Evaporite sedimentation in the precambrian as related to changes in biosphere and seawater chemistry, article 1: Evaporites of the Archean and lower Proterozoic

Stratigraphic position, geographic distribution, and composition of proved Archean to Lower Proterozoic evaporites are analyzed. Being of a limited geographic range on the Earth, as is established, evaporites of respective ages are represented predominantly by various pseudomorphs, glyptomorphs, and fanlike sedimentary structures after primary gypsum crystals. The limited distribution of Archean to Lower Proterozoic gypsum (anhydrite) is supposed to be a consequence of relatively low concentration of sulfates, as their reserve in seawater was below the present-day stationary level. The concentration was however sufficient for deposition of thin gypsum lenses and interlayers. Their absence in Archean and Lower Proterozoic successions can be explained by transformation of calcium sulfate into aragonite under influence of intensive anaerobic oxidation induced by activity of sulfate-reducing bacterial communities in evaporite basins. It is supposed that composition, pH, and pCO(2) of the Archean-Lower Proterozoic seawater favored deposition of biogenic limestones, although their accumulations in evaporite basins were of chemogenic origin. The known records insufficient for reconstruction of Archean and Lower Proterozoic and belts, but extensive and zones likely existed at those times. It is stated that concentration of sulfates in the World Ocean increased progressively to reach the stationary level in the Early Riphean.