Arctic Azolla Bloom Draws Down Atmospheric CO2

Around 49 million years ago, the freshwater fern Azolla colonized the nearly enclosed Arctic Ocean basin over approximately 800,000 years. River runoff created a freshwater surface layer atop the brackish sea, enabling Azolla to spread across an estimated four million square kilometers. Dead fronds sank into anoxic bottom waters where decomposition could not occur, sequestering carbon in sediments reaching at least eight meters thick. Atmospheric CO2 declined from approximately 3,500 to 650 parts per million during this interval.