The Azolla event, or The Little Fern That Could

*Republished from Palavar, volume 4, issue 1

http://palaverjournal.com/wp-content/Spring%202014/Spring%202014_Final/Spring%202014_Final.swf

Today, climate change is a subject of intense and heated debate.  It’s an issue that has several dimensions, each with serious implications for the entirety of life on Earth.  There is evidence beyond doubt that the global climate has changed drastically throughout the course of Earth’s history for several reasons, including the movement of tectonic plates, natural disasters, shifting ocean currents, and both the Earth’s eccentricity and its Milankovitch cycles around the sun.  The theory that is currently contested, however, is that humans are responsible for a recent increase in the planet’s temperature due to the industrial and agricultural emission of CO₂ and other greenhouse gases.  Climatologists almost unanimously agree that these emissions can and will have a significant effect on the global climate; however, some groups, most notably the American media, still convey doubt.  It is often difficult to make complex theories, with several variables, palpable enough so that we can understand them; climate change is not an exact science, after all.

                  One of the ideas that has been used to dispute the theory of anthropogenic climate change takes a somewhat humble view of the human race: how could humans, who are just one of millions of species, have enough impact to change something as enormous as the climate of an entire planet?  It does seem unlikely when viewed in this context.  But one group of organisms having an undue influence on the temperature of our planet is not without precedent; there is evidence to suggest that it’s happened before, about 50 million years ago to be precise¹.

                  The discovery was made back in 2006 by a team of scientists working for the International Ocean Discovery Program (IODP), an organization that regularly sends out ships to collect data from beneath the ocean floor.  While drilling in the Arctic Ocean, they were able to retrieve sediment cores, parts of which were up to 80 million years old (back when the dinosaurs were still around).  But they found something unexpected in the sediment; at about the 50 million year mark, there was a very dark layer of what was later identified as mostly the preserved cells and spores of a fern.  As if that wasn’t strange enough, the spores belonged to a fern that grows only in freshwater, such as in ponds and streams.  So what was this fern doing at the bottom of the Arctic Ocean?

                  Well, it turns out that the climate on Earth about 50 million years ago was very different from the one we have today.  During this time (Eocene Epoch), temperatures skyrocketed well beyond their usual norm.  Deciduous forests were flourishing in Antarctica, and amphibians and reptiles, such as alligators and turtles, inhabited the Canadian Arctic.  Tropical plants, such as palm trees, could be found as far north as Wisconsin and even Alaska²!  The Rocky Mountains were also beginning to form and were dotted with active volcanoes, and several new species of mammals began to evolve and assert their dominance on the landscape³.  Life was literally rampant!

The Arctic Ocean also looked a lot different; for one thing, it wasn’t covered in ice, but it was also much more landlocked than it is today.  If you look at a map of the Arctic, you can see that the Eurasian and North American continents enclose an area that is mostly isolated from everything but a broad stretch of the Atlantic Ocean.  During the Eocene, however, this enclosure was much more pronounced, and the Arctic Ocean had only two very small connections with other bodies of water. 

(Barke et al. 2012)

Several rivers also flowed from Asia and North America into the Arctic, and since the Arctic was buffeted by continents, this freshwater had nowhere to go; it therefore accumulated on the surface of the ocean (freshwater is less dense than saltwater).  This brings us back to our mystery, for which we can now reasonably connect the dots.  The high temperature of the Eocene, along with the layer of freshwater that covered the Arctic Ocean, made it possible for aquatic plants to colonize it. 

Enter the genus Azolla, an aquatic fern that has a wide distribution throughout the world.  Not only is Azolla ‘just’ a fern, but it’s a tiny one at that, with the leaves of some species measuring less than an inch in length⁴.  Today, Azolla is considered something of a super plant.  It’s used extensively in the production of rice, since it harbors bacteria in its leaves that can fix atmospheric nitrogen (an important ingredient in fertilizers).  It also grows extremely fast; in just 3-5 days, it can completely double in size⁵.  Finally, in addition to its other amazing qualities, this fern can act as a highly effective carbon sink.  Plants get most of their carbon from atmospheric CO₂, but Azolla is exceptionally good at doing this because of its high growth rate.  Now, from the spores found at the bottom of the Arctic, we know that Azolla colonized parts of the Arctic Ocean, as well as surrounding rivers and waterways.  As the old fern growth died, it would have sunk to the bottom of the ocean.  Normally, in fresh- and saltwater systems, there are sufficient quantities of bacteria at the bottom to degrade all of the dead organisms that ultimately find their way there, which releases any carbon stored by that organism back into the surrounding environment.  But the enclosure of the Arctic meant that there was very little mixing of water on the seabed, creating an anoxic (oxygen depleted) environment in which very few bacteria could grow⁶.  This is why we’re able to find such thick layers of Azolla in the sediment cores – there was nothing there to degrade it! 

So, the theory is that Azolla was able to store so much carbon in the ocean floor that it had quite a large effect on the Earth’s climate.  Scientists analyzing the amount of fern biomass in the sediment cores estimate that it was able to draw down as much as 188 parts per million (ppm) of CO₂ from the atmosphere over the course of about 1 million years⁷.  To put that into perspective, since 1960, humans have put a total of about 400 ppm of CO₂ into our atmosphere⁸! 

So what does this mean in regard to the climate?  Around the time Azolla was flourishing in the Arctic, global temperatures began to plummet; to explain why, scientists point to a decreasing level of atmospheric CO₂.  This seems to suggest that Azolla did, in fact, have an inordinate influence on global climate.  There were certainly other factors that contributed to this decline, especially since the Earth continued to cool long after the blooms of Azolla had died off.  There is evidence, for example, that indicates the weathering of the newly formed Himalayan Mountains may have locked away a significant amount of carbon².  Once CO₂ began to decline, however, it would have triggered a chain reaction that caused other greenhouse gases to become scarce as well, further accelerating planetary cooling.

It’s important to note the disparity in time between past changes in climate and those occurring today.  Earth’s climate has certainly changed several times, often drastically, in the past; however, these changes took place over millions of years, which allowed most organisms the time needed to adapt and evolve to the changing conditions.  In contrast, anthropogenic climate change could take place in the space of hundreds of years, and it is mostly unknown how this will affect plants and animals.  What we do know doesn’t look too good.  It’s certainly not unreasonable to conclude that if something as small as a fern could have an influence on the climate, then one of the most aggressively dominant species on the planet (you and me) could do it too.

Citations

¹Gradstein FM, Luterbacher HP, Ali JR, Brinkhuis H, Gradstein FM, Hooker JJ, Monechi S, Ogg JG, Powell J, Röhl U, Sanfilippo A, Schmitz B. (2004) The paleogene period. In: A Geologic Time Scale (eds Gradstein FM, Ogg JG, Smith AG). pp. 396. Cambridge University Press, Cambridge, UK.

Barke, J., Burgh, J., Cittert, J., Collinson, M., Pearce, M., Bujak, J., Clausen, C. Speelman, E., Kempen, M., Reichart, G., Lotter, A. & Brinkhuis, H. (2012)  Coeval Eocene blooms of the freshwater fern Azolla in and around Arctic and Nordic seas.  Palaeogeography, palaeoclimatology, Palaeoecology 337, 108-119.  

²Beerling, D. (2007). The emerald planet. Oxford: Oxford University Press.

³Department of Paleobiology, National Museum of Natural History, Smithsonian Institution. (n.d.). The Eocene. Retrieved from http://paleobiology.si.edu/geotime/main/htmlversion/eocene1.html

⁴Smith, A. R., Pryer, K. M., Schuettpelz, E., Korall, P., Schneider, H., & Wolf, P. G. (2008). Fern classification. In T. A. Ranker & C. H. Haufler (Eds.), Biology and evolution of ferns and lycophytes (pp. 417-467). Cambridge: Cambridge University Press.

⁵Wagner GM (1997) Azolla: a review of its biology and utilization. The Botanical Review 63, 1–26.

⁶Speelman, E., Damste, J. S., Marz, C., Brumsack, H., & Reichart, G. J. (2010). Arctic ocean circulation during the anoxic eocene azolla event. Geophysical Research Abstracts, 12.

⁷Speelman, E. N., Van Kempen, M. M. L., Barke, J., Brinkhuis, H., Reichart, G. J., Smolders, A. J. P., Roelofs, G. M., & Sangiorgi, F., De Leeuw, J. W., Lotter, A. F., Sinninghe Damste, J. S.  (2009). The eocene arctic azolla bloom: environmental conditions, productivity and carbon drawdown. Geobiology, 7, 155-170. doi: 10.1111/j.1472-4669.2009.00195.x

⁸Dr. Pieter Tans, NOAA/ESRL (www.esrl.noaa.gov/gmd/ccgg/trends/) and Dr. Ralph Keeling,

Scripps Institution of Oceanography (scrippsco2.ucsd.edu/).