Holocene Formation and Evolution of Horn Island, Mississippi, USA

Horn Island, one of the most stable barriers along the Mississippi-Alabama chain, provides critical habitat, helps regulate estuarine conditions in the Mississippi Sound, and reduces wave energy and storm surge for the mainland. This study integrates 2,200 km of high-resolution geophysics, 35 sediment cores, and 15 radiocarbon ages to better understand the formation and evolution of the island in response to sea-level rise, storms, and antecedent geology. The Biloxi and Pascagoula incised valleys converge at Horn Island and have played a profound role in the evolution of the system. Within the incised valleys, numerous shallow paleochannels between 4 and 9 meters below sea level exist only on the landward side of the island, indicating seaward transgressive and tidal ravinement. Sand released due to ravinement processes thus contributed to the formation of Horn Island. Based on radiocarbon ages, an ancestral island existed 8,000 years BP that was ephemeral, frequently overwashed, and unable to build a sandy shoreface. This time period occurs during known rapid rates of relative sea-level rise of 4 mm/yr. Approximately 4,500 years BP coinciding with a deceleration in sea-level rise to about 1 mm/yr, radiocarbon ages associated with Horn Island’s barrier complex and lower shoreface indicate progradation and island formation, in addition to lateral migration in a westward direction that continues to present day. Past sensitivity to rates of sea-level rise coupled with an exhausted sediment supply make the future of Horn Island uncertain