The above noted changes in the nature of accumulating sediments indicate that after the start of the Younger Dryas, the southern route for Laurentide Ice Sheet meltwater was largely blocked. On the rare occasions it could flow southward, glacial meltwater flowed through Lake Agassiz and sometimes the Great Lakes to the Mississippi River. As the water moved through either Lake Agassiz or other proglacial lakes, they completely trapped and removed any glacial outwash and the older, reworked organic material and reworked nanofossils that the outwash contained. As a result, the sediment carried by the Mississippi River after the start of the Younger Dryas consisted of illite and chlorite enriched sediments from the Great Lakes region that lacked any reworked nanofossils. These changes argue that the superfloods of MWF-5 which fed Meltwater Pulse B are related to either rare periods of southerly discharge of meltwater through Lake Agassiz, nonglacial periods of climate-enhanced discharge within the Mississippi River Basin, or a combination of both.
