HOLOCENE LOESS SEDIMENTATION AND SOIL FORMATION IN THE MATANUSKA VALLEY, SOUTHERN ALASKA: A MODERN ANALOG TO PAST DUST DEPOSITION

Although loess-paleosol sequences are among the most important records of Quaternary climate change and past dust deposition cycles, there are few modern examples of such sedimentation systems. One example exists in southern Alaska, however, where silts produced through grinding by the Matanuska and Knik glaciers are deposited as outwash, entrained by strong winds, and re-deposited as loess in the Matanuska Valley. Stratigraphic studies and 22 new AMS radiocarbon ages on charcoal and wood show that loess deposition began sometime after ~6,500 radiocarbon yr BP and has continued to the present. Over a downwind distance of ~40 km, loess thickness, sand content, and mean particle size decrease, whereas medium-to-fine silt content and clay content increase. Loess deposition was episodic at distances >5 km from the loess source. Stratigraphic complexity is at a maximum (i.e., the greatest number of loesses and paleosols) at intermediate distances from the loess source. As many as four separate phases of loess deposition occurred in the past ~6,000 yr at a locality ~10 km from the 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 with A/AC/C or A/E/C profiles whereas distal soils are Spodosols characterized by distinct A[O]/E/Bw [Bs]/C profiles. Chemical analyses show that distal Spodosols are more weathered than proximal Entisols and Inceptisols. Ratios of soluble Ca, K and Fe to insoluble Ti show decreases in surface horizons with distance from the source. The Matanuska Valley serves as a rare modern analog to loess systems of the past that record dust deposition cycles. Study of this region shows that particle size can vary over short distances, loess deposition can be episodic over limited time intervals, and soils on stabilized loess in a small area can show remarkable variability under the same vegetation.