Rejuvenated marsh and bay-bottom accretion on the rapidly subsiding coastal plain of U.S. Gulf coast: a second-order effect of the emerging Atchafalaya delta

Sedimentation processes in marshes and bays under the influence of the emerging Atchafalaya delta are described. The Atchafalaya delta is a major geological event in the Holocene history of the Mississippi River delta system because it represents the initial stages of a new delta cycle. The delta has resulted from the capture of the Mississippi River flow by the hydraulically more efficient Atchafalaya River. Using ¹³⁷Cs and ²¹⁰Pb dating techniques, maximum sedimentation of delta-flanking environments was found to occur in the bay bottoms and marshes closest to the emerging delta. Marshes directly under the influence of the emerging delta were accreting at rates as great as 1·4 cm y⁻¹ with appreciable mineral sediment inputs. In addition to increasing shoreline progradation, the added sediment is providing nutrients for increased net plant productivity which provides the organic source needed for vertical marsh accretion. Results show that the major area of coastal progradation will be in the immediate vicinity of the delta and along down-drift coasts. Updrift marshes (East Terrebonne marshes) away from the delta are accreting at a slower rate with smaller mineral sediment input and a larger percentage of organic material. These marshes are likely to continue experiencing rapid rates of deterioration.