THE ROLE OF CONTINENTAL MAGMATIC ARCS ON PHANEROZOIC pCO2 AND ICEHOUSE–GREENHOUSE TRANSITIONS

The Phanerozoic is characterized by shifts between major intervals of greenhouse-icehouse climates, with greenhouse conditions observed during the early Paleozoic and Mesozoic–early Cenozoic, and icehouse climates observed during the late Paleozoic and late Cenozoic. These climate states are thought to be governed by changes in the partial pressure of atmospheric carbon dioxide (pCO₂). On multimillion year timescales pCO₂ is controlled by the long-term carbon cycle: tectonic outgassing from volcanism and metamorphic degassing contributes the major CO₂ flux to the atmosphere, whereas silicate weathering and organic matter burial draws down CO₂. Recent studies postulate that continental arc magmatic systems may play the major role in pCO₂ variation and associated climatic transitions. As zircon is produced in high volumes along continental arc systems, the detrital zircon record when evaluated in a stratigraphic context can be used to track continental arc activity through time. Here we present a compilation of new and published single grain detrital zircon age-dates acquired from globally dispersed siliciclastic sedimentary rocks with depositional ages that span the last ~700 million years. The data are binned by geologic period and presented as (1) regionally and temporally differentiated age distributions and (2) as temporally differentiated, normalized global composite age distributions. Globally, zircon age distributions tend to be skewed towards relatively young detrital zircon ages during greenhouse climates, whereas age distributions during icehouse climates are broader with relatively low concentrations of young grains. Observed shifts in the relative abundance of young zircon grains suggest that continental volcanic arcs were spatially profuse during greenhouse climates and spatially reduced during icehouse climates. Based on this relationship, we propose that Phanerozoic atmospheric pCO₂ was principally controlled by the continental arc volcanic flux.