Holocene loess deposition and soil formation as competing processes, Matanuska Valley, southern Alaska

Abstract

Although loess-paleosol sequences are among the most important records of Quaternary climate change and past dust deposition cycles, few modern examples of such sedimentation systems have been studied. Stratigraphic studies and 22 new accelerator mass spectrometry radiocarbon ages from the Matanuska Valley in southern Alaska show that loess deposition there began sometime after ???6500 14C yr B.P. and has continued to the present. The silts are produced through grinding by the Matanuska and Knik glaciers, deposited as outwash, entrained by strong winds, and redeposited as loess. Over a downwind distance of ???40 km, loess thickness, sand content, and sand-plus-coarse-silt content decrease, whereas fine-silt content increases. Loess deposition was episodic, as shown by the presence of paleosols, at distances >10 km from the outwash plain loess source. Stratigraphic complexity is at a maximum (i.e., the greatest number of loesses and paleosols) at intermediate (10-25 km) distances from the loess source. Surface soils increase in degree of development with distance downwind from the source, where sedimentation rates are lower. Proximal soils are Entisols or Inceptisols, whereas distal soils are Spodosols. Ratios of mobile CaO, K2O, and Fe2O3 to immobile TiO2 show decreases in surface horizons with distance from the source. Thus, as in China, where loess deposition also takes place today, eolian sedimentation and soil formation are competing processes. Study of loess and paleosols in southern Alaska shows that particle size can vary over short distances, loess deposition can be episodic over limited time intervals, and soils developed in stabilized loess can show considerable variability under the same vegetation. ?? 2004 University of Washington. All rights reserved.