Keith F. Lynch kfl at KeithLynch.net I've long been baffled by the claim that the free oxygen in the atmosphere came from photosynthesis. If the atmosphere started out neutral, then oxygen in the atmosphere must be balanced by reduced matter (biomass, peat, coal, oil, natural gas, etc.). It seems implausible that there's enough combustible stuff that burning it all would consume all the oxygen.
--WDS: Interesting. But I think you're wrong, see below. First of all, the earth's overall elemental composition clearly is reductant-rich, not oxidant-rich -- just consider the iron core. So on a lifeless earth there would be no free O2. It is ludicrous to contend that there just happens to be a teeny tiny amount more O than it takes the oxidize the rest, left over, just enough to form the atmosphere. There also is no question the earth atmosphere was originally oxygen-free and we know from geological evidence roughly when the "great oxygenation event" occurred yielding massive iron oxide ore deposits, etc. There is about 235 grams of free O2 per square cm of earth surface. (If atmospheric pressure is 1 kg per square cm.) This amounts to about 10^18 kg = 10^6 gigatons of free O2. So the question of whether there is enough combustible stuff to get rid of all the O2 comes down to: if you took a square cm of earth and went down, say, 1 km, would that long prism of soil, rock (or ocean floor material) contain enough combustible stuff to consume 235 grams of O2? Which if it were carbon would mean >=88 grams? Wikipedia: "Soil plays a major role in the global carbon cycle, with the global soil carbon pool estimated at 2500 gigatons. This is 3.3 times the size of the atmospheric pool (750 gigatons) and 4.5 times the biotic pool (560 gigatons)." Wikipedia: The ocean floors are covered with ooze and sediments, ranging from 0.6 to 9 km thick, largely made of dead microbes. "Ooze is pelagic sediment that consists of at least 30% of microscopic remains of either calcareous or siliceous planktonic debris organisms. The remainder typically consists almost entirely of clay minerals." It has been found that ocean siliceous ooze will react with hot oxygen and will thereby lose mass from 3 to 16.5%. See tables in book "THE MINERAL RESOURCES OF THE SEA" Elsevier 1965 "loss on ignition." So: those numbers imply soil carbon is totally inadequate to use up all the free oxygen; burning it all it would only use up about 0.67% of the O2. However, ocean ooze combustible material seems more than enough to consume all oxygen, in fact it looks to me like enough to use up at least 5 times all free oxygen. So, that's where it went. The geologic "carbon cycle" is that carbonate rocks are baked in volcanos and emit CO2, which then gets grabbed by life and ends up as ocean ooze, which forms into carbonate rocks, etc. -- Warren D. Smith http://RangeVoting.org <-- add your endorsement (by clicking "endorse" as 1st step)