Shell We Date? ESR Dating Sangamon Interglacial Episode Deposits at Hopwood Farm, IL

• Published in: Radiation protection dosimetry Vol. 172; no. 1-3; p. 283
• Main Authors: Blackwell, Bonnie A B, Kim, Danny M K, Curry, B Brandon, Grimley, David A, Blickstein, Joel I B, Skinner, Anne R
• Format: Journal Article
• Language: English
• Published: England 01.12.2016
• Subjects: Animals; Biological Assay - methods; Electron Spin Resonance Spectroscopy - methods; Geologic Sediments - analysis; Geologic Sediments - chemistry; Ice Cover - chemistry; Illinois; Mollusca - chemistry; Mollusca - classification; Mollusca - radiation effects; Radiometric Dating - methods; Reproducibility of Results; Sensitivity and Specificity; Species Specificity
• ISSN: 1742-3406
• DOI: 10.1093/rpd/ncw213

During the Sangamon Episode, North America occasionally experienced warm climates. At Hopwood Farm, IL, a small kettle lake filled with sediment after the Illinois Episode glaciers retreated from southern Illinois. To date those deposits, 14 mollusc samples newly collected with associated sediment from three depths at Hopwood Farm were dated by standard electron spin resonance (ESR) dating. ESR can date molluscs from ~0.5 ka to >2 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 all molluscs contained ≤0.6 ppm U, their ages do not depend on the assumed U uptake model. Using five different species, ESR analyses for 14 mollusc subsamples from Hopwood Farm showed that Unit 3, a layer rich in lacustrine molluscs, dates at 102 ± 7 ka to 90 ± 6 ka, which correlates with Marine (Oxygen) Isotope Stage 5c-b. Thus, the period with the highest non-arboreal pollen at Hopwood also correlates with the European Brørup, Dansgaard-Oeschger Event DO 23, a time period when climates were cooling and drying somewhat over the same period.