2010 GSA - Malenda, Simpson, Wizevich, Tindall, and Boling

Morphological variation and experimental compaction in the Colorado Plateau biological soil crusts: Implications for their possible recognition in the rock record

Malenda, H. Fitzgerald, Simpson, Edward L., Wizevich, Michael C., Tindall, Sarah E., and Boling, Kenneth S., 2010, Morphological variation and experimental compaction in the Colorado Plateau biological soil crusts: Implications for their possible recognition in the rock record [abs]: Geological Society of America - Denver Annual Meeting (31 October –3 November 2010).

Biological soil crusts (BSCs) are a ubiquitous and crucial component of modern dryland ecosystems and probably the first community-type to colonize the Precambrian land surface. BSCs are complex symbioses of cyanobacteria, green algae, mosses, lichens, and fungi, and have adapted to intense UV radiation and drastic modifications in precipitation and temperature. BSCs, although they were certainly successful in the Precambrian, are underreported in the rock record conceivably due to the crusts’ inconspicuous appearance. This BSC study reports on the morphological variation in a modern setting (Grand Staircase-Escalante National Monument) and preliminary compaction experiments in order to aid the identification of BSC in the rock record.

Three BSC morphological types were indentified, described, and compacted. They are: (1) pinnacled, those dominated by cyanobacteria and high relief (3-5 cm), (2) smooth, also cyanobacteria dominated but with very low relief (< 1 cm), and (3) sporadic, pinnacled crusts buried by eolian and fluvial sediment with medium relief (1- 3 cm). Sporadic crusts differ from pinnacled by the reduction in photosynthetic organics in naturally covered areas. Samples from each morphology were covered with colored sand, can-cored, compacted. Compaction of the crusts decreased porosity, dampened crust amplitude and eliminated much of the microrelief. Globular soil peds, which range from 2 cm to 5 cm in diameter, have been observed immediately below modern crusts. These peds and other microstructures become less distinctive and flattened. Original lateral continuity of the mats was unaffected.

The overwhelming importance of BSCs in the colonization of the continental setting and their impact on later evolution demands more vigilant investigation in the rock record. Further systematic characterization of the range of preserved expressions of BSCs will aid in distinguishing them from other sedimentary structures in the rock record.

Kurt Friehauf - December 2010