Running out of gas: Zircon 18O-Hf-U/Pb evidence for Snowball Earth preconditioned by low degassing

Abstract

The general long-term stability of Earth's climate over geologic time was punctuated by dramatic excursions. Between ca. 2.5 and 0.5 billion years ago (Ga), these events included the globally extensive glaciations known as Snowball Earths, when ice extended to tropical latitudes. Such anomalous periods of time provide unique opportunities for understanding the mechanisms regulating planetary climate and habitability. However, the causes of these events remain enigmatic, in part because there is little information about fluxes in the global carbon cycle in deep time. We propose that the oxygen stable isotope composition in zircons (δ18Ozircon) contains information about past weathering conditions on the continents, imparted during the time between separation of parent material from the mantle (reflected in the Hf model age) and zircon crystallisation (the U/Pb age). A new compilation of coupled 18O-Hf-U/Pb isotopic data shows that the mean δ18Ozircon value varied particularly between 2.5 Ga and 0.5 Ga. The maximum in the δ18Ozircon record, which we interpret as a time of intense weathering, is associated with the Lomagundi Event (∼2.22-2.07 Ga), a dramatic carbon isotope excursion thought to reflect enhanced organic carbon burial facilitated by the release of phosphorous during rock weathering. The onset of the Neoproterozoic Snowball Earth events coincides with the minimum in δ18Ozircon, suggesting low silicate weathering rates at the time. This evidence suggests that long-term decreases in the rate of CO2 release to the atmosphere from solid Earth degassing may have preconditioned the global climate system for intense glaciations.