Abstract:
Columnar stromatolites representing more than a half of species described in Precambrian stromatolite assemblages reveal a regular trend of size variations during the Proterozoic and Early Paleozoic. Their dimensional parameters grew gradually during the Paleoproterozoic to attain peak values in the Early Riphean and to decline steadily afterward during the Middle-Late Riphean, Vendian, and Cambrian. Size variations are established based on statistically averaged maximum diameters of columns calculated for 230 taxa and on percentages of large, medium and small species occurring in successive units of stratigraphic scale. The units correspond to three Paleoproterozoic subdivisions (time span from 2.3 to 1.65 Ga) and to five subdivisions of the Riphean, Vendian and Early Paleozoic jointly spanning a comparable period of geologic time. The results of calculation depict a unimodal variation curve with one infliction point designating inversion of ascending and descending trends in the Early Riphean time. The inversion and cardinal changes in taxonomic composition of the entire stromatolite community across the Riphean lower boundary appear to be interrelated. Abiotic events, which certainly influenced diversity of all, especially columnar stromatolites, have no manifestation however in the size-variation curve lacking perceptible oscillations in both the ascending and descending branches. Consequently, dimension parameters of columnar stromatolites appear to be independent of direct influence of abiotic events. © MAIK "Nauka/Interperiodica" 2006.
