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Project Reports

• ALTERNATIVE EARTH 1 – Atmospheric Traces of Oxygenic Photosynthesis

  The precise timing of the onset of oxygenic photosynthesis is a matter of intense debate. Current estimates span over a billion years of Earth history, ranging from prior to 3.7 billion years ago (Ga), the age of the oldest sedimentary rocks, to 2.4—2.3 Ga, coincident with the first permanent rise of atmospheric oxygen at the so-called “Great Oxidation Event” (GOE). Even without consensus on when biological oxygen production emerged, pinpointing the evolution of this process is essential for understanding Earth’s planetary evolution. If oxygenic photosynthesis evolved early, well before the permanent rise of atmospheric oxygen, the transition to a more oxidizing world in the Proterozoic is likely to be a reflection of Earth’s tectonic history, such as the emergence and stabilization of continents and related shifts in the temporal patterns of volcanism and associated fluxes of reduced gas. Alternatively, biological evolution (specifically, the emergence of oxygenic photosynthesizers) may have directly triggered this switch in Earth states. We are exploring these alternative models and their implications for the systematics of planetary oxygenation on Earth from a combined experimental, empirical and theoretical perspective.

  ROADMAP OBJECTIVES: 4.1 7.2

• ALTERNATIVE EARTH 2 – Dramatic Oxygen Fluctuations

  Our studies of the middle portion of the Paleoproterozoic (2.2 to 2.0 Ga) are focused on whether Earth’s surface experienced a unidirectional oxygen rise or instead rose to high levels (potentially near-modern) and then crashed dramatically. More specifically, we are rigorously testing the idea that the middle Paleoproterozoic Earth was marked by high oxygen levels—in strong contrast to traditional arguments for far lower values. The resolution of this question is perhaps one of the most important issues in Earth history, as it points to the likelihood that the much later development of complex life was not solely contingent on high levels of oxygen at Earth’s surface. Work to date has focused on trying to place empirical constraints on ocean-atmosphere O₂ levels during the Paleoproterozoic and developing quantitative theoretical tools for understanding the dynamics of large shifts in ocean-atmosphere oxygen levels.

  ROADMAP OBJECTIVES: 4.1 7.2

• ALTERNATIVE EARTH 3 – Oxygen Stasis and the Rise of Eukaryotes

  The importance of a full understanding of the controls on ocean-atmosphere O₂ levels during the mid-Proterozoic is difficult to overstate. The evolution of O₂ levels in the mid-Proterozoic ocean-atmosphere system forms the backdrop for the initial emergence and subsequent evolutionary stasis of eukaryotic life. Furthermore, it provides the possibility of a remarkably long period of Earth’s history during which many of the links among tectonics, climate, and life may have been short-circuited and/or amplified in unusual ways. Finally, it provides the preface that is essential reading for any story about the proximate causes of the subsequent emergence of complex life in the late Neoproterozoic. The central question in this regard is whether ocean-atmosphere O₂ levels were low enough to inhibit the evolution and ecological emergence of complex multicellular life, or must we seek mechanisms strictly associated with internal biology to explain this event—or both? Our developing framework for very low oxygen levels during the mid-Proterozoic in the deep ocean, shallow ocean, and atmosphere is the baseline against which the dramatic environmental, climatic, and biotic events and triggers of the later Proterozoic should be assessed.

  ROADMAP OBJECTIVES: 4.1 4.2 7.2

• ALTERNATIVE EARTH 4 – the Rise of Complexity Amid Environmental Turmoil

  Climatic turmoil and major upheavals in global biogeochemical cycles characterize the latter part of the Proterozoic Eon, during the so-called Neoproterozoic (1,000–541 million years ago). The Neoproterozoic was marked by pronounced shifts in atmospheric composition—especially increased oxygen levels. This environmental instability provided the backdrop for the rise of complex life, including animals; however, limited empirical constraints have hindered attempts to untangle the cause-and-effect relationships among biological innovations, shifts in ecosystem complexity, and biogeochemical evolution. Likewise, still sparse coupled geochemical and paleontological records make it difficult to gauge whether the Neoproterozoic unfolded as a unidirectional march toward greater organismal complexity and higher oxygen levels, as traditionally envisioned, or whether dramatic swings in surface oxygen levels accompanied non-unidirectional ecological shifts. To resolve this debate, we are producing extensive, high-resolution records of oxygen levels and tracking the distribution, abundance, and impact of eukaryotic phytoplankton over this critical interval. Our central goal is to work synergistically with the Origins of Complexity NAI Team to better understand how the rise of complex life shaped planetary-scale biosignatures.

  ROADMAP OBJECTIVES: 4.1 4.2 7.2