Early Precambrian Processes

by: M. P. Coward and A. C. Ries (Geological Society Special Publication) The papers in this volume are grouped together, following particular themes or models of Early Precambrian processes. In the first paper Maarten de Wit & Andrew I-lynes discuss the onset of interaction between the hydrosphere and oceanic crust. According to these authors the Jamestown Ophiolite Complex, part of the Barberton greenstone belt of South Africa, represents very early ocean-like lithosphere, obducted onto a volcanic arc terrain by processes very similar to those which have emplaced modern ophiolites. The early metamorphism and degree of hydration are similar to modern sea-floor hydrothermal processes. Ken Eriksson discusses the relationships between crustal growth, surface processes and atmospheric evolution in the Early Precambrian. Models for the evolution of the Precambrian atmosphere range from those proposing little change through time to those advocating a stepwise increase in oxygen content. According to Ken Eriksson the evolution of the atmosphere to an oxidizing state, necessary for the formation of red beds and red palaeosols, was only achieved by 2.0 Ga following the escape of oxygen to the atmosphere from the oceans. Prior to 3.0 Ga, oxygen was consumed mainly in oceanic crust alteration. From 3.0 to 2.0 Ga it was consumed in the precipitation of Fe and Mn supplied to the oceans by fumerolic activity. Euan Nisbet reviews Archaean ecology and speculates on the development of a global scale biosphere. The earliest living communities may have consisted of chemotrophic bacteria that existed around hydrothermal systems.