HOLOCENE LOESS OF ILLINOIS REVEALED THROUGH OSL DATING

In Illinois most uplands are blanketed by late-Wisconsin Peoria Loess. The conventional wisdom is that little or no dust accumulation occurred during the Holocene (11.7 ka to present, ka is thousand years ago), because the retreating Laurentide Ice Sheet was no longer providing silt-sized particles as a regional loess source. In the recent effort to search Holocene-aged loess deposits in Illinois, we applied optically stimulated luminescence (OSL) dating to several of the thickest loess deposits in Illinois to determine whether any Holocene loess was preserved. These locations include three loess bluffs along the Mississippi River in westernmost Illinois, and two loess bluffs downwind of the Havana Lowland dunefield along the Sangamon River in central Illinois. Five OSL ages from the uppermost of the two sections in central Illinois (depths from 0.4-1.3 m from the ground surface) range from 16 to 23 ka, indicating that no dust had accumulated in central Illinois during the Holocene. In contrast, of eleven OSL ages from four sites (depths from 0.3 to 1 m) along the Mississippi River, nine ages fall within the Holocene, e.g. 3.5, 7.4, 10.2 ka. It is possible that some of these Holocene ages may be adversely affected by bioturbation or human activity, and in other studies these post-depositional processes have resulted in OSL ages that are younger than the sample’s burial ages. But two observations suggest that these ages most likely reflect their depositional time. First, multiple ages taken from the same site are in stratigraphic order, where the Holocene ages are found above Pleistocene loess, suggesting a continuation of dust deposition from the Pleistocene to the Holocene. Second, the age dated to16 ka at the depth of 0.4 m suggests bioturbation near the ground surface is insignificant in loess deposits. Therefore, these OSL ages and stratigraphic succession suggest loess was deposited during the Holocene in Illinois along the Mississippi River, with thicknesses that range from 0.5 to 1 m. Holocene loess provides an important archive on the region’s climate history, which helps understand the evolution of these landscapes over the last 12 ka.