Theory and Observation of Postglacial Sediment - Ocean Loading and Geoid Variability: Application to Subsidence in the Gulf of Mexico and other River Delta Systems and Enclosed Basins (Invited)

The causes of multi-decadal coastal subsidence are numerous. A host of causal relationships between anthropogenic activity and coastal subsidence and erosion are both compelling and well understood. A short list of these processes should include fluid extraction, reservoir compaction, fluid induced motion of shallow growth faults, decomposition of organic sediments (left in the wake of major water diversion projects) and sediment empoundment and/or diversion. Among natural causes are long-term aseismic vertical tectonics, large earthquakes, volume change of the ocean, changes in wind direction and strength (Fukumori et al. J. Phys. Oceanography, 37, 338-358, 2007) and global (long wavelength) glacial isostatic crustal and geoid change. We have determined that under some special circumstances sediment loading may also have a substantial impact on the active subsidence of the crust and mantle. Those special circumstances apply near river delta systems capable of large changes in volume and/or position of deposition, for these circumstances are precisely what is required to drive the gravitationally-layered, hydrostatically pre-stressed viscoelastic Earth away from gravitational equilibrium. Sediment-load-induced subsidence occurs over horizontal length scales, λ, comparable to thickness of the lithosphere: λ ≥ 40-60 km and has amplitudes ranging from of 0.5 to 8 mm/yr (Ivins et al. Geophys. Res. Lett., 34, 2007; Syvitski Sustainability Science, 3, 23-32, 2008). Here we discuss the theory, the necessary conditions for, and the present-day predicted amplitudes that apply to the cases of the Atchafalya -Mississippi River Delta system, the Yangzte River Delta, the Danube River Delta Plain in the Black Sea and the massive glaciomarine sediments of the Weddell Sea. In the case of the Gulf Coast in the US a variety of subsidence mechanisms operate simultaneously. We attempt to provide a model-based template for delineating the optimum terrestrial and space-based data sets that are best suited to ferreting out competing processes that cause present-day and past subsidence.