Freshwater fern Azolla in and around Arctic and Nordic seas

Coeval Azolla fossils in neighboring Nordic seas were thought to have been sourced from the Arctic. The recognition of a different Azolla species in the North Sea raised doubts about this hypothesis.

Here we show that no less than five Azolla species had coeval blooms and spread in the Arctic and NW European regions. A likely trigger for these unexpected Azolla blooms is high precipitation prevailing by the end of the warmest climates of the Early Eocene Climatic Optimum (EECO).