(Palaeo)-ecology and biogeochemistry of the freshwater fern Azolla and its importance in global biogeochemical cycles
During the Arctic Coring Expedition (ACEX) 302 of the Integrated Ocean Drilling Program (IODP) unique central Arctic drill cores have been recovered from the Lomonosov Ridge (Backman et al., 2006; Brinkhuis et al., 2006). Preliminary analyses of the laminated sediments has shown that enormous quantities of the free floating, freshwater fern Azolla grew and reproduced in situ in the mid Eocene (~48,5 Ma) Arctic Ocean for a period of at least 800 kyrs. Additional cores taken during commercial coring in the Arctic Basin and in the adjacent Nordic Seas have also been reported to contain huge amounts of Azolla remains (Brinkhuis et al., 2006).
Azolla is a free floating aquatic fern, which nowadays can be found in freshwater environments in temperate and tropical regions all over the world. It ranks among the fastest growing plants on Earth and due to its association with the nitrogen fixing cyanobacteria Anabaena azollae, it is independent of extern organic nitrogen. In that way Azolla is not only known to fix huge amounts of carbon, but as well to produce vast amounts of organic nitrogen. The nutrient, which mainly limits the growth of Azolla, is phosphorous.
From the presence of the Azolla blooms, covering the entire Arctic basin, interesting information about the prevailing environmental conditions can be derived. Due to its intolerance for saline water, its presence implies that at least the surface waters must have been fresh during extended episodes. Furthermore, it is believed that this event might have influenced global carbon, nitrogen and phosphorus cycles. Notably, the Azolla event occurred precisely at the transition from a Greenhouse to an Icehouse Earth.
From a biogeochemical cycling as well as from a global climate change perspective we thus identify a strong need for better constraining the capability of Azolla as a sink for atmospheric CO2 in a fresh Eocene Arctic Ocean, and assess its potential role as a regulator of regional and global nutrient cycles.