Notes

1-are very different from the stromatolites that are forming today x
are important because they trap sea sediments and use them in their metabolism and this purified the early oceans x
can be dated to 3.4 billion years old and may record some of the earliest life on Earth t
2-is thought to have originated as a by-product of photosynthesis by oceanic microbes t
3-produced oxygen which then dissolved into the ocean water t
4-were photosynthetic and produced oxygen which reacted with the iron that t
5-contain iron-rich sediments that were the result of the oxidation of iron x
can be as old as 2.5 billion years and record the clearing of iron from the ancient seas as oxygen was released by t
6- contained no free oxygen t
7-all of the above t
8-postulate that the gases from volcanic outgassing played a minor role in the establishment of the early atmosphere x
postulate that the chemical composition of out-gassing is essentially the same today as in the past t
9-because although volcanoes x
because it readily breaks down into free oxygen and the element sulfur x
because it is not a product of volcanic out-gassing
10-because it is the fossilized remains of the organisms that introduced oxygen into the oceans and atmosphere x
because although volcanoes today do produce S0₂, ancient volcanoes that gave rise to the atmosphere did not release S0₂ x