Salt marshes represent the interface between land and sea and are a vitally important ecosystem that protects the mainland from storms while providing a multitude of valuable services including nursery grounds for fish and shellfish, filtering land-derived wastes, and exporting nutrients and food sources to the coastal ocean. However, platform marshes are now endangered due to accelerating sea-level rise and diminished sediment supplies, which together may lead to their drowning. This project will study how major storms help the sustainability of marshes by transferring sediment from the coastal zone and depositing it in a widespread layer on top of the marsh. This sediment layer (an inch or more) helps the marsh to accrete vertically, fertilizes the marsh with nutrients, and enhances the productivity of the salt marsh grasses. This research will produce an understanding of how storms source these sediments and how characteristics of the nearshore and marsh system influence the direction of floodwaters and the resulting distribution and deposition of the suspended sediment. As a broader impacts activity, the investigators will design and facilitate a graduate-level course on the Interaction of Storms with the Coastline, covering aspects of hydrodynamics, sediment dynamics, geomorphology, ecology, human geography, and economics as they relate to storm impacts. Additional broader impacts include: 1) fostering new and existing relationships with local stakeholders (including the Georgia Coastal Ecosystems Long Term Ecological Research site) to facilitate data exchange, cultivate collaboration, and disseminate findings, 2) maintaining a project website presenting details of the motivation, progress, the graduate course, and providing links to data products; 3) training one postdoctoral researcher, one Ph.D. student, and at least two undergraduate students and two high school interns; and 4) direct outreach to the public through development of hands-on K-12 learning activities to be implemented at the Virginia Institute of Marine Science.
The study leverages a suite of sediment cores and marsh surface samples collected behind Sapelo Island, Georgia, in December 2017 to quantify the provenance of sediment deposits associated with Hurricane Irma (supported by a RAPID grant from the NSF Geomorphology and Land-use Dynamics Program). Post-Irma data will be supplemented with additional samples from potential sediment sources within nearshore, backbarrier (channel; bay floor; tidal flat; marsh-edge erosion), and fluvial environments. The relative sediment contributions from each of these sources to the storm deposit will be determined using a novel combination of sedimentologic (e.g., grain size, loss-on- ignition, foraminifera content) and geochemical (biomarker compositions, bulk-sediment and biomarker stable- and radio- isotopic compositions) proxies analyzed through principal component analysis and non- metric multi-dimensional scaling statistics. Relationships among storm-surge elevation, local flow patterns, deposit thickness, and sediment provenance will be analyzed using multi-dimensional statistical tools. These data will then be used in conjunction with a Delft3D model, which can simulate hydrodynamics, sediment transport, and resulting sedimentation patterns during Irma and other storm conditions, to: 1) explore the source of storm deposits across a range of backbarrier subenvironments, 2) determine the relative importance of nearshore, backbarrier, and fluvial reservoirs in providing sediment to saltmarshes during major storms, 3) quantify the processes, pathways, and mechanisms of sediment distribution to the marsh platform (e.g., storm-surge level and duration, wave exposure, local geomorphology, and river sediment discharge), and 4) evaluate the importance of storm sedimentation relative to long-term vertical marsh accretion.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
