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Project Progress

Molecular oxygen accumulates in the oceans and atmosphere when organic carbon is immobilized in rocks. After resurfacing, previously buried organic carbon is oxidized by weathering and other processes, thereby consuming the O₂ that had accumulated due to its burial. Various lines of evidence suggest that oxygen levels permanently rose in the Neoproterozoic. The apparent stability of this change can be explained by the oxygen-exposure time hypothesis of Hedges and colleagues, who showed that preservation rates of sedimentary organic matter decrease as the duration of time organic matter is exposed to oxygen increases. Demonstrated for oxygen exposure times of approximately a year or greater, this relation provides a mechanism of negative feedback via which perturbations to modern oxygen levels must dampen. In other words, a balance between O₂ accumulation and consumption provides a steady state, and this steady state is stable. Presumably the lower O₂ levels of the Proterozoic were similarly steady and stable. How, then, could oxygen levels have increased?

We (D. Rothman and T. Bosak at MIT) note that the oxygen-exposure time hypothesis implicitly assumes that a portion of sedimentary organic matter can only be degraded aerobically. We refer to this portion as refractory, and to the remainder as labile. We hypothesize that these two organic phases interact. In the absence of the labile fraction, the anaerobic degradation of refractory organic matter is slow and its preservation potential is enhanced. If the labile fraction is instead present, the refractory portion degrades at a faster rate, and preservation is diminished.

Our hypothesis implicitly assumes a role for priming and/or cometabolism, whereby apparently refractory organic matter is made more reactive due to the presence of labile organic matter. An important consequence is that rates of organic matter burial can increase with increasing oxygen exposure time, so long as the exposure time is less than that required for oxidation of the labile fraction. This provides a mechanism for positive feedback, whereby increases in oxygen lead to more oxygen, when oxygen levels are low.

In the past year we have succeeded in expressing this anti-priming hypothesis as a problem of dynamics.¹ We find that such a system can consist of two stable steady states,representing weakly and strongly oxygenated environments, separated by an unstable steady state. If the system is perturbed, by, e.g., a transient increase in burial rates, or a transient decrease in rates of oxidative weathering, the weakly oxic state and the unstable state vanish, and the environment rapidly transitions to the strongly oxic state. Known technically as a saddle-node bifurcation and popularly as passage through a tipping point, this mechanism provides for a stepwise permanent rise in oxygen levels without any permanent change in burial or weathering rates. It therefore indicates how transient changes in burial, as suggested by the Neoproterozoic carbonisotopic record, may have instigated permanent increases in oxygen levels.

We are also pursuing experimental analogs of this process. A graduate student (Robert Yi) working with Rothman and Bosak has established anaerobic enrichment cultures using microbes from the euxinic Green Lake, NY and is currently conducting experiments designed to probe the importance of priming in anaerobic environments.
Two other aspects of our work deserve mention.

*Bosak has started using organic remnants of Neoproterozoic eukaryotes to explore the cycling of carbon during the deposition of isotopically extreme carbonates between the Sturtian and the Marinoan glaciation (collaboration with K. Freeman at Penn State, J. Valley and K. Williford at U Wisconsin, F. Macdonald and A. Pearson at Harvard and S.@Pruss at Smith College).

*Rothman has completed theoretical studies of heterogeneous degradation kinetics,performed in collaboration with his graduate student D. Forney. This work has resulted in new methods for the analysis of decay experiments. We anticipate that these methods will be of much use for analyzing experimental realizations of the anti-priming hypothesis.

¹ This work has been presented at the 2012 NASA Astrobiology Science Conference in Atlanta, and at the Fermor Neoproterozoic Meeting of the Geological Society in London (September 2012).