Anatomy of Mississippi Delta growth and its implications for coastal restoration.

The decline of several of the world's largest deltas has spurred interest in expensive coastal restoration projects to make these economically and ecologically vital regions more sustainable. The success of these projects depends, in part, on our understanding of how delta plains evolve over time scales longer than the instrumental record. Building on a new set of optically stimulated luminescence ages, we demonstrate that a large portion (~10,000 km2) of the late Holocene river-dominated Mississippi Delta grew in a radially symmetric fashion for almost a millennium before abandonment. Sediment was dispersed by deltaic distributaries that formed by means of bifurcations at the coeval shoreline and remained active throughout the life span of this landform. Progradation rates (100 to 150 m/year) were surprisingly constant, producing 6 to 8 km2 of new land per year. This shows that robust rates of land building were sustained under preindustrial conditions. However, these rates are several times lower than rates of land loss over the past century, indicating that only a small portion of the Mississippi Delta may be sustainable in a future world with accelerated sea-level rise.

Denudation of the continental shelf between Britain and France at the glacial-interglacial timescale.

The erosional morphology preserved at the sea bed in the eastern English Channel dominantly records denudation of the continental shelf by fluvial processes over multiple glacial-interglacial sea-level cycles rather than by catastrophic flooding through the Straits of Dover during the mid-Quaternary. Here, through the integration of multibeam bathymetry and shallow sub-bottom 2D seismic reflection profiles calibrated with vibrocore records, the first stratigraphic model of erosion and deposition on the eastern English Channel continental shelf is presented. Published Optical Stimulated Luminescence (OSL) and 14C ages were used to chronometrically constrain the stratigraphy and allow correlation of the continental shelf record with major climatic/sea-level periods. Five major erosion surfaces overlain by discrete sediment packages have been identified. The continental shelf in the eastern English Channel preserves a record of processes operating from Marine Isotope Stage (MIS) 6 to MIS 1. Planar and channelised erosion surfaces were formed by fluvial incision during lowstands or relative sea-level fall. The depth and lateral extent of incision was partly conditioned by underlying geology (rock type and tectonic structure), climatic conditions and changes in water and sediment discharge coupled to ice sheet dynamics and the drainage configuration of major rivers in Northwest Europe. Evidence for major erosion during or prior to MIS 6 is preserved. Fluvial sediments of MIS 2 age were identified within the Northern Palaeovalley, providing insights into the scale of erosion by normal fluvial regimes. Seismic and sedimentary facies indicate that deposition predominantly occurred during transgression when accommodation was created in palaeovalleys to allow discrete sediment bodies to form. Sediment reworking over multiple sea-level cycles (Saalian-Eemian-early Weichselian) by fluvial, coastal and marine processes created a multi-lateral, multi-storey succession of palaeovalley-fills that are preserved as a strath terrace. The data presented here reveal a composite erosional and depositional record that has undergone a high degree of reworking over multiple sea-level cycles leading to the preferential preservation of sediments associated with the most recent glacial-interglacial period.