Modes of Contintental Sediment Storage and the History of Atmospheric Oxygen

Abstract:

Documenting the history of atmospheric oxygen levels, and the processes that have governed that history, are among the most fundamental of problems in Earth science. Diverse observations from sedimentary petrography, isotope geochemistry, stratigraphy and trace element geochemistry have led to a model wherein concentrations of oxygen experienced two significant rises: the first ‘Great Oxidation Event' near the Archean-Proterozoic boundary, and a second near the Proterozoic-Phanerozoic boundary. Despite ongoing debates over important details in the history of atmospheric O₂, there is widespread agreement that the burial and long-term storage of sedimentary organic matter derived from photosynthesis, which represents net O₂ production over consumption by respiration, is the primary driver of oxygenation of the atmosphere. In this regard, sedimentation on the continents is vitally important; today, >90% of buried organic matter occurs in sediments deposited on continental crust. Here we use 23,813 rock units, distributed among 949 geographic regions in North America, from the Macrostrat database to constrain patterns of sedimentation through Earth history. Sedimentary packages are low in number in the Archean, increase to a higher steady state value across the transition to the Proterozoic, and rise again across the Proterozoic-Phanerozoic boundary during the final stage in the formation of the Great Unconformity. Map-based data from polar Eurasia and Australia show qualitatively similar macrostratigraphic patterns of sediment abundance. The temporal similarities between continental sedimentation and the putative history of pO₂ are sensible in the context of organic carbon burial. A simple model of burial and weathering on North America predicts two significant rises in pO₂. These results suggest that the changing ability of the continents to serve as long-term organic carbon storage reservoirs, presumably due to geodynamic processes, has exerted a first-order control on the stepwise oxygenation of Earth's atmosphere.