Precambrian clastic sedimentation systems

Microorganisms have flourished in the oceans since at least 3.8 billion years (3.8 Gyr) ago1,2, but it is not at present clear when they first colonized the land. Organic matter in some Au/U-rich conglomerates and ancient soils of 2.3–2.7 Gyr age has been suggested as remnants of terrestrial organisms3,4,5. Some 2.7-Gyr-old stromatolites have also been suggested as structures created by terrestrial organisms6,7. However, it has been disputed whether this organic matter is indigenous or exogenic, and whether these stromatolites formed in marine or fresh water. Consequently, the oldest undisputed remnants of terrestrial organisms are currently the 1.2-Gyr-old microfossils from Arizona, USA8. Unusually carbonaceous ancient soils—palaeosols—have been found in the Mpumalanga Province (Eastern Transvaal) of South Africa9. Here we report the occurrences, elemental ratios (C, H, N, P) and isotopic compositions of this organic matter and its host rocks. These data show that the organic matter very probably represents remnants of microbial mats that developed on the soil surface between 2.6 and 2.7 Gyr ago. This places the development of terrestrial biomass more than 1.4 billion years earlier than previously reported.

Geochemical evidence for terrestrial ecosystems 2.6 billion years ago

Concentrically rolled-up silty mudrock laminae, 1–2 mm thick, are found in the uppermost of four, 40–110 cm thick interdune beds within a thick aeolian succession of the c. 1.8 Ga Makgabeng Formation, Waterberg Group, South Africa. These curved laminae are analogous to previously described “roll-ups,” biogenic structures generally ascribed to soft-sediment deformation or desiccation of microbial mats overlying either carbonate or siliciclastic sediments, within shallow- to deep-marine paleoenvironments. The Makgabeng roll-ups are thought to reflect desiccation of a microbial mat, followed by resedimentation of cohesive, discrete, curled mat fragments. Their alignment is considered to be the result of an extreme precipitation event. The significance of the South African example of roll-ups is that they appear to have formed within a fully terrestrial paleoenvironment, in one of the oldest known Precambrian deserts. Thus, they represent the oldest evidence for microbial colonisation of a terrestrial setting.

Muddy Roll-up Structures in Siliciclastic Interdune Beds of the c. 1.8 Ga Waterberg Group, South Africa

The nature of sedimentary mineral deposits has evolved during Earth's history in concert with changes in the oxidation (redo) state of the ocean-atmosphere system, biological evolution, and the growing importance of geologically young accumulations of ore-grade material. There is now strong evidence that the atmosphere and the oceans were anoxic, or essentially anoxic, before 2.4 Ga. Banded iron formations (BIF) and the detrital uranium ores formed prior to 2.4 Ga are consistent with such a state. The period between 2.4 and 2.0 Ga is called the Great Oxidation Event by some. Its ores bear unmistakable marks of the presence of atmospheric O{sub 2}. Between 1.8 and 0.8 Ga the Earth system seems to have been remarkably stable. Sedimentary ore deposits of this period were influenced by the presence of O{sub 2}. BIF, sedimentary manganese, and phosphorites disappeared ca. 1.8 Ga, but sedimentary exhalative (SEDEX) deposits and unconformity-type uranium deposits flourished, and nonsulfide zinc deposits put in an appearance. The period between 0.8 Ga and the end of the Proterozoic at 0.54 Ga was as turbulent or more so than the Paleoproterozoic. BIF returned, as did sedimentary manganese deposits and phosphorites. A further rise in the O{sub 2} content of themore » atmosphere and an increase in the sulfate concentration of seawater during this period brought the composition of the atmosphere and of seawater close to their present redox state. The last 540 m.y. of Earth's history have seen the system pass through two supercycles of roughly equal length. Climate, the redox stratification of the oceans ocean mixing, and the nature of sedimentary ores were influenced by tectonically and volcanically driven changes during these supercycles. The evolution of the higher land plants gave rise to coal deposits and sandstone-type uranium ores and was important for the formation of bauxites.« less

100th anniversary special paper: Sedimentary mineral deposits and the evolution of earth's near-surface environments

Chemostratigraphic methods (δ13Corg) for pinpointing the Permian–Triassic boundary in nonmarine rocks of Antarctica, Australia, and South Africa have drawn attention to unusual claystone breccias at this horizon of the greatest extinction in the history of life on Earth. These rocks differ from other breccias in having a high proportion of clasts with birefringence microfabrics (sepic plasmic fabrics) characteristic of soils, and can be called sepic pedoliths in the terminology of soil science. At many localities, earliest Triassic sepic pedoliths are thin (5–15 cm) beds, but some fill shallow paleochannels, and pinch out laterally. Other non-marine breccias at the Permian–Triassic boundary are dominated by fragments of coal, pedogenic carbonate nodules, and deeply weathered volcanic rock fragments. Earliest Triassic sepic pedoliths record an unusually severe and widespread episode of soil erosion associated with forest dieback at the Permian–Triassic boundary. Comparable sepic pedoliths are found in debris flows after clear-cutting of forests in western Oregon today. Studies of these modern deposits show that sepic soil peds do not withstand weathering for more than a few months. Such clayey clasts are held together mainly by roots and fungi, which decay rapidly in soils and streams. These modern analogs demonstrate that terminal Permian soil erosion was rapid and profound. Sepic pedoliths are also known at other times of mass extinction, such as the Triassic–Jurassic boundary, but are not yet known before the Devonian advent of trees and forested clayey soils.

J.E.J. Martini

Papers

A late Archaean-Palaeoproterozoic (2.6 Ga) palaeosol on ultramafics in the Eastern Transvaal, South Africa