Towards a comprehensive understanding of Mississippi Delta subsidence

The Mississippi Delta is a classic example of a rapidly subsiding, transgressive coastal environment that suffers the consequences of accelerated relative sea-level (RSL) rise, shoreline erosion, and wetland loss. A flurry of recent studies, based on a variety of methods that often measure different components of land subsidence over different timescales, has led to considerable debate about the premier driving forces that cause subsidence in this area. Current estimates of subsidence rates sometimes differ by several orders of magnitude. The availability of a new Holocene RSL record from the Mississippi Delta based on compaction-free basal-peat data, combined with an analysis of deformation rates of a buried swamp surface, allows the relative subsidence contribution of tectonics, glacio-isostasy, and compaction to be determined. The similarity of the RSL record with a compilation of RSL data from the Caribbean suggests that tectonic subsidence rates (primarily due to sediment loading) in significant portions of the delta are likely on the order of ~0.1 mm/yr only. In contrast, it can be shown that compaction rates of Holocene strata can be as high as ~5 mm/yr over millennial timescales, and likely more over shorter time intervals. Thus, the exceptionally high rates of land surface subsidence due to compaction constitutes the premier candidate that can account for rates of RSL rise of up to 10 mm/yr as recorded by tide gauges. Finally, a case is made that glacio-isostatic adjustment (by means of forebulge collapse) is a significant factor along the U.S. Gulf Coast, operating at rates of about 0.5 mm/yr. The findings discussed here are compared with a variety of recently published subsidence data (continuous GPS records, InSAR data, compaction modeling results). It is shown that these data can be comfortably reconciled, and the incompatibility of data sets alluded to above may be more apparent than real.