in the presence of atmosphere oxygen
Answers
Explanation:
Today, the atmosphere is ~21% free oxygen. How did oxygen reach these levels in the atmosphere? Here is a summary of the oxygen cycle:
Oxygen Production
Photochemical dissociation - breakup of water molecules by ultraviolet
Produced O2 levels approx. 1-2% current levels
At these levels O3 (Ozone) can form to shield Earth surface from UV
Photosynthesis - CO2 + H2O + sunlight = organic compounds + O2 - produced by cyanobacteria, and eventually higher plants - supplied the rest of O2 to atmosphere.
Oxygen Consumers
Chemical Weathering - through oxidation of surface materials (early consumer)
Animal Respiration (much later)
Burning of Fossil Fuels (much, much later)
Step 6: Addition of Oxygen to Soil and then Atmosphere
Throughout the Archean there was little to no free oxygen in the atmosphere (<1% of presence levels). What little was produced by cyanobacteria, was probably consumed by the weathering process. Once rocks at the surface were sufficiently oxidized, more oxygen could remain free in the atmosphere.
During the Proterozoic the amount of free O2 in the atmosphere rose from 1 - 10 %. Most of this was released by cyanobacteria, which increase in abundance in the fossil record 2.3 Ga. Present levels of O2 were probably not achieved until ~400 Ma.
Evidence from the Rock Record
Iron (Fe) is extremely reactive with oxygen. If we look at the oxidation state of Fe in the rock record, we can infer a great deal about atmospheric evolution.
Archean - Find occurrence of minerals that only form in non-oxidizing environments in Archean sediments: Pyrite (Fools gold; FeS2), Uraninite (UO2). These minerals are easily dissolved out of rocks under present atmospheric conditions
did you asked essay or poem