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

18. September 2017, von Tom Jaeppinen

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 (δ¹⁸O_zircon) 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 (δ¹⁸O_zircon) value varied particularly between 2.5 Ga and 0.5 Ga. The maximum in the (δ¹⁸O_zircon) 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 (δ¹⁸O_zircon), 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.
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δ¹⁸O_zircon calculated for ∆T_Hf-U/Pb 400 Ma, along with timing of major carbon cycle anomalies observed in the geologic record. The grey band represents ±2 s.d. (s.d.: standard deviation of the mean). Red bars indicate periods of supercontinent assembly (Cawood et al., 2013). Dark blue bars represent periods of glaciations in the late Archean and in the Neoproterozoic. The dark green bar covers the period of the Lomagundi event. Note that the δ¹⁸O_zircon values are plotted using the midpoint age of the ∆T_Hf-U/Pb window, but the actual δ¹⁸O_zircon values integrate processes (e.g., weathering conditions) occurring over longer periods of time, characterised by a mean ∆T_Hf-U/Pb duration of 225 Ma. To illustrate the effect on the comparison between the timing of geological events and the δ¹⁸O_zircon curve, lighter shaded areas around periods of geological events reflect uncertainty of the δ¹⁸O_zircon curve associated with plotting the curve for the midpoint between the Hf and U/Pb ages, given as half of the mean ∆T_Hf-U/Pb (112.5 Ma).