IS SHORELINE CANNIBALIZATION SUSTAINING LOUISIANA MARSHES? INSIGHTS FROM VERTICAL ACCRETION, MINERAL ACCUMULATION, AND LAND LOSS PATTERNS IN BARATARIA BASIN (Invited Presentation)

Over the past century, Louisiana has experienced extreme coastal land loss, with long term rates averaging 62 km² yr^-1. In an attempt to mitigate or reduce land loss, billions of dollars are earmarked for restoration projects that promote land reclamation and habitat stabilization, and defend against saline intrusion. In this study, twenty-five shallow cores covering a broad span of Barataria Basin, from freshwater to saline environments, were extracted in an effort to better understand spatial trends of marsh sustainability in coastal Louisiana marshes. Cores were processed for ¹³⁷Cs and ²¹⁰Pb radiochemistry, bulk density, grain size, and organic matter; results were subsequently compared to decadal geomorphic changes, such as elevation, subsidence, and land loss. Long-term vertical accretion rates from this study’s radiochemical analyses agree with previous work in the basin (0.5-1.5 cm yr^-1; averaging 0.67 ± 0.14 cm yr^-1), however do not show a clear spatial trend across the basin or with relative elevation. Mass accumulation rate does show a relatively robust spatial trend with greater values south of Lake Salvador compared to those north (1.85 ± 0.69 kg m^-2 yr^-1 versus 0.81 ± 0.35 kg m^-2 yr^-1, respectively), and sites to the south appear to have greater mineral mass accumulation and lower organic content (1.33 ± 0.65 kg m^-2 yr^-1 and 30 ± 12%, versus 0.54 ± 0.68 kg m^-2 yr^-1 and 52 ± 22%, respectively). This dichotomy of trends suggests that paucity of mineral sediments does not inhibit marsh vertical accretion in the northern area of the basin, but the accumulation of organic material dominates here. Elevated mineral accumulation rates in middle to southern regions of the basin appear to overlap with the areas that historically have experienced extensive land loss. It is hypothesized that material formerly comprising eroded marsh edges is the likely source of this material, advected onto the marsh platforms during storm or inundation events. This suggests that marsh cannibalization is an important process for maintaining elevation in these marshes, and future restoration projects that increase mineral sediment influx to the basin may alleviate this process.