The evolution of reefs

Category Other
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Wood, Rachel
Holding Date 10 September 2008

Reefs have long fascinated natural historians and geologists for their unearthly beauty as well as their ability to produce prodigious amounts of carbonate sediment. Darwin recognised that reefs offered more than their share of paradoxes: How do coral reef islands seemingly grow from great depths in the middle of the oceans? What controls the production of all this limestone? And why do so many of these reefs form necklaces strung across the Pacific? Darwin used his formulation of the accumulation of slow and gradual phenomena to account for the complex patterns of reef growth he observed from the Beagle. Indeed he solved the ‘coral reef problem’ long before final proof of the origin of reefs was forthcoming with the deep drilling of atolls in the 1950s. We now know that the 3 billion year history of reefs is closely tied with the history of the oceans: changing sea water chemistry has dictated the mineralogy of reef organisms, mass extinctions creating global perturbations in the carbon cycle have caused reef growth to cease, and evolutionary innovations such as increasingly novel methods of predation and symbioses have allowed reef organisms to exploit new ecological niches. For example, there has been a proliferation of traits with proven anti-predatory benefits since the Mesozoic, and some forms, such as branching corals appear not only to thrive, but actually require conditions of considerable disturbance for their survival in shallow tropical seas. Coral reefs worldwide are now undergoing dramatic and far-reaching change, and many of these changes are historically recent phenomena. Most notable are the increase of soft-bodied algal cover and biomass, and the decline of corals in the Caribbean. This phase-shift from coral to algae dominance has also led to reduced coral biodiversity and a precipitous decline in rates of calcification. That these changes are taking place is unequivocal; but the causes and controls are far less clear. They have all been variously attributed to anthropogenic impacts, either directly as a result of changing land use, overfishing and pollution, or to a multitude of indirect effects caused by global warming and ocean acidification. With the likelihood of accelerated degradation as a result of synergies between these causes, reefs clearly face an uncertain future.