Geologic history of reefs

Article By:

Stanley, George D., Jr. Department of Geosciences, University of Montana, Missoula, Montana.

Last reviewed:2014

DOI:https://doi.org/10.1036/1097-8542.YB150961

Reefs in the geologic sense are ancient marine ecosystems that have experienced profound changes during 500 million years of prosperity and collapse. Reefs have experienced high levels of carbon dioxide (CO₂) during their geologic history. The buildup of this greenhouse gas leads to global warming and ocean acidification. Ancient reefs responded to these changes, which can be seen by their frequent collapse followed by long periods in which reefs were absent (reef gaps). Reefs today and in the past were controlled by nutrient levels, sedimentation, sunlight, and temperature. They develop best in tropical settings between latitudes of 30°N and 30°S of the equator. This occurs as a result of an important relationship with symbiotic algae living in the tissues of corals and other reef organisms. These algae exploit photosymbiosis (a symbiotic relationship between two organisms, with one being able to perform photosynthesis), providing substantial nutritional benefit to their reef hosts while also greatly accelerating calcification rates. Photosymbiosis explains why reefs exist today and may well explain their successes and failures through time. Understanding the dynamics of ancient reef systems is central to understanding the current reef crisis.

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