Application of single-grain OSL dating to ice-proximal deposits, glacial Lake Benson, west-central Minnesota, USA

Abstract

a b s t r a c tGlacial Lake Benson formed in west-central Minnesota as the Des Moines lobe of the Laurentide ice sheet retreated north of a small moraine in the Minnesota River lowland. Although previous research has constrained the timing of glacial Lake Agassiz immediately to the north, little age control is available for the formation of glacial Lake Benson and ice-marginal positions to the south. In order to constrain the age of glacial Lake Benson and test the application of single-grain optically stimulated luminescence (OSL) dating to ice-marginal deposits, seven OSL samples were collected from a variety of depositional settings. These included deltaic deposits linked to specific lake levels, pro-glacial fluvial, ice-contact and supra-glacial deposits. Single-grain OSL results indicate evidence for incomplete resetting (partial bleaching) of the luminescence signal, as expected for glacial environments, and therefore ages were calculated using a minimum age model. OSL results constrain the timing of ice-margin retreat and lake formation to 14.4e14.8 ka. Analysis of single-grain equivalent dose distributions indicates that deposits created by glacial-dominated processes typically had higher over-dispersion (>50%) and greater positive skew (>0.9) than deposits originating from fluvial processes. These results suggest that water-lain deposits should be targeted for OSL sampling over those created by glacial processes when dating iceproximal settings.

Response to Comment on “Permanent human occupation of the central Tibetan Plateau in the early Holocene”

“…There is no scientific base for the suggestion of Zhang and Li to remove the high equivalent dose (De) values from the single-grain De distribution to reduce the overdispersion (OD) value and thus align our OSL age, which is based on single-grain measurements, with their multigrain age. The petrographic observations for sample CS-T-2 (composed of mostly subrounded to angular clasts and detrital grains) [ figure S5, D to F, of (3)] and our OD value of 37% (that would potentially be much higher if the Chusang quartz would reveal higher sensitivity) rather (and strongly) suggest that partial bleaching is compromising this sample and that a minimum age solution is appropriate (8)(9)(10)(11)(12)(13).…”

“…Interestingly, all samples yielded OSL ages of~21 thousand years. It is very likely that incomplete zeroing is affecting samples XZ-2 and 3 (as was the case for our sample CS-T-2), but with a multigrain approach (applied by Zhang and Li) as opposed to a singlegrain approach (applied by us), averaging effects in De determination will unavoidably result in age overestimation (8)(9)(10)(11)(12)(13)(14)(15). Zhang and Li argue that their 20 thousand year age must hold because sample XZ-1 was zeroed by heat, but they do not provide any proof for this assumption (neither thin-section observations nor a De versus depth profile for the hearth wall).…”

Morphology and stratigraphy of aeolian sand stringers in southeast Minnesota and western Wisconsin, USA

“…This later readvance of the Des Moines Lobe reached its furthest extent (near Des Moines, IA) ca. 18–17 ka (Dalton et al, 2020; Rittenour et al, 2015), or possibly as late as 16.2 cal kyr BP (Heath et al, 2018). The Des Moines Lobe retreated out of Iowa and south central Minnesota by ca.…”

“…A radiocarbon age of 14.6–13.8 cal kyr BP from wood found in proglacial lake sediments of Lake Minnesota, formed by the retreating Des Moines Lobe, also suggests ice was far removed from the area at the time the dated stratigraphic units at CC were deposited (Jennings et al, 2012). In addition, OSL ages from another Des Moines Lobe‐derived proglacial lake basin, Lake Benson, in west central Minnesota, indicate the Des Moines Lobe had retreated entirely from southern Minnesota by 14.8 ka (Rittenour et al, 2015).…”

Luminescence dating of sand wedges constrains the Late Wisconsin (MIS 2) permafrost interval in the upper Midwest, USA

“…1) at c. 16.5-17 ka. Ice remained on the Iowa landscape, but in reduced extent, until about 15 ka (Ruhe 1969;Clayton & Moran 1982;Hallberg & Kemmis 1986;Bettis et al 1996;Rittenour et al 2015;Heath et al 2018). The retreat chronology of the ice lobes that border the study area to the north, especially the Chippewa Lobe (Fig.…”