Stratigraphy, paleoceanography, and evolution of Cretaceous Pacific guyots: relics from a greenhouse Earth

Many guyots in the north Pacific are built of drowned Cretaceous shallow-water carbonates that rest on edifice basalt. Dating of these limestones, using strontium- and carbon-isotope stratigraphy, illustrates a number of events in the evolution of these carbonate platforms: local deposition of marine blackshales during the early Aptian oceanic anoxic event; synchronous developmentof oolitic deposits during the Aptian; and drowning at different times during the Cretaceous (and Tertiary). Dating the youngest levels of these platform carbonate shows that theshallow-water systems drowned sequentially in the order in which plate-tectonicmovement transported them into low latitudes south of the Equator (paleolatitude 0°-10° south). The chemistry of peri-equatorial waters, rich in upwelled nutrients and carbon dioxide, may have been a contributory factor to the suppression of carbonate precipitation on these platforms. However, oceanicanoxic events, thought to reflect high nutrient availability and increased productivity of planktonic organic-walled and siliceous microfossils, did not occasion platform drowning. Neither is there any evidence that relative sealevel changes were the primary cause of platform drowning, which is consistent with theestablished resilience of shallow-water carbonate systems when influenced by such phenomena. Comparisons with paleotemperature data show that platform drowning took place closer to the Equator during cooler intervals, such as the early Albian and Maastrichtian, and farther south of the Equator during warmer periods such asAlbian-Cenomanian boundary time and the mid-Eocene. Initiation of one carbonate platform relatively close to the Equator, at paleolatitudes more northerly thanthose where others drowned, took place during the cool early mid-Aptian. Thesecorrelations are in accord with an interpretation that excess warmth in shallowperi-equatorial waters proved inimical for many carbonate-secreting organismsliving on the platforms, allowing subsidence or eustatic sealevel rise to out pace sedimentation and guyots to form. A parallel may be drawn with the recent phenomenon of coral and for a miniferal bleaching, whereby photosynthetic symbionts succumb to prolonged high temperatures (G30°C) and the host organismdies. The fact that most Cretaceous guyots reside in the north Pacific may not besolely related to the age-distribution pattern of ocean floor but to their having run the gauntlet of a difficult and dangerous passage across the Equator. North Pacific guyots are relics from the Cretaceous (and Eocene) ‘‘greenhouse’’ Earth