General evolution of carbonate reefs

Reefs have shown some unifying features over their 3.5-billion-year history. Reefs have shown zonation in response to environmental gradients from their inception, and metazoan reefs have been differentiated into open surface and cryptic reef communities for their entire ~550-million-year history (Wood et al., 2002). Tectonics drives many aspects that control reef growth including sea level, circulation patterns, climate, and evolving seawater chemistry change over long timescales. Reefs have always been most numerous on passive continental margins at low latitudes where they typically form prograding or aggrading barriers (with growth during transgressive and highstand periods and erosion during longer low stand periods), or fringing reefs. By contrast, reef-forming toward the cratonward side of foreland basins is relatively rapid and reef geometries more aggradational. Finally, reefs forming in strike-slip basins and on thrust complexes are affected by the vagaries of local tectonic movements, which may be highly episodic but involve substantial displacements and are thus highly unpredictable (James and Wood, 2010). Climate is a major controlling force on the evolution of reefs on both short and long timescales, as the latitudinal range of carbonate-producing species is largely governed by temperature and carbonate supersaturation. Reef growth therefore shows cyclicity at all scales in response to shortterm oscillations (e.g., Milankovitch and glacial–interglacial cycles) as well as to longer-term climatic intervals, driven by slower, tectonically driven processes. Global climate has oscillated through greenhouse and icehouse phases, in concert with aragonite and calcite seas, respectively. During icehouse times of continental glaciation, high eustatic sea-level changes created aggrading reef growth, common subaerial exposure and conspicuous regional disconformities in reef complexes. These large changes in sea level caused ramps to have steep gradients, and platform tops to have significant depositional relief characterized by pinnacle reefs and erosional topography. Icehouse reefs are typically dominated by heterotrophs or autotrophs with aragonitic or high-Mg mineralogies. During greenhouse times, with little global ice, reef sequences were generated by small sea-level fluctuations. Reef cycles typically consist of very shallow-water facies, with regional-scale tidal flat caps and minor disconformities. Greenhouse reefs are often either compound or progradational as reef growth could easily match or outpace the fastest rates of sea-level rise. Ramps often have very low gradients, and platforms tend to be progradational, with minor topography. Greenhouse reefs can also have a more extensive range as carbonate settings extend into higher latitudes due to elevated global temperatures. Composition may also be more uniform, often dominated by low-Mg calcite skeletal components. Biological disturbance, that is bioturbation, boring, and excavatory predation and herbivory, has clearly escalated since the Mesozoic. Reef biotas have responded with the proliferation of traits with proven anti-predatory benefits, particularly regeneration after partial mortality. Indeed some modern dominant reef taxa, such as branching corals and coralline algae, appear not only to thrive, but actually require conditions of considerable disturbance for their survival in shallow tropical seas (Wood, 1999, 2002). Many modern reefs are largely reduced to rubble and sand via physical abrasion and bioerosion. Modern coral reefs are also dominated by branching forms due to their high diversity and abundance, propensity to proliferate via fragmentation, and resilience to taphonomic destruction. Many reefs prior to the Jurassic show the preservation of intact, in situ frameworks. Most epibenthic, soft-sediment dwelling organisms typical of Paleozoic reefs appear to have become largely absent from shallow marine tropical reef biotas during the late Paleozoic to early Mesozoic, perhaps due to intolerance of the rise of deep burrowing taxa and excavatory attack (Wood, 2010). Scleractinian corals, in particular branching taxa, show a marked increase in the proportion of forms with complex modularity from the Eocene onward, even though corals displayed the full range of morphological forms and corallite size by the Late Triassic. Highly defended, thick crusts in coralline algae become more dominant, and branching forms also become noticeably less conspicuous on reefs from the Eocene onward. This major reorganization of the coral reef ecosystem coincides with the rapid appearance and radiation of herbivorous and coral-eating reef fish, but this remain to be tested experimentally (Wood, 2002).