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Fern Azolla key role in Eocene Arctic climate

On 5 November 2010 Eveline Speelman will defend her PhD thesis. Her Darwin research project was focussed on Eocene Arctic climate change and the role of the fern Azolla. Speelman has shown that the symbiotic relationship between Azolla and Anabaena Azollae dates back to the Eocene and that it contributed to the fast growth of the fern Azolla. The Arctic ocean freshened considerably during this period.


No ice-sheets

During a period in the Eocene, 49 Myrs ago, the was not covered with ice-sheets, but with enormous quantities of the aquatic floating fern Azolla. This freshwater fern currently ranks amongst the fastest growing plants on earth and lives in symbiosis with nitrogen-fixating cyanobacteria.


Symbiotic relationship

Organic geochemical analyses of Eocene Arctic sediments and extant Azolla filiculoides resulted in the discovery of unique biomarkers for both Azolla and its symbiont Anabaena azollae. The finding of these lipids in the Eocene Arctic sediments indicates that the symbiotic relationship dates back to the Eocene and that it contributed to the past fast growth of Azolla.


Ocean water freshened

Through analyses of the hydrogen isotopic composition (dD) of the novel biomarkers for Azolla and isotope modelling we found out that the surface waters of the Eocene Arctic Ocean freshened considerably (salinity: 0 – 6). Enhanced moisture transport to the Northern high latitudes, resulting from the prevalence of a reduced meridional temperature gradient in a warm greenhouse world, led to the influx of excess freshwater runoff into the Arctic. 

At the same time, the inflow of saline deeper waters explains the strong stratification and corresponding oxygen depleted bottom water conditions of the Arctic.


Azolla bloom

Combined with the enhanced influx and regeneration of phosphorus, this led to the sustained production of Azolla during a period of 160.000 to 1.200.000 yrs in the Eocene. Enormous quantities of organic material derived from Azolla were subsequently stored in the Arctic sediments, which resulted in a world-wide decrease in atmospheric pCO2, thereby contributing to the transition from the Eocene greenhouse to our present-day temperate climate.


Promotor: Prof. dr. ir. Jaap Sinninghe Damste

Co-promotor: Dr. Gert-Jan Reichart

Date: 5 November 2010
Time: 10.30 a.m.
Location: Academiegebouw, Domplein 29, Utrecht.

Darwin program 1070: The DARWIN Azolla project – (palaeo) ecology and biogeochemistry of the freshwater fern Azolla and its importance in global biogeochemical cycles