see very high sedimentation rates, with possibility of 1m deposition decreased but remained very high. Layers of coarse material
EPN
Cores Location Transect Roanoke River 1954 Elevation (meters) < -3
are not present anymore. It is
EPS Mean discharge ( ^3/s) 300000
-0.3 - 0
-3 - -2
0- 1
-2 - -0.9 -0.9 - -0.5
1-2 2-3
-0.5 - -0.3
>3
AWa 200000
Dam
150000
Fig.5 Elevation and bathymetry map of the study area. The map is a
Fig.7. Sumberged tree stumps on the Side-Scan Sonar imagery data.
composite of three Digital Elevation Models (3, 6 and 30 m cell sizes).
Roanoke River
Sea-level Rise Horton et al., 2009 presented sea-level rise curves for North Carolina Region. RSL was
decrease to 0.2 cm/yr. 0
Chowan River
raising more rapidly (ca. 5 mm/year) at 10.5 and 11 kyr BP, at 3.6-4.2 kyr BP (ca. 0.82
Albemarle Sound Albemarle Sound is part of Albemarle and Pamlico Estuary System and is separated from the Atlantic Ocean by Outer Banks. The exchange with the world ocean is currently restricted to a narrow passage of Croatan and Roanoke Sounds. Tides are negligible in the western part of the Sound. The Albemarle Sound is a drowned river valley estuary that was ooded and valley reamined an open-marine embayment, with estuarine conditions to the west, until approximately 3kyr BP when Outer Banks started to form (Fig. 9).
M ea n discharg e ( ^3/s)
Fig.14 Lead-210 geochronology results from EPN core 1954
mm/year), and 2.5-2.8 (ca. 1.14 mm/year). Kemp et al., 2011 shows two accelerations
Fig.15 Mean Discharge from Roanoke Rapids Dam
ACKNOWLEDGEMENTS
in SLR. During Medieval warming- AD 950-AD 1400 (ca. 0.6 mm/year) and AD 1880- AD 1920 (ca. 2.1 mm/year)
2010
Estimated Rate of SLR in NC (mm/year)
back lled during the Holocene sea-level rise (Fig.8). Previous studies (Culver et al.,2008) showed that ooded paleo- Roanoke
post-dam changes in type of Since 1970’ sedimentation rates
50000
of the Roanoke River
change in sediments loads, and materials delivered to the delta.
100000
Fig.6 Side-Scan Sonar data swath collected at the mouth
associated with post-dam ow regulations (Fig. 15), post-dam
250000
I would like to thank NOAA- funding agency, my advisors Brent A. McKee and Antonio B. Rodriguez for their endless support, Joseph Roberts
Aligator River
14
for help with building the GIS data base, Ethan J. Theuerkauf for his amazing help with the eld work, Patricia Rodriguez for running the grain OBX
12
size and LOI analyses, and other people who contributed to the project: Emily A. Elliott, Robin Mattheus, John Biddle, Sam Perkins, Sherif Ghrobrial and Kristen Jarman.
10
Literature 1. M. Wolman, A cycle of sedimentation an d erosion in urban river channels., Geogra ska Annaler 49, 385–395 (1967).
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2. R. B. Jacobson, D. J. Coleman, Stratig raphy and Recent evolution of Maryland Piedmont ood plains, American Journal of Science 286, 617–637 (1986).
Depositional wind driven
6 Depositional open estuary
4 2 Backstepping 0 0
Fig.8 Paleo-Roanoke valley ca 20 Kyr (Mallinson et al., 2005)
Fig.9 Paleogeographic reconstruction for the Albemarle embayment in Holocene (Culver et al. 2008)
Fig.10
500
Backstepping 1000
1500
2000
Backstepping
2500 3000 Cal. years BP
3500
4000
4500
Horton et a l. (2009) Regio n 1
Kemp et a l. (2011)
Horton et a l. (2009) gure 5
= et a l. (2009) Reg ion 2
5000
Time 1. Paleo- Roanoke Valley during the Last Glacial Maximum ca. 18 kyr BP(in blue). In the study area the Roanoke had oodplains inundated only the time of overbanking. Image based on Mallinson et al., 2010. Time 2. Drowned Paleo-Roanoke Valley. Ca. 4 kyr BP. The early estuary was a tidal system due to the connectivity with the open ocean. Tidal ats and bars could become a foundation of future islands. The drowned paleo-Roanoke valley had increased accommodation and was accumulating large amounts of alluvial sediments. Image based on Culver et al., 2008 and Horton et al., 2009. Time 3. Albemarle Sound. Ca. 1kyr BP. The inlet in the Outer Banks has closed, changing the characteristics of the system from tidal to wind dominated estuary. Image based on Culver et al., 2008. Time 4. Due to increase in sediment supply associated with land usechange islands trapped sediments lling up the delta. Maximum extend about 60 years BP. Time 5. Improved agriculture practices and series of impoundments within the watershed resulted in decrease of sediment supply followed erosion and back-stepping a bayhead delta since late 1800s.
3. C. R. Hupp, E. R. Schenk, J. M. Richter, R. K. Peet, P. A. Town send, Bank erosion alon g the dam-regulated lower Roanoke River, Nor th Carolina, The Geolo gical Society of America 451, 97–108 (2009). 4. P. M. Brown, J. A. Miller, F. M. Swain, Structural and Stratig raphic Framework, and Spatial Distribution of Permeability of the Atlantic Coastal Plain, North Carolina to New York, U.S. Geological Survey Professional Paper 796, 79 (1972). 5. R. H. Meade, T. R. Yuz yk, T. J. Day, in The Geology of North America, Vol.O-1, Sur face Water Hydrology, G. M. Wolman, S. R. Rigg s, Ed s. (GSA, Boulder, CO, 1990), pp. 255–280. 6. S. J. Culver et al., Micropaleontolog ic record of late Pliocene and Quatern ary paleoenvironments in the northern Albemar le Embaymen t, Nor th Carolina, U.S.A., Palaeogeography, Palaeoclimatology, Palaeoecology 264, 54–77 (2008). 7. B. P. Horton et al., Holocene sea-level c hanges along the North Carolina Coastline and th eir implications for g lacial isostatic ad just ment mod els, Quaternary Science Reviews 28, 1725–1736 (2009). 8. A. C. Kemp et al., Climate related sea-level variations over t he past two millennia., Proceedings of the National Academy of Scien ces of th e United States of Amer ica 108, 11017–22 (2011). 9. C. A. Nittrouer, R. W. Sternberg, R. Carpenter, J. T. Bennett, The Use of Pb-210 Geochronology as a Sedimentological Tool: Application to t he Washington Continental Shelf, Marine Geology 31, 297–316 (1979). 10. B. A. McKee, C. A. N ittrouer, D. J. D eMaster, Concepts of sediment deposition and accumulation applied to th e continental shelf near the mouth of the Yangtze R iver, Geo logy 2, 631–633 (1983). 11. D. J. Demaster, B. A. Mckee, C. A. Nittrouer, Q. Jiangchu, C. Guodon g, Rates of sediment accumulatio n and particle reworking based on rad iochemical measurements from continental shelf deposits in the East China Sea, Contin ental Shelf Research 4, 143–158 (1985). 12. P. G. Appleby, F. Old eld, in Uranium Series Disequilibrium: Applications to Earth , Marine an d Environmental Sciences, M. Ivanovich, R. S. Har mon, Eds. (Oxford Sciences Publication s, New York, 1992), pp. 731–778.
