Cretaceous history of Pacific Basin guyot reefs: a reappraisal based on geothermal endo-upwelling
Abstract The mid-Cretaceous histories (origin, growth, death) of algal-rudist-stromatoporoid reef communities located on many Pacific Basin guyots are complex and controversial. These shallow water, tropical communities originated on volcanic edifices extruded during the Barremian-Albian, grew upward during edifice subsidence/transgression throughout the Aptian, Albian and Cenomanian and several of them died almost synchronously near the Cenomanian-Turonian boundary. During their periods of origin and growth, we postulate that the reef ecosystems received dissolved oxygen by wave surge and nutrients by geothermal endo-upwelling. By this process oceanic waters of intermediate depth (approx. 500–1500 m) were: (a) drawn into the weathered and fractured volcanic summit and lower part of the older reef and driven upward through the porous framework by the remnant geothermal gradient of the volcanic foundation and (b) emerged at the reef surface to support the high metabolism of the living community. The death of most of the reefs near the Cenomanian-Turonian boundary approximately coincides with the most intense oceanic anoxic event (OEA) in Pacific Ocean history. During this OAE the chemistry of the endo-upwelled fluids arriving at the reef surface changed from nutrient/oxygen-rich to dysoxic-anoxic-toxic, and killed the community. Additionally, the combination of foundation subsidence and global transgression reduced aeration of interstitial reef waters by wave turbulence. The post-mortem history of the guyot summits consisted of: (a) drowning to prevent revival of the reef communities, (b) deposition of Turonian-Cenozoic fossiliferous pelagic sediments in the former atoll lagoons and dead reef debris and (c) minor erosion (submarine; karstification?) of the dead reef frameworks.