Biological Implications of Atmospheric Chemical Change

Recall: the acid rain produced by a "large" comet impact would have lowered the pH of the mixing zone (the top 75 M of the ocean) to 7.8 (see Atmospheric Chemical Change). Calcareous phytoplankton only existed in the photic zone ( the farthest distance that light can travel down the ocean's water column, approximately 100M). The skeletons of calcareous phytoplankton cannot survive a ph lower than 8. This acid rain caused the destruction of many of the calcareous phytoplankton and other calcareous organisms (Prinn and Fegley, 1987).

Even siliceous organisms would have been destroyed in some areas. On the continents many of the insoluble elements would have become soluble due to the acid rain. The elements would have eventually found their way to the ocean where they would have precipitated out. It is in these river deltas that the pH would possibly be low enough to destroy siliceous organisms.

This would severely affect the marine food chain as a decrease in producers (phytoplankton) would cause an exponential decrease in the organisms that feed off them (see diagram).

Life on the continents would also have been seriously affected. The global acid rain (with a pH of 0-1.5 ) would have caused defoliation of plants and asphyxiation of lunged animals (Prinn and Fegley, 1987). This loss in plant life would cause a decline in herbivores. All eggs exposed to the acid rain would have been destroyed.

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