The Paleocene-Eocene Thermal Maximum, a period of dramatic climate change that occurred about 55 million years ago, has been the target of intensive studies of late. That's because it may provide clues as to what could happen as the concentrations of carbon dioxide in our own atmosphere continue to rise. The PETM saw a slower, but (so far, at least) more sustained rise in carbon dioxide concentrations, which triggered significant changes in the climate and oceans, with major extinctions and other disruptions of ecosystems.
The rise in atmospheric carbon dioxide occurred in what was, in geological terms, a very short period of time, in the area of 10,000 years. It stayed high for about 100,000 years, but then returned to its pre-PETM levels almost as suddenly as it rose. So, while some researchers are working to understand where the carbon came from, a new paper looks into a related question: how did it all get removed from the atmosphere?
The Earth's carbon cycle has a variety of sources, sinks, and storage sites. Volcanic activity and fires act as a net source, placing carbon in the atmosphere. Plant growth and weathering of rocks removes it. And lots of carbon ends up sequestered in forests, ocean sediments, and carbonate deposits. The net balance between these sources and sinks ultimately determines the levels of carbon dioxide in the atmosphere.
At the onset of the PETM, something changed dramatically, suggesting one of the places that carbon was stored suddenly released its contents into the atmosphere. The influx into the atmosphere was rich in a light isotope of carbon, which is preferentially used in living matter, suggesting it came from a source like the bacterial communities present in ocean sediments (either directly or though the methane clathrates they form).
Once it was in the atmosphere, however, various sinks should have started to reestablish an equilibrium. This might be expected to cause a gradual draw down of atmospheric CO2 levels. Instead, isotope records suggest nothing of the sort; atmospheric carbon dioxide remained high for over 100,000 years before decreasing back to the Eocene baseline in the span of roughly 20,000 years.
Generally, the explanation for this has been enhanced weathering. Rocks react with CO2 in the air and gradually remove it. Assuming a higher rate of weathering, exponential decay could potentially create something close to the precipitous decline seen in the records. But the authors test this assumption and find that, even assuming the rates were twice as high as we see in modern times, it's still not possible to get all the carbon back out in the same time period. Another potential option, recycling by the Earth's crust, may also have contributed, but probably not sufficiently to explain the trajectory of the decrease.
This really leaves one good option: sequestration by the biosphere, which can incorporate the carbon into clathrates, sediments, and other sites of storage. That could also explain the suddenness of the change seen in the isotope ratio, since sequestration by living organisms would preferentially deplete the light carbon isotope.
But the rate of sequestration would have to be absolutely enormous; the authors calculate an amount of carbon equal to the total stored in today's terrestrial biosphere was removed during this span. As the authors point out, the PETM had given ecosystems a lot to recover from. The oceans underwent a significant extinction event, and the acidification caused by added carbon dioxide dissolved a lot of the carbonate sediments. Things weren't as fatal on land, but a lot of terrestrial ecosystems were disrupted, with Gulf Coast ecosystems migrating as far north as Montana. Much more of the Earth's surface was ice-free than it is now, which would also provide the ecosystems with more land on which to proliferate.
In fact, the authors suggest that the disruptions caused by the PETM could have led to the release of sequestered carbon as tropical forests and continental interiors dried out. All of this could have kept supplying carbon to the atmosphere even after the initial burst, ensuring that levels stayed elevated for so long. In effect, the boom of the biosphere that might have contributed to the end of the PETM was simply a reverse of one of the processes that helped kick it off.