RESEARCH FOCUS During Snowball

Frank A. Corsetti Department of Earth Sciences, University of Southern California, Los Angeles, California 90089, USA

INTRODUCTION AND BACKGROUND glacial events. The simple leiospheres occur below and above the glacial Neoproterozoic “” is arguably one of the most fascinat- strata, thus seem to have “survived” Snowball Earth (e.g., Grey et al., 2003; ing times in Earth history (e.g., Hoffman and Schrag, 2002): geologic and Moczydłowska, 2008; Riedman et al., 2014, and references therein). In paleomagnetic evidence (e.g., Evans, 2000) suggests the presence of sum, the microbiological data would suggest a somewhat underwhelming at low (but not without controversy: Allen and Etienne [2008], response of the biosphere to presumed extreme conditions during Snow- Eyles and Januszczak [2007]). Recent interest in Snowball Earth can be ball Earth, but the fossil record itself is somewhat underwhelming. traced to Hoffman et al. (1998), who sparked interest in this concept which In a potentially exciting development, Maloof et al. (2010) reported led to a multitude of publications (at the time of this writing, Hoffman et -grade body fossils from pre-Marinoan strata in Australia. The pres- al. [1998] had been cited by 1751 scientific articles). ence of before the Marinoan glacial interval implies that sponge- Most workers think that there were two snowball glacial events, the grade survived a Snowball Earth, and biomarker data (Love et al., Sturtian (ca. 710 to ca. 670 Ma; Fanning and Link, 2004; Rooney et al., 2009) may support this view. The appearance of fossil sponges at this time 2014) and the Marinoan (ending at ca. 635 Ma). In many sections, glacial is consistent with molecular clock estimates of the divergence of major deposits, some of which contain -rich sediments (e.g., Young, 2002), eukaryotic clades (Sperling et al., 2010; Parfrey et al., 2011), but the find appear to immediately overlie platform sediments, and are awaits further confirmation. themselves directly overlain by carbonate deposits (usually called “cap ”). Cap carbonates display a pronounced negative iso- Biomarker Data from Glacial Strata topic signature (see Halverson et al. [2010] for a comprehensive review). Complex organic produced by and/or attributable to certain The origin of the d13C excursion is, of course, debated (e.g., Shields, clades—biomarkers—can be preserved in ancient sediments (but see 2005). Other geochemical proxies have been measured in preglacial units Rashby et al. [2007] for a cautionary tale). Olcott et al. (2005) reported and the postglacial “cap” carbonates, but this discussion will focus on the extractable biomarkers from the synglacial Vazante Formation (Brazil), record of life itself before, during, and after Snowball Earth times, to see and concluded that they indicate a complex and productive what fossils can tell us about their environments. at this locality during Snowball Earth, comprised of photosynthetic and The Snowball Earth events are clearly interesting from a climatological heterotrophic as well as . The geologic age of the unit point of view, but a survey of the literature suggests that it is the biotic was questioned by Azmy et al. (2008), then defended by Marshall et al. response to such extreme events that captures much of the imagination of (2009), thus remains somewhat controversial. Love et al. (2009) found scientists and the public. Nearly every Snowball Earth–related publica- 24-isopropylcholestanes within Marinoan-aged glacial strata in Oman, tion mentions the effects that a global glaciation would, could, or should interpreted as hydrocarbon remains of C30 sterols produced by marine have had on life. If Earth experienced a global glaciation, how did life , indicating the presence of the Metazoa during a Snow- survive (especially eukaryotic life)? What was the biotic response to such ball Earth event (but see Antcliffe [2013]… nothing related to Snowball extreme glaciation? Is there a relation between the origin of animals and Earth appears to be uncontroversial!). Thus, sponge-grade metazoans Snowball Earth, etc.? Conversely, one might ask, given our knowledge of may have survived during Snowball Earth, consistent with molecular the limits of life, can paleontological evidence constrain the environments clock estimates and potential fossil records. during Snowball Earth? There are vanishingly little paleontological data, as compared to, for example, carbon isotopic data on cap carbonates. The Fossil Data from Glacial Strata life forms present in the time bracketing the Snowball Earth intervals were Until the report of benthic macroalgae from shales bracketed by diamic- predominantly microscopic and soft-bodied—not a good prospect for fos- tites of the Nantuo Formation in South China (Ye et al., 2015), the syn- silization. Corsetti et al. (2006) reviewed the paleontological record for glacial fossil record was sparse and microscopic. Filamentous microfos- Snowball Earth, showing that few fossils were known from within the gla- sils were described from dropstone-laden shales (Abu Mahara Group, cial deposits. Most data originated from immediately prior to, or after, the Oman,) including forms that exhibit a false-branching habit reminiscent extreme glaciation. With a few notable exceptions, the situation remains of multi-trichomous (Butterfield and Grotzinger, 2012). similar today, with very little data originating from within the glacial Syn-glacial deposits of Sturtian age (Australia, ) contain domi- deposits themselves. Therefore the paper by Ye et al. (2015, p. 507 in this nantly simple leiosphere acritarchs (Riedman et al., 2014), similar to the issue of ) brings important new paleontological evidence to our taxa directly below and above the glacial units. Other fossils have been understanding of Snowball Earth. reported, but the context of the syn-glacial microbiota from the Kings- ton Peak Formation (Corsetti et al., 2003) has recently been questioned Fossil Data Bracketing the Glacial Intervals (Macdonald et al., 2013), and other structures from syn-glacial strata are Organic-walled , predominantly from shales were relatively considered by some dubiofossils (e.g., Bavlinella, discussed in Butter- diverse by standards before the (see Ried- field and Grot­zinger [2012]). man et al., 2014, and references therein), including ornamented acritarchs (organic walled microfossils considered by most to represent eukaryotes). DISCUSSION Diversity declined to include simple leiosphere forms (unornamented Given the productivity of modern ice margin environments (e.g., Per- spheres that may have eukaryotic or bacterial affinities) before the Stur- rette et al., 2011), and the propensity of microbial forms to survive freez- tian glaciation (Nagy et al., 2009; Riedman et al., 2014), with this decline ing and thawing, it is not a stretch to envision a microbial biosphere sur- therefore decoupled from the extreme conditions hypothesized during the viving an extreme glaciation—possibly even photosynthetic microbiota,

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/43/6/559/3549104/559.pdf by guest on 28 September 2021 given our knowledge of microbiota in ice-covered Antarctic lakes (e.g., Hoffman, P.F., Kaufman, A.J., Halverson, G.P., and Schrag, D.P., 1998, A Neo- Andersen, et al., 2011). The filamentous microfossils with possible cya- proterozoic Snowball Earth: Science, v. 281, p. 1342–1346, doi:10.1126​ nobacterial affinities would suggest did take place during /science​.281​.5381.1342. Hoffman, P.F., and Schrag, D.P., 2002, The Snowball Earth hypothesis: Testing Snowball Earth, but the leiosphere-dominated assemblages (Riedman et the limits of global change: Terra Nova, v. 14, p. 129–155, doi:10.1046/j​ ​ al. 2014) carry less certain information, other than that marine microbial .1365​-3121​.2002.00408.x. life was present in the water column during Snowball Earth. Ye et al. Love, G.D., Grosjean, E., Stalvies, C., Fike, D.A., Grotzinger, J.P., Bradley, A.S., (2015) provide the most convincing evidence to date for photosynthesis Kelly, A.E., Bhatia, M., Bowring, S.A., Condon, D.J., and Summons, R.E., 2009, Fossil steroids record the appearance of Demospongiae during the during glacial times, and by large, benthic eukaryotes, no less—a form period: , v. 457, p. 718–721, doi:10.1038/nature07673. that would likely have required significant light and open water to survive Macdonald, F.A., Prave, A.R., Petterson, R., Smith, E.F., Pruss, S.B., Oates, K., versus microbial forms. Waechter, F., Trotzuk, D., and Fallick, A.E., 2013, The Laurentian record Although still sparse, the emerging picture of Neoproterozoic Snow- of Neoproterozoic glaciation, tectonism, and eukaryotic evolution in Death ball Earth painted by the fossil record is one of a relatively complex Valley, California: Geological Society of America Bulletin, v. 125, p. 1203– 1223, doi:10.1130/B30789.1. ecosystem that likely required times of open water and less extreme con- Maloof, A.C., Rose, C.V., Beach, R., Samuels, B.M., Calmet, C.C., Erwin, D.H., ditions than typically conjured when pondering Snowball Earth. Simply Poirier, G.R., Yao, N., and Simons, F.J., 2010, Possible -body fossils stated, benthic macroalgae need light and space (open water) (Ye et al., in pre-Marinoan from South Australia: Nature Geoscience, v. 3, 2015). As filter feeders, the record of sponges (Maloof et al. [2010] and p. 653–659, doi:10.1038/ngeo934. Marshall, A.O., Corsetti, F.A., Sessions, A.L., and Marshall, C.P., 2009, Raman Love et al. [2009]) would suggest a complex food web, as well. 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