CHEMICAL AND ISOTOPIC SIGNATURES FOR SEDIMENT SOURCES ASSOCIATED WITH LARGE HURRICANES IN COASTAL LOUISIANA, U.S.A

Inorganic sediment in coastal wetlands is largely derived from terrigenous sources; however, modern sediment transport in many coastal regions has been altered by flood-control measures (levees, channels, diversions). Although annual flooding of the Mississippi River no longer supplies overbank sediment to coastal wetlands in Louisiana, large hurricanes may periodically transport substantial quantities of sediment from offshore to estuarine environments. To test this hypothesis, we collected 60-cm push-cores from several sites near the 2005 path of Hurricane Katrina (Barataria Bayou, Breton Sound). Chemical, radioisotope (Cs-137 and Pb-210), and Sr isotope (Sr-87/Sr-86) data from 1-cm core intervals were determined to look for offshore components attributable to Category 3 or larger hurricanes.

Initial results are consistent with published estimates of an average 5-cm of mud deposited by the Katrina storm surge. Profiles of Ca, Sr, and Mn concentration, as well as Ca/Sr, decrease over the uppermost 10 cm in both acid-leach (0.1 to 1N HNO3) and bulk-sediment fractions. Concentrations of As, Ba, Cd, Li, Ni, Pb, Rb, U, and V also show distinct differences between shallower (0–5 cm) and deeper (10-30 cm) intervals. Unlike normal accumulation profiles, Pb-210 activities are significantly lower (5-6 dpm/g) in 0- to 5-cm depth intervals compared to higher values (8-10 dpm/g) in 10- to 25-cm depth intervals. Cs-137 profiles show a similar jump from low, uniform values of 0.2 dpm/g at 0–5 cm depths to higher values of ~0.6 dpm/g at 10–15 cm depths before peaking at a depth of 25 cm (bomb-test fallout deposited in the early 1960’s C.E.). The range of Sr isotope ratios in acid leachates at all depths is small (0.70932–0.70940), and only slightly higher than modern seawater. Bulk sediment has more radiogenic Sr and shows greater variability, with the lowest values at 0-5 cm depths (0.7142-0.7156) and higher values in deeper samples (0.7162).

Mud attributed to Hurricane Katrina has more marine-like chemical and isotopic signatures and longer residence times than immediately underlying sediment. Results are consistent with onshore transport of offshore sediment during large storm surges. Similar data from deeper intervals may allow reconstruction of historic and prehistoric sediment budgets in coastal estuaries.