Quantifying Holocene lithospheric subsidence rates underneath the Mississippi Delta

The pattern of Holocene relative sea-level (RSL) change on the US Gulf Coast has been a long-standing subject of debate, featuring opposing scenarios of continuous submergence versus one or more Holocene RSL highstands. The significance of this debate is that the relative role of eustasy, glacial isostatic adjustment (GIA), and lithospheric flexural subsidence associated with Mississippi Delta sediment loading remains unresolved. Here we present a new RSL record from the Louisiana Chenier Plain, > 100 km west of the Mississippi Delta margin, based on AMS 14C dated marsh basal peat. This enables us to provide – for the first time – constraints on Holocene lithospheric subsidence rates underneath one of the worldʹs major deltas. Our new record conclusively shows that no middle Holocene RSL highstands occurred on the central US Gulf Coast. Rather, it exhibits a pattern of progressive RSL rise comparable to that from the Mississippi Delta, suggesting that long-wavelength GIA is a dominant deformational process driving lithospheric subsidence in the entire region by means of forebulge collapse. Nevertheless, a 0.15 ± 0.07 mm/yr differential rate of subsidence between the Chenier Plain and key portions of the Mississippi Delta (including the New Orleans metropolitan area) exists. This shows that while the sediment loading effect is real, it is about an order of magnitude smaller than recent studies have postulated.