OPTICALLY STIMULATED LUMINESCENCE (OSL) DATING OF GREAT LAKES SHORELINES: EVALUATING METHODOLOGICAL ACCURACY FOR PAST 4000 YEARS FOR LAKE SUPERIOR, USA

Raised strandplains commonly occur in embayments along Great Lakes coastlines, but in Lake Superior these records of late Holocene lake-level variations remain largely undeciphered because of chronologic limitations. We report the application of OSL dating, specifically single aliquot regeneration (SAR) protocols, to yield ages for beach-ridge sequences in Tahquamenon and Grand Traverse Bays, MI. AMS radiocarbon ages on plant macrofossils from basal peats in swales between beach ridges provide independent chronologic control. However, the accumulation of organics in swales is contingent on local hydrology, which is sensitive to climate variability. Direct OSL dating of beach-ridge sediment provides an alternative means of estimating the age of past shorelines.

SAR-OSL ages were determined on 150-250mm quartz grains extracted from littoral sediments. Thirty individual SAR ages were calculated for each sample and then averaged to yield a final age. Intersample age distribution was normal, though some samples showed positive skewing which reduced precision to ~10%. Samples from modern foreshore, backshore, and dune environments gave SAR ages between 170 and 50 yrs, which indicates that OSL can provide century-scale resolution for dating Holocene raised sediments.

Beach ridges at ~1 m above current lake level at Tahquamenon and Grand Traverse Bays yield ages of c. 900 and 500 years respectively, which are consistent with measured rates of isostasy. OSL dates of the lakeward 23 ridges at Tahquamenon Bay between 0.9 and 2 ka, which is ~1 ka younger than corresponding 14C ages on detrital organics as well as pollen data. The OSL chronology suggests a ca. 2 ka discontinuity in strandline formation, though not evident in foreshore elevations or strandline orientation, between ridges 27 and 37 at Tahquamenon Bay. OSL age estimates at the Grand Traverse Bay sequence post-date the high Algoma stand, ca. 3 ka, in contrast to AMS dates. Our research explores potential reasons for contrasting ages between OSL, AMS and bulk sediment samples as well as methods for resolving the OSL chronology of strandplain development. Our results indicate that OSL has potential to provide reliable ages for raised beach sequences and may be used to evaluate rates of glacioisostacy, lake level change and progradation rates in the Great Lakes.