Contents lists available at ScienceDirect Marine Geology journal homepage: www.elsevier.com/locate/margo Invited review article Mississippi River subaqueous delta is entering a stage of retrogradation Jillian M. Maloney a, ⁎ , Samuel J. Bentley b,d , Kehui Xu c,d , Jeffrey Obelcz e , Ioannis Y. Georgiou f , Michael D. Miner g a Department of Geological Sciences, San Diego State University, 5500 Campanile Dr., San Diego, CA 92182, United States b Department of Geology & Geophysics, Louisiana State University, Baton Rouge, LA 70803, United States c Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, LA 70803, United States d Coastal Studies Institute, Louisiana State University, Baton Rouge, LA 70803, United States e NRC Postdoctoral Fellow, U.S. Naval Research Laboratory, 1005 Balch Blvd., Stennis, MS 39556, United States f Department of Earth and Environmental Sciences, University of New Orleans, New Orleans, LA 70148, United States g U.S. Department of Interior, Bureau of Ocean Energy Management, New Orleans, LA 70123, United States ARTICLE INFO Editor: E. Anthony Keywords: Mississippi Mississippi Delta Source-to-sink Sediment supply Sediment deposition ABSTRACT The subaqueous delta of the Mississippi River, the largest river system in the conterminous U.S., has entered a stage of retrogradation caused by multiple natural and anthropogenic activities. Since the 1950s, the suspended sediment load of the Mississippi River has decreased by ~50% due primarily to the construction of > 50,000 dams in the Mississippi basin. The impact of this decreased sediment load has been observed in subaerial en- vironments, but the impact on sedimentation and geomorphology of the subaqueous delta front has yet to be examined. To identify historic trends in sedimentation patterns, we compiled bathymetric datasets, including historical charts, industry and academic surveys, and National Oceanic and Atmospheric Administration hy- drographic data, collected between 1764 and 2009. The progradation rate (measured at the 10 m depth contour) of Southwest Pass, which receives 69% of the suspended sediment load reaching Head of Passes, has decreased from ~67 m/yr between 1874 and 1940 to ~26 m/yr between 1940 and 1979, with evidence of further de- celeration from 1979 to 2009. At South Pass and Pass a Loutre, the delta front has entered the destructive phase, with the 10 m contour retreating at rates > 20 m/yr at both passes since 1979. Advancement of the delta front also decelerated in deeper water (in some areas out to ~180 m depth). Except locally, where mudflow lobes are advancing, deeper contours show a pattern of decreasing progradation rate between 1874–1940 and 1979–2005 time periods. Furthermore, based on differences measured between available bathymetric datasets, the sediment accumulation rate across the delta front decreased by ~73% for the same period. The retention rate of Mississippi River sediment on the delta front ranged from 67 to 81% for the time periods assessed, with total sediment load stored on the delta front equal to 317 ± 54 Mt/yr from 1874 to 1940, 145 ± 25 Mt/yr from 1940 to 1979, and 87 ± 15 Mt/yr from 1979 to 2005. We document for the first time that the Mississippi River delta front has entered a phase of retrogradation, which will likely be accelerated by future upstream activities that divert a portion of the sediment load to the upper delta for coastal protection and restoration projects. The decline of the subaqueous Mississippi River Delta has critical implications for biogeochemical cycling, sub- aqueous mass wasting, and sediment dispersal to the coastal ocean. 1. Introduction Deltas are an important part of the source-to-sink pathway where terrestrial sediments are dispersed into the marine environment. Transport and deposition of sediment within and away from the delta are important for global carbon cycling, marine ecosystems, pollutant dispersal, and natural resources. Recent research on global deltas has shown that anthropogenic impacts to river systems are influencing patterns of sediment distribution at the river mouth (e.g., Bergillos et al., 2016; Blum and Roberts, 2009; Couvillion et al., 2011; Fan et al., 2006; Yang et al., 2017). Here, we examine changes in sedimentation patterns on the subaqueous Mississippi River delta, which is formed where the Mississippi River empties into the northern Gulf of Mexico (Fig. 1). The Mississippi River is ranked seventh in the world in both water discharge and suspended sediment load (Milliman and Meade, 1983; Meade, 1996) and the Mississippi River delta is one of the most https://doi.org/10.1016/j.margeo.2018.03.001 Received 10 December 2016; Received in revised form 6 December 2017; Accepted 1 March 2018 ⁎ Corresponding author. E-mail address: jmaloney@mail.sdsu.edu (J.M. Maloney). Marine Geology 400 (2018) 12–23 Available online 10 March 2018 0025-3227/ © 2018 Elsevier B.V. All rights reserved. T