Criteria For Distinguishing Microbial Mats and Earths

Abstract

Microbial earths are communities of microscopic organisms living in well-drained soil. Unlike aquatic microbial mats and stromatolites, microbial earths are sheltered from ultraviolet radiation, desiccation, and other surficial hazards within soil cracks and grain interstices. Currently, such ecosystems are best known in small areas of unusually cold, hot, or saline soils unfavorable to multicellular plants and animals. During the Precambrian, microbial earths may have been more widespread, but few examples have been reported. This review outlines a variety of features of modern microbial earths that can be used to distinguish them from aquatic microbial mats and stromatolites in the fossil record. Microbial earths have vertically oriented organisms intimately admixed with minerals of the soil, whereas microbial mats are laminated and detachable from their mineral substrate as flakes, skeins, and rollups. Microbial earths have irregular relief, healed desiccation cracks, and pressure ridges, whereas microbial mats have flexuous, striated domes, and tufts. Microbial earths form deep soil profiles with downward variations in oxidation, clay abundance, and replacive nodular subsurface horizons, whereas microbial mats form as caps to unweathered, chemically reduced sedimentary layers. Microbial earths develop increasingly differentiated soil profiles through time, whereas microbial mats build upward in laminar to domed increments. Microbial earths are found in nonmarine sedimentary facies, whereas microbial mats form in lacustrine, floodplain, and marine sedimentary facies. Microbial mats and stromatolites are known back to the oldest suitably preserved sedimentary rocks in the 3458 Ma Apex Chert and 3430 Ma Strelley Pool Formation (respectively) of the Pilbara region of Western Australia. The geological antiquity of microbial earths extends back to 2760 Ma in the Mount Roe paleosol of the Hamersley Group near Whim Creek, Western Australia.

Ubiquitous occurrence of basaltic-derived paleosols in the Late Archean Fortescue Group, Western Australia

“…Nevertheless, our results support the presence of significant microbial land colonization by 2.76 Ga, as previously documented by (Rye and Holland, 2000;Stueken et al, 2012;Watanabe et al, 2000Watanabe et al, , 2004. Possibly, the occurrence of organic material in the MR#1 and MR#2 paleosols could reflect a significant production of organic acids through microbial activity, which may have also participated in the leaching of ferric oxides (Ohmoto, 1996;Retallack, 2012).…”

“…The presence of terrestrial methanotrophs, suggested by the occurrence of 13 C-depleted carbonaceous material atop the diaspore-pyrophyllite boudins (see Section 9), may be further support for oxidizing conditions during weathering. Also, the presence of microbial activity may have been responsible for significant leaching of ferric oxides through the production of organic acids (Ohmoto, 1996;Retallack, 2012). The net gain of Al is best attributed to lateral migration of aluminum in soil profiles as documented in modern, intensely weathered bauxites (Valeton, 1972).…”

La vida temprana en la Tierra y los primeros ecosistemas terrestres

“…Aunque existen análogos fósiles de CGC (Simpson et al, 2010;Beraldi-Campesi et al, 2011;Retallack, 2009Retallack, , 2011Sheldon, 2012), no se sabe cuál fue su composición microbiana. Comparativamente, cualquier costra biológica del suelo es menos resistente que los tapetes microbianos Figura 2.…”

Reply to comments on Retallack 2011: Problematic megafossils in Cambrian Palaeosols of South Australia

“…(2012). Abundant life, at least at the microbial level, is the most likely explanation for many documented palaeosol features: surficial enrichment in clay, segregation of calcite nodules and gypsum crystals, and depletion of alkali and alkaline earth elements and phosphorus within the palaeosol profiles (Retallack 2008, 2012 a ).…”

“…2012). Funisia and Coronacollina may also have been soil organisms because they are found on cracked surfaces (‘old elephant skin’) characteristic of Ediacaran soil crusts (Retallack 2012 a , b ). Features of the fossils considered relict bedding by Jago et al .…”