Accumulating evidence about snowball Earth

A hot topic in glacial sedimentology, the snowball Earth hypothesis involves the premise that several global ice ages have transpired during Earth's history. Although this theory is contested, it provides an explanation for the observation that diamictites, so-called 'glacial deposits', reside in Palaeoproterozoic and Neoproterozoic rocks across several continents.

Researchers have marshalled evidence for two major glacial activity intervals in Earth's history, the first occurring in the Palaeoproterozoic (approximately 2.4 Bya) and the second occurring during the Neoproterozoic (between 800 Mya and 600 Mya (Kirshvink et al., 2000)). Three main snowball Earth events have been reported from the Neoproterozoic: the Sturtian (approximately 725710 Mya), the Marinoan (approximately 635-600 Mya), and the Gaskiers (approximately 580 Mya (Narbonne, 2005)). These catastrophic events mark a time when global temperatures plummeted to -50 °C, causing the oceans to freeze, with sea ice thicknesses exceeding 1 km (a theory discussed in a recent paper by Eyles and Januszczack (2004)). These changes are thought to have halted the global hydrological cycle and terminated almost all biological activity (Kirshvink et al., 2002). As evidenced by the geological record, these conditions are thought to have continued for approximately 5 million years, until an equally catastrophic, volcanically generated 'greenhouse effect' caused a total reversal (Hoffman et al., 1998).

The most-cited data supporting the snowball Earth hypothesis are the aforementioned diamictites (Hoffman et al., 1998; Kirschvink et al., 2000; Hoffman and Schrag, 2002; Maconin et al., 2004). According to the dictum that current depositional environments may be used to interpret past environments, the similarity between the Neoproterozoic diamictites and present-day glacial tills is used as hard evidence for a global glaciation between 800 Mya and 600 Mya. Located unconformably atop the diamictites are 'cap carbonates', extensive carbonate rock deposits, termed 'Ccs' (Hoffman and Schrag, 2002; Sankaran, 2003;

Sheilds, 2005). Carbonate sequences typically comprise depositions that represent post-glacial sea-level rises, starting with warm, shallow-water deposits that grade to cool, deep-water shales (Sheilds, 2005). Unique to the Neoproterozoic Ccs, however, are the extreme and abnormal thicknesses that they have attained. In the Otavi group (Namibia), for instance, the carbonate sequence ranges in thickness from 300 to 400 m (Hoffman and Schrag, 2002). These thick sequences are explained as resulting from overturn in an anoxic, deep ocean coupled with accelerated chemical weathering from extraordinary greenhouse conditions that followed global glaciation (Sheilds, 2005).

Additional evidence for several anomalous geological occurrences that can be accounted for by the snowball Earth hypothesis also resides within the diamictite successions, themselves. Analyses involving S13C ratios (i.e., ratios quantifying 12C levels relative to 13C levels preserved in strata, which serve as markers for biomass levels) for these successions have revealed an extreme dip in biological activity at several points during the Neoproterozoic (W. Vincent, personal communication). In successions preceding the three main snowball Earth events during the Neoproterozoic, ¿13C levels dip well below normal and start to rise again afterward (Halverson et al., 2002, 2005). Perhaps most interestingly for the hypothesis presented here, the last snowball Earth event ended coincidentally with the Cambrian explosion, which also can be documented by changing ¿13C ratios (Hoffman and Schrag, 2002).

From a biological perspective, snowball Earth events would have imposed strong selection regimes on organisms. As described by Vincent et al. (2004), the initial freeze-up phase associated with snowball Earth events would have resulted in a mass extinction and a reduction in genetic diversity; thermophilic (heat-loving) organisms would have been able to survive only near deep-sea hydrothermal vents or geothermal hot springs and volcanoes. Such extreme conditions also would have favoured 'psychrotrophic' (cold-tolerant) organisms and 'psychrophilic' (cold-loving) organisms.

Research involving psychrotrophic and psychrophilic organisms currently is burgeoning progress and has spawned new-found interest in the Canadian high Arctic. Among the most-ambitious research efforts in this remote region is NASA's Haughton Mars project on Devon Island. Researchers consider this unique ecosystem, situated in the Haughton crater, as an analogue site for the ecological conditions that prevail on the red planet. They seek to identify, collect, and study the organisms living in the frigid environment. Elucidating the mechanisms by which these organisms survive will provide researchers with explanations for how organisms might evolve or might have evolved in similar ecosystems, like those found on Mars and Europa (see Chapter 15).

Fig. 11.2. Ward Hunt Island, Nunavut, Canada. The island is surrounded by the Ward Hunt Ice Shelf to the west (right) and the Markham Ice Shelf to the east (left). (Image captured by A. Pontefract.)

More domestically (from an institutional perspective), the Origins Institute, in a joint effort with Université Laval, has been involved in fieldwork conducted on Ward Hunt Island in Nunavut, Canada, for the past year (A. Pontefract, unpublished data; Figure 11.2). The island is located at 83° N latitude, several kilometres off the coast of Ellesmere Island. Ward Hunt Island was chosen as an ideal analogue site for a snowball Earth environment because extremely low temperatures are experienced by organisms living at that latitude. In addition, the island is surrounded by the Ward Hunt and Markham ice shelves (perennial ice-floats that reach up to 100 m in thickness), and therefore constitutes a unique ecosystem (D. Mueller, personal communication; Figure 11.3). The biota thereon live directly on the ice or snow or in meltwater pools in which sediment is present.

Researchers are interested primarily in determining the temperature tolerances for these organisms, with specific reference to cyanobacteria. Many organisms residing on the ice shelf are psychrotrophic rather than psychrophilic (as hypothesized (A. Pontefract, unpublished data; Vincent et al. 2004), Figure 11.4). Typical optimal growth temperatures for cyanobacteria range between 10 °C and 16 °C, with significant growth well past 20 °C (A. Pontefract, unpublished data). Secondary research interests include determining how pigment composition changes with varying temperature regimes and varying ultraviolet b (UVb) radiation levels and exploring microbes' abilities to survive the freeze-thaw process and extended dark-dark photoperiods.

Fig. 11.3. Ward Hunt Ice Shelf viewed aerially north-west from Ward Hunt Island. Crests and troughs are created on the surface through aerial ablation, creating a series of rivers running parallel to the coast-line. (Image captured by A. Pontefract.)
Fig. 11.4. Collecting microbial mat samples on Markham Ice Shelf. (Image captured by Denis Sarrazin.)

Among the more-surprising observations, however, was the discovery that metazoans, such as tardigrades, rotifers, and nematodes, live on the ice shelves! To survive thereon, these animals must withstand high-saline conditions and desiccation in addition to extremely low temperatures (A. Pontefract, unpublished data; Vincent et al. 2004). These extreme conditions constitute the basis on which hypotheses that the origin of metazoans was delayed until after the last snowball Earth event, when environmental conditions returned to 'normal', are postulated (Hoffman and Schrag, 2002; Kirshvink, 2002). The Metazoan presence on these ice shelves warrants that this idea should be re-evaluated and raises the possibility that the metazoan lineage underwent several 'revitalizations'.

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