Talk Description
There is a growing interest to base the Archean-Proterozoic boundary on the synchronous and potentially global surface event. The rise of atmospheric oxygen in the early Paleoproterozoic closely followed the assembly and emergence of large landmasses and associated emplacement of Large Igneous Provinces (LIPs), was bracketed by 3 to 4 Snowball Earth events, and led to the largest and longest positive carbon isotope excursion in seawater composition in Earth’s history, the Lomagundi Event (LE). Since each of these events could be globally synchronous and widespread, they hold an eminent potential to define a GSSP that deserves to be considered. Assembly of the supercraton Superia occurred over a protracted period that started in the late Neoarchean and continued to the early Paleoproterozoic, potentially ending at ~2.3 Ga with the Arrowsmith Orogeny in NW Canada. Associated emplacement of LIPs affected all landmasses, but their ages cluster into several discreet groups that individually are not expressed on all continents (Ernest, 2014). The initiation of the GOE, as defined by the disappearance of mass-independently fractionated sulphur (MIF-S) from sedimentary records (Holland, 2002), has been constrained between ~2.45 and 2.43 Ga (e.g., Warke et al., 2020). However, the long-term disappearance of MIF-S has been recently considered to be either globally asynchronous (Philippot et al., 2018) or to correspond to a series of rises and falls in atmospheric oxygen in association with global glaciations (Gumsley et al., 2017; Bekker et al., 2020; Poulton et al., 2021). The Paleoproterozoic glaciations are generally envisioned to have a global extent since there is strong evidence for glaciation at sea level near paleoequator (Evans et al., 1997). However, diamictites in the Boolgeeda Iron Formation (Martin, 1999) and the Koegas Subrgroup (Polteau et al., 2006), if indeed glacial, would correspond to regional-scale glaciations leading to the Snowball events. Although three stratigraphic horizons with glacial diamictites are recorded in the Huronian and Snowy Pass supergroups of Ontario and Wyoming, respectively, other Paleoproterozoic successions bear evidence for two, one, or no glaciations. Furthermore, synchronicity of Paleoproterozoic ice ages, in contrast to the Neoproterozoic glaciations, has to be tested with high-precision geochronology. Finally, the LE that was inferred to last between ~2.22 and 2.06 Ga (Karhu and Holland, 1996), might be mistaken for shorter-lived, but similar magnitude excursions leading to the LE and in its aftermath before ~2.0 Ga. Pending future work testing synchronicity of the early Paleoproterozoic events with high-precision geochronology, it seems premature at this stage to define the GSSP based on any of these events that could be multiple and asynchronous worldwide. In contrast, the conventional approach based on the numerical value for the Archean-Proterozoic boundary avoids potential confusion and provides an independent framework to test synchronicity and global extent of the early Paleoproterozoic events.