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

Abstract

Large parts of the upper Midwest, USA were impacted by permafrost during the Last Glacial Maximum (LGM). Even though permafrost persisted as the Laurentide Ice Sheet began to recede, direct age control of this interval is largely lacking. To better temporally constrain the permafrost interval in western Wisconsin, we identified two sites, outside the Late Wisconsin (MIS 2) glacial limit, that contain relict, ice‐wedge pseudomorphs, initially interpreted to be sand wedges, hosted within well‐drained outwash deposits. The pre‐Wisconsin (>MIS 5) host material commonly displays up‐turned bedding near the contact with the wedges, indicative of well‐formed features. The wedges are filled with well‐sorted, gravel‐free, medium and fine sands, and lack evidence of post‐formational disturbance, pointing to an aeolian sand infill and confirming them as sand wedges. Ventifacts on nearby uplands attest to windy conditions here in the past. Optically stimulated luminescence (OSL) ages on five sand wedges indicate that they filled with sand between c. 19.3 and 18.3 ka at the southerly site and between c. 15.1 and 14.7 ka at the northerly site, which is closer to the LGM margin. Sand wedges at the latter site were wider and had more complex morphologies, possibly suggesting a longer interval of formation and/or more intense permafrost. We also examined a site along a ridge crest, between the two wedge sites, which displayed interbedded loess and sand, dated by OSL to 12.7 ka. Together, these results point to dry, cold, windy conditions in west‐central Wisconsin, within 100 km of the LGM limit. At this time, aeolian sands were being transported across a landscape with (at least scattered) permafrost. The OSL results suggest multiple phases, or perhaps time‐transgressive, sand‐wedge formation, associated with permafrost between c. 19 and 15 ka, with dry, windy (and likely, cold) conditions persisting until at least 12.7 ka.

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

“…The eight depositional OSL ages at ECC and in Mataitis (2020) also fit the chronological period we hypothesize as a regional‐scale sandy aeolian depositional pulse at CC and GC in southeastern Minnesota. In this portion of western Wisconsin, loess deposition was occurring between 18 and 13 ka (Schaetzl et al, 2014), with sediments derived from western sources (Schaetzl et al, 2021; Scull & Schaetzl, 2011). Early loess deposition may have covered much of the landscape, but currently loess deposits are not found in this area beyond the top of isolated bedrock uplands and/or east–southeastern sides of bedrock uplands.…”

“…14–10.6 ka (Rawling et al, 2008); and a ridge crest with intercalated loess and aeolian sands ca. 12.7 ka (Schaetzl et al, 2021). These data further support our hypothesis of a regional sandy aeolian pulse during the Late Pleistocene and Early Holocene that impacted southeastern Minnesota and western Wisconsin far beyond the ice margin.…”

“…This may represent a change to warmer, drier conditions (Zanner, 1999) during the interval of regional sandy aeolian deposition that waned ca. 13–9 ka, eventually quiescing due to increasing vegetation and soil moisture as climate changed (Schaetzl et al, 2021). Levels of pedogenesis at GC and ECC indicate stabilization and reduced sediment inputs during much of the Middle Holocene and Late Holocene.…”

“…GC is ~65 km east of the former ice margins of the Des Moines Lobe (Lusardi et al, 2011) and ECC is ~50 km southwest of the Chippewa Lobe terminal moraine of the Laurentide ice sheet (Figure 1) (Dalton et al, 2020). The locations of GC, ECC, and CC were not covered in glacial ice during the LGM but were likely influenced by permafrost conditions, possibly until ~14–12 ka (Clayton et al, 2001; Loope, 2012; Mason, 2015; Schaetzl et al, 2021; Zanner, 1999).…”

Testing the potential of using coarse-grain feldspars for post-IR IRSL dating of calcium sulphate-wedge growth in the Atacama Desert

“…Luminescence dating has been applied on fillings of relict sand wedges and composite-wedge pseudomorphs to obtain absolute ages and palaeoclimatic information of these periglacial geomorphological features (e.g. Porter et al, 2001;Bateman et al, 2008;Buylaert et al, 2009a;Schaetzl, et al, 2021).…”