Sedimentological Analysis of Spatial Variations in Seagrass Blue Carbon Deposition

<p dir="ltr" style="line-height: 2.4; text-indent: 36pt; margin-top: 0pt; margin-bottom: 0pt;"><span style="font-size: 12pt; font-family: Arial,sans-serif; color: #000000; background-color: transparent; font-weight: 400; font-style: normal; font-variant: normal; text-decoration: none; vertical-align: baseline; white-space: pre-wrap;">Seagrass beds support healthy estuaries and are among the most efficient carbon sinks. High-salinity seagrass commonly forms behind barrier islands in an area frequently impacted by storms. Storm processes can remove seagrass as new inlets form and erode back-barrier areas, bury seagrass as washover sand is deposited on the meadow, or form new seagrass habitat by depositing sand and elevating the bed. The research community mainly assesses seagrass carbon stocks from surface samples; however, to constrain the entire stock, from the vegetated surface to the substrate at depth, a depositional facies model is required. There is a lack of information on seagrass depositional facies and the relationship between carbon burial in seagrass beds and the frequently shifting depositional environment of many temperate meadows.&nbsp;</span> <span id="docs-internal-guid-33b89558-7fff-6e54-70c8-5e8a12b8b3bf"></span> <p dir="ltr" style="line-height: 2.4; text-indent: 36pt; margin-top: 0pt; margin-bottom: 0pt;"><span style="font-size: 12pt; font-family: Arial,sans-serif; color: #000000; background-color: transparent; font-weight: 400; font-style: normal; font-variant: normal; text-decoration: none; vertical-align: baseline; white-space: pre-wrap;">North Carolina has the largest seagrass area (mainly </span><span style="font-size: 12pt; font-family: Arial,sans-serif; color: #000000; background-color: transparent; font-weight: 400; font-style: italic; font-variant: normal; text-decoration: none; vertical-align: baseline; white-space: pre-wrap;">Zostera</span><span style="font-size: 12pt; font-family: Arial,sans-serif; color: #000000; background-color: transparent; font-weight: 400; font-style: normal; font-variant: normal; text-decoration: none; vertical-align: baseline; white-space: pre-wrap;"> and </span><span style="font-size: 12pt; font-family: Arial,sans-serif; color: #000000; background-color: transparent; font-weight: 400; font-style: italic; font-variant: normal; text-decoration: none; vertical-align: baseline; white-space: pre-wrap;">Halodule</span><span style="font-size: 12pt; font-family: Arial,sans-serif; color: #000000; background-color: transparent; font-weight: 400; font-style: normal; font-variant: normal; text-decoration: none; vertical-align: baseline; white-space: pre-wrap;">) on the east coast of North America and ~80% of that area is positioned on high-salinity sand flats behind the Outer Banks barrier island chain.&nbsp; We collected four core transects separated between 3 and 42 km in Core Sound, extending across patchy and continuous seagrass beds from the back barrier shoreline into the estuary. The southern transects sampled a 1.83 km-wide meadow, and a 506 m-wide meadow from areas that show no evidence of washover or flood tidal delta deposition since 1954 CE. The northern transects sampled a 933.5 m-wide meadow, and a 1.28 km-wide meadow from areas with an active flood-tidal delta in 1943 CE. Seagrass initially colonized sandflat and is characterized by a fining upward unit of bioturbated sand at the base to a poorly sorted organic muddy sand at the top. All transects sampled older seagrass beds below the modern meadow near the island margin. The older beds were thinner than the surface beds; however, some had similar carbon stocks. Cross sections from all sites suggest rapid deposition of sand buried a former seagrass bed, preserved a portion of its carbon stock, and seagrass subsequently recolonized that sandy substrate. This work underscores the great resilience of seagrass beds to storm burial events in the past and the need to define carbon stock as the entire seagrass depositional system as opposed to surface beds.</span>