Deglacial Record in the Illinois River Valley Explains Asynchronous Phases of Meltwater Pulses and Clay Mineral Excursions in the Gulf of Mexico
Thursday, 18 December 2014
One prominent event of the Bølling/Allerød (B/A) interstadial was the large meltwater release to global oceans. The Laurentide Ice Sheet (LIS) is usually considered the main source. But, the large LIS meltwater discharge conflicts with the marine record showing an active North Atlantic meridional overturning circulation (AMOC) during the B/A interval. Continuous dune-lacustrine successions in the Illinois River Valley (IRV) have shown complete records of the last deglacial chronozones. Their grain-size distributions and accurate B/A age 14C dates of plant fossils from 15 m deep lacustrine sediment in the IRV suggest that most of the IRV and parts of the adjacent upland were inundated by water. The inundation was caused by a sediment dam interpreted to have been constructed and followed by a breach at the confluence of the Mississippi and Illinois Rivers during the B/A interval due to sediment mobilization by the large meltwater release. The grain size distributions correlate with meltwater pulses and mineralogical excursions in sediments from the Gulf of Mexico (GOM) very well. The blockage and release of illite and chlorite rich fine-grained sediments from the Lake Michigan basin changed the relative abundance of clay minerals and thus the ratio of smectite/(illite + chlorite) in the sediment of the GOM. This finding explains why the meltwater episodes from the LIS and the associated detrital discharges are not synchronous in the sediments in the GOM. The finding also ties meltwater pulses and associated detrital discharges in the GOM closely to the LIS discharges via the Mississippi River Valley on chronozonal scales. Three arguments can be made from this result: 1) unaffected AMOC during B/A interval resulted potentially from the hyperpycnal inflow into the GOM floor; 2) limited volume of the meltwater discharge did not significantly influence the AMOC; and 3) the freshwater input into the GOM from the LIS at this particular location did not significantly influence the AMOC.