INTRODUCTION AND BACKGROUND

Neoproterozoic “Snowball Earth” is arguably one of the most fascinating times in Earth history (e.g., Hoffman and Schrag, 2002): geologic and paleomagnetic evidence (e.g., Evans, 2000) suggests the presence of ice at low latitudes (but not without controversy: Allen and Etienne [2008], Eyles and Januszczak [2007]). Recent interest in Snowball Earth can be traced to Hoffman et al. (1998), who sparked interest in this concept which led to a multitude of publications (at the time of this writing, Hoffman et al. [1998] had been cited by 1751 scientific articles).

Most workers think that there were two snowball glacial events, the Sturtian (ca. 710 to ca. 670 Ma; Fanning and Link, 2004; Rooney et al., 2014) and the Marinoan (ending at ca. 635 Ma). In many sections, glacial deposits, some of which contain iron-rich sediments (e.g., Young, 2002), appear to immediately overlie carbonate platform sediments, and are themselves directly overlain by carbonate deposits (usually called “cap carbonates”). Cap carbonates display a pronounced negative carbon isotopic signature (see Halverson et al. [2010] for a comprehensive review). The origin of the δ13C excursion is, of course, debated (e.g., Shields, 2005). Other geochemical proxies have been measured in preglacial units and the postglacial “cap” carbonates, but this discussion will focus on the record of life itself before, during, and after Snowball Earth times, to see what fossils can tell us about their environments.

The Snowball Earth events are clearly interesting from a climatological point of view, but a survey of the literature suggests that it is the biotic response to such extreme events that captures much of the imagination of scientists and the public. Nearly every Snowball Earth–related publication mentions the effects that a global glaciation would, could, or should have had on life. If Earth experienced a global glaciation, how did life survive (especially eukaryotic life)? What was the biotic response to such extreme glaciation? Is there a relation between the origin of animals and Snowball Earth, etc.? Conversely, one might ask, given our knowledge of the limits of life, can paleontological evidence constrain the environments during Snowball Earth? There are vanishingly little paleontological data, as compared to, for example, carbon isotopic data on cap carbonates. The life forms present in the time bracketing the Snowball Earth intervals were predominantly microscopic and soft-bodied—not a good prospect for fossilization. Corsetti et al. (2006) reviewed the paleontological record for Snowball Earth, showing that few fossils were known from within the glacial deposits. Most data originated from immediately prior to, or after, the extreme glaciation. With a few notable exceptions, the situation remains similar today, with very little data originating from within the glacial deposits themselves. Therefore the paper by Ye et al. (2015, p. 507 in this issue of Geology) brings important new paleontological evidence to our understanding of Snowball Earth.

Fossil Data Bracketing the Glacial Intervals

Organic-walled microfossils, predominantly from shales were relatively diverse by Proterozoic standards before the Sturtian glaciation (see Riedman et al., 2014, and references therein), including ornamented acritarchs (organic walled microfossils considered by most to represent eukaryotes). Diversity declined to include simple leiosphere forms (unornamented spheres that may have eukaryotic or bacterial affinities) before the Sturtian glaciation (Nagy et al., 2009; Riedman et al., 2014), with this decline therefore decoupled from the extreme conditions hypothesized during the glacial events. The simple leiospheres occur below and above the glacial strata, thus seem to have “survived” Snowball Earth (e.g., Grey et al., 2003; Moczydłowska, 2008; Riedman et al., 2014, and references therein). In sum, the microbiological data would suggest a somewhat underwhelming response of the biosphere to presumed extreme conditions during Snowball Earth, but the fossil record itself is somewhat underwhelming.

In a potentially exciting development, Maloof et al. (2010) reported sponge-grade body fossils from pre-Marinoan strata in Australia. The presence of sponges before the Marinoan glacial interval implies that sponge-grade animals survived a Snowball Earth, and biomarker data (Love et al., 2009) may support this view. The appearance of fossil sponges at this time is consistent with molecular clock estimates of the divergence of major eukaryotic clades (Sperling et al., 2010; Parfrey et al., 2011), but the find awaits further confirmation.

Biomarker Data from Glacial Strata

Complex organic molecules produced by and/or attributable to certain clades—biomarkers—can be preserved in ancient sediments (but see Rashby et al. [2007] for a cautionary tale). Olcott et al. (2005) reported extractable biomarkers from the synglacial Vazante Formation (Brazil), and concluded that they indicate a complex and productive ecosystem at this locality during Snowball Earth, comprised of photosynthetic and heterotrophic bacteria as well as eukaryotes. The geologic age of the unit was questioned by Azmy et al. (2008), then defended by Marshall et al. (2009), thus remains somewhat controversial. Love et al. (2009) found 24-isopropylcholestanes within Marinoan-aged glacial strata in Oman, interpreted as hydrocarbon remains of C30 sterols produced by marine demosponges, indicating the presence of the Metazoa during a Snowball Earth event (but see Antcliffe [2013]… nothing related to Snowball Earth appears to be uncontroversial!). Thus, sponge-grade metazoans may have survived during Snowball Earth, consistent with molecular clock estimates and potential fossil records.

Fossil Data from Glacial Strata

Until the report of benthic macroalgae from shales bracketed by diamictites of the Nantuo Formation in South China (Ye et al., 2015), the syn-glacial fossil record was sparse and microscopic. Filamentous microfossils were described from dropstone-laden shales (Abu Mahara Group, Oman,) including forms that exhibit a false-branching habit reminiscent of multi-trichomous cyanobacteria (Butterfield and Grotzinger, 2012). Syn-glacial deposits of Sturtian age (Australia, Svalbard) contain dominantly simple leiosphere acritarchs (Riedman et al., 2014), similar to the taxa directly below and above the glacial units. Other fossils have been reported, but the context of the syn-glacial microbiota from the Kingston Peak Formation (Corsetti et al., 2003) has recently been questioned (Macdonald et al., 2013), and other structures from syn-glacial strata are considered by some dubiofossils (e.g., Bavlinella, discussed in Butterfield and Grotzinger [2012]).

DISCUSSION

Given the productivity of modern ice margin environments (e.g., Perrette et al., 2011), and the propensity of microbial forms to survive freezing and thawing, it is not a stretch to envision a microbial biosphere surviving an extreme glaciation—possibly even photosynthetic microbiota, given our knowledge of microbiota in ice-covered Antarctic lakes (e.g., Andersen, et al., 2011). The filamentous microfossils with possible cyanobacterial affinities would suggest photosynthesis did take place during Snowball Earth, but the leiosphere-dominated assemblages (Riedman et al. 2014) carry less certain information, other than that marine microbial life was present in the water column during Snowball Earth. Ye et al. (2015) provide the most convincing evidence to date for photosynthesis during glacial times, and by large, benthic eukaryotes, no less—a form that would likely have required significant light and open water to survive versus microbial forms.

Although still sparse, the emerging picture of Neoproterozoic Snowball Earth painted by the fossil record is one of a relatively complex ecosystem that likely required times of open water and less extreme conditions than typically conjured when pondering Snowball Earth. Simply stated, benthic macroalgae need light and space (open water) (Ye et al., 2015). As filter feeders, the record of sponges (Maloof et al. [2010] and Love et al. [2009]) would suggest a complex food web, as well. It is important to remember that the fossils represent organisms living in the environment, and if we understand how to interpret their environmental requirements, we can use these fossils to inform us on the environmental settings during Neoproterozoic Snowball Earth, and help guide climate and geochemical modeling to understand the Earth system during such interesting times.