CONTINUOUS HOLOCENE RELATIVE SEA-LEVEL RISE ALONG THE CENTRAL US GULF COAST: IMPLICATIONS FOR MISSISSIPPI DELTA SUBSIDENCE RATES

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 signficance 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. Our 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, operating at a rate of ~0.45 mm/yr. Nevertheless, a ~0.15 mm/yr differential rate of subsidence between the Chenier Plain and key portions of the Mississippi Delta exists, and the long-term lithospheric subsidence rate underneath the New Orleans metropolitan area is < ~0.25 mm/yr. This shows that while the deltaic sediment loading effect is real, it is about an order of magnitude smaller than recent studies have postulated.