Understanding subsidence in the Mississippi Delta region due to sediment, ice, and ocean loading: Insights from geophysical modeling

The processes responsible for land surface subsidence in the Mississippi Delta (MD) have been vigorously debated. Numerous studies have postulated a dominant role for isostatic subsidence associated with sediment loading. Previous computational modeling of present-day vertical land motion has been carried out in order to understand geodetic data. While the magnitudes of these measured rates have been reproduced, the model parameter values required have often been extreme and, in some cases, unrealistic. In contrast, subsidence rates in the MD on the 103 year timescale due to delta loading estimated from relative sea level reconstructions are an order of magnitude lower. In an attempt to resolve this conflict, a sensitivity analysis was carried out using a spherically symmetric viscoelastic solid Earth deformation model with sediment, ice, and ocean load histories. The model results were compared with geologic and geodetic observations that provide a record of vertical land motion over three distinctly different timescales (past 80 kyr, past 7 kyr, and past ~15 years). It was found that glacial isostatic adjustment is likely to be the dominant contributor to vertical motion of the Pleistocene and underlying basement. Present-day basement subsidence rates solely due to sediment loading are found to be less than ~0.5 mm yr−1. The analysis supports previous suggestions in the literature that Earth rheology parameters are time dependent. Specifically, the effective elastic thickness of the lithosphere may be <50 km on a 105 year timescale, but closer to 100 km over 103 to 104 year timescales.