Robert J. Stanton Jr.
Nutrient models for the development and location of ancient reefs

Geo.Alp 3, 2006, p. 191–206

Abstract
The primary function of reef builders, from a geological perspective, is the in situ production of carbonate sediment at a relatively high rate to create a biogenic carbonate body with relief above the sea floor. Reef-building metazoans fall into two broad categories based on the mechanism by which they produce skeletal carbonate — those with primary (autotrophic or photosymbiotic) production through the aid of photosynthetic algae or bacteria, and those with secondary (heterotrophic) production. In modern reefs, carbonate production by photosymbiotic organisms is strongly correlated with low-nutrient, oligotrophic settings and efficient metabolism, whereas carbonate production by heterotrophic reef builders depends on high concentrations of organic nutrients.
Photosymbiotic reef builders are largely responsible for the construction of modern tropical shallow-water reefs, and this appears to have been the case back into the Mesozoic. This conclusion is based on the geologic range of scleractinian corals, the dominant constructors of modern shallow–water reefs. However, the existence of photosymbiotic reef builders in the geologic record cannot otherwise be definitively established. Heterotrophs are responsible for modern deep- and cold-water reefs as well as contributing to the growth of shallow-water reefs. In the absence of firm evidence for photosymbiotic carbonate producers in the geologic record, they are postulated to have been the dominant carbonate-producing metazoans responsible for ancient reefs as well.
In addition to metazoans, photoautotrophs (algae and bacteria growing independently of a metazoan host) were also major carbonate contributors to ancient reefs and mounds. Because carbonate production by both photoautotrophs and heterotrophs depends on nutrient supply rather than the oligotrophic conditions optimal for modern photosymbionts, explanations for the sites and characteristics of ancient reefs must focus on the phenomena and settings that result in sources of abundant nutrients at the geographic scale of reef growth. These nutrient sources consist of localized terrestrial influx, cold seeps, endo-upwelling, and the oceanographic phenomena of upwelling, oxygen minimum zones, and internal waves. Analysis of ancient reefs in terms of nutrient source provides the opportunity, through an alternative paradigm than that provided by modern shallow-water reefs, to understand fundamental controls on growth and distribution of reefs in the geologic record.


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