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