The Ups and Downs of Mississippi Delta

Land-surface dynamics in the Mississippi delta region are linked to vertical motion of the landscape, Gulf of Mexico (GOM) sea-level change, and rates of deposition. This paper introduces a new element to vertical motion, a cyclical flexural uplift and subsidence due to sediment unloading and loading at glacial-interglacial timescales. During the last glacial period, when sea level was low, meltwater discharge drove incision of the lower Mississippi valley, with valley filling and delta construction during Holocene sea-level rise. 1d steady-state and 3d visco-elastic modeling shows that sediment volumes removed and replaced were sufficient to induce flexural isostatic uplift of >9 m along valley margins, followed by subsidence of the same magnitude, with effects dissipating only over distances of >100–150 km along the GOM coast.

Recognition of cyclical uplift and subsidence refutes recent widely publicized interpretations of delta stability, and suggests late Holocene relative sea-level curves from the delta region may contain no sea-level signal at all. Moreover, our modeling does not verify Holocene sea-level curves from other GOM coast locations, and detailed study of sea-level change at varying distances from the delta is necessary to test isostatic models. However, our modeling shows that records of sea-level change should vary alongshore to the extent suggested by published data due to the Mississippi load anomaly. More broadly, valley incision and filling is a common fluvial response to glacio-eustasy, and cyclical uplift and subsidence should be common to alluvial-deltaic systems elsewhere. The magnitude and timing will reflect the details of valley unloading and loading history. For the Mississippi, the magnitude and timing of flexural uplift and subsidence in the delta region is directly coupled to the response of the continental-scale Mississippi sediment dispersal system to glaciation, deglaciation and meltwater routing, and global sea-level change.