SALTMARSH EVENT SEDIMENTATION: A COMPARATIVE STUDY OF HURRICANE IRMA DEPOSITS ACROSS THE SOUTHEAST U.S.A

Saltmarshes provide a range of ecosystem services and offer protection to mainland communities from storm impacts. Storms can also cause widespread marsh erosion along the backside of barrier islands and partially bury them through overwash activity. However, storms may also play a critical role in helping marshes maintain their elevation relative to sea level through the contribution of inorganic sediment. This study examines sediment deposits associated with Hurricane Irma on saltmarshes behind Amelia Island in FL, Sapelo Island in GA, and Hilton Head Island and Cape Romain in SC. The maximum height of storm surges varied at the sites from 2.4 (Amelia) to 1.48 m (Hilton head). Likewise, surge duration ranged from 18.5 (Amelia Island) to 7.2 hrs (Hilton Head). Two and half months after the hurricane, a suite of 16 sediment cores (15 cm in diameter, 60 cm depth) were collected along two transects (two cores per transect) at each of these four sites. The cores were sectioned, sampled, and analyzed for changes in stratigraphy, bulk density, and loss on ignition (LOI). Results reveal storm deposits at all locations ranging in thickness from 2 to 5 cm, and characterized by oxidized, laminated, organic-poor (lack of rootlets), low-density (“fluffy”) mud. These storm layers, clearly deposited during Irma, have a bulk density of 0.4-0.8 g/cm³ and LOI of 15-25%. The peat layers immediately underlying the storm deposit have bulk density and LOI values that are approximately half and twice that of the storm unit, respectively. In contrast to the storm layer, the underlying sediment exhibits an organic content spike that gradually decreases down core with sharp contacts between oxygen-rich and anoxic mud layers occurring at depth of 20-40 cm. Areas that experienced a higher storm surge and/or longer sustained water elevation have generally thicker deposits. Prior to compaction, Irma-associated sediments represent years to decades of non-storm vertical marsh accretion, likely scaling with the magnitude of the local storm impact. Although saltmarshes are among the most threatened environments due to climate change and accelerating sea-level rise, the predicted greater storm frequency and magnitude may actually benefit saltmarshes due to increased sedimentation.