COMPLEX RESPONSE OF GULF COAST BARRIERS AND BAYS TO HOLOCENE SEA-LEVEL RISE DUE TO EPISODIC ICE SHEET RETREAT

The current rate of eustatic rise, which averages 3.3 mm/yr and ranges between 2 and 4 mm/yr globally, is nearly seven times the average rate for the past 4,000 years in the Gulf of Mexico (0.5 mm/yr). It is generally believed that the rate will at least double by the end of this century, exceeding the average rate for the past 7,500 years. Detailed reconstructions of barrier evolution in western Louisiana and Texas during this time interval have revealed a complex and diachronous history, with some barriers experiencing growth while others rapidly retreated landward. The exact timing of recessional events remains problematic due to the incomplete nature of the offshore record. Bays of the Gulf Coast also experienced dramatic change as they shifted landward within their incised river valleys. Flooding events are manifest as abrupt landward shifts and re-organization of estuarine environments. Some of these abrupt changes were a threshold response to the overall rise in sea level caused by reductions in sediment supply. But, the apparent synchronous nature of some events implies that the overall sea level rise was punctuated by rapid increases that were below the resolution of sea-level curves. Indeed, the episodic nature of Holocene sea level rise is consistent with our understanding of Antarctic ice sheet evolution, which is generally believed to be the main source of the middle to late Holocene sea level rise, and a likely contributor to future rise.