SHELL WE DATE? ESR DATING MOLLUSCS FROM THE SANGAMONIAN INTERGLACIAL DEPOSITS AT HOPWOOD FARM, IL

During the Sangamonian Interglacial, North America experienced unusually warm climates. At Hopwood Farm, IL, a small kettle lake filled with sediment after the Illinoian glaciers retreated from southern IL. The Hopwood Farm kettle, a lacustrine deposit ~ 6 m deep and ~ 1.5 km wide, sits south of the Wisconsinan ice margin on the Illinoian Till Plain in central Illinois. The kettle likely started as a permanent boreal forest lake, filling up during the late Middle Illinoian through the Sangamonian. After final desiccation, during which carbonate was deposited, Wisconsinan loess buried the lake. Today, the lake is gone, but the deposit contains lacustrine sediment with many late Illinoian and Sangamonian fauna.

Few geochronological methods can reliably date sites older than 50 ka. Since the Sangamonian at Hopwood Farm exceed the ¹⁴C dating limit, seven different absolute dating methods have been attempted at Hopwood Farm, without reaching a consensus about the age. For example, amino acid racemization (AAR) on molluscs, ²³⁰Th/²³⁴U on enamel and dentine, ²³¹Pa/²³⁵U on dentine, ESR on tooth enamel, plus coupled ESR-²³⁰Th/²³⁴U and coupled ESR-²³¹Pa/²³⁵U all yielded ages that did not agree well. Each method gave slightly, or drastically, different ages, ranging from 11 to 180 ka.

To solve the controversy, 14 mollusc samples newly collected with associated sediment from three depths at Hopwood Farm were dated by standard ESR. ESR can date molluscs from 0.5 ka to 2.6 Ma in age with 5-10% precision, by comparing the accumulated radiation dose with the total radiation dose rate from the mollusc and its environment. Because the molluscs had little U, their ages do not depend on the U uptake model. For Unit 3, the molluscs dated from 98-90 ± 6 ka, which correlates best with Marine Isotope Stage (MIS) 5c, or possibly, MIS 5b. These ESR ages also agreed well with the coupled ESR-²³⁰Th/²³⁴U ages on the teeth and the most recent AAR analyses on molluscs given their stratigraphic relationships.