Secular Variations in the Phanerozoic Reef Ecosystem
Published:January 01, 2002
Quantitative attributes of more than 3000 pre-Quaternary Phanerozoic reefs areevaluated numerically based on a comprehensive computer database. The complex reef ecosystem is subdivided into several components, which are analyzed separately at different stratigraphic resolutions (stages, supersequences). The evolution of geometrical, environmental, paleontological, and petrographical reef attributes is tested for systematic temporal trends and correlations, and potential controls on reef evolution are evaluated by comparing changes in reef parameters with changing earth system parameters such as paleoclimate, sea level, and chemical evolution of the oceans.
While there is no significant secular trend in the number of reef per unit time or in mean reef thickness, several other reef attributes tend to either decrease or increase through time. A significant temporal increase of bioerosion, debris potential, the relative abundance of platform margin reefs, and aragonitic or high magnesium calcite reef builders is evident, whereas contents of micrite and marine cement decrease significantly during the Phanerozoic. Overall within-reef diversity of reef builders shows a weak increase through the Phanerozoic, which is in contrast to the pronounced apparent increase in Phanerozoic global marine diversity.
The average paleolatitude of reefs decreased through the Phanerozoic, suggesting that reefs progressively became better adapted to high-latitude settings. The latitudinal range of tropical pre-Quaternary reefs does not seem to be related to paleoclimatic changes, except for the Cenozoic. However, particular reef attributes such as reef dimensions, content of marine cement and micrite, reservoir quality, preferential environmental setting, and prevailing reef builders change with paleolatitude in all time slices and can thus be used for paleogeographic analyses. The patterns of global reef distribution indicate that Paleozoic reefs were probably more nutrient-opportunistic than Mesozoic and especially Cenozoic reefs. The Phanerozoic history of reef building can statistically be divided into two intervals: a Paleozoic interval and a Mesozoic-Cenozoic interval.
Correlations between various reef attributes are common both for the analysis of individual reefs and for reef attributes averaged over stages and supersequences. The strongest and most meaningful correlations in the separate analysis point to the following: (1) Diversity and ecological succession are greatest in reefs with a high debris potential and in shelf/platform-margin reefs. This relation underscores the role of disturbance in maintaining ecological diversity. (2) Shelf-margin reefs are the significantly largest and most diverse reefs with the highest debris potential, and the greatest reservoir potential. (3) High micrite content is associated with microbial abundance, low debris potential, and low bioerosion intensity, indicating that micrite formation by bioerosion was not a common process but was more often related to microbial activity. The most remarkable correlations in averaged reef attributes are positive relations between the number of reef sites, mean reef diversity, mean debris potential, and the abundance of reefal reservoirs. Thus, factors favoring global development of high diversity also favor the global expansion of reefs.
Correlations between reef attributes and earth system parameters are numerous and multifaceted, but are rarely significant on a 99% confidence level, if detrended values are analyzed. Reef abundance and reef diversity are rarely correlated with global physicochemical factors. On long time scales, reef abundance appears to be governed mostly by the diversity of potential reef builders, such as corals, sponges, bryozoans, and rudists. Reef diversity is weakly inversely correlated with the 87Sr/86Sr ratio in sedimentary apatite/carbonate, suggesting that enhanced continental weathering may lower reef diversity on long time scales. A supersequence-based principal component analysis of most variables discussed in this paper indicates that four major and largely independent factors control most of Phanerozoic reef development: (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. (2) Carbonate productivity/continental freeboard: 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 loading of measured 87Sr/86Sr and δ34S values in sediments. (4) Climate: Paleoclimate explains the relative importance of algal reefs, and influences reef reservoir potential.
Unexplained variance in the data set is likely controlled by two hypothetical factors: (1) short-term events that have the potential to fundamentally change reef attributes without being accompanied by long-term physicochemical changes; and (2) unmeasured or geographically unresolved geological variables such as storm intensity through time, climatic variations at different latitudes, and regional tectonosedimentary development.
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Phanerozoic Reef Patterns
Detecting patterns and processes of ecosystem evolution is among the main challenges of an integrated earth system science in the 21st century. The evolution of reefs reflects changes triggered by evolutionary innovations and variations in global and regional controls at different scales. The prime fossil record of Phanerozoic reefs offers the opportunity to trace these patterns through space and time. Phanerozoic Reef Patterns presents a comprehensive and up-to-date review on the history of reef building in the last 540 million years. A selection of internationally respected reef specialists presents a database on ancient reefs that is hardly available for any other ecosystem. The thoroughly documented patterns are analyzed with respect to global change, whose impact on living reefs is intensely discussed today. Phanerozoic Reef Patterns stands out from recent reviews on reef evolution by its careful qualitative and quantitative approach based on a comprehensive and multifaceted databank, by the strong focus on data, by a complete and unified coverage of the Phanerozoic from the Early Cambrian to the late Neogene, by emphasizing paleogeographic reef distributions presented on 32 newly developed color maps, and by a detailed index that makes the book a valuable research tool.