NUTRIENTS FROM LAND AND PHANEROZOIC REEF DEVELOPMENT

Modern reefs are sensitive to elevated nutrient concentrations owing to the adaptation of reef-building zooxanthellate corals to oligotrophic settings. The history of reef building, however, includes periods, when nutrient-opportunistic reef builders dominated the global reef factory. This could be attributed to elevated nutrient concentrations in the oceans caused by enhanced influx from the continents. The hypothesis is tested with a paleogeographic database on Phanerozoic reefs and published information on geochemical proxies and terrestrial evolution.

Using the Phanerozoic Strontium isotope curve as a rough proxy for continental weathering and influx of terrestrial nutrients through time, several significant correlations indicate that changes in terrestrial weathering indeed have a significant influence on reef composition. Positive excursions of the Sr curve coincide with a reduction of the mean alpha diversity in reefs, an increase in aragonitic reef builders, and a rise of microbial mounds.

Qualitatively, major ecological changes in terrestrial vegetation consistently coincided with major changes in reefal ecosystems: (1) the Late Devonian rise of deeply rooting trees and enhanced chemical weathering was followed by a major reef crisis and a bloom of nutrient-opportunistic microbial reef builders; (2) the Carboniferous spread of tropical rain forests coincided with a global expansion of nutrient-opportunistic algal reefs; (3) the latest Carboniferous-Permian poleward shift of land-plant productivity due to heating and drying of the tropics agrees with a restriction of nutrient-opportunistic microbial-bryozoan reefs to subtropical settings; (4) a major rise in terrestrial productivity in the Cretaceous was accompanied by a global decline of oligotrophic scleractinian coral reefs; (5) the Neogene expansion of grasslands coincided with a major bloom of coral-algal reefs both in the tropics and subtropics. These patterns suggest close connections between terrestrial and marine ecosystems that operate via evolutionary innovations, climate change, and nutrient fluxes. More rigorous tests are needed but currently difficult owing to many uncertainties in the processes of recent terrestrial-marine teleconnections and the nutrient requirements of ancient reef builders.