Origins and Cycling of CO2 from Earth during the Archean and Proterozoic Eons
Earth acquired essentially its entire carbon inventory very early. Most carbon arrived as volatile components trapped within planetesimals that formed the planet. As Earth approached its ultimate size, the greater energy associated with large impacts caused substantial amounts of volatiles to be lost to space. Because the redox state of the upper mantle has been relatively constant for at least the past 3.7 billion years, CO2 and CO32- species have dominated mantle carbon inventories since the early Archean. The cycling of carbon between its reservoirs in the atmosphere, ocean, crust and mantle has responded to major long-term evolutionary trends, e.g., increasing solar luminosity, declining sizes and rates of impacts, declining radiogenic heat flow, and the stabilization of large continents. The major changes have occurred principally in the relative sizes of these carbon reservoirs and in the carbon fluxes that linked them. Today, rates of carbon exchange between the mantle and crust are slower (~10%) than global sedimentary carbon cycling which, in turn, is much slower (~0.1%) than global biological carbon cycling. The hotter Archean mantle must have influenced significantly the inventory of carbon in the crust, oceans and atmosphere. Higher Archean rates of crustal production sustained higher mantle carbon outgassing rates. A hotter upper mantle retained any subducted carbon with greater difficulty. All of this indicates that the Archean crustal carbon inventory actually might have exceeded the modern crustal inventory. The enormous size of the mantle, together with more vigorous Archean mantle-crust exchange, probably allowed the mantle to control crustal volatile inventories and constrain the redox state of surface environments to a greater extent than it does today. This control weakened over time, following the decay of mantle radionuclides and declining heat flow. Also, the tectonic reworking of ancient crust during the late Archean and early Proterozoic led to more stable continents with more extensive stable shallow marine platforms that became major sites for the deposition and long-term preservation of carbonates and organic carbon. The rise of pervasive photosynthetic microbial communities transformed life into a major player in carbon cycling. Biological productivity enhanced sedimentary organic carbon burial rates, it contributed to the oxidation of the oceans and atmosphere, and ultimately it helped to modulate atmospheric CO2 levels.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009