APPLICATION OF OSL DATING TO GLACIAL DEPOSITS IN SOUTHERN MASSACHUSETTS: REFINING THE CHRONOLOGY AND ADDRESSING QUESTIONS RELATED TO SOLAR RESETTING IN GLACIAL ENVIRONMENTS

The late Wisconsin advance and retreat of the southeastern margin of the Laurentide ice sheet created many iconic ice marginal features such as moraines, outwash plains, and glacial-lake deposits in southern New England. Accurate dating of these deposits is key to reconstructing the glacial and deglacial history of the region. However, due to limited organic materials for radiocarbon dating in some locations, intricate and fluctuating variables associated with ¹⁰Be dating of boulders, and lack of correlateable varves, the timing of ice-marginal positions and rates of ice-front recession are not well constrained.

In this study we implemented use of optically stimulated luminescence (OSL) dating on quartz sand-sized grains from proglacial deposits that could be linked to ice-margin positions in this region. Samples from ice-contact heads of outwash on Nantucket and Martha’s Vineyard yielded very scattered probability distributions of equivalent doses, as expected. Minimum-age models using the OSL equivalent doses produced OSL ages that were older than the anticipated ages of these deposits. Similar older ages were calculated from local ice-contact outwash on the Vineyard even when using central-age and minimum-age models. Although a sample from a mid position in the Mashpee plain of Cape Cod yielded a uniform distribution and age equal to the expected age, the sample of sand from beneath thick diamicton at the surface of the Buzzard’s Bay moraine had an age that is too old, even though the equivalent dose distribution indicated the grains were more uniformly bleached. All of the erroneously old ages of ice-margin deposits can be attributed to incomplete and highly varied bleaching during transport and deposition in ice-contact environments.

In contrast, deltaic topset beds in the Connecticut River valley gave ages that are younger than correlated ice-retreat ages based on varve correlations. These ages may reflect near-surface mixing of sediments or variables associated with ground water history or other dose-related factors. While pro-glacial deposits are not ideal for complete solar resetting of the luminescence signal, results indicate that with careful sample site selection, OSL dating provides reliable and reproducible results for these geomorphic features and depositional settings.