Glacio-eustasy during the early Bajocian peak cooling events (Middle Jurassic)

The Middle Jurassic is characterized by cold climate conditions following the early Toarcian super-warmth. The presence of perennial ice caps during this time interval, and therefore a better appreciation of the magnitude of this cooling, remains however an unsettled issue. Here, we demonstrate that ample (ca. 30 m) and rapid (Milankovitch-scale; 40–100 kyr) sea-level fluctuations observed in the lower Bajocian (Propinquans and Humphriesianum ammonite-zones) of the Central High Atlas Basin of Morocco coincide with transient seawater cooling (as documented by a newly acquired high-resolution belemnite δ 18 O dataset from South-East France). This positive correlation uniquely points at the rhythmic growth and decay of polar ice caps during the Middle Jurassic, whose volume could have reached up to half of the current Antarctic ice volume. • High-resolution sequence stratigraphy analysis reveals two episodes and rapid and ample sea-level changes during the late Early Bajocian. • Belemnite δ 18 O dataset indicate that these episodes are concomitant with two cooling episodes. • Implies Milankovitch-paced transient growth and decay of polar ice sheets.