Micro-Halocline Enabled Nutrient Recycling May Explain Extreme <em>Azolla</em> Event in the Eocene Arctic Ocean

<div><p>In order to understand the physicochemical mechanisms that could explain the massive growth of <em>Azolla arctica</em> in the Eocene Arctic Ocean, we carried out a laboratory experiment in which we studied the interacting effects of rain and wind on the development of salinity stratification, both in the presence and in the absence of a dense <em>Azolla</em> cover. Additionally, we carried out a mesocosm experiment to get a better understanding of the nutrient cycling within and beneath a dense <em>Azolla</em> cover in both freshwater and brackish water environments. Here we show that <em>Azolla</em> is able to create a windproof, small-scale salinity gradient in brackish waters, which allows for efficient recycling of nutrients. We suggest that this mechanism ensures the maintenance of a large standing biomass in which additional input of nutrients ultimately result in a further expansion of an <em>Azolla</em> cover. As such, it may not only explain the extent of the <em>Azolla</em> event during the Eocene, but also the absence of intact vegetative <em>Azolla</em> remains and the relatively low burial efficiency of organic carbon during this interval.</p>