Middle Pleistocene age of the Nome River glaciation, northwestern Alaska

During the middle Pleistocene Nome River glaciation of northwestern Alaska, glaciers covered an area an order of magnitude more extensive than during any subsequent glacial intervals. The age of the Nome River glaciation is constrained by laser-fusion ⁴⁰Ar ³⁹Ar analyses of basaltic lava that overlies Nome River drift at Minnie Creek, central Seward Peninsula, that average 470,000 ± 190,000 yr (±1σ). Milligram-size subsamples of the lava were dated to identify and eliminate extraneous ⁴⁰Ar enrichments that rendered the mean of conventional KAr dates on larger bulk samples of the same flow too old (700,000 ± 570,000 yr). While the ⁴⁰Ar ³⁹Ar analyses provide a minimum limiting age for the Nome River glaciation, maximum ages are provided by a provisional KAr date on a basaltic lava flow that underlies the Nome River drift at nearby Lave Creek, by paleomagnetic determinations of the drift itself at and near the type locality, and by amino acid epimerization analysis of molluscan fossils from nearshore sediments of the Anvilian marine transgression that underlie Nome River drift on the coastal plain at Nome. Taken together, the new age data indicate that the glaciation took place between 580,000 and 280,000 yr ago. The altitude of the Anvilian deposits suggests that eustatic sea level during the Anvilian transgression rose at least as high as and probably higher than during the last interglacial transgression; by correlation with the marine oxygen-isotope record, the transgression probably dates to stage 11 at 410,000 yr, and the Nome River glaciation is younger still. Analyses of floor altitudes of presumed Nome River cirques indicate that the Nome River regional snowline depression was at least twice that of the maximum late Wisconsin. The cause of the enhanced snowline lowering appears to be related to greater availability of moisture in northwestern Alaska during the middle Pleistocene.