Pleistocene glacial history of the southern Ahklun Mountains, southwestern Alaska: Soil-development, morphometric, and radiocarbon constraints

Four new AMS ¹⁴C ages, glacial mapping, and measurements of soil development, loess thickness, and moraine morphology constrain the age and extent of at least three Quaternary advances by outlet lobes of an ice cap over the Ahklun Mountains of southwestern Alaska. The relative-age data are from 107 sites correlated to 25 ice-marginal positions in the Kanektok, Goodnews, Togiak, and Kulukak River valleys and along the southeastern flank of the range. Radiocarbon ages provide minimum ages for six - and a maximum age for one - of the former ice margins. Soil and morphometric parameters subdivide the ice limits into three relative-age groups. One to three pre-Wisconsin advances, probably middle Pleistocene in age, are represented by drift with relatively thick B horizons (60 ± 5 cm, with Bt horizons), thick loess caps (80 ± 12 cm), and broad moraines (135 ± 134 m) with gentle side slopes (7 ± 5°). An extensive early Wisconsin (sensu lato, s.1.) advance, > 39.9 ka, and three associated stillstands or readvances are characterized by intermediate soil and morphometric parameters (Bw and weak Bt horizons with thicknesses of 40 ± 11 cm; loess thicknesses of 69 ± 46 cm; crest widths of 38 ± 13 m; and slope angles of 14 ± 4°). The maximum late Wisconsin advance, > 16.9 ka, and two readvances or stillstands are associated with thin (20 ± 5 cm), weakly to moderately developed Bw horizons, thin loess caps (28 ± 8 cm), narrow crest widths (28 ± 9 m), and steep slope angles (18 ± 3°). The data confirm that early-Wisconsin glaciers in southeast Beringia were much more extensive than late Wisconsin glaciers, which were apparently limited by availability of moisture.