Late Pleistocene luminescence chronology of loess deposition in the Missouri and Mississippi river valleys, United States
The loess stratigraphy of the mid-continental U.S. is an important proxy record for the advance of the North American ice sheet into the catchment of the Mississippi, Missouri and Ohio rivers. One of the most outstanding problems is deciphering the age of the loess deposits in this area during the Late Pleistocene. We used multiple-aliquot additive dose procedures under infrared stimulated luminescence (IRSL) and single aliquot regeneration (SAR) protocols to resolve ages of loess at the Loveland Silt type locality, Iowa, the Pleasant Grove School section, Illinois, and the Bonfils Quarry section, Missouri. Radiocarbon dated levels in the Peoria Loess and Roxana Silt were used to test the accuracy of the IRSL and SAR methods. SAR on polymineral and quartz extracts yielded underestimates in age by 20–55%, whereas IRSL gave concordant ages to the 14C control and was used to date the loess sequences. The oldest loess, the Bonfils Silt, gave IRSL ages between 159 and 264 ka and are considered non-finite estimates because the luminescence response was near saturation with additive β dose. The Loveland Silt at the type locality exhibited considerable luminescence in growth with additive β dose and yielded the mean age of 159±14 ka (n=4). The Teneriffe Silt gave a mean IRSL age of 93±5 ka (n=4) which, overlaps at two σ with a previous TL age and indicates deposition sometime between 100 and 80 ka. The Roxana Silt yielded stratigraphically consistent IRSL ages between ca. 60 and 30 ka. Luminescence and radiocarbon dating indicate that there were four distinct periods of loess deposition in the Mississippi and Missouri river basins at ca. 180–140 ka, 100–80 ka, 60–30 ka, and 25–12 ka which are concordant with periods of meltwater input to the Gulf of Mexico, another proxy of glaciation in mid-continental North America.