OPTICALLY STIMULATED LUMINESCENCE DATING OF LATE HOLOCENE STRANDPLAIN SEQUENCES ADJACENT TO LAKE SUPERIOR

Chronologic control on the timing of beach ridge formation in strandplains adjacent to the Great Lakes has relied on 14C dating ages from basal organics in wetlands that commonly form between swales. We evaluate 14C ages from four strandplains adjacent to Lake Superior. Creation of a reliable 14C-based age model for strandplains in Lake Superior is confounded by (1) clustering conventional and AMS 14C ages within portions of the strandplains (2) scatter of 10’s to 1000’s of years in ages from nearby swales, and (3) a general lack of peat accumulation or preservation. Optically stimulated luminescence is introduced as an alternate geochronometer. The single aliquot regeneration (SAR) technique is used to date littoral sediments from beach ridges in the same strandplains previously dated by 14C methods. Beach ridges that yield SAR ages <2000 yr B.P. show general agreement with corresponding 14C ages on swale organics at Grand Traverse and Tahquamenon Bays, Michigan. Significant variability in 14C ages >2000 yr B.P. complicates comparison to SAR ages at these sites. SAR provides ages for ridges in strandplains at Au Train, Michigan and Batchawana Bay, Ontario where a lack of organic material precluded use of 14C dating methods. All sites consistently show a decrease in the rate of ridge formation ~1400 cal. yr. B.P., likely reflecting separation of Lake Superior from lakes Huron and Michigan. This study shows that SAR is a credible alternative to 14C methods for dating Great Lakes and other coastal strandplains.