Research plan

 

Our research is coordinated around three major research themes

1) Reconstructing the history of environmental change 

2) Identifying major episodes of biological innovation
 

3) Ecosystem evolution and controls of Earth surface chemistry

A detailed account is given below of each of these major research themes.  We recognize that new research directions will likely emerge in response to progress within our group and within the community at large.

 

1) Reconstructing the history of environmental change

To understand the relationship between environmental change and biological evolution, we must understand how the chemistry of the environment has changed through time. There is accumulating evidence that the Earth’s progression from an early chemically-reducing planet surface to the present well-oxidized surface has been punctuated by at least two major oxidation events. The first of these events occurred around 2.3 to 2.4 billion years ago (Ga) and the second around 0.6 to 0.7 Ga (Karhu and Holland, 1996; Knoll, 1992; Canfield and Teske, 1996; Bekker et al., 2004). The prevailing view is usually one of singular episodes of change, but in reality the evolution has probably been much more complicated and may indeed differ substantially from present models. Our understanding of Earth-surface chemical evolution is critical as episodes of rapid biological evolution are often stimulated by environmental change (see Knoll, 2003). Therefore, we seek to constrain the history of ocean and atmosphere chemistry.

We are interested in times when the Earth system was in apparent relative stasis, as well as times of transition or profound instability. During times of relative stasis we aim to define levels of atmospheric oxygen as well as the chemistry of the ocean. For example, were the oceans oxic, sulfidic or iron-containing? The chemistry of the oceans is related directly to levels of oxygen (a fully oxic ocean requires elevated atmospheric oxygen) and also to the workings of other major biogeochemical cycles. Thus, an iron-containing ocean indicates a limited sulfur cycle, while a sulfidic ocean indicates an active sulfur cycle, and the activity of the sulfur is driven by the concentrations of seawater sulfate (Canfield, 1998). Our broader goal, then, is to understand what combination of factors controlled the chemical state of the surface environment.

Times of transition are defined by important changes in the chemistry of the Earth surface, and these times are associated with reorganized elemental cycling. Widely discussed is the pervasive oxidation of the Earth surface at about 2.3 billion years ago (e.g. Karhu and Holland, 1996; Holland, 2002). However, even in this case, atmospheric oxygen levels are unclear, the chemical state of the oceans is unexplored, and any relationship with biological evolution is unknown.

Our main source of information is sedimentary rocks which contain an integrated record of ocean chemistry and its relationship to biological processes.

 

2) Major episodes of biological innovation

There is a long way from the first microbes inhabiting simple ecosystems on the early Earth to the abundant and conspicuous life on the present Earth. This path has been marked by a variety of key innovations including the evolution of oxygen-producing photosynthesis, the inclusion of organelles into early eukaryote cells, and the development of multicellularity among eukaryotes.  Indeed, multicellularity led to cell differentiation, functional complexity, and an increase in organism size.  This was a prelude to the development of conspicuous life as is presently abundant at the Earth surface.  Running parallel with these developments was the evolution of new metabolisms, taking advantage of the changing chemistry of the environment. For example, there was an apparent rapid expansion of metabolisms using oxygen in association with the evolution of oxygenic photosynthesis (e.g. Canfield and Raiswell, 1999).

With this backdrop, our center will explore the fossil record for major episodes of biological innovation.

 

3) Ecosystem evolution and controls of Earth surface chemistry

Marine ecosystems have evolved through Earth history. Factors defining ecosystem structure are biological innovation, providing the cast of characters able to populate an ecosystem, as well as ocean and atmosphere chemistry, dictating which characters will be major players within the ecosystem. For example, early in Earth history the deep ocean contained ferrous iron (e.g. Canfield, 1998), which could have been used by a phototrophic Iron oxidizing population as the ferrous iron was advected to the surface ocean. This type of photosynthesis does not produce oxygen (anoxygenic photosynthesis), but rather oxidizes ferrous iron to its oxidized equivalent, ferric iron oxides (Widdel et al., 1993). The introduction of oxygen-producing cyanobacteria would have initially added ecosystem complexity, but deep oceans were not immediately oxidized and anoxygenic phototrophic populations likely still prospered as they do in the modern Black Sea, for example (Repeta et al., 1989). Thus, even after cyanobacteria evolved, Earth-surface chemistry remained much different from today. Why was this so? Our goal is to explore experimentally, and through mathematical modeling, how biological innovation has influenced the dynamics of ecosystem structure, and the surface chemistry of the Earth.

This goal is relevant for understanding the progress of Earth surface oxidation on the modern Earth, and for appreciating the prospects for an equally oxidizing planet elsewhere in the heavens. Our starting point is the history of biological and Earth surface evolution as elucidated in the first two points.

 

References

Bekker, A., H.D. Holland, P.-L. Wang, D. Rumble III, H.J. Stein, J.L. Hannah, L.L. Coetzee, and N.J. Beukes. 2004. Dating the rise of atmospheric oxygen. Nature 427: 117-120.

Canfield, D.E. 1998. A new model for Proterozoic ocean chemistry. Nature 396: 450-453.

Canfield, D.E. and R. Raiswell. 1999. The evolution of the sulfur cycle. American Journal of Science 299: 697-723.

Canfield, D.E. and A. Teske. 1996. Late Proterozoic rise in atmospheric oxygen concentration inferred from phylogenetic and sulphur-isotope studies. Nature 382: 127-132.

Holland, H.D. 2002. Volcanic gases, black smokers, and the great oxidation event. Geochimica et Cosmochimica Acta 66: 3811-3826.

Karhu, J.A. and H.D. Holland. 1996. Carbon isotopes and the rise of atmospheric oxygen. Geology 24: 867-870.

Knoll, A.H. 1992. Biological and biogeochemical preludes to the Ediacaran radiation. pp. 53-84. In J.H. Lipps and P.W. Signor (eds.), Origin and Early Evolution of the Metazoa. Plenum Press, New York.

Knoll, A.H. 2003. The geological consequences of evolution. Geobiology 1: 3-14.

Repeta, D.J., D.J. Simpson, B.B. Jørgensen, and H.W. Jannasch. 1989. Evidence for anoxygenic photosynthesis from the distribution of bacteriochlorophylls in the Black Sea, Nature, pp. 69-72.

Widdel, F., S. Schnell, S. Heising, A. Ehrenreich, B. Assmus, and B. Schink. 1993. Ferrous iron oxidation by anoxygenic phototrophic bacteria. Nature 362: 834-835.