Prospects for Complete Middle Pleistocene Loess Records in Interior Alaska: A Role for Tephrochronology

Loess records in unglaciated Yukon and Alaska (eastern Beringia) are exceptional repositories for paleoenvironmental studies. The volcanic ash (tephra) beds found within the loess provide a means to date and correlate these deposits across this region. However, the middle Pleistocene (~780-130 ka) is poorly represented and/or has not been fully recognized at most previously examined sites. This is problematic because important events took place in the middle Pleistocene, including the transition from 40 to 100 ka interglacial-glacial cycles, the evolution and dispersion of steppe fauna, and interglacials that are thought to have been longer and warmer than the Holocene. However, studies at several sites in the interior of Alaska in recent years demonstrate that middle Pleistocene loess deposits are widespread across the interior of Alaska, some of which are relatively continuous. Here we focus on loess and tephra exposures at Gold Hill (<70 ka to ~3 Ma) near Fairbanks, the Palisades (<125 ka to >2 Ma) in west-central Alaska, and Birch Creek (<125 to >220 ka) and Chester Bluff (~70 to 780 ka) in east-central Alaska. Multiple tephra beds are present in these sections, and allow correlation of sites to one another, strengthening their respective chronologies. The tephra beds also highlight unconformities, which are common in loess deposits but often difficult to identify by lithostratigraphy alone. The improved chronologic control will allow more robust interpretation of high-resolution paleoenvironmental proxy records from these sites, including a 5-cm-resolution magnetic susceptibility profile through ~30 m of Gold Hill loess, from the ~1 Ma old AT tephra to several metres above the ~80 ka VT tephra. Dated tephra beds present in this sequence, such as GI (~560 ka), HP (~610 ka) and SP (~870 ka), provide critical chronostratigraphic control for this magnetic susceptibility record.