RECONCILING HOLOCENE SEA-LEVEL HISTORY ON THE US GULF COAST: IS THE MISSISSIPPI DELTA THE ROSETTA STONE?

Published Holocene relative sea-level (RSL) curves for the US Gulf Coast are in mutual conflict, with some characterized by a smooth RSL rise akin to widely accepted eustatic sea-level curves vs. others, including several recent ones, that are characterized by a conspicuous “stair-step” pattern with prolonged (millennium-scale) RSL stillstands alternating with rapid rises. In addition, recent work on the Texas Gulf Coast has revitalized the notion of a Middle Holocene RSL highstand, estimated at about 2 m above present mean sea level. An extensive sampling program in the Mississippi Delta (Louisiana) focuses on the collection of basal peats that accumulated during the initial transgression of the pre-existing, consolidated Pleistocene basement. We preferentially select plant macrofossils indicative of brackish to saline conditions from these basal peats and subject them to AMS 14C dating. The first data set of approximately 30 sea-level index points from a ~20 km2 study area in the eastern part of the delta provides conclusive evidence that RSL rise followed a smooth, negative exponential trend for the time interval 8000–3000 cal yr BP, thus invalidating the occurrence of RSL stillstands. Although a true Middle Holocene highstand never occurred in this subsiding setting, the high level of detail of our time series enables a rigorous test of this hypothesis. Correction of our data set for a subsidence rate of 1 m ka-1 (we assume a linear subsidence rate compared to the tectonically relatively stable adjacent Texas Gulf Coast) leads to sea levels of 1-2 m above present during the time interval 6000–4000 cal yr BP. However, this model also implies a RSL position near –2 m around 8000 cal yr BP, which is inconsistent both with data of this age from Texas, as well as with eustatic sea-level data. We therefore conclude that a Middle Holocene highstand for the US Gulf Coast is highly unlikely, and that the entire area is still responding glacio-isostatically, by means of forebulge collapse, to the melting of the Laurentide Ice Sheet. Our results demonstrate the power of continuous time series of RSL change from subsiding settings.