The Influence of Sediment Isostatic Adjustment on Sea Level Change and Land Motion Along the US Gulf Coast

Sea level rise presents a hazard for coastal populations, and the Mississippi Delta (MD) is a region particularly at risk due to the high rates of land subsidence. We apply a gravitationally self-consistent model of glacial and sediment isostatic adjustment (SIA) along with a realistic sediment load reconstruction in this region for the first time to determine isostatic contributions to relative sea level (RSL) and land motion. We determine optimal model parameters (Earth rheology and ice history) using a new high-quality compaction-free sea level indicator database. Using the optimal model parameters, we show that SIA can lower predicted RSL in the MD area by several meters over the Holocene and so should be taken into account when modeling these data. We compare modeled contemporary rates of vertical land motion with those inferred using GPS. This comparison indicates that isostatic processes can explain the majority of the observed vertical land motion north of latitude 30.7 degrees N, where subsidence rates average about 1mm/yr; however, subsidence south of this latitude shows large data-model discrepancies of greater than 3mm/yr, indicating the importance of nonisostatic processes. This discrepancy extends to contemporary RSL change, where we find that the SIA contribution in the Delta is on the order of 10(-1)mm/yr. We provide estimates of the isostatic contributions to 20th and 21st century sea level rates at Gulf Coast Permanent Service for Mean Sea Level tide gauge locations as well as vertical and horizontal land motion at GPS station locations near the MD. Plain Language Summary We have modeled solid Earth deformation and sea level change in response to past ice-ocean and ocean loading as well as sediment loading in the Mississippi Delta region. We find that sea level change and Earth deformation related to sediment loading is nonnegligible, but that it is a small signal when compared to other processes contributing to land subsidence (e.g., sediment compaction). We estimate part of the compaction signal by comparing our modeled results to GPS records and find that the Earth deformation component of the vertical motion recorded at GPS stations is roughly a tenth that of compaction.