FROM COOLHOUSE TO SUPERGREENHOUSE AND BACK AGAIN: DEEP-SEA PALEOTEMPERATURE CHANGES AND PLANKTIC FORAMINIFERAL TURNOVER EVENTS DURING THE LAST 50 M.Y. OF THE CRETACEOUS

The most reliable estimates of Cretaceous marine paleotemperatures have been obtained from oxygen isotope analyses of monospecific planktic and benthic foraminifer samples exhibiting no apparent test recrystallization or cement infilling. Sediments yielding such ‘glassy’ foraminiferal preservation tend to be found on passive margins, interior seaways and oceanic plateaus where burial depths were relatively shallow, fluxes of calcium carbonate were low and clay sedimentation rates were relatively high. Efforts to obtain oxygen isotope data from glassy foraminifera have met good success with studies of deep-sea and land-based sections from low, middle and high latititudes, resulting in a relatively complete paleotemperature history that spans nearly 50 m.y. from the late Aptian through the late Maastrichtian. These records reveal a long-term global climate cycle where the late Aptian-early Albian was relatively cool, the late Albian through the Cenomanian experienced long-term and gradual warming, the Turonian experienced extremely warm ‘Supergreenhouse’ temperatures, the Coniacian-early Campanian continued with very warm temperatures, and the middle Campanian-late Maastrichtian experienced temperatures similar to those of the late Aptian-early Albian. Two major planktic foraminiferal extinction events occurred within this long-term climate cycle. The first occurs at the Aptian/Albian boundary where there is a dramatic reduction in planktic foraminiferal shell size, shell ornamentation, species diversity and standing population at the same time as abrupt shifts in oxygen, carbon and strontium isotopes. The cause of this extinction and associated geochemical shifts is not yet understood. The second planktic foraminiferal extinction event occurred just below the Cenomanian/Turonian boundary. Disappearance of only deep-dwelling species at the same time as abrupt bottom water warming suggests that this extinction was caused by greenhouse warming. A much denser array of sections yielding well preserved foraminiferal assemblages is needed to better understand these and other global climate changes, ocean geochemistry shifts and species turnover events that occurred during the Cretaceous.