Jeffrey Duxbury

We present the first integrated regional subaqueous Mississippi River sediment budget to better understand how declines in sediment load from the Mississippi River over the 20th and 21st centuries contributed to the development and retrogradation of the submarine Mississippi River Delta Front (MRDF). This retrogradation can be partly attributed to downslope mass transport, which may move sediment more rapidly than replenishment from proximal fluvial sources. ²¹⁰Pb/¹³⁷Cs geochronology from 67 sediment cores was utilized to calculate apparent sediment mass accumulation rates (MAR) during the 20th and early 21st centuries. ²¹⁰Pb and ¹³⁷Cs regional maps documenting MARs from 1953 to 2025 were created to document sediment accumulation across the MRDF, which identified up to 73.8 Mt/y. A regional mass-accumulation budget for the subaqueous delta of this river-dominated system, when compared to mean 1939–2010 sediment discharge rates, indicates an approximate time-averaged retention of 25% of fluvial sediment discharge from major historical outlets on the MRDF. Furthermore, patterns of accumulation record the effects of major anthropogenic reductions in fluvial sediment load, as well as the complex interactions between oceanic and gravity-driven processes.

Sediment cores were collected from a mudflow lobe (80 m water depth) offshore of the Mississippi River’s Southwest Pass in 2017 to better understand the sedimentology near the lobe entraining the SS Virginia shipwreck (sunk by a German U-boat in 1942) and surrounding Mississippi River delta front. Core analyses included ²¹⁰Pb/¹³⁷Cs geochronology, granulometry, and X-radiography. Sediment accumulation rates (SAR) calculated from excess ²¹⁰Pb activity in multicores are 0.22–0.29 cm/y at seabed depths less than 20 cm and 0.29–0.51 cm/y at depths greater than 20 cm. Accumulation rates for ¹³⁷Cs have been ~0.15 to ~0.37 cm/y since 1954 and 1963, respectively. Sediment accumulation rates from ²¹⁰Pb, ¹³⁷Cs geochronology and indicators of relative sedimentation and bioturbation from X-radiographs suggest that rates of sediment accumulation near the Virginia have declined since the mid-20th century. This may be explained by the multi-decade downslope mass transport of the mudflow lobe in which the shipwreck is embedded and decreases in sediment supply delivered offshore from the Mississippi river. Mass transport calculations of the Virginia lobe derived from core properties and published lobe advection rates suggest downslope mass transport is far higher than sediment resupply from the Mississippi river, consistent with recent studies of delta retreat.

Sediment cores were collected from a mudflow lobe (80 m water depth) offshore of the Mississippi River’s Southwest Pass in 2017 to better understand the sedimentology near the lobe entraining the SS Virginia shipwreck (sunk by a German U-Boat in 1942) and surrounding Mississippi River Delta Front. Core analyses included 210Pb/137Cs geochronology, granulometry, and X-radiography. Sediment accumulation rates (SAR) calculated from excess 210Pb activity in multicores are 0.22–0.29 cm/yr at seabed depths less than 20 cm and 0.29–0.51 cm/year at depths greater than 20 cm. 137Cs accumulation rates are ~0.15 to ~ 0.37 cm/year since 1954 and 1963 respectively. Sediment accumulation rates from 210Pb and 137Cs geochronology and indicators of relative sedimentation and bioturbation from X-radiographs suggest that rates of sediment accumulation near Virginia have declined since ca. the mid-20th century. This may be explained by the multi-decade downslope mass transport of the mudflow lobe in which the shipwreck is embedded, and decreases in sediment supply delivered offshore from the Mississippi River. Mass-transport calculations of the Virginia lobe derived from core properties and published lobe advection rates suggest downslope mass transport is far higher than sediment resupply from the Mississippi River, consistent with recent studies of delta retreat.