Low-latitude seasonality of Cretaceous temperatures in warm and cold episodes

The Cretaceous period (144–65 million years ago) is often taken as a good example of Earth's climate system in a greenhouse mode, with Arctic temperatures as high as 20°C in the mid-Cretaceous. But our knowledge of sea-surface temperatures (SSTs) at the time is limited, as it is based largely on microfossils that provide no information on sub-annual variation. The oxygen isotope composition within shells of large-sized rudist bivalves from the Caribbean and Mediterranean is a dataset with a difference: the variation within individual shells is a measure of seasonal SST variation. Rudists are extinct bivalves that lived in warm, shallow oceans at low latitudes; some developed bizarre, occasionally large shells. Oxygen isotope data from these shells show that during warm Cretaceous episodes seasonal variation was low, but during cooler periods the variation was similar to that we experience today, and is compatible with the presence of polar ice sheets.