Biological structures in Archean and lower Proterozoic rocks of the Kursk Magnetic Anomaly and Kola Peninsula

Abstract

The integral duration of the Archean and Proterozoic eonothems approximates 4 Ga. The corresponding sediments of that time interval, which constitutes approximately 85% of the Earth’s history, are poorly characterized by signs of biological activity. Such a feature provided grounds for most researchers to neglect the influence of biological processes on sedimentation during that time. Nevertheless, some authors [2, 3] report solid evidence in favor of direct participation of living organisms in the formation of some rocks, confirming inferences by V.I. Vernadsky who believed that both the sedimentary and granite shells of the Earth represent products of past biospheres [1]. The formation of the Earth’s hydrosphere (approximately 4.4 Ga ago) and atmosphere (at first, oxygenfree, although not reducing [7]) was an important prerequisite for the appearance of life and, simultaneously, for the commencement of sedimentation. It is thought that the chemical composition of the Archean and Early Proterozoic atmosphere and hydrosphere differed sharply from that in the recent and Phanerozoic epochs [8]. It was oxygen-free and characterized by high contents of carbon dioxide, which influenced processes of hypergenesis. The chemical weathering during the Archean and Early Proterozoic was more intense than in subsequent epochs. Nevertheless, significant quantities of pelitic material, in addition to dissolved components of weathered rocks, should have been transported to sedimentation basins. Along with dissolved compounds of K, Na, Ca, Mg, and other elements, clayey particles of colloidal sizes should have been dispersed through the entire basin, preventing the formation of relatively pure (with respect to chemical and mineral compositions) beds of quartz, iron oxides, organic carbon, and other minerals and their associations. In fact, these rocks are highly differentiated as exemplified by jaspilite sequences. In our opinion, this fact provides logical grounds for the assumption that biological activity was a determining factor in the formation of quartz in most Archean and Lower Proterozoic pararocks (gneisses and jaspilites). Indeed, only living organisms can extract and accumulate various chemical components, which are frequently characterized by background concentrations, from the surrounding medium. The rocks constituting Archean‐Lower Proterozoic metamorphic complexes in the study regions experienced significant alterations and transformations through all the stages of catagenesis, metagenesis, and several phases of metamorphism. Based on investigations of siliceous rocks [4] genetically related to the metasomatic replacement of calcium carbonate in chalky rocks by silica, we hoped to define some kinds of biological traces using electron microscope methods, although we did not know what the traces should look like. Our work was based on the assumption of a primary biogenic carbonate or biogenic opal composition of quartz layers.