Establishment of a Subsidence Superstation in the Mississippi Delta: Integrating sediment core, SET, GPS and vertical strainmeter data to understand subsidence

There is a great need for coordinated efforts to monitor and better understand subsidence rates in low-elevation coastal zones by integrating different, complementary techniques at carefully selected sites. We present recent efforts to establish a subsidence superstation in the Mississippi Delta. The site is 2 km from the river near Myrtle Grove, Louisiana, at a CRMS (Coastwide Reference Monitoring System) site. The CRMS site consists of a surface elevation table (SET) and marker horizon established in 2008. The surface elevation relative to a rod driven to refusal (26 m) and the sedimentation above the marker horizon is measured semiannually. Adjacent to this site we have added three borehole optical fiber strainmeters that have been providing continuous records of displacement between the near-surface and depths of 10, 26, and 42 m. The instruments provide unprecedented resolution for compaction studies (see Hatfield et al. abstract). We regularly record teleseismic events with amplitudes <1 μm. The records also show a number of days-long compaction and rebound events of less than 1 mm, resulting from changes in the weather and water level. We have attached GPS to each of the wells. For the deepest well, the GPS is anchored to the bottom of the well with the base of the optical strainmeter. For the other two wells, the GPS is anchored to the upper casing of the well. While drilling the wells, a 5" diameter continuous core was collected reaching the Pleistocene boundary at 37 m depth (see Bridgeman et al. abstract). The silty uppermost 10 m, comprised of proximal overbank deposits, reveal up to 5-6 m of subsidence over the past 3000 years. In contrast, the surficial sediments ( 70 cm) are almost entirely organic matter and show little subsidence. The SET shows only 0.4 mm/yr for a 7.4 yr time window. Over the first year, the strainmeters show no long-term compaction or extension greater than ± 0.5 mm. Precise processing of the available GPS data indicates the sites subside at subtly different rates, but a longer time series will be required to confirm this result. The current low-density organic deposition is not driving significant compaction, but this may change when the planned nearby Mid-Barataria diversion becomes operational. We may see significant change in compaction once clastic sediment fluxes increase after the diversion.