Diercks, A-R, et al. 2018 Scales of seafloor sediment resuspension in the northern Gulf of Mexico. Elem Sci Anth, 6: 32. DOI: https://doi. org/10.1525/elementa.285 IntroductionThe sedimentation of large amounts of oil via marine snow and its accumulation on the deep seafloor (>1,200 m) during and after the Deepwater Horizon (DwH) oil spill (Passow et al., 2012;Valentine et al., 2014;Brooks et al., 2015;Chanton et al., 2015;Daly et al., 2016;Joye, 2016;Joye et al., 2016;Passow, 2016) raised questions regarding the distribution and re-distribution processes of freshly sedimented material (marine snow) on the seafloor. Once on the seafloor, marine snow contributes to unconsolidated fluffy sediment layers (Gardner, 1978;Gardner et al., 1984;1985;Walsh et al., 1988;Pilskaln et al., 1998;Newell et al., 2005) that are subject to resuspension and the production of benthic nepheloid layers (BNLs).Resuspension leads to the re-invigoration of degradation processes, which would impact the degradation rates of the oil associated with marine snow following the DwH accident (Ziervogel et al., 2016). Additionally, resuspension leads to lateral transport and redistribution of the material that sank to the seafloor. After the DwH accident such re-distribution processes make it especially difficult to estimate the total amount of Macondo oil that reached the seafloor (Passow and Hetland, 2016).BNLs, which are formed when the frictional stress of water motion strips sediment off the seafloor, therewith carrying particles into the overlying water layer, exist near the seafloor, but may reach tens to hundreds of meters upward into the water column (McCave et al., 1976). The thickness of the BNL extending above the seafloor scales with the strength of the bottom currents and the particle composition. Bottom currents >10 cm s -1 may cause resuspension events (Gardner et al., 2017), especially when low density phytodetritus or fine silt covers sediments, but large benthic storms reach 20 cm s -1 (Gardner et al., 1985). Besides locally resuspended material, particles in the BNL also include aggregates settling from the upper ocean as Time-series data of size-specific in-situ settling speeds of marine snow in the benthic nepheloid layer (moored flux cameras), particle size distributions (profiling camera), currents (various current meters) and stacked time-series flux data (sediment traps) were combined to recognize resuspension events ranging from small-scale local, to small-scale far-field to hurricane-scale. One smallscale local resuspension event caused by inertial currents was identified based on local high current speeds (>10 cm s -1) and trap data. Low POC content combined with high lithogenic silica flux at 30 m above bottom (mab) compared to the flux at 120 mab, suggested local resuspension reaching 30 mab, but not 120 mab. Another similar event was detected by the changes in particle size distribution and settling speeds of particles in the benthic nepheloid layer. Flux data indicated two other small-scale events, which occurre...

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Coastal systems in the Gulf of Mexico are threatened to reduced sediment supply, storm impacts and relative sea level rise (RSLR). The geologic record can provide insights of geomorphic threshold crossings (formation, progradation, transgression, destruction) to these forcing mechanisms to predict future barrier evolution to climate change. The stratigraphic framework and antecedent topography directly influence coastal evolution over geologic timescales. This study synthesizes ~2100km of geophysical data, 700+ sediment cores, and 63 radiocarbon dates to regionally map two sequence boundaries, multiple ravinement surfaces and fourteen depositional facies. One marine isotope stage (MIS) 6 valley's fill provided up to 300 x10 6 m 3 of sand to modern systems through transgressive ravinement during the Holocene. Repeated storm breaches or tidal inlets correspond to paleotopographic low's in the MIS 2 surface. A Holocene geomorphic evolutionary model was created for Petit Bois and Dauphin Island from available data, highlighting RSLR rates and sediment supply. As the MIS 2 surface was flooded, tidal/wave scour supplied sand to migrating marine shoals. These rapidly transgressing shoals converted drowned paleovalleys to estuaries starting about 9ka. Islands formed in their modern positions about 6ka, when sediment supply was high and RSLR rates were 2mm/yr. Between 4ka-1750 CE. Islands prograded due to RSLR rates of 1-0.4mm/yr and sufficient sand supply from alongshore and inner shelf sources. Currently, the islands experience RSLR rates of 3.61 mm/yr and reduced sediment supply resulting in barrier degradation. This study provides geologic evidence of coastal geomorphic thresholds related to RSLR, sediment supply and antecedent topography. iii ACKNOWLEDGMENTS I would like to thank my advisor, Dr. Davin Wallace for his guidance and great support through this project. I would also like to thank my committee members Dr. Michael Miner and Dr. Jessica Pilarczyk, and lab mates, Clayton Dike, Nina Gal, and Shara Gremillion for their helpful and insightful feedback and guidance. This project was funded by the Bureau of Ocean and Energy Management, as part of a Gulf of Mexico sand resource mapping effort. Thank you to colleagues at the University of Alabama, the Geological Survey of Alabama and USGS for allowing core sampling. The USGS and UTIG shared archive geophysical data. Mollusk bivalve identification was greatly aided by Dr. Jennifer Walker. Dr. John Anderson (Rice) and Dr. Ervin Otvos (USM) also provided valuable insight. iv DEDICATION This thesis is dedicated to friends, family and loved ones especially Bob, Ellen, Joey and Alli. Their love, appreciation and support allowed me to pursue my scientific passion. v