THE LOWER MISSISSIPPI RIVER DURING THE PAST GLACIAL-INTERGLACIAL CYCLE: UPSTREAM VS. DOWNSTREAM CONTROLS AND SOURCE-TO-SINK SEDIMENT FLUX

The Prairie Complex is a Pleistocene allostratigraphic unit along the borders of the Lower Mississippi Valley that features a number of river terraces with a floodplain morphology that resembles that of the modern, meandering Lower Mississippi River. Vigorous debate on the age of Prairie Complex strata has persisted for decades, and the lack of chronostratigraphic data has precluded any conclusive inferences about how the Lower Mississippi River responded to upstream (climate) and downstream (sea level) controls. This has been an obstacle to a comprehensive understanding of the role of the alluvial reach of this continental-scale sediment-dispersal system in a source-to-sink context.

Nearly 30 new OSL ages provide new constraints on the age of Prairie Complex strata and show that a large proportion of the preserved deposits formed during Marine Isotope Stage (MIS) 5, in particular during MIS 5a, about 80 kyr ago. Combined with a recently published chronology for the MIS 4-2 braided-stream surfaces in the Lower Mississippi Valley, a relatively complete and unprecedented record now exists for the Lower Mississippi River during the past glacial-interglacial cycle. This allows us to infer that the sea-level rise during MIS 5 led to widespread aggradation that extended at least 600 km inland. In addition, the new OSL chronology shows that the incision and floodplain abandonment, including the transformation from meandering to braided channel morphology, that occurred due to the sea-level fall at the MIS 5a/4 transition (associated with the growth of the Laurentide Ice Sheet) was relatively rapid and proceeded within about 10 kyr or less. In other words, this large river responded surprisingly rapidly to external forcing, faster than most theoretical studies have predicted. Finally, our findings demonstrate that considerable sediment storage occurred during periods of rising sea level and sea-level highstands, and that the transfer of sediment to the deep-marine realm must have increased substantially during the period around the Last Glacial Maximum, when sea level dropped below the shelf edge.