The snowball Earth hypothesis: testing the limits of global change

Abstract

The gradual discovery that late Neoproterozoic ice sheets extended to sea level near the equator poses a palaeoenvironmental conundrum. Was the Earth's orbital obliquity > 60° (making the tropics colder than the poles) for 4.0 billion years following the lunar‐forming impact, or did climate cool globally for some reason to the point at which runaway ice‐albedo feedback created a `snowball' Earth? The high‐obliquity hypothesis does not account for major features of the Neoproterozoic glacial record such as the abrupt onsets and terminations of discrete glacial events, their close association with large (> 10‰) negative δ13C shifts in seawater proxies, the deposition of strange carbonate layers (`cap carbonates') globally during post‐glacial sea‐level rise, and the return of large sedimentary iron formations, after a 1.1 billion year hiatus, exclusively during glacial events. A snowball event, on the other hand, should begin and end abruptly, particularly at lower latitudes. It should last for millions of years, because outgassing must amass an intense greenhouse in order to overcome the ice albedo. A largely ice‐covered ocean should become anoxic and reduced iron should be widely transported in solution and precipitated as iron formation wherever oxygenic photosynthesis occurred, or upon deglaciation. The intense greenhouse ensures a transient post‐glacial regime of enhanced carbonate and silicate weathering, which should drive a flux of alkalinity that could quantitatively account for the world‐wide occurrence of cap carbonates. The resulting high rates of carbonate sedimentation, coupled with the kinetic isotope effect of transferring the CO2 burden to the ocean, should drive down the δ13C of seawater, as is observed. If cap carbonates are the `smoke' of a snowball Earth, what was the `gun'? In proposing the original Neoproterozoic snowball Earth hypothesis, Joe Kirschvink postulated that an unusual preponderance of land masses in the middle and low latitudes, consistent with palaeomagnetic evidence, set the stage for snowball events by raising the planetary albedo. Others had pointed out that silicate weathering would most likely be enhanced if many continents were in the tropics, resulting in lower atmospheric CO2 and a colder climate. Negative δ13C shifts of 10–20‰ precede glaciation in many regions, giving rise to speculation that the climate was destabilized by a growing dependency on greenhouse methane, stemming ultimately from the same unusual continental distribution. Given the existing palaeomagnetic, geochemical and geological evidence for late Neoproterozoic climatic shocks without parallel in the Phanerozoic, it seems inevitable that the history of life was impacted, perhaps profoundly so.

The Late Cryogenian Warm Interval, NE Svalbard: Chemostratigraphy and genesis

“…These features imply that that deposition at the base of E3 begins during the transition to a highstand, consistent with Sturtian cap carbonates globally (Kennedy et al, 1998;. This pattern contrasts sharply with the falling δ 13 C profile recorded in a transgressive systems tract at the base of Marinoan cap carbonates (Kennedy et al, 1998;Hoffman and Schrag, 2002;Halverson et al, 2004).…”

“…The most comprehensive studies have been performed on thick carbonate successions in South Australia (e.g. McKirdy et al, 2001;Giddings and Wallace 2009a, b;Rose et al, 2012;Hood & Wallace, 2014, 2015Wallace et al, 2015) and Namibia (Halverson et al, 2002(Halverson et al, , 2005Hoffman and Schrag, 2002;Hurtgen et al, 2002;Hoffman and Halverson, 2008;Hoffman, 2011), and mixed carbonate-siliciclastic sediments of NW Canada (Hofmann et al, 1990;Narbonne and Aitken, 1995;Day et al, 2004).…”

New records of late Ediacaran microbiota from Poland

“…The Earth's frozen stage was abruptly switched into greenhouse conditions due to the accumulation of carbon dioxide from subaerial volcanism as is depicted by precipitation of cap carbonates worldwide. In these harsh climatic and environmental conditions, very limited refugia for bacteria and simple eukaryotes were suggested to be in meltwater pools, ice cracks, open leads or hydrothermal veins (Hoffman et al, 1998;Hoffman and Schrag, 2002). Accordingly, the acclaimed worst catastrophe in history left the Earth almost lifeless because of extinction of the photosynthesising biota.…”

C, O, and Sr isotopic variations in Neoproterozoic-Cambrian carbonate rocks from Sete Lagoas Formation (Bambuí Group), in the Southern São Francisco Basin, Brazil

“…Neoproterozoic carbonate sequences overlying glacial deposits, also known as cap carbonates, have been widely used as regional and global chemostratigraphic correlation tools due to their characteristic negative carbon isotopic signature and lack of reliable biostratigraphy (Kennedy, 1996;Kaufman et al, 1997;Hoffman et al, 1998;Jacobsen & Kaufman, 1999;Hoffman & Schrag, 2002). During the Cryogenian, the Sturtian (~720 Ma) and Marinoan (~635 Ma) low-latitude global glaciations (Halverson et al, 2005, Hoffman & Li, 2009) isolated the atmosphere of oceans, thus creating low bio-productivity of photosynthetic marine organisms, and also generated 12 C accumulation in the oceans, as proposed in the "Snowball Earth" hypothesis (Kirschvink, 1992;Hoffman et al, 1998).…”