TWO STORMS FOR THE PRICE OF ONE: A SEABED-IMPACT STUDY OF TROPICAL STORM ISIDORE AND HURRICANE LILI, NORTHERN GULF OF MEXICO

A box core study was conducted in the Mississippi Sound and the Mississippi inner shelf to investigate the impacts of Hurricane Lili and Tropical Storm Isidore. The Mississippi Sound is a large (~2100 km²) bar-built estuary that parallels the Mississippi Coast. This region is regularly impacted by both tropical storms and hurricanes. Recent data indicate that major hurricanes create preservable event layers in the Mississippi sound and represent 8-26% of the sediment column. Smaller tropical and extratropical storms and estuarine processes probably deposit the other 74-92% of the sediment column. However, the seabed effects of weaker tropical systems in this region have been investigated only rarely. Tropical Storm Isidore made landfall west of the Mississippi Bird Foot Delta on September 26 and Hurricane Lili came ashore near Marsh Island on October 3. Box cores were collected on October 11, one week after Hurricane Lili made landfall. Cores have been analyzed using radioisotope geochronology (⁷Be and ²¹⁰Pb), X-radiography, and granulometry.

Wave data and core analyses indicate that there was probably intense re-working of the sediment on the shelf with moderate re-working in the Sound. Peak near-bottom orbital velocities (linear wave theory, data from WAVCIS 13 and NDBC 42007) during Isidore varied from >0.5 m s^-1 in the Sound to >1.4 m s^-1 on the shelf. Lili produced velocities ranging from >0.3 m s^-1 in the Sound to >0.8 m s^-1 on the shelf. Within the Mississippi Sound, X-radiographs and ⁷Be data indicate formation of variable event layers 1-4 cm thick on the muddy seabed. X-radiographs and ⁷Be data from the shelf display a muddy event layer ~10 cm thick off shore and very little deposition on the inner shelf. This indicates the shelf probably experienced intense erosion near shore and rapid deposition further offshore.