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Desert surface crusts and fissures

/(With contribution from M.A.J. Williams. Revised December 2004)/

Geoindicator


NAME: Desert surface crusts and fissures

BRIEF DESCRIPTION: These are thin (mm to few cm), transient surface layers in playas and depressions in arid and semi-arid regions. Some are silt and clay drapes on clay pans, and former lake beds, and others form when surface water evaporates leaving behind salts, lime and silica. Sand dunes and plains may also be covered by a carpet of complex mosaic of highly specialized organisms known as biological (cryptogamic, cryptobiotic, microbiotic, microphytic) soil crusts, which bind together loose particles to protect the surface from erosion. The formation of persistent deep (to several m), polygonal cracks in the mud and silt floors of closed basins and depressions may indicate the onset of aridification or severe drought. Surfaces may contain other desiccation features such as sedimentary dikes, evaporite deposits (especially gypsum and halite), adhesion ripples and large salt polygons.

SIGNIFICANCE: Desert surface crusts are important because they protect the underlying fine material from wind erosion. They reduce rain-water infiltration and can prevent the emergence of vascular plant seedlings. Their appearance or disappearance may indicate changes in aridity. Biological soil crusts can be used as indicators of dryland ecosystem health, and can help to fix atmospheric nitrogen and contribute to soil organic matter. The formation of salt crusts is at the heart of the major problem of surface salinization affecting many of the world.s drylands [see groundwater quality].

HUMAN OR NATURAL CAUSE: The formation of surface crusts is related primarily to natural causes, but the persistence of crusts and fissures can be altered by human activities, such as burning, overgrazing, off-road vehicles, or groundwater extraction or excessive recharge.

ENVIRONMENT WHERE APPLICABLE: Arid to semi-arid terrains

TYPES OF MONITORING SITES: Playas, sabkhas, vegetated sand dunes and sand plains in arid regions

SPATIAL SCALE: patch / mesoscale

METHOD OF MEASUREMENT: Field measurements of feature size, depth and extent, supplemented by ground surveys, air photos, and satellite images.

FREQUENCY OF MEASUREMENT: Fissures: 1-50 years. Crusts: 5-10 years

LIMITATIONS OF DATA AND MONITORING: Surface features may not be preserved.

APPLICATIONS TO PAST AND FUTURE: The thickness, composition and number of evaporitic, clay or biological crusts may indicate past patterns of local and regional precipitation and temperature.

POSSIBLE THRESHOLDS: NA

KEY REFERENCES:

Belnap, J, J.H.Kaltenecker, R.Rosentreter, J. Williams, S. Leonard, and D. Eldridge 2001. Biological Soil Crusts: Ecology and Management. US Dept Interior, Bureau of Land Management: Technical Reference 1730.2, 118p

Rosen, M.R. (ed) 1994. Paleoclimate and basin evolution of playa systems. Geological Society of America Special Paper 289.

Talbot, M.R. & M.A.J.Williams 1978. Erosion of fixed dunes in the Sahel, Central Niger. Earth Surface Processes 3:107-113.

OTHER SOURCES OF INFORMATION: Geological surveys, desert research institutes, INQUA, IGA.

RELATED ENVIRONMENTAL AND GEOLOGICAL ISSUES: Changes in quality of shallow groundwater.

OVERALL ASSESSMENT: Surface crusts and fissures in deserts are good indicators of rapid changes in precipitation, temperature, and grazing pressure.

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