Earth snowball

It is clear from the records of ice ages (see, e.g., Fig. 1-2) that Earth can have and has had climates that are different from our current state. Other, more extreme possibilities have been suggested, each of which could be stable for considerable periods of fime. The frozen "snowball Earth" already mentioned is analogous to present-day Mars, where its CO2 green-... 

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Snowball Earth Global glaciation events during which all, or nearly all, of Earth s surface, including the surface ocean, was covered by ice. 

Fig. 3.26 5 C-values for marine carbonates over time. Note persistent mean values of 0-3%c and anomaleous variability at 2.3 to 2.0 Ga and 0.8 to 0.6 Ga correlative with snowball earth episodes (Shields and Veizer, 2002)...

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Walker G. (2003). Snowball Earth The Story of the Great Global Catastrophe that Spawned Life as We Know It. Crown Publishers, Washington, D.C., 269 pp. 

One of the great virtues of the FREZCHEM model is its ability to examine complex chemistries. The number of independent components for the systems examined in this chapter range from four to eight. Earth seawater consisting of Na+, K+, Mg2+, Ca2+, Cl-, SO4-, and alkalinity has seven independent components (six salts and water). The most complex system evaluated is the snowball Earth seawater (eight independent components), which in addition to the above seven components also includes Fe2+. This ability to cope with complexity makes models like FREZCHEM more realistic in describing natural systems than simpler binary and ternary diagrams we demonstrate this point with data from Don Juan Pond, the most saline body of water on Earth. 

An aspect of seawater freezing that will play a role in our discussion of a snowball Earth (Sect. 5.1.3) is the quantity of water that remains unfrozen at subzero temperatures. For validation of this facet of the model, see Fig. 3.17. For a seawater system starting with 1.0kg water at 0°C, ice starts... 

Fig. 5.6. A summary of model inputs and outputs for snowball Earth and hothouse Earth scenarios. For minerals in boxes, down arrows signify precipitation, and up arrows signify dissolution...

We assumed fixed 02(g) partial pressures of 0.00 and 0.02 bars for snowball Earth and hothouse Earth, respectively. A low model value is necessary for snowball Earth because otherwise ferrous iron is oxidized to ferric iron, whose solubility is too low to account for banded iron formations. It is in the late Neoproterozoic era that atmospheric O2 rose to modern values (Holland, 2004). According to Canfield and Teske (1996), between 1.05 and 0.64 Ga ago, O2 levels were 5-18% of the modern value (0.2 bars). For hothouse Earth, we choose a level of 10% of the modern value. For our simulations, it is immaterial whether we used 0.002, 0.02, or 0.2 bars of O2 pressure. All are sufficient to completely oxidize ferrous to ferric iron. 

Table 5.2. Comparison of snowball Earth and hothouse Earth equilibrium oceanic compositions at 1 bar of pressure (assuming the ocean is saturated with dolomite)...

In our model, cap carbonates are explained as a result of high carbonate alkalinity in an ice-covered ocean, regardless of the specific mechanism for melting the snowball Earth. Ice could melt for a variety of reasons (e.g., greenhouse gases, a positive perturbation in solar forcing, a decrease in planetary albedo), but cap carbonates are expected to be produced as a result of the transition from high to low oceanic carbonate alkalinity. 

The third example of model outputs deals with gas hydrates (Table A.4), which was part of our snowball Earth simulations (Fig. 5.6). The Pco2, in this case, was set equal to 0.12 bars and was independent of total pressure this served to prevent Pco2 from becoming too high and beyond the validity... 

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