Many high elevation surfaces in the Canyonlands region of the central Colorado Plateau are mantled by a thin layer of fine-grained sediment that harbors much of the regional biologic fertility.

Samples of sediment from potholes on slickrock buttes, mesa tops, and grassland steppes (settings that receive no alluvial sediments) differ in many physical and chemical respects from the associated bedrock (11 sites in coarse sandstone, two in limestone).

The biologic soil crust (BSC; the upper 0.5 cm in our sampling) differs greatly in some magnetic and chemical properties from the underlying sediment. At our sites the BSC has been a stable surface for several decades, as inferred from the mature population of cyanobacteria (Microcoleus vaginatus and Scytonema nostoc) and lichen (Collema).

(click for larger image- 35K) Paricles in magnetic extract from sediment on Cedar Mesa sandstone. Particle in center is titanomagnetite (30 µm diamteter).

(click for larger image- 39K) Specular hematite grain (120 µm across) from the Kayenta Fm.

RESULTS

1. Magnetic properties:
Surficial sediment & soil has much higher magnetic susceptibility (MS) than associated bedrock. The BSC has higher MS than underlying sediment. Magnetic mineral extracts reveal that the magnetic signal is caused by detrital Fe-Ti oxide minerals (magnetite and titanomagnetite) that formed originally in igneous rocks. These minerals are absent in the bedrock.

2. Geochemistry:
The surficial sediment is enriched in most analyzed elements (e.g., Fe, Ti, Zr, Al, P, Pb, Zn, Mg, Na, Ca, Sr, Nd, V, Mn) compared to associated sandstone. Zr content resembles the MS profile. The BSC is enriched in Zr relative to Ti, compared to underlying sediment, but Fe/Ti remains constant.

3. Particle-size analysis:
Silt plus clay typically composes about 30-60% of the sediment, providing a rough estimate for eolian material. Sand is locally derived. The BSC is not enriched in silt relative to underlying sediment.

Magnetic and XRF chemical data. Averages from sites in surficial mantle on sandstone substrate.

INTERPRETATIONS & SUMMARY

1. The fine-grained sediment that mantles much of the Colorado Plateau contains eolian silt, at least some of which has been transported some tens to hundreds of km.

2. The biologic soil crust (BSC) acts as a natural dust trap that records a change in eolian dust source over the past several decades. The magnetic (magnetite enriched relative to titanomagnetite) and chemical (high Zr relative to Ti) patterns suggest enhanced dustfall from source areas more silicic than those that yielded silt to the underlying sediment. Most igneous rocks on the Colorado Plateau are basaltic lavas that occur at the margins of the Plateau.

Likely dust-source regions may be areas off the Colorado Plateau, including the Mojave Desert, much of which is underlain by granite and other felsic rocks.

The dust record from the Colorado Plateau may reflect increased human disturbance of deserts off the Plateau over the past several decades. Can changing dust composition affect ecosystem processes, including perhaps the invasion of exotic species?

3. The eolian dust in the surficial deposits and soils renders them especially vulnerable to wind erosion. Land-management policies can benefit from these results.

4. Magnetic petrology is a rapid, quantitative method to detect eolian dust in arid-land surficial deposits. We are using the method to identify eolian components over different surfaces of the Mojave ecosystem. Even in this geologically and magnetically complex setting, we find strong magnetic mineral contrasts between silt in soil and the underlying bedrock, indicating an eolian origin for the silt.