Morphological and chemical record of the organic particles in precambrian sediments

Abstract

Summary A review is presented of the currently available morphological and chemical evidence of life in the Precambrian, with special reference to the interrelationships between microbial activity and biomineral accumulation. Fossil organic microstructures can be detected in thin sections of the rock under the light microscope and examined in demineralized sections under the transmission electron microscope (TEM). They are chemically analyzed in microprobes and spectrophotometer microscopes. On the basis of such studies, the interaction of microorganisms with the formation of minerals in cherts, carbonates and from sediments can be traced back to early Archean times, 3800 million years (MY) ago. The stratiform gold and uranium ores of the lower Proterozoic (2700–2800 MY) indicate anoxic conditions during deposition. Certain bacteria and fungi are apparently involved in the formation of the facies. The banded iron formations occur most often in the subsequent time span 2300–1700 MY ago. The deposits are interpreted as being produced by the activity of cyanobacteria and iron bacteria. During this period, the first red beds appeared indicating that the atmosphere became increasingly oxygenated. Copper deposits can be interpreted as a marker of the younger Proterozoic 1200−650 MY ago as far as their biological significance is concerned. At this time, the first diversification of eukaryotic phytoplankton occured. It is suggested that the evolution of eukaryotic algae was accompanied by an increase in copper, coupled with a decrease in iron in the cell.