Development and reworking of a seasonal flood deposit on the inner continental shelf off the Atchafalaya River

Abstract

Sediment cores and water column measurements of suspended sediment and flow conditions were taken on the continental shelf off the Atchafalaya River in Louisiana to examine the development and reworking of a seabed flood layer with seasonal variations in river discharge and hydrodynamics. Five stations in water depths of 5–23 m were occupied on the Atchafalaya inner shelf on four cruises from October 1997 to March 1999 representing a range of freshwater input and wave energy conditions. Downcore profiles of the short half-life (53 d) cosmogenic radiotracer 7Be showed a three to fivefold increase in seabed inventory and an increase in depth of penetration during the 1998 high Atchafalaya discharge period (April) at two inshore stations (5–7 m water depth). X-radiograph evidence of the absence of biological mixing at these sites suggests that the 7Be data is recording the deposition of a 1–3 cm thick annual flood deposit. The organic carbon contents and stable carbon isotopic compositions of this flood deposit are distinct and reflect the increased terrestrial influence of the riverine sediment flux. 210Pb and 137Cs sediment profiles indicate that this seasonal deposit is two to six times the long-term (e.g., decadal) accumulation at these sites. Passage of cold fronts on 3–7 d timescales interrupts the formation of these flood deposits, particularly during the rising to early high discharge period (December–March). The depth of sediment resuspension landward of 10 m water depth during these events may reach 1 cm and decreases offshore. Offshore stations (∼20 m water depth) show only a small increase in deposition during the high Atchafalaya discharge period. Redistribution of sediment from shallower parts of the shelf during the remainder of the year is likely a major supplier to these areas. A station east of the Atchafalaya mouth exhibits no correlation with discharge and no long-term accumulation, indicating minimal influence from the Mississippi discharge 150 km to the east.