Print this pageComparative Study of Early Proterozoic Carbonates (older than or equal to 2.22-2.06 Ga): Evidence for a Greenhouse Event
BEKKER, ANDREY and JUHA A. KARHU
In comparison to the Paleoproterozoic ice-house event at about 2.3-2.4 Ga, the succeeding period has not been paleoclimatically defined on a global scale, as paleoclimatic evidence is scarce and interbasinal correlation is difficult. New carbon isotope data for carbonates in North America, Scotland, Baltica, Africa indicate that the delta{13}C[carb] underwent a large positive excursion between 2.22 and 2.06 Ga. These {13}C-enriched carbonates are dolomites and they commonly contain pseudomorphs, after gypsum and anhydrite. Some contain copper deposits and red beds associated with sabkha-type facies. Black shales with high organic carbon content are interlayered with these carbonates. This evidence argues for a warm arid climate during deposition of the {13}C-enriched carbonates in stratified ocean with anoxic bottom. The glacial event most likely indicates a drop in the level of atmospheric CO[2]. Onset of the succeeding greenhouse event and a probable. Build-up of CO[2] in the atmosphere may have been due to breakup of the Kenorland and accompanying volcanic degassing, consequential sea level rise, and decrease in the weathering rate compared with that during existence of the supercontinent. The subsequent delta{13}C[carb] excursion probably was related to high rates of organic matter burial, which resulted in high O[2] production. Increase in the O[2] level may have led to the formation of the stratospheric ozone shield. Both ozone and O[2] may have added to the greenhouse effect.