Monday, September 2, 2013

Snowball Earth: A Frozen Planet

Some say the world will end in fire,
Some say in ice.
From what I’ve tasted of desire
I hold with those who favor fire.
But if it had to perish twice,
I think I know enough of hate
To say that for destruction ice
Is also great
And would suffice.
- Robert Frost, Fire and Ice (1923)

Rodinia is the name of a supercontinent that existed during the early Neoproterozoic era. It formed around 1100 Mya and involved the assembly of virtually all continental masses known to exist on Earth at that time. Rodinia was entirely barren since it existed at a time before life colonized dry land. Nevertheless, the continental margins of Rodinia may have played an important role in the development of life from ocean to land. Continental crust is considerably thicker than oceanic crust and the existence of a supercontinent insulated the underlying mantle. This fostered the development of a mantle superplume beneath Rodinia and eventually led to widespread continental rifting that resulted in the breaking up of Rodinia between 800 to 600 Mya (Meert and Torsvik, 2003).

Figure 1: Palaeogeographic reconstruction of the Rodinia supercontinent. The model posits two rifting events, one along the present-day western margin of Laurentia between 800 to 700 Mya, and a second along the present-day eastern margin of Laurentia between 600 to 550 Mya. (Meert and Torsvik, 2003)

Figure 2: Palaeogeographic reconstruction of the break-up and dispersal of the Rodinia supercontinent at 720 Mya. (Z.X. Li et al. 2008)

As Rodinia was starting to break-up, most of its continental masses happened to be grouped along the Equator. Such a continental configuration led to the development of a very unique climatic situation on Earth, characterised by intense evaporation and tropical rainfall on these continental areas. The intense rainfall washed out carbon dioxide from the atmosphere and produced carbonic acid which weathered exposed rocks on the continents. As a result, carbon dioxide, a greenhouse gas, was transferred from the atmosphere into the Earth’s crust and the Earth started to cool (Donnadieu et al., 2004). In addition, a tropical distribution of continents helped cool the Earth further because tropical continents are more reflective than open oceans and so absorb less of the Sun’s energy. In comparison, most of the Sun’s energy that is absorbed by present-day Earth occurs over tropical oceans.

As the Earth cooled, ice started to advance beyond the polar regions. When ice advanced to within 30° of the Equator a positive feedback mechanism known as the ice-albedo feedback ensured that the increased reflectivenss due to the formation of ice led to further cooling and the formation of yet more ice. This went on until the whole Earth became ice covered, probably right up to the Equator, creating what is commonly known as a “Snowball Earth”. During the Neoproterozoic era, the Earth may have experienced at least two snowball events at ~740 and ~635 Mya (Trindade and Macouin, 2007).

Figure 3: Artist’s impression of a Snowball Earth. Credit: Walter Myers

During a snowball event, global temperatures fell so low that the Equator may have been as cold as present-day Antarctica. The low temperatures were maintained by the high reflectivity of ice which reflected most of the incoming solar energy back into space. Since the Earth was almost completely ice covered, carbon dioxide could no longer be drawn out of the atmosphere by weathering of rocks. Over several million years, volcanos constantly emitted carbon dioxide which accumulated in the atmosphere. Evidence suggests that when the build-up of carbon dioxide exceeded ~10 percent of the atmosphere, the greenhouse effect became strong enough to thaw the Earth from a snowball state (Bao et al., 2008).

Figure 4: This chart illustrates a hypothetical depiction of a snowball event in terms of global mean surface temperature and ice cover (pale blue) on a palaeogeographic representation of the Earth at ~750 Mya. Note the abrupt onset and termination of glaciation at low-latitudes and the hot aftermath due to high levels of atmospheric carbon dioxide. The gradual temperature rise during the Snowball Earth event was due to the increasing greenhouse effect caused by the build up of atmospheric carbon dioxide from volcanic emissions.

There are pieces of good evidence to support the Snowball Earth hypothesis and that ice cover did extend into the tropics. Glacier deposits from the Neoproterozoic era with estimated palaeolatitudes based on palaeomagnetic data show a global distribution with a large fraction of deposits within 10° of the Equator. When an ice sheet moves over the ocean, rocks carried within the ice can become dislodged and fall onto the sediments on the ocean floor. These rocks become incorporated into the oceanic sediments and are known as dropstones. The presence of dropstones near the Equator indicates sea-level glaciation in the tropics during the Snowball Earth event.

Figure 5: A dropstone of quartzite embedded within sedimentary layers.

After the Snowball Earth event, the Earth’s surface is expected to become very warm due to the huge amount of carbon dioxide still present in the atmosphere. Elevated sea surface temperatures drove torrential rains that dissolve carbon dioxide and washed it out of the atmosphere as a weak carbonic acid. This weathered rocks on the continents and resulted in the release of large amounts of calcium that precipitated to form layers of carbonate sedimentary rocks. In fact, in the geological record, layers of carbonate rocks are indeed found to lie directly on top of glacier deposits. These layers of carbonate rocks are known as cap carbonates. The transition from glacier deposits to cap carbonates is abrupt and points towards a catastrophic collapse of the snowball state where the climate flipped rapidly from very cold to very hot.

Figure 6: Glacial deposits and cap carbonates in the Tillite Group of the East Greenland Caledonides.

Photosynthetic life is already known to exist billions of years before the Snowball Earth events of the Neoproterozoic era. There are a number of ways in which photosynthetic life in the ocean can continue to survive on a completely ice covered planet. One way involves the difference in how ice moves over land and ocean. Ice on land tends to be more locked while ice on the ocean tends to move more freely. Near continental margins, the difference in ice movement creates tension and can cause cracks to develop in the ice, opening up exposed bodies of ocean surface where photosynthetic life can thrive.

After the Snowball Earth event, the huge amount of erosional products being washed off the continents into the ocean by torrential rains fuelled a proliferation of photosynthetic life in the oceans and a corresponding leap in the amount of atmospheric oxygen. The increase in atmospheric oxygen concentration may have led to the rapid emergence of a huge variety of large, multi-cellular life, in what is now known as the Cambrian explosion around 550 Mya.

Figure 7: Artist’s impression of an Earth-like exoplanet. Credit: Scott Richard

The study of Earth-like exoplanets may help shed light on Earth’s geological past. For example, by studying a sample of Earth-like exoplanets in snowball states, it can show whether the grouping of continental masses along the Earth’s Equator during the Neoproterozoic era played a key role in kicking the Earth into a snowball state. However, mapping the continental distribution of an ice covered exoplanet is challenging and determining if one is indeed an Earth-like planet in a snowball state is going to be tricky. If the build up of atmospheric carbon dioxide created a sufficiently strong greenhouse effect to warm the Earth out of a snowball state, then Earth-like exoplanets in snowball states can be expected to show elevated levels of atmospheric carbon dioxide.

- Meert and Torsvik, “The making and unmaking of a supercontinent: Rodinia revisited”, Tectonophysics 375 (2003) 261-288
- Z.X. Li et al., “Assembly, configuration, and break-up history of Rodinia: A synthesis”, Precambrian Research 160 (2008) 179-210
- Donnadieu et al., “A ‘snowball Earth’ climate triggered by continental break-up through changes in runoff”, Nature 428, 303-306 (18 March 2004)
- Trindade and Macouin, “Palaeolatitude of glacial deposits and palaeogeography of Neoproterozoic ice ages”, Comptes Rendus Geoscience 339 (2007) 200-211
- Bao et al. (2008), “Triple oxygen isotope evidence for elevated CO2 levels after a Neoproterozoic glaciation”, Nature, 453 (7194), 504-506