Through analyses of unique microlaminated sediments of Arctic drill cores, recovered from the Lomonosov Ridge in the central Arctic Ocean during Integrated Ocean Drilling Program (IODP) Expedition 302, it has been shown that enormous quantities of the free floating freshwater fern Azolla grew and reproduced in situ in the Arctic Ocean during the middle Eocene (Brinkhuis et al. 2006). In the late-middle Eocene the Arctic Ocean was almost completely enclosed as the Fram Strait was not fully open yet (Scotese 1988) and sea surface temperatures were high around ~13 °C (Brinkhuis et al. 2006). The presence of the freshwater fern Azolla, both within the Arctic Basin and in adjacent Nordic seas, suggests that at least the sea surface waters were frequently dominated by fresh- to brackish water during an interval of 800 kyr, facilitating basin wide stratification. In effect, around this time (~49-48 Ma), the transition from a greenhouse towards the modern icehouse Earth began (Zachos et al., 2001; Tripati et al., 2005), possibly facilitated by atmospheric pCO2 changes (Pearson and Palmer 2000).
The sustained growth of Azolla, ranking amongst the fastest growing plants on earth, in a major anoxic oceanic basin might have contributed to decreasing atmospheric pCO2-levels. However, what the consequences of these enormous Azolla blooms were for regional and global nutrient cycles and to which degree the Arctic Basin became fresh is still largely unknown. Comparing samples of extant Azolla, including its nitrogen fixating cyanobacterial symbionts, with samples from the Arctic Azolla interval revealed the presence of a group of highly specific biomarkers. These well-preserved long chain mid-chain ω-20 hydroxy wax constituents may serve as paleoindicators for the Eocene Arctic Azolla interval. Compound specific stable hydrogen isotope δD values of these biomarkers should provide insight into the degree of mixing between high salinity (isotopically heavy) deeper and low salinity surface water (isotopically light). The results of these isotope analyses are evaluated using climate modeling experiments. The Community Climate System Model (CCSM3) and Community Atmosphere Model (CAM3), developed by NCAR, have been set up to run with an offline isotope code, which tracks phase changes of water and thus the isotopic composition of incoming precipitation and run-off. Initial results of runs with Eocene boundary conditions, including Eocene topography, bathymetry, vegetation patterns and 2000ppm CO2, show the presence of an intensified hydrological cycle with precipitation exceeding evaporation in the Arctic region. The combined notion of the δD values of freshwater input into the Arctic Ocean and the fractionation of extant Azolla with respect to δD in the defined biomarkers, enables quantification of Eocene Arctic salinity, which in turn will shed light on the degree of (seasonal) mixing and hence potential organic carbon storage and nutrient recycling.