The Late Cretaceous period is described as being a “cool greenhouse” climate, with temperatures somewhat warmer than today and elevated atmospheric CO2 levels of 400-1000 ppm. Exactly how much warmer, and the spatial pattern of that warming, remain under debate due to disagreements between different temperature proxies and model output. I will present new clumped isotope temperature data from the Maastrichtian age (72-66 Ma) and compare it to existing proxy data and new global climate model simulations. The clumped isotope proxy also measures the isotopic composition of seawater (δ18Oseawater), a quantity that has not previously been constrained for the Cretaceous and is assumed to be -1‰ in most paleoclimate studies. With this new data we can directly test whether these assumptions are valid. This talk will have three focal points – a global look at equator-to-pole temperature and δ18Oseawater gradients, a regional study of δ18Oseawater and salinity in the disappearing Western Interior Seaway, and an in-depth look at climate on the Antarctic Peninsula at the end of the Cretaceous. This last vignette will look at the climate impacts of Deccan Traps volcanism and the Chicxulub meterorite impact event that, together or separately, wiped out the dinosaurs and caused the most famous of the “big five” mass extinctions.