RESULTS OF EXPERIMENTAL AEOLIAN SALTATION AND IMPLICATIONS FOR THE INTERPRETATION OF LOESS DEPOSITS

The Quaternary record contains abundant loess deposits, and loess is increasingly recognized in the deep-time geologic record. However, the origin of the silt particles composing loess remains debated, with some arguing for the primacy of glacial grinding, and others suggesting that aeolian saltation of sand in dune systems plays a significant role. The issue has been further complicated by conflicting results from experimental studies conducted over the last ~60 years. Understanding the relative contributions of these two environments bears on the paleoclimatic significance of loess deposits, and the origin and character of dust on other planets, notably Mars.

Experiments were conducted in a new device designed to simulate aeolian abrasion of sand grains to better understand the potential silt production of desert dune fields. These experiments differ from previous work by 1) using a larger abrading mass, and 2) measuring the air velocity in the device. We subjected 100 g of dune sand to 8 days of saltation abrasion at approximately 25 m/s (wind-storm conditions). The resultant silt amounts to 0.0037% of the original starting mass, lower than nearly all previous experiments. The produced silt exhibits modes of 35-46 µm (by volume %) as measured with a Malvern laser particle size analyzer. This silt is finer than the coarse modes found near desert margins, finer than Chinese Loess Plateau modes, and in the range of the Peoria loess in North America.

Quantification of velocity in the device enables scaling of results to desert dimensions, allowing estimation of silt production at the geological scale. Based on assessment of Automated Surface Observation Station (ASOS) data from Wyoming, Texas, Oklahoma and China, wind velocities exceeding 20 m/s prevail for < 400 hours annually. The scaling method employs a desert area comparable to the Namib (~30,000 km²) and a target loess deposit 10,000 km² and 1 m thick. Applying the experimental rate of silt production indicates that sourcing a 1-m-thick loess deposit by aeolian abrasion of sand would require ~1.6 My (using the experimentally derived rate of 1g/m²/y). This rate is too low to produce a geologically significant deposit; hence we posit that aeolian abrasion of sand is an ineffective means of producing the silt that forms loess.