Did a single species of Eocene Azolla spread from the Arctic Basin to the southern North Sea?

Recent Arctic drilling has revealed that the freshwater surface-floating heterosporous fern Azolla arctica Collinson et al. (Azollaceae, Salviniales) bloomed and reproduced in the Arctic Ocean on a massive scale during the early Middle Eocene. These blooms have been suggested to have been capable of significant drawdown of atmospheric CO(2) paving the way to Cenozoic climatic cooling. Sites of similar age across the Arctic and Nordic Seas also contain Azolla fossils suggestive of an area much larger than the Arctic Ocean being affected by Azolla blooms, as far south as Denmark. Here we investigate the Danish occurrences known from the Lillebaelt Clay Formation, transitional Ypresian/Lutetian in age (latest Early Eocene to earliest Middle Eocene). The Lillebaelt Clay is a marine deposit rich in diverse organic-walled dinoflagellate cysts yet conspicuously characterized by abundant co-occurring and interconnected fully mature Azolla megaspores and microspore massulae. Perhaps surprisingly, we find that multiple morphological and ultrastructural characters distinguish the Danish Azolla species from Azolla arctica and it is here described as Azolla jutlandica sp. nov. Therefore, contrary to expectations given the overlapping age of these assemblages, it appears that not a single Azolla species has spread from the Arctic to the Southern North Sea either through freshwater spills from the Arctic Ocean or as a result of rapid spread due to highly invasive biology. Apparently Northern Hemisphere middle and high latitude conditions near the termination of a period known as the Early Eocene Climatic Optimum (EECO) were suitable for proliferation of two different Azolla species, one in the Arctic Ocean and one in the southern North Sea. (C) 2009 Elsevier B.V. All rights reserved.