Summary

Atmospheric O2 concentration has likely varied considerably over Phanerozoic time. Theoretical modeling, based on the abundance of organic carbon and pyrite sulfur in sedimentary rocks, and the carbon and sulfur isotopic composition of carbonates and sulfates in sedimentary rocks, indicates that atmospheric O2 attained values during the Permo-Carboniferous as high as 35% O2. The RROD isotopic modeling assumes that carbon isotope fractionation during photosynthesis globally increases with increasing O2 level. This agrees with measurements of the carbon isotopic composition of Permo-Carboniferous fossil plants combined with laboratory experiments and calculations on the effect of O2 on photosynthetic carbon isotope fractionation during plant growth. The principal cause of the high O2 values was the rise of large vascular land plants that brought about increased O2 production due to the increased global burial of microbially resistant, lignin-rich organic matter in sediments.

Higher levels of O2 are consistent with the presence of Permo-Carboniferous giant insects, and preliminary experiments with Drosophila indicate that insect body size can increase with elevated O2. Earlier work purported to show that such high O2 levels would have led to forest fires so frequent that terrestrial life could not have persisted over geologic time. However, this conclusion is based on burning experiments that consisted only of the ignition of paper strips. Paper is high in cellulose and does not char like natural forest materials. New experimental data, based on fire spreading during the burning of pine wood and pine needles, shows that slightly damp forest materials could resist burning at 35% O2 and that the degree of moisture is more important to burning than the level of O2. Nevertheless, elevated O2 should have led to more frequent and extensive forest fires, and evidence for this is that Permo-Carboniferous plants have morphologies suggestive of adaptation to fire damage.

Organic Gardening

Organic Gardening

Gardening is also a great way to provide healthy food for you and your loved ones. When you buy produce from the store, it just isnt the same as presenting a salad to your family that came exclusively from your garden worked by your own two hands.

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