Ma. Allison et al., Development and reworking of a seasonal flood deposit on the inner continental shelf off the Atchafalaya River, CONT SHELF, 20(16), 2000, pp. 2267-2294
Sediment cores and water column measurements of suspended sediment and flow
conditions were taken on the continental shelf off the Atchafalaya River i
n Louisiana to examine the development and reworking of a seabed flood laye
r with seasonal variations in river discharge and hydrodynamics. Five stati
ons 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 fre
shwater input and wave energy conditions, Downcore profiles of the short ha
lf-life (53 d) cosmogenic radiotracer Be-7 showed a three to fivefold incre
ase 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 Be-7 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. Pb-210
and Cs-137 Sediment profiles indicate that this seasonal deposit is two to
six times the long-term (e.g., decadal) accumulation at these sites, Passag
e of cold fronts on 3-7 d timescales interrupts the formation of these floo
d 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 s
tations (similar to 20 m water depth) show only a small increase in deposit
ion during the high Atchafalaya discharge period. Redistribution of sedimen
t from shallower parts of the shelf during the remainder of the year is lik
ely a major supplier to these areas. A station east of the Atchafalaya mout
h exhibits no correlation with discharge and no lone-term accumulation, ind
icating minimal influence from the Mississippi discharge 150 km to the east
. (C) 2000 Elsevier Science Ltd. All rights reserved.