CAREER: Dynamic sea-level trends during the past two millennia

Relative sea level (RSL; the height of the sea-surface with respect to the land) changes across space and through time in response to a wide range of physical processes that operate on timescales from minutes to millennia and on local to global spatial scales. Projections of future climate indicate that changes to the strength and/or position of ocean currents and prevailing winds will cause RSL rise along much of the U.S. Atlantic coast to exceed the global mean by pushing existing ocean water toward the coast (termed dynamic sea-level change). The relationship between coastal RSL and ocean/atmospheric circulation can be observed on timescales of months to a few years in instrumental datasets, but it remains unclear if such changes can be sustained on longer timescales. This project will reconstruct RSL changes during the past ~2000 years from Prince Edward Island to South Carolina using coastal sediment to explore if and why dynamic sea-level changes occurred over multiple decades and centuries. These reconstructions will support coastal management by helping to inform and constrain projections of 21st century regional-scale RSL rise. Public scientific literacy will be promoted through an original art exhibition (in collaboration with the School of the Museum of Fine Arts) that explores the causes and ramifications of regional sea-level rise.

Instrumental datasets (e.g., tide-gauge measurements) and model projections demonstrate that dynamic sea-level changes along the Atlantic coast of North America have a spatial expression that is characteristic of the underlying physical cause(s). Since these datasets are short and fragmentary, proxy RSL reconstructions are needed to quantify dynamic sea-level change on multi-decadal to centennial timescales. This project will produce high resolution and near-continuous RSL reconstructions spanning the past ~2000 years from two sites in each of Prince Edward Island, Maine, and South Carolina using foraminifera preserved in dated sequences of salt-marsh sediment. These locations were chosen because of their sensitivity to dynamic sea-level change in model simulations and to complement the distribution of existing RSL reconstructions. The vertical distribution of salt marshes (and foraminifera that live on them) is fundamentally linked to tide levels and therefore sea level. Under conditions of RSL rise salt marshes accumulate sediment to maintain their position in the tidal frame. The resulting sedimentary sequences contain the identifiable remains of plants and foraminifera that lived on paleo salt marsh surfaces and are a detailed archive of RSL change. Along much of the Atlantic coast of North America ongoing glacio-isostatic adjustment drives long-term RSL rise and the creation of accommodation space (approximately 1–3 m for the study areas in the past ~2000 years) that is filled by salt-marsh sediment. Dynamic sea-level changes overprint this underlying RSL trend and can be identified in sufficiently detailed RSL reconstructions. An age-depth model will be created for each of the six cores using radiocarbon dating of plant remains and recognition of pollution trends and events of known age in downcore profiles of elemental and isotopic abundance. The height of former RSL will be reconstructed using transfer functions trained on the observable relationship between assemblages of foraminifera and tidal elevation on salt marshes in the three study areas. The resulting RSL reconstructions will have the temporal and vertical resolution necessary to reliably identify relatively small and short-lived dynamic sea-level variability. Analysis of these new records alongside existing reconstructions using a spatio temporal model will identify regional patterns of coherent sea level variability in the North Atlantic Ocean from which the role of ocean and/or atmospheric dynamics as a driver of sea-level trends will be quantified. This long-term perspective on ocean/atmosphere conditions, climate forcing, and sea-level rise will generate insight into the causes and magnitude of past, present, and future changes in the coupled climate-sea level system.

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.