Geomorphic Response of the Georgia Bight Coastal Zone to Accelerating Sea Level Rise, Southeastern USA

Abstract

Synthesis of geologic and chronologic data generated from Holocene sedimentary sequences recovered along the inner continental shelf, shoreface, and modern coastal zone of the Georgia Bight reveal a synchronous sequence of paleoenvironmental events that occurred in response to rate of sea level rise tipping points. During the early Holocene (11.7–8.2 cal kyr BP), the paleoshoreline was overstepped and submerged by rapidly rising seas that averaged ~5 mm yr−1. Rates of rise during the middle Holocene (8.2–4.2 cal kyr BP) averaged ~2 mm yr−1 and this deceleration resulted in the formation of coastal environments and sedimentary sequences that were subsequently reworked as the shoreface continued its landward and upward migration. The modern coastal zone emerged commensurate with the late Holocene (4.2–0 cal kyr BP), when the rate of sea level rise averaged <1 mm yr−1. Analysis of water level data collected at six NOAA tide gauge stations located along the Georgia Bight coast indicates the rate of relative sea level rise has increased from a historical average of 3.6 ± 0.2 mm yr−1 (<1972 to 2022) to 6.6 ± 0.8 (1993 to 2022) and during the 21st century it has averaged 9.8 ± 0.3 mm yr−1 (2003 to 2022). The current rate of sea level rise is nearly double the early Holocene rate of rise. Based upon a novel application of the principle of uniformitarianism (i.e., the past is the key to the future), the likely geomorphic trajectory of the Georgia Bight coastal zone under conditions of 21st century accelerating sea level rise will be one of increasing instability (e.g., coastal erosion) and flooding (e.g., overwash, breaching). Evidence of an emerging instability within the coastal zone has been previously reported throughout the region and supports the trajectory of geomorphic change proposed herein. This will ultimately result in the submergence of existing landscapes and replacement by estuarine and marine environments, which may hasten in pace and scale given the current rate of sea level rise is expected to continue accelerating throughout this century. These findings have not been previously reported and should be considered by coastal practitioners responsible for conceptualizing risk, as well as the formulation and implementation of adaptation action plans designed to mitigate threats to the built and natural environment induced by climate change.

Horizontal Rates of Wetland Migration Appear Unlikely to Keep Pace with Shoreline Transgression under Conditions of 21st Century Accelerating Sea Level Rise along the Mid-Atlantic and Southeastern USA

“…The rates within the study domain ranged between 2.3 mm yr −1 (Northwest Florida area 5) and 3.5 mm yr −1 (Delaware Bay areas 1B and 1C) and averaged 2.8 mm yr −1 . The rate of sea level rise commensurate with all geological wetland migration rates was obtained from the western Atlantic submergence curve of [46] as reported in [47]. This value (0.8 mm yr −1 ) is similar to the late-Holocene rates published for North Carolina [50] and northeastern Florida [51].…”

“…The elevations were obtained from tide stations located proximal to each of the five regions of focus (Figure 1). Given that 21st-century rates of sea level rise along the mid-Atlantic and southeast US coast are currently tracking with the Intermediate High and High trajectories of NOAA [47,48], this study used the median High value to derive elevations and rates of rise. The correlation between the rates of horizontal marsh migration and sea level rise was tested.…”

“…nd = no data. a A = reference authors, B = [46] as calculated in [47], C = Key West as reported in [48], D = Cedar Key as reported in [48]. 2 Average value derived from Figure 1 of [33].…”