CHEMICAL WEATHERING IN LOESS-DERIVED PALEOSOLS OF CENTRAL ALASKA: PALEOCLIMATIC IMPLICATIONS

Thick loess sequences containing paleosols are present in much of central Alaska, particularly near Fairbanks, where four sections were studied in detail. Paleosols can be recognized by morphological criteria such as color, texture and horizon sequences. Ages are uncertain, but based on the presence of tephras ("PH," <600 ka; Old Crow, 160-120 ka; Dome, >56 ka), radiocarbon dating, and 10-Be accumulation, it is likely that paleosols date to mid-Pleistocene (700-200 ka) interglacials, the last (~120 ka) interglacial, and mid-Wisconsin (60-30 ka) interstadials. Ratios of mobile (Na, Ca, K, Si) to immobile (Ti, Zr) elements indicate that paleosols are characterized by significant chemical weathering. Values of Na/Ti and Si/Ti are lower in paleosols compared to subjacent loess, indicating probable depletion of sodic plagioclase and hornblende. Values of K/Ti do not show such a systematic depletion in paleosols, because this index represents abundances of more-resistant minerals, such as K-feldspar and muscovite. Values of Ca/Ti show enrichments in surface horizons (biocycling, or plant decay additions?) but depletions in subsurface horizons of most paleosols. However, one mid-Wisconsin interstadial soil has high Ca/Ti subsurface values, indicating carbonate accumulation, confirmed by X-ray identification of calcite. The amount of depletion of primary minerals in central Alaskan paleosols is similar to the degree of mineral weathering in modern soils of central Alaska. This observation indicates that past interglacial and interstadial periods in central Alaska had temperature and moisture regimes that were at least as favorable to mineral weathering as today.