11 Oxygen and Evolution on Early Earth I

Tuesday, 9 August 2005
9:00 AM - 12:00 PM

THE LATE ARCHEAN TO EARLY PALEOPROTEROZOIC EARTH SYSTEM AND THE RISE OF ATMOSPHERIC OXYGEN

Paper number: 11-6
Program: THEME SESSIONS
Time: 11:00 AM – 11:20 AM

Authors

Abstract

Late Archean to Early Paleoproterozoic terranes worldwide record a linkage between global tectonics, changing sea levels and environmental conditions that is broadly similar to Phanerozoic tectonics. The cycle started at ~2.78 Ga with the breakup of a pre-existing continent (Vaalbara) and one of the most prodigious periods of generation juvenile continental crust during a period of plume breakout (~2.72 to 2.65 Ga), that may have been orders of magnitude more intense than similar events during the Phanerozoic, accompanied by high sea levels. During this period, cratons formed by accretion of granitoid-greenstone terranes started to aggregate into larger continents (e.g. Kenorland). Lower sea levels between ~2.65 and 2.55 Ga were followed by a second (~2.51 to 2.45 Ga) period of plume breakout resulting in high sea levels and deposition of banded iron formations on the margins of the Pilbara and Kaapval cratons. Cratons in South Australia, Antarctica, India,and China record convergent margin magmatism and metamorphism between 2.56 and 2.42 Ga. Continued aggregation of continental fragments formed the Earth's first supercontinent by ~2.4 Ga with a return to low sea levels and a period of relative tectonic quiescence, before the supercontinent started to breakup from ~2.32 Ga.

Although oxygenic photosynthesizers may be as old as 2.71 Ga, periods of plume breakout (2.72 to 2.65 Ga and 2.51 to 2.45 Ga) would have enhanced the reduced conditions typical of the Archean biosphere. The initial rise of atmospheric O2 to > ~10-5 PAL appears to have occurred between 2.45 and 2.40 Ga. The decreased flux of reduced gasses following plume breakout coupled with the filling of crustal oxygen sinks produced during plume breakout and supercontinent assembly may have resulted in the global flux of reduced gasses falling below that of photosynthetic oxygen production leading to a rise in atmospheric O2 accompanied by loss of the CH4-rich greenhouse atmosphere resulting in the Earth's first widespread glaciation. Detrital pyrite and uraninite in these 2.45 to 2.40 Ga sediments suggests that oxidation of continental crust had not yet occurred. The oldest evidence of extensive oxidative weathering is associated with 2.32 to 2.22 Ga glacial deposits that accompanied the initial stages of rifting of the supercontinent.