Sedimentary Processes and Instability on the Mississippi River Delta Front near the Shipwreck of the SS Virginia

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.

2017年，从密西西比河西南水道近海的一个泥流舌状体（水深80米）采集了沉积物岩芯，以更好地了解卷入弗吉尼亚号沉船（1942年被一艘德国潜艇击沉）的舌状体附近以及密西西比河三角洲前缘周围的沉积学情况。岩芯分析包括²¹⁰Pb/¹³⁷Cs年代学、粒度分析和X射线照相。根据多芯中过量²¹⁰Pb活度计算出的沉积物堆积速率（SAR）在海底深度小于20厘米处为0.22 - 0.29厘米/年，在深度大于20厘米处为0.29 - 0.51厘米/年。自1954年和1963年以来，¹³⁷Cs的堆积速率分别约为0.15至约0.37厘米/年。来自²¹⁰Pb、¹³⁷Cs年代学的沉积物堆积速率以及X射线照片中相对沉积和生物扰动的指标表明，自20世纪中叶以来，弗吉尼亚号附近的沉积物堆积速率已经下降。这可能是由于包含沉船的泥流舌状体数十年来的下坡物质搬运以及密西西比河向近海输送的沉积物供应减少所致。根据岩芯特性和已公布的舌状体平流速率得出的弗吉尼亚舌状体的物质搬运计算表明，下坡物质搬运远高于密西西比河的沉积物补给，这与近期关于三角洲后退的研究结果一致。