BioDose
2015
Shellwedate?ESRdatingthesangamonianinterglacialdepositsat
HopwoodFarm,Illinois
Danny M.K. Kim1, Bonnie A.B. Blackwell1,2, B. Brandon Curry3, David A. Grimley3, Joel I.B. Blickstein1,
Anne R. Skinner1,2
1
ARFK Science Research Institute, Glenwood Landing, New York, USA
Williams College, Dept of Chemistry, Williamstown, Massachusetts, USA
3
Quaternary Geology Section, Illinois State Geological Survey, Urbana-Champaign, Illinois, USA
2
Study Goal: To understand how climate will change as global warming progresses, we must understand
changes after earlier glaciations. At Hopwood Farm, IL, the pollen in the deposits suggests that lake formed
after the glacier retreat 128 ka (ka = 1000 years ago). Other dating methods did not give reasonable ages.
This study used electron spin resonance (ESR). For Unit 3, the molluscs ranged from ~ 90-110 ka. ESR
provided dates consistent with the stratigraphy from the Sangamonian, a time when global temperatures
were warm. Studies like this provide us with examples to predict climate shifts.
Abstract: 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. To date those deposits over the last 25 years, tooth enamel and gar scales were analyzed
by electron spin resonance (ESR); 230Th/234U, 231Pa/235U, coupled ESR-230Th/234U and coupled ESR231Pa/235U dating were tried on tooth enamel and dentine; amino acid racemization (AAR) analyses were
used on molluscs twice. 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 510% 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 with Oxygen Isotope Stage (OIS)
5d, and agreed well stratigraphically with the coupled ESR-230Th/234U ages on the teeth and the most
recent AAR analyses on molluscs.
Conclusion: ESR analyses for 14 mollusc subsamples from Hopwood Farm showed that Unit 3 dates at 102
± 7 ka to 90 ± 5 ka. Unit 3 correlates with MIS 5d from 101.8 ± 6.8 ka to 90.4 ± 5.5 ka. Pollen analysis
suggests that, during MIS 5d, Hopwood Farm experienced high temperatures and higher precipitation.
Today, the area around Hopwood Farm experiences warm, wet summers and cool, dry winters. Now that 20
reliable dates exist, its climatic conditions can be used to better predict what central Illinois may experience
in the net few hundred years as climates continue to warm from rising anthropogenic CO2.
Acknowledgements: Jon A. Florentin, Williams College; Andrés Montoya, RFKSRI ; Israt Ahmed,
RFKSRI; Sarah Sass, RFKSRI; Vincent Feng, RFKSRI; Alice Pedruczny, McMaster U.; McMaster Reactor
Staff Funding: Illinois State Geological Survey, McMaster U. Nuclear Reactor, Williams College, RFK,
SRI, NSF, FLHS Science Dept.