THE MISSISSIPPI DELTA AS A NATURAL LABORATORY FOR HIGH-RESOLUTION SEA-LEVEL RESEARCH

The U.S. Gulf Coast in general, and the Mississippi Delta in particular, offer unique opportunities to study Holocene relative sea-level (RSL) changes due to the low tidal range, favorable oceanographic conditions, and the widespread presence of salt-marsh peat encroaching on a consolidated, mostly Pleistocene basement. An overview is presented of investigations carried out over the past six years that have led to the following findings: [1] Basal-peat records that cover most of the Holocene show a negative exponential trend of RSL rise suggestive of glacio-isotatic adjustment of this area (forebulge collapse associated with the melting of the Laurentide Ice Sheet). These RSL records show a striking similarity with recent compilations from the Caribbean, suggesting surprisingly slow tectonic subsidence rates. These observations are difficult to reconcile with indicators for higher than present sea levels elsewhere along the Gulf Coast during the Middle Holocene. [2] Recent investigations have provided the first sea-level record that can be tied to the 8.2 ka cooling event, reflected by an abrupt rise of sea level around this time. However, this rise is likely to have been less than 1.2 m, thus constraining the amount of meltwater that was released into the North Atlantic Ocean during the final demise of Lake Agassiz. [3] A new RSL record for the time period 600 to 1600 AD based on 27 sea-level index points suggests that dm-scale fluctuations may have been superimposed on the overall rising trend. These fluctuations suggest a striking correlation with paleotemperature records for the Northern Hemisphere, featuring an acceleration of RSL rise during the Medieval Warm Period followed by deceleration during the Little Ice Age.