USING THE SEDIMENTARY SIGNATURE OF HURRICANE KATRINA IN THE PEARL RIVER MARSH, LOUISIANA TO INFER PROVENANCE OF THE DEPOSITED SEDIMENTS: IMPLICATIONS FOR ACCRETION

Because the contribution of tropical cyclone-generated deposition to the vertical accretion of Gulf Coast marshes is an important factor in assessing the potential for coastal retreat, we investigated the sedimentary effects of Hurricane Katrina on the Pearl River marsh, a heavily-vegetated wetland covering the lower end the Pearl River, located on the Louisiana-Mississippi border. Hurricane Katrina, a category 3 storm, tracked north across the marsh on August 29, 2005, inundating much of the marsh under a ~5 m storm surge. In September 2015, we extracted seven sediment cores along a coast-parallel transect at the lower end of the marsh and collected 13 marsh and ten riverine/estuarine surface samples along parallel downstream transects extending from the bottomland hardwood forest to the open water. Six of the cores contain a brown clastic layer, with an average thickness of four centimeters, the top of which occurs at an average depth of 10 cm. This layer, which we identified as deposition associated with Hurricane Katrina, based on previously published literature and stratigraphic correlation, displays a consistent signature across the transect, closely resembling the sedimentological and geochemical (wet and dry bulk density, grain size, elemental concentrations, isotopic values and n-alkanes ratios) profiles of the riverine sediments, specifically, the material collected from the riverbed near the mouth of river, whereas samples from the organic intervals above and below the event layer closely resemble the marsh samples. This suggests that, as the storm-deposited sediments likely originated in the river bed, this material represents net sediment gain for the marsh, utilizing material that would otherwise be lost to the system. The calculated post-event accretion rate, based on the depth of the top of the layer is ~1 cm/yr, which increases to ~1.4 cm/year if the layer is included. Centennial- to millennial-scale sedimentation rates, based on two long cores from the immediate vicinity, are as much as an order of magnitude lower, with the rapid rate increase having occurred within the last two centuries. Possible explanations for this increased rate include land clearance, nutrient supply from agricultural runoff and increased hydroperiods, resulting from rising sea level and/or increased river discharge