Cretaceous Thermal Maximum, also known as Cretaceous Thermal Optimum, was a period early in the Late Cretaceous notable for its dramatic increase in global temperatures.
Cretaceous Thermal Maximum Wikipedia
Carbon dioxide levels rose to over 1000 parts per million (compared with recent pre-industrial average of 280 ppm), which resulted in a significant increase in the greenhouse effect, and elevated temperatures. In the seas, the glassy form foraminifera predominated, a key indicator of higher temperatures. The Cretaceous Thermal Maximum occurred during the Cenomanian/Turonian transition based from the fact that there was a major disruptions to global climate. It was the most extreme disruption of the carbon cycle recorded in the past 100 million years.
Late Cenomanian sea surface temperatures in the equatorial Atlantic Ocean (~33 °C) were substantially warmer than today (~27-29 °C). They may have been as high as 36 °C. The interval of peak warmth in the Turonian has been attributed to very high atmospheric CO2 and possibly enhanced by the changing geography of the oceans including an "Atlantic Gateway". However, during the Turonian age, several pronounced but relatively short-lived cooler intervals punctuate this otherwise remarkably stable interval of extreme warmth and shows that rapid tropical sea surface temperature changes occurred during the Cretaceous thermal maximum, and implies that even the mid-Cretaceous "super-greenhouse" climate may have been less stable than previously thought.