Abstract

We have studied a series of Toyeh (post-Isotope Stage 2: <12 ka) soil profiles on high shoreline features in the Lake Lahontan area, selected to provide a transact from upwind to downwind of the Carson Sink, a major postpluvial source of dust. At two localities we also studied Churchill (post-Isotope Stage 6: <130 ka) soil profiles on identical parent materials. The Churchill profiles, despite having been exposed to weathering for about ten times as long as the Toyeh profiles, contain less than twice as much silt and clay increases systematically as the clay as the paired Toyeh profiles. Furthermore, in profiles of each age the amount of silt and clay increases systematically as the area of upwind interpluvial playa increases. Carbonate and soluble salts increase downwind, but Churchill profiles do not contain significantly more than Toyeh soils.

Eolian dust fall was the source of fines and salts in these soils. Apparently, this influx was concentrated during two brief intervals, or "pulses," one during the Holocene, and one during the previous (Isotope Stage 5) interpluvial. Leaching during the Sehoo pluvial removed salts from Churchill soils; carbonate and soluble salts now observed in the Churchill soils represent a Toyeh overprint on the Churchill profile.

These pulses of fine sediment and accompanying salts are tantamount to the "soil-forming intervals" that Morrison described in the Lahontan area. Episodic eolian dustfall provides fine particles of silicates, calcite, and soluble salts that infiltrate into the clast-supported alluvial piedmont sediments. Several hundred playas exist in the Basin and Range, each providing directional dust contributions that influence local and regional soil development. We counsel caution in assuming constant, gradual development of soil properties without due consideration of paleoenvironmental influences on soil-forming factors.

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