Carbon Cycle: Life, Stability and Carbon

How stable is the balance that enables complex organisms to live on Earth? Might small changes combine to alter it substantially, perhaps creating a more hostile environment? These are crucial questions, but there are no clear answers. ESI-affiliated faculty members Daniel Rothman and Roger Summons, and their collaborator John Hayes of the Woods Hole Oceanographic Institution, are doing work that may help.

One of the most important influences on climate is the global carbon cycle, the series of transformations that circulate carbon atoms among the atmosphere, the oceans, living organisms, organic matter and inorganic matter. Many climate models rest on the assumption that the carbon cycle is generally stable-that the chemical pathways along which carbon moves do not change much over long periods. But Rothman, Hayes and Summons have found that at a crucial time in Earth history the carbon cycle was inherently unstable, and that it became stable at around the same time that complex life began to flourish. They believe that the stabilization of the carbon cycle and the widespread appearance of complex organisms are intimately linked. Their work points to the possibility that future changes in the carbon cycle could have far-reaching consequences for Earth's ability to support complex life. It also provides new tools for probing how the carbon cycle responds to, and helps to create, variations in environmental conditions.

The three scientists make an unusual team. Summons and Hayes are specialists in biogeochemistry, skilled at interpreting the chemical signals left in rocks by organisms that lived long ago. But Rothman is a theoretical geophysicist, and much of his work has been in purely physical science, he studies nonlinear dynamical systems, such as those that show the peculiar order-within-disorder described by chaos theory.

In nonlinear dynamics, it sometimes happens that the individual characteristics of a system-say the velocity and position of a pendulum that is being shaken-each seem to vary randomly over time. Often, however, when the characteristics are compared directly to one another (for example, by graphing the velocity of a pendulum versus its position, rather than by graphing either of them versus time), one can perceive subtle patterns. Those patterns may then make it possible to understand why the system behaves as it does. For example, they may help in determining the system's overall stability, its tendency to return to a particular state after being perturbed out of that state.

Drawing on Rothman's experience with such analyses, and Hayes's and Summons's experience with ancient chemical signals, the team examined characteristics of the global carbon cycle that have fluctuated significantly over time. (For example, they examined the ratio of carbon 13 to carbon 12 found in oceanic rocks and the degree to which changes in that ratio were due to photosynthesis.) If one simply looks at the variation of each of these quantities over time, they seem to fluctuate without regularity (although in some cases, as in the plot above, the fluctuations seem to change in character-say, becoming more or less rapid or intense-at various times). But the team found that when the variables were compared directly with one another the resulting plots followed relatively simple patterns, resembling the patterns found in other dynamical systems.

Those patterns led the team to a surprising realization: before the appearance of complex organisms, the carbon cycle behaved like an unstable system; after complex organisms began to flourish, the carbon cycle had stabilized.

What caused that stabilization? It may well have been the new complex organisms themselves. These were among the first creatures to make shells or to eliminate waste in the form of fecal pellets. Sinking fecal pellets, and shells from dead organisms, created a new pathway by which carbon could travel
into the deep ocean; the sinking pellets and shells brought oxygen downward along with the carbon, making the deep ocean's chemistry more hospitable to complex life. The resulting proliferation of complex organisms increased the strength and importance of the new chemical pathways, eventually driving the carbon cycle to a new, steadier state.

The team's work does not indicate what kinds of events might make the carbon cycle unstable again. But it does illuminate fundamental, unexpected characteristics that must be taken into account in modeling Earth's climate. Approximations suitable in today's more stable carbon cycle may be inappropriate to use when studying the ancient Earth. The work also hightens awareness of the fact that organisms within an ecosystem can themselves have important effects on the biogeochemical mechanisms that sustain that ecosystem. Thus lessons learned by one team of ESI scientists may help shape the work of others, bringing their diverse skills to bear in new collaborations.