Seasonal storage of fine-grained sediment in the lowermost Mississippi and Atchafalaya Rivers and an analysis of associated terrestrial organic carbon

Geophysical and geological data collected during cruises conducted on the lowermost Mississippi and Atchafalaya Rivers from 1999 through 2004 investigated the seasonal storage patterns of fine-grained sediment and the associated terrestrial organic carbon (TOC). We found that in the estuarine section of the Mississippi River, storage was dominated by estuarine trapping associated with salt-water wedge intrusion, with rapid rates of accumulation (mm-cm/day), whereas the Atchafalaya showed little wedge intrusion or storage due to the presence of un-dredged river-mouth bars. Above the estuarine region, bathymetric trapping was observed as the major mechanism of seasonal sediment storage, and as a result of the spring freshet, up to 40% of the stored mass was eroded. As a result of total confinement of the river by a complete levee system, channel down-cutting was measured below New Orleans. The layers being eroded are peats yielded radiocarbon ages of roughly 2100--4200 yBP, and are therefore identified as a previously unknown source of old organic carbon being supplied to the Gulf of Mexico. Finally, we investigated the lignin characteristics of the TOC being transported in association with the sand dominated bed-load as compared to the mud dominated suspended load. We found a statistically significant difference between the two lignin populations, as a result of hydrodynamic sorting in the high-energy Mississippi, while samples in the lower energy Atchafalaya showed a more mixed signal as a result of no sorting.