investigating the development of a bay …altered by human activity, including construction,...

INVESTIGATING THE DEVELOPMENT OF A BAY PROMONTORY SALT MARSH IN GREENBACKVILLE, VA THROUGH VIBRACORING

Greenbackville Point is a promontory salt marsh on the eastern shore of Virginia just south of the Maryland border. It is located on the western side of Chincoteague Bay with Assateague and Chincoteague Islands to the east. Greenbackville Point afforded easy access to open bay water and the many resources in the Mid-Atlantic region, and was the home of a thriving town, Franklin City, in the late 1800’s to mid-1900’s. Hotels, oyster shucking houses, train tracks, and docks were built in and around the salt marsh altering the natural ecosystem and Chincoteague Bay was dredged for oysters almost to the point of population collapse. Despite the economic convenience, the morphology of the point also put the city in an exposed position, directly impacted by significant high-energy storm events. The town was mostly destroyed by flooding in 1962 when a Nor’easter hit. There are only a few houses left today lining the perimeter of the marsh.

PurposeAs sea level rose during the Holocene, coastal processes formed a series of barrier islands in the Mid-Atlantic Region, and bays and estuaries developed behind them. Salt marsh ecosystems formed in the brackish mixing zones adjacent to the mainland and the bayside of these barrier islands. Despite their importance, little is known about the geologic origin and history of salt marshes along Chincoteague Bay’s mainland coast. Along its shoreline, the bay has numerous salt marshes that form a series of prominent points or peninsulas. Salt marshes commonly form in embayments, making salt marsh formation on points that extend outward into the bay more enigmatic. We are investigating the formation of bay promontory salt marshes by interpreting environmental change in a series of sediment cores collected by vibracore in Greenbackville, Virginia.

Research GoalsDetermine the geologic history of the Greenbackville/Franklin City salt marsh.

Provide a model for formation of bay promontory salt marshes in Chincoteague Bay.

Inform ongoing salt marsh restoration projects.

References and Acknowledgments

Marsh Diagram: Integration and Application Network (IAN), University of Maryland, http://ian.umces.edu/imagelibrary/displayimage-823.html Aerial photographs: Google Earth, 2014, Study Area basemap: GeoMapAppField and Lab assistance was provided by: John Kusnierz, Chris Bochicchio, Nicole Delong, Casey Michalowski, Nick Mathews, Dr. Tom Betts.

Adam COOPER1 (acoop545@live.kutztown.edu), Thomas BOND1, Adrienne OAKLEY1, Sean CORNELL2, and Eric WINK1

(1) Department of Physical Sciences, Kutztown University, (2) Department of Geography and Earth Science, Shippensburg University

Stratigraphic Key

Peat: Sediment containing greater than 50% total organic content Organic Rich Clay: Sediment containing greater that 25% total organic matter and has no visible individual grains

Organic Rich Silt: Back bay muds/ silts containing greater than 10-25% total organic material

Silt / Back Bay Muds: Fine grained, grey sediment with <10% organic matter

Massive Quartz Sand: Non-bedded, coarse grained quartz sand Bedded Quartz Sand: Horizontally bedded, coarse grained quartz sand

Fine Grained Sand: Non-bedded, finer-grained quartz sand

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Unit 1

Unit 1

Unit 1

Unit 2A

Unit 2B

Unit 2B

Unit 3

Unit 3Unit 3

Lower Sands:Massive overwash or

Coastal plane

Bay / Marine Deposit

Bay / Marine Deposit

Salt Marsh

VC 9VC 3

VC 13

VC 6

Paleo Shoreline

Coastal Plain Accretion

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ern S

hore

line

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CB VC-3(1994)

Unit 1: Coarse grained quartz sand likely part of the coastal plain, but may represent a massive overwash fan. Sands are overlain by a thin layer of organic rich clay representing localized stabilization by vegetation.

Unit 2: Silt and clay sized particles deposited in a marine back bay environment representing relative sea level rise (transgression). This unit is separated into two layers based on the percent of organic material present.

Unit 3: Organic rich clays and peat deposited in a salt marsh environment.

This overall sequence indicates that the marsh grew outward into Chincoteague Bay at least 170 m prior to any historical records.

Current sea level rise is causing erosion along the coast of Chincoteague Bay. As seen in the aerial photograph above, in 1994 the location of CB VC-3 was within the salt marsh. Uppermost marsh sediments (Unit 3) were removed by erosion.

Transgression Sea level rise or Subsidence

Regression Prograding MarshIncreased Sediment Supply?

Stratigraphic Interpretation

Future Work

GBV VC-3 GBV VC-9 GBV VC-13 GBV VC-6 CB VC-3

Geologic Cross Section

Chincoteague Bay

We will continue analysis of the 13 cores extracted from the salt marsh:

- Macro Fossil analysis will allow us to distinguish between near coastal and back bay depositional environments-Loss on Ignition and Bulk Density for the remaining cores will allow us to refine the sediment stratigraphic units, make comparisons between cores, and correlate sediment units across the marsh-Radiometric Dating using 14C will provide age controls for the cores which we will use to determine sedimentation rates and the rate at which the marsh grew into the bay.

1. What is the origin of the lower sand layer? Massive overwash vs. Coastal Plain

2. Does marsh expansion coincide with development of the coastal plain?

We will create a model for formation of bay promontory salt marshes in Chincoteague Bay and use our understanding of the geologic history of the marsh to inform restoration efforts.

Salt Marsh

Assateague Island Chincoteague

Island

Chincoteague Bay N

CB VC 3

Root

Shells and Shell Fragments

Organic Material

Bioturbation

Symbol Key

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h (c

m)

Study Area

Unit 2

Unit 3

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Importance of Salt Marshes

mud

sand

native marsh grass

dead native marsh grass

erosion and scouring

Blue Heron

sediment

Protected areas

Restoration

1

2

3 Reduce atmospheric emissions

Sea level rise results in increased erosion of salt marshes

Reclamation of land for industry and urban expansion results in physical displacement of salt marsh and changed hydrology

Invasive strains of Phragmites displace native species

Salt marsh ecosystems provide important nursery habitat for fisheries species

Salt marsh ecosystems act as a nutrient filter

Estuarine ecosystems support high primary production

concrete

3

3

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Key Features/Services Major Threats Management Priorities

Unimpacted Impacted

Icon Key

Salt marsh Ecosystems

A salt marsh is a coastal ecosystem in the upper coastal intertidal zone between land and open salt water or brackish water that is regularly flooded by the tides. These ecosystems and their plant communities play important roles in trapping sediments, minimizing erosion, improving water quality, and providing important nursery habitats for a myriad of species. Coastal salt marshes link the mainland to bay estuaries. They act as a buffer between fresh and salt water, filter runoff, and trap contaminants. The Greenbackville salt marsh is an impacted system that has been altered by human activity, including construction, dredging of mosquito ditches to drain the marsh, and pollution from septic discharge.

Methods

Extraction by Vibracore: A concrete vibrator was used to drive a 3 inch diameter aluminum pipe into the marsh sediment. The pipe was capped to provide suction and pulled up with a tripod and winch.

Core Description: Cores were split and photographed at high-resolution and described for color, grain size, and bedding.

Loss on Ignition: 1 cm3 subsamples at 5 cm intervals were dried for 24 hours at 100 C, combusted at 550 C and 1000 C for 3 hours each. Mass lost during combustion provides proxies for organic and carbonate content.

0 20 40 60 80Organic Matter %

0.0 0.5 1.0 1.5 2.0 2.5

Bulk Density (gm/cm3)

0.0 0.5 1.0 1.5 2.0 2.5

0 20 40 60 80

Bulk Density (gm/cm3)

Organic Matter %