ESTUARINE RESPONSE TO THE 8.2 KA COOLING EVENT ACROSS THE NORTHERN GULF OF MEXICO AND IMPLICATIONS FOR THE FUTURE

Estuaries along the northern Gulf of Mexico, including Mobile Bay, Sabine Lake, Galveston Bay, Corpus Christi Bay, and Baffin Bay, show rapid landward shifts in estuarine environments (back stepping) around 8200 cal yr. BP. Paleogeographic maps of the estuaries based on seismic data and cores, and constrained chronologically by radiocarbon dates, depict bay-head delta retreat in excess of 25 m/yr. During these events the subaerial delta plain flooded to create sub-tidal upper and middle bay environments. Abrupt sea-level rise associated with the 8200 cal yr. BP climate-change event caused these estuaries and others around the world to reorganize abruptly. In the northern Gulf of Mexico, the sea-level rise associated with the event was only ~0.1 m over a sub-decadal time period. This rise was superimposed on a background decelerating rate of sea-level rise of ~1.0 to 0.5 m/century. The widespread bay-head delta back-stepping, documented here, may be an appropriate analog for possible abrupt future climate change in response to global warming and shows that estuaries fringed by low-gradient shorelines are highly sensitive to low amplitude and sudden sea-level rise. However, examples from the geologic record ignore the impacts of humans. Changing sediment sources and the modification and/or disruption of established sediment-transfer routes throughout the coastal zone has a strong impact on sediment fluxes to estuarine systems (including barrier islands) through rivers, creeks, and man-made canals and channels. Human modifications have the potential to dramatically change the evolution of estuarine shorelines, which should be fringed by salt marsh but are rapidly being replaced by bulkheads, riprap, and rock sills. In addition to analogs from the geologic record, the impact of human modifications to low elevation and low gradient coasts must be taken into account when projecting future shoreline positions to accelerated sea-level rise.