MICROBIAL CRUSTS AS THE EARLIEST LIFE ON LAND

Extremely pure ancient quartz sandstones have long challenged geologists. How could 95% pure quartz sand be distilled from parent rocks containing less than 25% quartz? Long favored was physical purification by repeated abrasion and winnowing, for which there is much textural evidence, while chemical purification in single or a few episodes was less popular. But first-cycle pure sands documented by Johnsson and others (1988; 1991) within the tropical Orinoco River basin have greatly strengthened the latter. Importance of chemical weathering is also supported by the increasing recognition of mature paleosols beneath several ancient pure sandstones. The Chemical Index of Alteration for these paleosols as well as for pelites interstratified with such sandstones range as high as 96-98, which rivals that of the most chemically mature soils and shales of any age.

There is a paradox, however. Physical purification through multi-cycling seemed to be supported by apparent absence of pre-Silurian macroscopic land vegetation, which implied much more intense past wind and braided fluvial abrasion. But how can the mature paleosols be explained without stabilizing vegetation? Very flat landscapes offer a possible rationale, but transport of fluvial quartz sand and pebbles required sufficient topographic relief to support streams with competent velocities. The solution we suggest is the Precambrian appearance of subaerial biological (microbial) crusts or mats such as characterize many present-day arid and other extreme environments. Microbial soils are complex communities of cyanobacteria, green and brown algae, microfungi, lichens, mosses, and liverworts in varying proportions. Sticky sheaths and filaments bind soil surfaces and the organisms enhance weathering biochemically. Botanists have argued that such microbial communities must have been the first land organisms, having evolved from marginal marine or lacustrine microbial mats. Precambrian evidence in the form of fossil filaments and the geochemistry of organic caps on paleosols support their appearance as early as the late Archean.