5-5 What the Clay? Investigating the Origin of a Unique Sediment Layer Found in Vibracores from a Salt Marsh in Greenbackville, VA.

Session: Nearshore and Estuarine Research: Dynamics and Future Resiliency in the Coastal Zone (Posters)

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Abstract:

Greenbackville Point is a promontory salt marsh jutting into Chincoteague Bay on the eastern shore of Virginia on the Southern Delmarva Peninsula. The marsh, currently experiencing rapid erosion, contains a 46k year-record of environmental change in 2-3 meters of sediment. Sedimentological and geochemical analysis of vibracores taken along a cross section of the salt marsh reveal local-level responses to climate and consequent sea-level changes since the late Pleistocene. Here we analyze a mystery clay layer found only in vibracores taken from the back marsh region. This layer lies directly beneath a large unconformity which separates the organic sandy silt layer deposited in a lacustrine environment (46-28 ka cal BP, MIS3) from bedded and massive quartz sands (fining upward) deposited in a high energy coastal plain environment (Late-Holocene to ~1800 y cal BP). Previous work suggests that this erosion surface was created during the Last Glacial Maximum, ~21,000 ybp. This project aims to determine the environment of deposition of the clay and its origin through microfossil and pollen identification, XRD analysis, LOI, and SEM imaging.

Preliminary XRD results of the fine separates documents the presence of variable mixtures of kaolinite, illite, chlorite, and montmorillonite which is consistent with a paleosol but is not uniquely conclusive. Unlike the yellow-orange palosols sampled 94 km to the south on the Chesapeake Bay coast of the Delmarva, these clays are a light greyish blue. Lack of oxidized iron, presence of brackish diatoms, and potential fecal pellets imaged by the SEM suggest a brackish water origin. 

Determining the origin of the clay layer will build on previous work in the region, help to refine the geological history of Greenbackville over the last 46,000 years, and place it into context of sea level and environmental changes. In addition, understanding how Greenbackville has responded to sea level changes in the past will help us predict how the salt marsh will react to modern sea level rise and help inform future projects to protect and restore this impacted coastal habitat.

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