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-wa ter carbonates that rest on edifice basalt, Dating of these limestones, usi ng strontium- and carbon-isotope stratigraphy; illustrates a number of even ts in the evolution of these carbonate platforms: local deposition of marin e black shales during the early Aptian oceanic anoxic event; synchronous de velopment of 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 the shall ow-water systems drowned sequentially in the order in which plate-tectonic movement transported them into Tow latitudes south of the Equator (paleolat itude similar to 0 degrees-10 degrees south), The chemistry of peri-equator ial waters, rich in upwelled nutrients and carbon dioxide, may have been a contribute factor to the suppression of carbonate precipitation on these pl atforms. However, oceanic anoxic events, thought to reflect high nutrient a vailability and increased productivity of planktonic organic-walled and sil iceous; microfossils, did not occasion any changes were the primary cause o f platform drowning, which is consistent with the established resilience of shallow-water carbonate systems when influenced by such phenomena. Comparisons with paleotemperature data show that platform drowning took pla ce closer to the Equator during cooler intervals, such as the early Albian and Maastrichtian, and farther south of the Equator during warmer periods s uch as Albian-Cenomanian boundary time and the mid-Eocene. Initiation of on e carbonate platform relatively close to the Equator, at paleolatitudes mor e northerly than those where others drowned, took place during the cool ear ly mid-Aptian, These correlations are in accord with an interpretation that excess warmth in shallow peri-equatorial waters proved inimical for many c arbonate-secreting organisms living on the platforms, allowing subsidence o r eustatic sealevel rise to outpace sedimentation and guyots to form. A par allel may be drawn with the recent phenomenon of coral and foraminiferal bl eaching, whereby photosynthetic symbionts succumb to prolonged high tempera tures (> 30 degrees C) and the host organism dies. The fact that most Creta ceous guyots reside in the north Pacific may not be solely related to the a ge-distribution pattern of ocean floor but to their having run the gauntlet of a difficult and dangerous passage across the Equator. North Pacific guy ots are relies from the Cretaceous (and Eocene) "greenhouse" Earth.