USING STRATIGRAPHY AND OPTICALLY STIMULATED LUMINESCENCE DATING TO QUANTIFY SUBSIDENCE IN AN ABANDONED SUBDELTA

Understanding the rates and drivers of subsidence in deltas is essential to managing land in these naturally vulnerable areas. Faulting, compaction of Holocene deposits, and anthropogenic activities such as fluid withdrawal are known drivers of rapid subsidence in the Mississippi Delta, however their relative contributions are debated.

Here we present rates of subsidence of the transition from mouthbar sand to overbank mud at 1-8 m below the surface along lower Bayou Lafourche, an abandoned Mississippi River course. This approach potentially captures contributions of sediment compaction below the stratigraphic boundary, of subsurface fluid withdrawal, and of faulting or local tectonic processes. The contribution of GIA is removed by subtracting a relative sea-level rise term, calculated using the depth occurrence of basal peat that formed in the Mississippi Delta during Lafourche activity. We find that rates, averaged over 600 to 1500 yrs, are generally <2 mm/yr. Rates decrease coastward and are positively correlated with the thickness of the overbank mud, but not the thickness of underlying Holocene strata. This suggests that the weight of the overburden exerts a first-order control on the measured subsidence, and the elevation change we observe is largely due to compaction. Previous studies have demonstrated that load-driven compaction is highest at the onset of loading and decreases exponentially with time. This suggests that present-day rates in the unit we observe are likely lower than the millennial to centennial average produced by our method, because sedimentation in Lafourche largely ceased 600 years ago. Two faults crosscut our study area; the mouthbar sand to overbank mud transition is not offset by the faults. This indicates that these faults do not play a major role in the high surface subsidence rates observed in our study area.

The strength of the study is that it offers a new approach to measure subsidence rates that is potentially applicable to a large portion of a delta. However, our measurement did not capture present-day subsidence rates and elevation changes in the shallowest Holocene sediments. A comprehensive assessment of Mississippi Delta subsidence must incorporate data produced with different measurement techniques and record durations, to parse out the main contributors to subsidence.