DOWNSTREAM PLUMBING OF THE LAURENTIDE ICE SHEET: MELTWATER DISCHARGE RECORDS FROM THE LOWER MISSISSIPPI VALLEY

Meltwater influx into ocean basins has influenced sea-surface temperatures, ocean circulation and global climate. Accurate reconstructions of the timing, volume and routing of meltwater are needed to improve models of deglacial climate and ice dynamics. The lower Mississippi River served as the ultimate southern drainage route for meltwater derived from the Laurentide Ice Sheet (LIS). As such, geomorphic landforms and sedimentary deposits in the lower Mississippi valley (LMV) act as a gage of the timing, and to a lesser extent, the magnitude of meltwater discharge.

During the LGM, and throughout much of the last glaciation, meltwater derived from the southern margin of the LIS and pro-glacial lakes was funneled into the LMV via the Missouri, upper Mississippi and Ohio Rivers. The timing and volume of meltwater discharge is recorded in a series of large (10s km wide by 100's km long) braid belts that range in age from MIS 4 to MIS2. The oldest braid belt (64-50 ka) records aggradation following incision from MIS 5a-4 sea level fall and the switch in channel morphology from meandering (MIS 5 Prairie Complex) to braided in response to initial MIS 4 glaciation of the upper catchment and influx of meltwater. Seven successively lower braid belts formed between 64ka and 11ka in response to fluctuations in meltwater and sediment discharge. Glacio-eustatic sea level controlled the elevation to which the river was graded, causing late-glacial braid belts to dip below the Holocene floodplain in the southern LMV. The switch to a meandering channel occurred in the earliest Holocene due to final meltwater withdrawal from the drainage basin. In addition to the braid-belt record, evidence for timing of meltwater discharge will be summarized from loess records from adjacent uplands and slackwater deposits in small tributary valleys. These terrestrial chronologies will then be compared to marine records of freshwater influx and sea-level rise from the Gulf of Mexico and elsewhere.