Low-latitude glaciation in the Cretaceous greenhouse: reviewing the cryosphere reach during an archetypal hothouse Earth

The traditional "Hothouse–Icehouse" dichotomy and the prevailing "Cretaceous greenhouse" narrative fail to accurately represent the geological record. Geological evidence reveals an unknown Late Cretaceous glaciation (82.8–80.96 Ma, the Campanian Barrika glaciation), with tidewater glaciers grounded at an unusually low palaeolatitude (35°N), at a time when Mesozoic temperatures have been modelled near their highest. The Barrika glaciation constitutes the last known low-latitude glaciation on Earth since the Last Paleozoic Ice Age (LPIA), which reached 30°N. The Barrika glaciation is characterized by a remarkably well-preserved glaciomarine record of subtropical tidewater glaciers associated with outlets of an extensive ice cap in Iberia. Our multiproxy analysis reveals five distinct glaciomarine units, indicative of glacial advances and retreats with a 360-kyr spacing. Calving fronts of tidewater glaciers delivered large icebergs to the palaeo-Atlantic Ocean. This glaciation correlates with a peak of ultra-depleted δD ice-sheet-related meltwater signals from Antarctica and other independent indicators of global change. This discovery of a low-latitude glaciation during a purported 'hothouse' period fundamentally challenges simplified Cretaceous climate models. It underscores the critical need for refined paleoclimate proxies and integrated Earth system modelling to fully comprehend such transient yet significant glacial episodes. The robust multiproxy workflow developed for the Barrika glaciation offers a powerful tool for identifying other unknown glaciations in deep-time greenhouse stages. Despite its generally warm reputation, the 77.06-million-year-long Cretaceous Period surprisingly records the lowest latitudinal glaciation since the Paleozoic. Remarkably, 55% of this time shows evidence of meltwaters linked to Antarctic ice sheets, with ice-rafted debris and glacial deposits present for 53% of the period. Glendonites, indicators of cold conditions, are found in 24% of Cretaceous time, and glacio-eustasy played a significant role in short-term sea-level changes for a striking 86% of the period. Collectively, this evidence of an active Cretaceous cryosphere is strengthened by evidence of permafrost in plateaus and high-altitude deserts, coupled by robust geochemical palaeoclimate proxies. Our findings suggest that the conventional 'hothouse–icehouse' scheme applied on deep-time climate requires reconsideration, pointing instead to a much more complex Earth climate evolution that will require a thorough re-evaluation of geochemical proxies used during the Mesozoic.