CONTROLS OF PHANEROZOIC REEF EVOLUTION: A QUANTITATIVE APPROACH

Quantitative attributes of more than 3000 pre-Quaternary Phanerozoic reefs are evaluated numerically based on a global database. The complex reef ecosystem is subdivided into several components, which are analyzed separately. The evolution of geometrical, environmental, paleontological and petrographical reef attributes is tested for systematic trends and correlations with published physico-chemical earth system parameters using a time-slice approach. While there is no significant secular trend in the abundance, diversity or thickness of reefs, several other reef attributes tend to either decrease or increase through time. A significant temporal increase in bioerosion, debris potential, the relative abundance of platform margin reefs, and aragonitic and high-magnesium calcitic reef builders is evident, whereas micrite and marine cement content significantly decrease during the Phanerozoic.

Correlations between reef attributes and earth system parameters are numerous and multifaceted. A principal component analysis of quantified reef attributes and published earth system parameters (e.g. sea level, climate, isotope geochemistry) reveals that four individual factors explain most of the variance in the dataset. 1) Evolution/recruitment of macrofauna: This factor is intrinsic to the reef ecosystem and explains reef diversity, and the development of many compositional and constructional reef types. No published earth system parameter is closely associated with this factor. 2) Continental freeboard/preservation: This factor explains reef abundance, reef carbonate production and reef dimensions. 3) Nutrient level: This factor explains the mineralogical composition of reef builders, the prevalent trophic mode and some compositional attributes. The inference that this factor represents nutrient level is based on the high factor loading of measured 87Sr/86Sr and d34S values in sediments. 4) Climate: Paleoclimate explains the relative importance of algal reefs, and influences reef reservoir potential.

Unexplained variance in the dataset is likely controlled by short-term events that have the potential to fundamentally change reef attributes without being accompanied by long-term physico-chemical changes.