An evaluation of mobile mud dynamics in the Mississippi River deltaic region

Large rivers are the primary interface between terrestrial and ocean environments. A relatively small number of rivers account for a disproportionate amount of the freshwater and suspended materials that are delivered to the coastal ocean. Sediment delivery to these coastal systems plays a key role in the global carbon cycle since deltas and continental shelves are considered to be the main repositories of organic carbon in marine sediments. Particulate material in these environments are typically deposited and resuspended several times before permanent accumulation or transport off the shelf. This sediment cycling is an important component influencing biogeochemical processes that occur in coastal environments. During two cruises in April and October 2000 on the shelf adjacent to the Mississippi River, water and sediment samples were collected for analysis of suspended solids and particle reactive radionuclides ((210)Pb, (137)Cs, (7)Be and (234)Th) to evaluate the transport and fate of terrestrial and marine material. A comparison of the distribution of these tracers provides insight about the pathways and residence times of particulate materials on the shelf. Inventories of these short-lived radiotracers showed variations of more than two orders of magnitude, indicating dramatic variations in sediment deposition between sampling events. Short-lived radiotracers indicate that river-borne materials are transported less than similar to 30 km from the river mouth before initial deposition. However, seasonal variations in (7)Be and (137)Cs indicate significant remobilization of sediment and potential export of sediment out of the study area during the high energy (e.g., wind/wave) winter months. In addition, depth profiles of (7)Be and excess (234)Th indicate sediment deposition rates between 0.8 and 3.9 cm month(-1) (0.4-2.1 g cm(-2) month(-1)) at two shelf locations (near river and open shelf environments). These rates are much greater than those observed on decadal time scales (1.3-2.0 cm year(-1) or 0.6-1.5 g cm(-2) year(-1)) via (210)Pb at the same sites. This further substantiates active sediment reworking and potential export off the shelf as has been observed in other river-dominated ocean margins. (C) 2004 Elsevier B.V. All rights reserved.