IS IT A DATE? THE SANGAMONIAN INTERGLACIAL DEPOSITS AT HOPWOOD FARM, IL

Few Quaternary geochronological methods can reliably date sites older than 50 ka. 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 Illinois. Although the Sangamonian units at Hopwood Farm did exceed the ¹⁴C dating limit, nonetheless over 25 years, eight different absolute dating methods were attempted to date these deposits at Hopwood Farm, without reaching a consensus about their age: Amino acid racemization (AAR) analyses were used on molluscs twice. Tooth enamel and gar scales were analyzed by standard electron spin resonance (ESR), while isochron analyses were tested on tooth enamel. On tooth enamel and dentine, ²³⁰Th/²³⁴U, ²³¹Pa/²³⁵U, coupled ESR-²³⁰Th/²³⁴U and coupled ESR-²³¹Pa/²³⁵U dating were tried. Each method gave slightly, or sometimes drastically, different ages, ranging from 11 to 180 ka. Meanwhile, pollen analyses and the ostracode fauna had suggested that Unit 3 had represented a very warm period, likely correlated with the Sangamonian Interglacial period. To solve the controversy, 14 mollusc samples newly collected with associated sediment from three depths at Hopwood Farm were dated by standard ESR in 2013. 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 101.8 ± 6.8 ka to 90.4 ± 5.5 ka, which correlates with Marine (Oxygen) Isotope Stage (MIS) 5d-c. The mollusc ages from the study agree well with the ESR LU ages on tooth enamel and gar scales, and with coupled ESR-²³⁰Th/²³⁴U ages on enamel.  If not coupled with ESR, ²³⁰Th/²³⁴U and ²³¹Pa/²³⁵U dates did not provide reliable dates for the Hopwood teeth, due to significant secondary U uptake. The newest AAR results also yielded consistent ages. The excellent agreement among these four chronometers suggests that the age for the Hopwood Farm deposits has finally been settled. Therefore, the Sangamonian Interglacial deposits at Hopwood Farm correlate with MIS 5, and also with the Eemian in Europe.