WHAT COMES NEXT? THE FINAL PUSH TO THE FUTURE LATE HOLOCENE SEA-LEVEL HIGHSTAND

Given the vast human population, economic investments, and resources found along the modern coasts, there is well-warranted concern about accelerating rates of sea-level rise and future coastal inundation. Rates of sea-level rise have increased over the last 120 years, from global rates of approximately zero in the 19^th century, to an average rate in the 20^th century of approximately 1.7 mm/y, to the present rate of 3.4 mm/y. Collapse of the already thinning West Antarctic Ice Sheet, and accelerated melting of the Greenland Ice Sheet could raise sea level by +3 to +6 m. Observations of past highstands, most notably during Marine Isotope Stage (MIS) 5e, suggest that global sea level attained a height of approximately +2 m (relative to modern SL) for several millennia, followed by a rapid rise to approximately +6 m. It is quite possible that a similar trend is occurring during the Holocene, but is likely accelerated by anthropogenic processes. In eastern North Carolina and Virginia, we have a record of rapid relative sea-level (RSL) changes during MIS 5e, 5a, and 3. The MIS 5e/5a paleoshoreline (the Suffolk Shoreline) is a highstand prism stranded up to 120 km inland from the modern ocean shoreline. MIS5e and 5a transgressive to highstand deposits occur seaward of the paleoshoreline, on the coastal plain and beneath the estuaries. These deposits contain a record of past conditions occurring during rapid rise and highstand that may provide an analog for future conditions. Modeling is being used to understand the changes to hydrodynamics of this coastal region during barrier island fragmentation and inundation of lowlands that occur as sea level rises. Initial findings of the modeling indicate increased tidal ranges and wave energy in estuarine systems, accompanying and amplifying (i.e., serving as a positive feedback to) barrier island fragmentation and transgression, which may explain the broad distribution of MIS5a tidal deposits in the coastal plain. If the past conditions are a good analog, we can expect a transition in northeastern North Carolina from the broad estuaries and wave-dominated barrier islands, to a tidally dominated shallow inner shelf system, with fewer estuaries confined mainly to trunk river valleys.