Reconstruction of the Arctic Ocean environment during the Eocene Azolla interval using geochemical proxies and climate modeling. Geologica Ultraiectina (331)

Abstract

With the realization that the Arctic Ocean was covered with enormous quantities of the aquatic floating fern Azolla 49 Myrs ago, new questions regarding the Eocene conditions facilitating these blooms arose. This dissertation describes the reconstruction of paleo-environmental conditions facilitating the large-scale occurrence of the freshwater fern Azolla in the Early/Middle Eocene Arctic and how this bloom might have affected global climate. Comparison of organic geochemical analyses of Eocene Arctic sediments and extant Azolla filiculoides resulted in the discovery of unique biomarkers for both Azolla and its nitrogen fixating cyanobacterial symbiont Anabaena azollae. Lipids identified as 1,?20 C30–C36 diols and structurally related mid-chain hydroxy compounds can now serve as palaeo-environmental indicators for the Arctic Azolla interval and as markers for past occurrences of Azolla in general. The finding of heterocyst glycolipids in the Eocene sediments indicates that diazotrophic cyanobacteria played a major role in adding newly fixed nitrogen to surface waters in the past-stratified Arctic. Moreover, the similar distribution of heterocyst glycolipids in the Eocene Arctic sediments and extant Azolla suggests that the symbiotic relationship between Azolla and diazotrophic cyanobacteria of the family of Nostocaceae was already established in the Early/Middle Eocene and that Azolla arctica production mainly depended on phosphorus supply. Through analyses of the hydrogen isotopic composition (?D) of the novel biomarkers for Azolla and isotope modeling the water sources for and the salinity of the Eocene Arctic basin could be constrained. Model simulations using an isotope-enabled atmospheric general circulation model were validated using ?D values of paleo-precipitation obtained through ?D analyses of terrestrially derived n-alkanes found in sediments from the Azolla interval. The Eocene simulation accurately predicted the occurrence of less depleted precipitation, with ?D values ranging only between 0 and -140‰ (compared to Present-day 0 to -300‰), supporting the prevalence of a strongly reduced meridional temperature gradient during the Azolla interval. The hydrogen isotopic composition of the biomarkers for Azolla encountered in Eocene Arctic sediments fell in the same range as precipitation ?D reconstructed using n-alkanes extracted from the same sediments. This indicates that virtually no mixing could have occurred between deeper saline waters (isotopically heavy) and the freshwater (isotopically light), thus showing that the Arctic surface waters freshened considerably (salinity: 0 to 6) as a result of enhanced moisture transport to Northern high latitudes. Using trace metal budgets also the inflow of saline deepwater into the Arctic was constrained. Combining sedimentary phosphorus data, modeled freshwater fluxes and the reconstructed deepwater inflow in a new mass balance box model showed that a combination of enhanced regeneration of phosphorus and upwelling of phosphorus-rich deepwater is needed to explain the sustained growth of Azolla in the Eocene Arctic. Sustained production and subsequent burial of organic matter during a period of 160.000 to 1.000.000 yrs led to storage of 0.9 – 3.5 1018 g carbon in the Arctic, which resulted in a 55 to 470 ppm drawdown of atmospheric pCO2. This indicates that the Arctic Azolla blooms likely contributed to lowering atmospheric pCO2 levels in the Early/Middle Eocen