Coastal response to accelerated sea-level rise (>4 mm/yr) based on early-mid Holocene coastal evolution in the northwestern Gulf of Mexico

There is growing consensus that the rate of sea level rise by the end of this century will reach, and possibly exceed, 5 mm/yr. Predictions as to how sea-level rise will impact coasts often rely on passive inundation models that simply flood the coastal landscape. However, the geological record clearly shows that coastal response to past sea-level rise was more complex, mainly due to differences in sediment supply and subsidence. Ultimately, coastal submergence and erosion depend on whether coastal environments are capable of aggrading as fast as sea-level rises, and this is largely dependent on sediment supply and, in the case of wetlands, vegetation growth. It has been 7000 years since sea level was rising at a rate of 5 mm/yr in the northern Gulf of Mexico. After approximately 4000 cal yrs BP the rate of rise decreased to less than 1 mm/yr. The rate has more than doubled in historical time. An analysis of shoreline and bayline change in Texas and western Louisiana during the past 9000 years shows that coastal retreat was quite episodic, with episodes of widespread and pronounced change that lasted a few centuries. During these episodes, the larger bays of the region (Calcasieu Lake, Sabine Lake, Galveston Bay, Matagorda Bay and Corpus Christi Bay) experienced major re-organization of estuarine environments. Within the limits of radiocarbon precision (a few centuries due to poorly constrained regional carbon reservoir variations) these events appear to have been contemporaneous. This begs the question, where these events caused by short-lived increases in the rate of rise or do they reflect a threshold response of coastal systems to an overall rise that averaged 4.0 mm/yr? This was a time when the West Antarctic ice sheet was experiencing its final stages of retreat from the inner continental shelf and inland passages, which could have resulted in rapid sea-level events of a few decimeters, below the resolution of Gulf of Mexico sea-level curves. These results indicate that coastal inundation models, which focus on the magnitude of sea-level rise, greatly underestimate the impact of accelerated sea-level rise that will likely occur by the end of this century.