Primary production, isotopes, extinctions and the atmosphere

Phytoplankton photosynthesis has controlled the atmospheric carbon dioxide-oxygen balance since the early Precambrian when algal abundance became sufficient to convert the reducing atmosphere to an oxygenic one. Later phytoplankton maxima indicated in the geologic record coincide with and are here suggested to have caused extensive biogenic calcareous deposition, and evolutionary diversification of the contemporaneous biota. In contrast, the onset of decreased productivity in the geologic past triggered a general biotic turnover on both land and sea.

Times of phytoplankton extinctions coincide with similar periods affecting animal taxa rather than terrestrial plants. These periods of severely reduced microfloras and low productivity resulted in atmospheric oxygen depletion and increased pCO2, 12C enriched limestones, formation of 32S enriched sulfates, and submarine dissolution of carbonates. Selective extinctions of animal taxa were due to quantitative variations in the interrelated processes of carbon fixation (the food source for the marine environment), and the photosynthetic consumption of carbon dioxide and production of oxygen.

The phytoplankton abundance may have been controlled by contemporaneous continental physiography, through its effect on climate, atmospheric circulation and oceanic upwelling. Low continents, equable climates, little influx of land-derived materials and lessened effectiveness of upwelling allowed the sinking of nutrients to exceed their renewal, resulting in marked reduction of phytoplankton, with the attendant effects of the decreased productivity and oxygen depletion crossing ecologic and systematic boundaries.

Continental rejuvenation with resultant climatic changes and increased oceanic circulation stimulated renewed phytoplankton diversification and growth, and allowed a renewed expansion of dependent animal taxa.