Skip to Main Content

Major Phanerozoic crises, resulting in extinction of marine organisms, are suggested to be the culmination of a gradual process of selection caused by periods of decreasing marine primary productivity. Nutrient deficiencies were the immediate cause of the phytoplankton decrease but changes in the contemporaneous land biota provided the ultimate cause.

Five threshold crises resulting from major evolutionary changes are suggested to have each had wide ranging effects on both the physical and organic worlds. Bacterial photosynthesis allowed use of solar energy to fix organic carbon; cyanobacterial oxygen-producing photosynthesis resulted in an oxygenic atmosphere, but required contemporaneous organisms to develop oxygen-mediating enzymes, or inhabit only anoxic areas. The Devonian spread of vascular plants produced a land biomass that retained on land much organic carbon and associated nitrogen and phosphorus, the postulated decrease in transport seaward of such nutrients gradually starving the phytoplankton of a renewed supply to replace that lost to the marine sediments; the resultant decreased productivity caused the Late Devonian invertebrate extinctions. The Carboniferous fern-gymnosperm floras further increased the land biomass, while coals permanently sequestered organic matter, repetition of the Devonian nutrient starvation causing the late Paleozoic marine crisis. By Late Cretaceous, the spread of angiosperm deciduous floras expanded the soil cover, producing another major increase in nutrient retention on land. These land plant innovations progressively led to the present world distribution of organic carbon, over 62% in soil, nearly 17% in land biomass, and 20% dissolved in the oceans, with less than 2% of this carbon and other nutrients occurring in the upper 200 m where it is available for recycling by phytoplankton. The remaining 1% forms the oceanic biomass. Extinctions accompanied periods of temporary but major reductions in nutrient supply. As nutrient levels were replenished or better adapted organisms evolved, diversification occurred.

Physical catastrophes, such as meteoric impacts, may have affected selected organisms or local areas, but the timing and selectivity of most extinctions are not compatible with such a sudden event.

You do not currently have access to this chapter.

Figures & Tables




Citing Books via

Related Book Content
Close Modal
This Feature Is Available To Subscribers Only

Sign In or Create an Account

Close Modal
Close Modal